update lua API to 1-based indices; add renderer

.github/workflows/ci.yaml

@@ -11,5 +11,6 @@ jobs:
       - uses: rhysd/action-setup-vim@v1
         with:
           neovim: true
-          version: v0.10.1
+          version: v0.11.0
+          arch: 'x86_64'
       - run: make test

.gitignore

@@ -1 +1,4 @@
+/.lux/
+lux.lock
 *.src.rock
+*.aider*

.luacheckrc

@@ -0,0 +1,2 @@
+-- :vim set ft=lua
+globals = { "vim" }

Makefile

@@ -3,7 +3,8 @@ PLENARY_DIR=~/.local/share/nvim/site/pack/test/opt/plenary.nvim
 all: lint test
 
 lint:
-	selene .
+	lua-language-server --check=lua/u/ --checklevel=Error
+	lx check
 
 fmt:
 	stylua .

README.md

@@ -1,64 +1,302 @@
 # u.nvim
 
-🚨🚨 **BRANCH NOTICE: further development is happening on the `v2` branch. In the future, `v2` will be merged into `master`. If you want to pin to an older version of this library, please refer to a specific commit, or the `v1` branch.** 🚨🚨
+Welcome to **u.nvim** - a powerful Lua library designed to enhance your text
+manipulation experience in NeoVim, focusing on text-manipulation utilities.
+This includes a `Range` utility, allowing you to work efficiently with text
+selections based on various conditions, as well as a declarative `Render`-er,
+making coding and editing more intuitive and productive.
 
-🚨🚨[CLICK HERE FOR v2](https://github.com/jrop/u.nvim/tree/v2)🚨🚨
-
-Welcome to **u.nvim** – a powerful Lua library designed to enhance your text manipulation experience in NeoVim, focusing primarily on a context-aware "Range" utility. This utility allows you to work efficiently with text selections based on various conditions, in a variety of contexts, making coding and editing more intuitive and productive.
-
-This is meant to be used as a **library**, not a plugin. On its own, `u.nvim` does nothing. It is meant to be used by plugin authors, to make their lives easier based on the variety of utilities I found I needed while growing my NeoVim config.
+This is meant to be used as a **library**, not a plugin. On its own, `u.nvim`
+does nothing. It is meant to be used by plugin authors, to make their lives
+easier based on the variety of utilities I found I needed while growing my
+NeoVim config. To get an idea of what a plugin built on top of `u.nvim` would
+look like, check out the [examples/](./examples/) directory.
 
 ## Features
 
-- **Range Utility**: Get context-aware selections with ease. Replace regions with new text. Think of it as a programmatic way to work with visual selections (or regions of text).
+- **Rendering System**: a utility that can declaratively render NeoVim-specific
+  hyperscript into a buffer, supporting creating/managing extmarks, highlights,
+  and key-event handling (requires NeoVim >0.11)
+- **Signals**: a simple dependency tracking system that pairs well with the
+  rendering utilities for creating reactive/interactive UIs in NeoVim.
+- **Range Utility**: Get context-aware selections with ease. Replace regions
+  with new text. Think of it as a programmatic way to work with visual
+  selections (or regions of text).
 - **Code Writer**: Write code with automatic indentation and formatting.
-- **Operator Key Mapping**: Flexible key mapping that works with the selected text.
-- **Text and Position Utilities**: Convenient functions to manage text objects and cursor positions.
+- **Operator Key Mapping**: Flexible key mapping that works with the selected
+  text.
+- **Text and Position Utilities**: Convenient functions to manage text objects
+  and cursor positions.
 
 ### Installation
 
-lazy.nvim:
+This being a library, and not a proper plugin, it is recommended that you
+vendor the specific version of this library that you need, including it in your
+code. Package managers are a developing landscape for Lua in the context of
+NeoVim. Perhaps in the future, `lux` will eliminate the need to vendor this
+library in your application code.
+
+## Signal and Rendering Usage
+
+### Overview
+
+The Signal and Rendering mechanisms are two subsystems of u.nvim, that, while
+simplistic, [compose](./examples/counter.lua) [together](./examples/filetree.lua)
+[powerfully](./examples/picker.lua) to create a system for interactive and
+responsive user interfaces. Here is a quick example that show-cases how easy it
+is to dive in to make any buffer an interactive UI:
+
+<details>
+<summary>Example Code: counter.lua</summary>
+
 ```lua
--- Setting `lazy = true` ensures that the library is only loaded
--- when `require 'u.<utility>' is called.
-{ 'jrop/u.nvim', lazy = true }
+local tracker = require 'u.tracker'
+local Buffer = require 'u.buffer'
+local h = require('u.renderer').h
+
+-- Create an buffer for the UI
+vim.cmd.vnew()
+local ui_buf = Buffer.current()
+ui_buf:set_tmp_options()
+
+local s_count = tracker.create_signal(0)
+
+-- Effect: Render
+-- Setup the effect for rendering the UI whenever dependencies are updated
+tracker.create_effect(function()
+  -- Calling `Signal:get()` in an effect registers the given signal as a
+  -- dependency of the current effect. Whenever that signal (or any other
+  -- dependency) changes, the effect will rerun. In this particular case,
+  -- rendering the UI is an effect that depends on one signal.
+  local count = s_count:get()
+
+  -- Markup is hyperscript, which is just 1) text, and 2) tags (i.e.,
+  -- constructed with `h(...)` calls). To help organize the markup, text and
+  -- tags can be nested in tables at any depth. Line breaks must be specified
+  -- manually, with '\n'.
+  ui_buf:render {
+    'Reactive Counter Example\n',
+    '========================\n\n',
+
+    { 'Counter: ', tostring(count), '\n' },
+
+    '\n',
+
+    {
+      h('text', {
+        hl = 'DiffDelete',
+        nmap = {
+          ['<CR>'] = function()
+            -- Update the contents of the s_count signal, notifying any
+            -- dependencies (in this case, the render effect):
+            vim.schedule(function()
+              s_count:update(function(n) return n - 1 end)
+            end)
+            -- Also equivalent: s_count:set(s_count:get() - 1)
+            return ''
+          end,
+        },
+      }, ' Decrement '),
+      ' ',
+      h('text', {
+        hl = 'DiffAdd',
+        nmap = {
+          ['<CR>'] = function()
+            -- Update the contents of the s_count signal, notifying any
+            -- dependencies (in this case, the render effect):
+            vim.schedule(function()
+              s_count:update(function(n) return n + 1 end)
+            end)
+            -- Also equivalent: s_count:set(s_count:get() - 1)
+            return ''
+          end,
+        },
+      }, ' Increment '),
+    },
+
+    '\n',
+    '\n',
+    { 'Press <CR> on each "button" above to increment/decrement the counter.' },
+  }
+end)
 ```
 
-## Usage
+</details>
+
+### `u.tracker`
+
+The `u.tracker` module provides a simple API for creating reactive variables.
+These can be composed in Effects and Memos utilizing Execution Contexts that
+track what signals are used by effects/memos.
+
+```lua
+local tracker = require('u.tracker')
+
+local s_number = tracker.Signal:new(0)
+-- auto-compute the double of the number each time it changes:
+local s_doubled = tracker.create_memo(function() return s_number:get() * 2 end)
+tracker.create_effect(function()
+  local n = s_doubled:get()
+  -- ...
+  -- whenever s_doubled changes, this function gets run
+end)
+```
+
+**Note**: circular dependencies are **not** supported.
+
+### `u.renderer`
+
+The renderer library renders hyperscript into a buffer. Each render performs a
+minimal set of changes in order to transform the current buffer text into the
+desired state.
+
+**Hyperscript** is just 1) _text_ 2) `<text>` tags, which can be nested in 3)
+Lua tables for readability:
+
+```lua
+local h = require('u.renderer').h
+-- Hyperscript can be organized into tables:
+{
+  "Hello, ",
+  {
+    "I am ", { "a" }, " nested table.",
+  },
+  '\n', -- newlines must be explicitly specified
+
+  -- booleans/nil are ignored:
+  some_conditional_flag and 'This text only shows when the flag is true',
+  -- e.g., use the above to show newlines in lists:
+  idx > 1 and '\n',
+
+  -- <text> tags are specified like so:
+  -- h('text', attributes, children)
+  h('text', {}, "I am a text node."),
+
+  -- <text> tags can be highlighted:
+  h('text', { hl = 'Comment' }, "I am highlighted."),
+
+  -- <text> tags can respond to key events:
+  h('text', {
+    hl = 'Keyword',
+    nmap = {
+      ["<CR>"] = function()
+        print("Hello World")
+        -- Return '' to swallow the event:
+        return ''
+      end,
+    },
+  }, "I am a text node."),
+}
+```
+
+Managing complex tables of hyperscript can be done more ergonomically using the
+`TreeBuilder` helper class:
+
+```lua
+local TreeBuilder = require('u.renderer').TreeBuilder
+
+-- ...
+renderer:render(
+  TreeBuilder.new()
+    -- text:
+    :put('some text')
+    -- hyperscript tables:
+    :put({ 'some text', 'more hyperscript' })
+    -- hyperscript tags:
+    :put_h('text', { --[[attributes]] }, { --[[children]] })
+    -- callbacks:
+    --- @param tb TreeBuilder
+    :nest(function(tb)
+      tb:put('some text')
+    end)
+    :tree()
+)
+```
+
+**Rendering**: The renderer library provides a `render` function that takes
+hyperscript in, and converts it to formatted buffer text:
+
+```lua
+local Renderer = require('u.renderer').Renderer
+local renderer = Renderer:new(0 --[[buffer number]])
+renderer:render {
+  -- ...hyperscript...
+}
+
+-- or, if you already have a buffer:
+local Buffer = require('u.buffer')
+local buf = Buffer.current()
+buf:render {
+  -- ...hyperscript...
+}
+```
+
+## Range Usage
 
 ### A note on indices
 
-I love NeoVim. I am coming to love Lua. I don't like 1-based indices; perhaps I am too old. Perhaps I am too steeped in the history of loving the elegance of simple pointer arithmetic. Regardless, the way positions are addressed in NeoVim/Vim is (terrifyingly) mixed. Some methods return 1-based, others accept only 0-based. In order to stay sane, I had to make a choice to store everything in one, uniform representation in this library. I chose (what I humbly think is the only sane way) to stick with the tried-and-true 0-based index scheme. That abstraction leaks into the public API of this library.
+<blockquote>
+<del>
+I love NeoVim. I am coming to love Lua. I don't like 1-based indices; perhaps I
+am too old. Perhaps I am too steeped in the history of loving the elegance of
+simple pointer arithmetic. Regardless, the way positions are addressed in
+NeoVim/Vim is (terrifyingly) mixed. Some methods return 1-based, others accept
+only 0-based. In order to stay sane, I had to make a choice to store everything
+in one, uniform representation in this library. I chose (what I humbly think is
+the only sane way) to stick with the tried-and-true 0-based index scheme. That
+abstraction leaks into the public API of this library.
+</del>
+</blockquote>
+
+<br />
+<b>This has changed in v2</b>. After much thought, I realized that:
+
+1. The 0-based indexing in NeoVim is prevelant in the `:api`, which is designed
+   to be exposed to many languages. As such, it makes sense for this interface
+   to use 0-based indexing. However, many internal Vim functions use 1-based
+   indexing.
+2. This is a Lua library (surprise, surprise, duh) - the idioms of the language
+   should take precedence over my preference
+3. There were subtle bugs in the code where indices weren't being normalized to
+   0-based, anyways. Somehow it worked most of the time.
+
+As such, this library now uses 1-based indexing everywhere, doing the necessary
+interop conversions when calling `:api` functions.
 
 ### 1. Creating a Range
 
-The `Range` utility is the main feature upon which most other things in this library are built, aside from a few standalone utilities. Ranges can be constructed manually, or preferably, obtained based on a variety of contexts.
+The `Range` utility is the main feature upon which most other things in this
+library are built, aside from a few standalone utilities. Ranges can be
+constructed manually, or preferably, obtained based on a variety of contexts.
 
 ```lua
 local Range = require 'u.range'
-local start = Pos.new(0, 0, 0) -- Line 1, first column
-local stop = Pos.new(0, 2, 0) -- Line 3, first column
+local start = Pos.new(0, 1, 1) -- Line 1, first column
+local stop = Pos.new(0, 3, 1) -- Line 3, first column
 
 Range.new(start, stop, 'v') -- charwise selection
 Range.new(start, stop, 'V') -- linewise selection
 ```
 
-This is usually not how you want to obtain a `Range`, however. Usually you want to get the corresponding context of an edit operation and just "get me the current Range that represents this context".
+This is usually not how you want to obtain a `Range`, however. Usually you want
+to get the corresponding context of an edit operation and just "get me the
+current Range that represents this context".
 
 ```lua
 -- get the first line in a buffer:
-Range.from_line(0, 0)
+Range.from_line(bufnr, 1)
 
 -- Text Objects (any text object valid in your configuration is supported):
 -- get the word the cursor is on:
-Range.from_text_object('iw')
+Range.from_motion('iw')
 -- get the WORD the cursor is on:
-Range.from_text_object('iW')
+Range.from_motion('iW')
 -- get the "..." the cursor is within:
-Range.from_text_object('a"')
+Range.from_motion('a"')
 
 -- Get the currently visually selected text:
--- NOTE: this does NOT work within certain contexts; more specialized utilities are more appropriate in certain circumstances
+-- NOTE: this does NOT work within certain contexts; more specialized utilities
+-- are more appropriate in certain circumstances
 Range.from_vtext()
 
 --
@@ -70,7 +308,8 @@ function MyOpFunc(ty)
   local range = Range.from_op_func(ty)
   -- do something with the range
 end
--- Try invoking this with: `<Leader>toaw`, and the current word will be the context:
+-- Try invoking this with: `<Leader>toaw`, and the current word will be the
+-- context:
 vim.keymap.set('<Leader>to', function()
   vim.g.operatorfunc = 'v:lua.MyOpFunc'
   return 'g@'
@@ -79,7 +318,8 @@ end, { expr = true })
 --
 -- Commands:
 --
--- When executing commands in a visual context, getting the selected text has to be done differently:
+-- When executing commands in a visual context, getting the selected text has
+-- to be done differently:
 vim.api.nvim_create_user_command('MyCmd', function(args)
   local range = Range.from_cmd_args(args)
   if range == nil then
@@ -90,14 +330,15 @@ vim.api.nvim_create_user_command('MyCmd', function(args)
 end, { range = true })
 ```
 
-So far, that's a lot of ways to _get_ a `Range`. But what can you do with a range once you have one? Plenty, it turns out!
+So far, that's a lot of ways to _get_ a `Range`. But what can you do with a
+range once you have one? Plenty, it turns out!
 
 ```lua
 local range = ...
 range:lines() -- get the lines in the range's region
 range:text() -- get the text (i.e., string) in the range's region
-range:line0(0) -- get the first line within this range
-range:line0(-1) -- get the last line within this range
+range:line(1) -- get the first line within this range
+range:line(-1) -- get the last line within this range
 -- replace with new contents:
 range:replace {
   'replacement line 1',
@@ -140,14 +381,15 @@ cw:write('}')
 
 #### Custom Text Objects
 
-Simply by returning a `Range` or a `Pos`, you can easily and quickly define your own text objects:
+Simply by returning a `Range` or a `Pos`, you can easily and quickly define
+your own text objects:
 
 ```lua
-local utils = require 'u.utils'
+local txtobj = require 'u.txtobj'
 local Range = require 'u.range'
 
 -- Select whole file:
-utils.define_text_object('ag', function()
+txtobj.define('ag', function()
   return Range.from_buf_text()
 end)
 ```
@@ -159,27 +401,39 @@ Access and manipulate buffers easily:
 ```lua
 local Buffer = require 'u.buffer'
 local buf = Buffer.current()
+buf.b.<option>        -- get buffer-local variables
+buf.b.<option>  = ... -- set buffer-local variables
+buf.bo.<option>       -- get buffer options
+buf.bo.<option> = ... -- set buffer options
 buf:line_count()      -- the number of lines in the current buffer
-buf:get_option '...'
-buf:set_option('...', ...)
-buf:get_var '...'
-buf:set_var('...', ...)
 buf:all()             -- returns a Range representing the entire buffer
 buf:is_empty()        -- returns true if the buffer has no text
 buf:append_line '...'
-buf:line0(0) -- returns a Range representing the first line in the buffer
-buf:line0(-1) -- returns a Range representing the last line in the buffer
-buf:lines(0, 1) -- returns a Range representing the first two lines in the buffer
-buf:lines(1, -2) -- returns a Range representing all but the first and last lines of a buffer
-buf:text_object('iw') -- returns a Range representing the text object 'iw' in the give buffer
+buf:line(1)           -- returns a Range representing the first line in the buffer
+buf:line(-1)          -- returns a Range representing the last line in the buffer
+buf:lines(1, 2)       -- returns a Range representing the first two lines in the buffer
+buf:lines(2, -2)      -- returns a Range representing all but the first and last lines of a buffer
+buf:txtobj('iw')      -- returns a Range representing the text object 'iw' in the give buffer
 ```
 
 ## License (MIT)
 
 Copyright (c) 2024 jrapodaca@gmail.com
 
-Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
 
-The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
 
-THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

examples/counter.lua

@@ -0,0 +1,65 @@
+local tracker = require 'u.tracker'
+local Buffer = require 'u.buffer'
+local h = require('u.renderer').h
+
+-- Create an buffer for the UI
+vim.cmd.vnew()
+local ui_buf = Buffer.current()
+ui_buf:set_tmp_options()
+
+local s_count = tracker.create_signal(0, 'counter_signal')
+
+-- Effect: Render
+-- Setup the effect for rendering the UI whenever dependencies are updated
+tracker.create_effect(function()
+  -- Calling `Signal:get()` in an effect registers the given signal as a
+  -- dependency of the current effect. Whenever that signal (or any other
+  -- dependency) changes, the effect will rerun. In this particular case,
+  -- rendering the UI is an effect that depends on one signal.
+  local count = s_count:get()
+
+  -- Markup is hyperscript, which is just 1) text, and 2) tags (i.e.,
+  -- constructed with `h(...)` calls). To help organize the markup, text and
+  -- tags can be nested in tables at any depth. Line breaks must be specified
+  -- manually, with '\n'.
+  ui_buf:render {
+    'Reactive Counter Example\n',
+    '========================\n\n',
+
+    { 'Counter: ', tostring(count), '\n' },
+
+    '\n',
+
+    {
+      h('text', {
+        hl = 'DiffDelete',
+        nmap = {
+          ['<CR>'] = function()
+            -- Update the contents of the s_count signal, notifying any
+            -- dependencies (in this case, the render effect):
+            s_count:schedule_update(function(n) return n - 1 end)
+            -- Also equivalent: s_count:schedule_set(s_count:get() - 1)
+            return ''
+          end,
+        },
+      }, ' Decrement '),
+      ' ',
+      h('text', {
+        hl = 'DiffAdd',
+        nmap = {
+          ['<CR>'] = function()
+            -- Update the contents of the s_count signal, notifying any
+            -- dependencies (in this case, the render effect):
+            s_count:schedule_update(function(n) return n + 1 end)
+            -- Also equivalent: s_count:schedule_set(s_count:get() - 1)
+            return ''
+          end,
+        },
+      }, ' Increment '),
+    },
+
+    '\n',
+    '\n',
+    { 'Press <CR> on each "button" above to increment/decrement the counter.' },
+  }
+end)

examples/filetree.lua

@@ -0,0 +1,468 @@
+--------------------------------------------------------------------------------
+-- File Tree Viewer Module
+--
+-- Future Enhancements:
+-- - Consider implementing additional features like searching for files,
+--   filtering displayed nodes, or adding support for more file types.
+-- - Improve user experience with customizable UI elements and enhanced
+--   navigation options.
+-- - Implement a file watcher to automatically update the file tree when files
+--   change on the underlying filesystem.
+--------------------------------------------------------------------------------
+
+--- @alias FsDir { kind: 'dir'; path: string; expanded: boolean; children: FsNode[] }
+--- @alias FsFile { kind: 'file'; path: string }
+--- @alias FsNode FsDir | FsFile
+--- @alias ShowOpts { root_path?: string, width?: number, focus_path?: string }
+
+local Buffer = require 'u.buffer'
+local Renderer = require('u.renderer').Renderer
+local TreeBuilder = require('u.renderer').TreeBuilder
+local h = require('u.renderer').h
+local tracker = require 'u.tracker'
+
+local logger = require('u.logger').Logger.new 'filetree'
+
+local M = {}
+local H = {}
+
+--------------------------------------------------------------------------------
+-- Helpers:
+--------------------------------------------------------------------------------
+
+--- Splits the given path into a list of path components.
+--- @param path string
+function H.split_path(path)
+  local parts = {}
+  local curr = path
+  while #curr > 0 and curr ~= '.' and curr ~= '/' do
+    table.insert(parts, 1, vim.fs.basename(curr))
+    curr = vim.fs.dirname(curr)
+  end
+  return parts
+end
+
+--- Normalizes the given path to an absolute path.
+--- @param path string
+function H.normalize(path) return vim.fs.abspath(vim.fs.normalize(path)) end
+
+--- Computes the relative path from `base` to `path`.
+--- @param path string
+--- @param base string
+function H.relative(path, base)
+  path = H.normalize(path)
+  base = H.normalize(base)
+  if path:sub(1, #base) == base then path = path:sub(#base + 1) end
+  if vim.startswith(path, '/') then path = path:sub(2) end
+  return path
+end
+
+--- @param root_path string
+--- @return { tree: FsDir; path_to_node: table<string, FsNode> }
+function H.get_tree_inf(root_path)
+  logger:info { 'get_tree_inf', root_path }
+  --- @type table<string, FsNode>
+  local path_to_node = {}
+
+  --- @type FsDir
+  local tree = {
+    kind = 'dir',
+    path = H.normalize(root_path or '.'),
+    expanded = true,
+    children = {},
+  }
+  path_to_node[tree.path] = tree
+
+  H.populate_dir_children(tree, path_to_node)
+  return { tree = tree, path_to_node = path_to_node }
+end
+
+--- @param tree FsDir
+--- @param path_to_node table<string, FsNode>
+function H.populate_dir_children(tree, path_to_node)
+  tree.children = {}
+
+  for child_path, kind in vim.iter(vim.fs.dir(tree.path, { depth = 1 })) do
+    child_path = H.normalize(vim.fs.joinpath(tree.path, child_path))
+    local prev_node = path_to_node[child_path]
+
+    if kind == 'directory' then
+      local new_node = {
+        kind = 'dir',
+        path = child_path,
+        expanded = prev_node and prev_node.expanded or false,
+        children = prev_node and prev_node.children or {},
+      }
+      path_to_node[new_node.path] = new_node
+      table.insert(tree.children, new_node)
+    else
+      local new_node = {
+        kind = 'file',
+        path = child_path,
+      }
+      path_to_node[new_node.path] = new_node
+      table.insert(tree.children, new_node)
+    end
+  end
+
+  table.sort(tree.children, function(a, b)
+    -- directories first:
+    if a.kind ~= b.kind then return a.kind == 'dir' end
+    return a.path < b.path
+  end)
+end
+
+--- @param opts {
+---   bufnr: number;
+---   prev_winnr: number;
+---   root_path: string;
+---   focus_path?: string;
+--- }
+---
+--- @return { expand: fun(path: string), collapse: fun(path: string) }
+local function _render_in_buffer(opts)
+  local winnr = vim.api.nvim_buf_call(
+    opts.bufnr,
+    function() return vim.api.nvim_get_current_win() end
+  )
+  local s_tree_inf = tracker.create_signal(H.get_tree_inf(opts.root_path))
+  local s_focused_path = tracker.create_signal(H.normalize(opts.focus_path or opts.root_path))
+
+  tracker.create_effect(function()
+    local focused_path = s_focused_path:get()
+
+    s_tree_inf:update(function(tree_inf)
+      local parts = H.split_path(H.relative(focused_path, tree_inf.tree.path))
+      local path_to_node = tree_inf.path_to_node
+
+      --- @param node FsDir
+      --- @param child_names string[]
+      local function expand_to(node, child_names)
+        if #child_names == 0 then return end
+        node.expanded = true
+
+        local next_child_name = table.remove(child_names, 1)
+        for _, child in ipairs(node.children) do
+          if child.kind == 'dir' and vim.fs.basename(child.path) == next_child_name then
+            H.populate_dir_children(child, path_to_node)
+            expand_to(child, child_names)
+          end
+        end
+      end
+      expand_to(tree_inf.tree, parts)
+      return tree_inf
+    end)
+  end)
+
+  --
+  -- :help watch-file
+  --
+  local watcher = vim.uv.new_fs_event()
+  if watcher ~= nil then
+    --- @diagnostic disable-next-line: unused-local
+    watcher:start(opts.root_path, { recursive = true }, function(_err, fname, _status)
+      fname = H.normalize(fname)
+
+      local dir_path = vim.fs.dirname(fname)
+      local dir = s_tree_inf:get().path_to_node[dir_path]
+      if not dir then return end
+
+      s_tree_inf:schedule_update(function(tree_inf)
+        H.populate_dir_children(dir, tree_inf.path_to_node)
+        return tree_inf
+      end)
+    end)
+  end
+  vim.api.nvim_create_autocmd('WinClosed', {
+    once = true,
+    pattern = tostring(winnr),
+    callback = function()
+      if watcher == nil then return end
+
+      watcher:stop()
+      watcher = nil
+    end,
+  })
+
+  local controller = {}
+
+  --- @param path string
+  function controller.focus_path(path) s_focused_path:set(H.normalize(path)) end
+
+  function controller.refresh() s_tree_inf:set(H.get_tree_inf(opts.root_path)) end
+
+  --- @param path string
+  function controller.expand(path)
+    path = H.normalize(path)
+    local path_to_node = s_tree_inf:get().path_to_node
+
+    local node = path_to_node[path]
+    if node == nil then return end
+
+    if node.kind == 'dir' then
+      s_tree_inf:update(function(tree_inf2)
+        H.populate_dir_children(node, path_to_node)
+        tree_inf2.path_to_node[node.path].expanded = true
+        return tree_inf2
+      end)
+      if #node.children == 0 then
+        s_focused_path:set(node.path)
+      else
+        s_focused_path:set(node.children[1].path)
+      end
+    else
+      if node.kind == 'file' then
+        -- open file:
+        vim.api.nvim_win_call(opts.prev_winnr, function() vim.cmd.edit(node.path) end)
+        vim.api.nvim_set_current_win(opts.prev_winnr)
+      end
+    end
+  end
+
+  --- @param path string
+  function controller.collapse(path)
+    path = H.normalize(path)
+    local path_to_node = s_tree_inf:get().path_to_node
+
+    local node = path_to_node[path]
+    if node == nil then return end
+
+    if node.kind == 'dir' then
+      if node.expanded then
+        -- collapse self/node:
+        s_focused_path:set(node.path)
+        s_tree_inf:update(function(tree_inf2)
+          tree_inf2.path_to_node[node.path].expanded = false
+          return tree_inf2
+        end)
+      else
+        -- collapse parent:
+        local parent_dir = path_to_node[vim.fs.dirname(node.path)]
+        if parent_dir ~= nil then
+          s_focused_path:set(parent_dir.path)
+          s_tree_inf:update(function(tree_inf2)
+            tree_inf2.path_to_node[parent_dir.path].expanded = false
+            return tree_inf2
+          end)
+        end
+      end
+    elseif node.kind == 'file' then
+      local parent_dir = path_to_node[vim.fs.dirname(node.path)]
+      if parent_dir ~= nil then
+        s_focused_path:set(parent_dir.path)
+        s_tree_inf:update(function(tree_inf2)
+          tree_inf2.path_to_node[parent_dir.path].expanded = false
+          return tree_inf2
+        end)
+      end
+    end
+  end
+
+  --- @param root_path string
+  function controller.new(root_path)
+    vim.ui.input({
+      prompt = 'New: ',
+      completion = 'file',
+    }, function(input)
+      if input == nil then return end
+      local new_path = vim.fs.joinpath(root_path, input)
+
+      if vim.endswith(input, '/') then
+        -- Create a directory:
+        vim.fn.mkdir(new_path, input, 'p')
+      else
+        -- Create a file:
+
+        -- First, make sure the parent directory exists:
+        vim.fn.mkdir(vim.fs.dirname(new_path), 'p')
+
+        -- Now create an empty file:
+        local uv = vim.loop or vim.uv
+        local fd = uv.fs_open(new_path, 'w', 438)
+        if fd then uv.fs_write(fd, '') end
+      end
+
+      controller.refresh()
+      controller.focus_path(new_path)
+    end)
+  end
+
+  --- @param path string
+  function controller.rename(path)
+    path = H.normalize(path)
+    local root_path = vim.fs.dirname(path)
+    vim.ui.input({
+      prompt = 'Rename: ',
+      default = vim.fs.basename(path),
+      completion = 'file',
+    }, function(input)
+      if input == nil then return end
+
+      local new_path = vim.fs.joinpath(root_path, input);
+      (vim.loop or vim.uv).fs_rename(path, new_path)
+      controller.refresh()
+      controller.focus_path(new_path)
+    end)
+  end
+
+  --
+  -- Render:
+  --
+  local renderer = Renderer.new(opts.bufnr)
+  tracker.create_effect(function()
+    --- @type { tree: FsDir; path_to_node: table<string, FsNode> }
+    local tree_inf = s_tree_inf:get()
+    local tree = tree_inf.tree
+
+    --- @type string
+    local focused_path = s_focused_path:get()
+
+    --- As we render the tree, keep track of what line each node is on, so that
+    --- we have an easy way to make the cursor jump to each node (i.e., line)
+    --- at will:
+    --- @type table<string, number>
+    local node_lines = {}
+    local current_line = 0
+
+    --- The UI is rendered as a list of hypserscript elements:
+    local tb = TreeBuilder.new()
+
+    --- Since the filesystem is a recursive tree of nodes, we need to
+    --- recursively render each node. This function does just that:
+    --- @param node FsNode
+    --- @param level number
+    local function render_node(node, level)
+      local name = vim.fs.basename(node.path)
+      current_line = current_line + 1
+      node_lines[node.path] = current_line
+
+      local nmaps = {
+        h = function()
+          vim.schedule(function() controller.collapse(node.path) end)
+          return ''
+        end,
+        l = function()
+          vim.schedule(function() controller.expand(node.path) end)
+          return ''
+        end,
+        n = function()
+          vim.schedule(
+            function()
+              controller.new(node.kind == 'file' and vim.fs.dirname(node.path) or node.path)
+            end
+          )
+          return ''
+        end,
+        r = function()
+          vim.schedule(function() controller.rename(node.path) end)
+          return ''
+        end,
+        y = function()
+          vim.fn.setreg([["]], H.relative(node.path, tree.path))
+          return ''
+        end,
+      }
+
+      if node.kind == 'dir' then
+        --
+        -- Render a directory node:
+        --
+        local icon = node.expanded and '' or ''
+        tb:put {
+          current_line > 1 and '\n',
+          h(
+            'text',
+            { hl = 'Constant', nmap = nmaps },
+            { string.rep('  ', level), icon, ' ', name }
+          ),
+        }
+        if node.expanded then
+          for _, child in ipairs(node.children) do
+            render_node(child, level + 1)
+          end
+        end
+      elseif node.kind == 'file' then
+        tb:put {
+          current_line > 1 and '\n',
+          h('text', { nmap = nmaps }, { string.rep('  ', level), '󰈔 ', name }),
+        }
+      end
+    end
+    render_node(tree, 0)
+
+    -- The following modifies buffer contents, so it needs to be scheduled:
+    vim.schedule(function()
+      renderer:render(tb:tree())
+
+      local cpos = vim.api.nvim_win_get_cursor(winnr)
+      pcall(vim.api.nvim_win_set_cursor, winnr, { node_lines[focused_path], cpos[2] })
+    end)
+  end, 's:tree')
+
+  return controller
+end
+
+--------------------------------------------------------------------------------
+-- Public API functions:
+--------------------------------------------------------------------------------
+
+--- @type {
+---   bufnr: number;
+---   winnr: number;
+---   controller: { expand: fun(path: string), collapse: fun(path: string) };
+--- } | nil
+local current_inf = nil
+
+--- Show the filetree:
+--- @param opts? ShowOpts
+function M.show(opts)
+  if current_inf ~= nil then return current_inf.controller end
+  opts = opts or {}
+
+  local prev_winnr = vim.api.nvim_get_current_win()
+
+  vim.cmd 'vnew'
+  local buf = Buffer.from_nr(vim.api.nvim_get_current_buf())
+  buf:set_tmp_options()
+
+  local winnr = vim.api.nvim_get_current_win()
+  vim.api.nvim_feedkeys(vim.api.nvim_replace_termcodes('<C-w>H', true, true, true), 'x', false)
+  vim.api.nvim_win_set_width(0, opts.width or 30)
+  vim.api.nvim_create_autocmd('WinClosed', {
+    once = true,
+    pattern = tostring(winnr),
+    callback = M.hide,
+  })
+
+  vim.wo[0][0].number = false
+  vim.wo[0][0].relativenumber = false
+
+  local bufnr = vim.api.nvim_get_current_buf()
+
+  local controller = _render_in_buffer(vim.tbl_extend('force', opts, {
+    bufnr = bufnr,
+    prev_winnr = prev_winnr,
+    root_path = opts.root_path or H.normalize '.',
+  }))
+  current_inf = { bufnr = bufnr, winnr = winnr, controller = controller }
+  return controller
+end
+
+--- Hide the filetree:
+function M.hide()
+  if current_inf == nil then return end
+  pcall(vim.cmd.bdelete, current_inf.bufnr)
+  current_inf = nil
+end
+
+--- Toggle the filetree:
+--- @param opts? ShowOpts
+function M.toggle(opts)
+  if current_inf == nil then
+    M.show(opts)
+  else
+    M.hide()
+  end
+end
+
+return M

examples/notify.lua

@@ -0,0 +1,137 @@
+local Buffer = require 'u.buffer'
+local TreeBuilder = require('u.renderer').TreeBuilder
+local tracker = require 'u.tracker'
+local utils = require 'u.utils'
+local Window = require 'my.window'
+
+local TIMEOUT = 4000
+local ICONS = {
+  [vim.log.levels.TRACE] = { text = '󰃤', group = 'DiagnosticSignOk' },
+  [vim.log.levels.DEBUG] = { text = '󰃤', group = 'DiagnosticSignOk' },
+  [vim.log.levels.INFO] = { text = '', group = 'DiagnosticSignInfo' },
+  [vim.log.levels.WARN] = { text = '', group = 'DiagnosticSignWarn' },
+  [vim.log.levels.ERROR] = { text = '', group = 'DiagnosticSignError' },
+}
+local DEFAULT_ICON = { text = '', group = 'DiagnosticSignOk' }
+
+--- @alias Notification {
+---   kind: number;
+---   id: number;
+---   text: string;
+--- }
+
+local M = {}
+
+--- @type Window | nil
+local notifs_w
+
+local s_notifications_raw = tracker.create_signal {}
+local s_notifications = s_notifications_raw:debounce(50)
+
+-- Render effect:
+tracker.create_effect(function()
+  --- @type Notification[]
+  local notifs = s_notifications:get()
+
+  if #notifs == 0 then
+    if notifs_w then
+      notifs_w:close(true)
+      notifs_w = nil
+    end
+    return
+  end
+
+  vim.schedule(function()
+    local editor_size = utils.get_editor_dimensions()
+    local avail_width = editor_size.width
+    local float_width = 40
+    local win_config = {
+      relative = 'editor',
+      anchor = 'NE',
+      row = 0,
+      col = avail_width,
+      width = float_width,
+      height = math.min(#notifs, editor_size.height - 3),
+      border = 'single',
+      focusable = false,
+    }
+    if not notifs_w or not vim.api.nvim_win_is_valid(notifs_w.win) then
+      notifs_w = Window.new(Buffer.create(false, true), win_config)
+      vim.wo[notifs_w.win].cursorline = false
+      vim.wo[notifs_w.win].list = false
+      vim.wo[notifs_w.win].listchars = ''
+      vim.wo[notifs_w.win].number = false
+      vim.wo[notifs_w.win].relativenumber = false
+      vim.wo[notifs_w.win].wrap = false
+    else
+      notifs_w:set_config(win_config)
+    end
+
+    notifs_w:render(TreeBuilder.new()
+      :nest(function(tb)
+        for idx, notif in ipairs(notifs) do
+          if idx > 1 then tb:put '\n' end
+
+          local notif_icon = ICONS[notif.kind] or DEFAULT_ICON
+          tb:put_h('text', { hl = notif_icon.group }, notif_icon.text)
+          tb:put { '  ', notif.text }
+        end
+      end)
+      :tree())
+    vim.api.nvim_win_call(notifs_w.win, function()
+      -- scroll to bottom:
+      vim.cmd.normal 'G'
+      -- scroll all the way to the left:
+      vim.cmd.normal '9999zh'
+    end)
+  end)
+end)
+
+local _orig_notify
+
+--- @param msg string
+--- @param level integer|nil
+--- @param opts table|nil
+local function my_notify(msg, level, opts)
+  vim.schedule(function() _orig_notify(msg, level, opts) end)
+  if level == nil then level = vim.log.levels.INFO end
+  if level < vim.log.levels.INFO then return end
+
+  local id = math.random(math.huge)
+
+  --- @param notifs Notification[]
+  s_notifications_raw:schedule_update(function(notifs)
+    table.insert(notifs, { kind = level, id = id, text = msg })
+    return notifs
+  end)
+
+  vim.defer_fn(function()
+    --- @param notifs Notification[]
+    s_notifications_raw:schedule_update(function(notifs)
+      for i, notif in ipairs(notifs) do
+        if notif.id == id then
+          table.remove(notifs, i)
+          break
+        end
+      end
+      return notifs
+    end)
+  end, TIMEOUT)
+end
+
+local _once_msgs = {}
+local function my_notify_once(msg, level, opts)
+  if vim.tbl_contains(_once_msgs, msg) then return false end
+  table.insert(_once_msgs, msg)
+  vim.notify(msg, level, opts)
+  return true
+end
+
+function M.setup()
+  if _orig_notify == nil then _orig_notify = vim.notify end
+
+  vim.notify = my_notify
+  vim.notify_once = my_notify_once
+end
+
+return M

examples/picker.lua

@@ -0,0 +1,949 @@
+local utils = require 'u.utils'
+local Buffer = require 'u.buffer'
+local Renderer = require('u.renderer').Renderer
+local h = require('u.renderer').h
+local TreeBuilder = require('u.renderer').TreeBuilder
+local tracker = require 'u.tracker'
+
+local M = {}
+
+local S_EDITOR_DIMENSIONS =
+  tracker.create_signal(utils.get_editor_dimensions(), 's:editor_dimensions')
+vim.api.nvim_create_autocmd('VimResized', {
+  callback = function()
+    local new_dim = utils.get_editor_dimensions()
+    S_EDITOR_DIMENSIONS:set(new_dim)
+  end,
+})
+
+--- @param low number
+--- @param x number
+--- @param high number
+local function clamp(low, x, high)
+  x = math.max(low, x)
+  x = math.min(x, high)
+  return x
+end
+
+--- @generic T
+--- @param arr `T`[]
+--- @return T[]
+local function shallow_copy_arr(arr) return vim.iter(arr):totable() end
+
+--------------------------------------------------------------------------------
+-- BEGIN create_picker
+--
+-- This is the star of the show (in this file, anyway).
+-- In summary, the outline of this function is:
+-- 1. Setup signals/memos for computing the picker size, and window positions
+-- 2. Create the two windows:
+--   a. The picker input. This is where the filter is typed
+--   b. The picker list. This is where the items are displayed
+-- 3. Setup event handlers that respond to user input
+-- 4. Render the list. After all the prework above, this is probably the
+--    shortest portion of this function.
+--------------------------------------------------------------------------------
+
+--- @alias SelectController {
+---   get_items: fun(): T[];
+---   set_items: fun(items: T[]);
+---   set_filter_text: fun(filter_text: string);
+---   get_selected_indices: fun(): number[];
+---   get_selected_items: fun(): T[];
+---   set_selected_indices: fun(indicies: number[], ephemeral?: boolean);
+---   close: fun();
+--- }
+--- @alias SelectOpts<T> {
+---   items: `T`[];
+---   multi?: boolean;
+---   format_item?: fun(item: T): Tree;
+---   on_finish?: fun(items: T[], indicies: number[]);
+---   on_selection_changed?: fun(items: T[], indicies: number[]);
+---   mappings?: table<string, fun(select: SelectController)>;
+--- }
+
+--- @generic T
+--- @param opts SelectOpts<T>
+function M.create_picker(opts) -- {{{
+  local is_in_insert_mode = vim.api.nvim_get_mode().mode:sub(1, 1) == 'i'
+  local stopinsert = not is_in_insert_mode
+
+  if opts.multi == nil then opts.multi = false end
+
+  local H = {}
+
+  --- Runs a function `fn`, and if it fails, cleans up the UI by calling
+  --- `H.finish`
+  ---
+  --- @generic T
+  --- @param fn fun(): `T`
+  --- @return T
+  local function safe_run(fn, ...)
+    local ok, result_or_error = pcall(fn, ...)
+    if not ok then
+      pcall(H.finish, true, result_or_error)
+      error(result_or_error .. '\n' .. debug.traceback())
+    end
+    return result_or_error
+  end
+
+  --- Creates a function that safely calls the given function, cleaning up the
+  --- UI if it ever fails
+  ---
+  --- @generic T
+  --- @param fn `T`
+  --- @return T
+  local function safe_wrap(fn)
+    return function(...) return safe_run(fn, ...) end
+  end
+
+  --
+  -- Compute the positions of the input bar and the list:
+  --
+
+  -- Reactively compute the space available for the picker based on the size of
+  -- the editor
+  local s_editor_dimensions = S_EDITOR_DIMENSIONS:clone()
+  local s_picker_space_available = tracker.create_memo(safe_wrap(function()
+    local editor_dim = s_editor_dimensions:get()
+    local width = math.floor(editor_dim.width * 0.75)
+    local height = math.floor(editor_dim.height * 0.75)
+    local row = math.floor((editor_dim.height - height) / 2)
+    local col = math.floor((editor_dim.width - width) / 2)
+    return { width = width, height = height, row = row, col = col }
+  end))
+
+  -- Reactively compute the size of the prompt (input) bar
+  local s_w_input_coords = tracker.create_memo(safe_wrap(function()
+    local picker_coords = s_picker_space_available:get()
+    return {
+      width = picker_coords.width,
+      height = 1,
+      row = picker_coords.row,
+      col = picker_coords.col,
+    }
+  end))
+
+  -- Reactively compute the size of the list view
+  local s_w_list_coords = tracker.create_memo(safe_wrap(function()
+    local picker_coords = s_picker_space_available:get()
+    return {
+      width = picker_coords.width,
+      height = picker_coords.height - 3,
+      row = picker_coords.row + 3,
+      col = picker_coords.col,
+    }
+  end))
+
+  --
+  -- Create resources (i.e., windows):
+  --
+
+  local w_input_cfg = {
+    width = s_w_input_coords:get().width,
+    height = s_w_input_coords:get().height,
+    row = s_w_input_coords:get().row,
+    col = s_w_input_coords:get().col,
+    relative = 'editor',
+    focusable = true,
+    border = vim.o.winborder or 'rounded',
+  }
+  local w_input_buf = Buffer.create(false, true)
+  local w_input = vim.api.nvim_open_win(w_input_buf.bufnr, false, w_input_cfg)
+  vim.wo[w_input][0].cursorline = false
+  vim.wo[w_input][0].list = false
+  vim.wo[w_input][0].number = false
+  vim.wo[w_input][0].relativenumber = false
+
+  -- The following option is a signal to other plugins like 'cmp' to not mess
+  -- with this buffer:
+  vim.bo[w_input_buf.bufnr].buftype = 'prompt'
+  vim.fn.prompt_setprompt(w_input_buf.bufnr, '')
+
+  vim.api.nvim_set_current_win(w_input)
+  tracker.create_effect(safe_wrap(function()
+    -- update window position/size every time the editor is resized:
+    w_input_cfg = vim.tbl_deep_extend('force', w_input_cfg, s_w_input_coords:get())
+    vim.api.nvim_win_set_config(w_input, w_input_cfg)
+  end))
+
+  local w_list_cfg = {
+    width = s_w_list_coords:get().width,
+    height = s_w_list_coords:get().height,
+    row = s_w_list_coords:get().row,
+    col = s_w_list_coords:get().col,
+    relative = 'editor',
+    focusable = true,
+    border = 'rounded',
+  }
+  local w_list_buf = Buffer.create(false, true)
+  local w_list = vim.api.nvim_open_win(w_list_buf.bufnr, false, w_list_cfg)
+  vim.wo[w_list][0].number = false
+  vim.wo[w_list][0].relativenumber = false
+  vim.wo[w_list][0].scrolloff = 0
+  tracker.create_effect(safe_wrap(function()
+    -- update window position/size every time the editor is resized:
+    w_list_cfg = vim.tbl_deep_extend('force', w_list_cfg, s_w_list_coords:get())
+    vim.api.nvim_win_set_config(w_list, w_list_cfg)
+  end))
+
+  -- Now that we have created the window with the prompt in it, start insert
+  -- mode so that the user can type immediately:
+  vim.cmd.startinsert()
+
+  --
+  -- State:
+  --
+
+  local s_items_raw = tracker.create_signal(opts.items, 's:items')
+  local s_items = s_items_raw:debounce(100)
+  local s_selected_indices = tracker.create_signal({}, 's:selected_indices')
+  local s_top_offset = tracker.create_signal(0, 's:top_offset')
+  local s_cursor_index = tracker.create_signal(1, 's:cursor_index')
+
+  local s_filter_text_undebounced = tracker.create_signal('', 's:filter_text')
+  w_input_buf:autocmd('TextChangedI', {
+    callback = safe_wrap(
+      function() s_filter_text_undebounced:set(vim.api.nvim_get_current_line()) end
+    ),
+  })
+  local s_filter_text = s_filter_text_undebounced:debounce(50)
+
+  --
+  -- Derived State:
+  --
+
+  local s_formatted_items = tracker.create_memo(function()
+    local function _format_item(item)
+      return opts.format_item and opts.format_item(item) or tostring(item)
+    end
+
+    local items = s_items:get()
+    return vim
+      .iter(items)
+      :map(function(item) return { item = item, formatted = _format_item(item) } end)
+      :totable()
+  end)
+
+  -- When the filter text changes, update the filtered items:
+  local s_filtered_items = tracker.create_memo(
+    safe_wrap(function()
+      local formatted_items = s_formatted_items:get()
+      local filter_text = vim.trim(s_filter_text:get()):lower()
+
+      --- @type string
+      local filter_pattern
+      --- @type boolean
+      local use_plain_pattern
+      if #formatted_items > 250 and #filter_text <= 3 then
+        filter_pattern = filter_text
+        use_plain_pattern = true
+      elseif #formatted_items > 1000 then
+        filter_pattern = filter_text
+        use_plain_pattern = true
+      else
+        filter_pattern = '('
+          .. vim.iter(vim.split(filter_text, '')):map(function(c) return c .. '.*' end):join ''
+          .. ')'
+        use_plain_pattern = false
+      end
+      filter_pattern = filter_pattern:lower()
+
+      --- @type table<integer, string>
+      local formatted_strings = {}
+      --- @type table<integer, string>
+      local matches = {}
+
+      local new_filtered_items = vim
+        .iter(formatted_items)
+        :enumerate()
+        :map(
+          function(i, inf) return { orig_idx = i, item = inf.item, formatted = inf.formatted } end
+        )
+        :filter(function(inf)
+          if filter_text == '' then return true end
+          local formatted_as_string = Renderer.markup_to_string({ tree = inf.formatted }):lower()
+
+          formatted_strings[inf.orig_idx] = formatted_as_string
+          if use_plain_pattern then
+            local x, y = formatted_as_string:find(filter_pattern, 1, true)
+            if x ~= nil and y ~= nil then matches[inf.orig_idx] = formatted_as_string:sub(x, y) end
+          else
+            matches[inf.orig_idx] = string.match(formatted_as_string, filter_pattern)
+          end
+
+          return matches[inf.orig_idx] ~= nil
+        end)
+        :totable()
+
+      -- Don't sort if there are over 500 items:
+      if #new_filtered_items <= 500 then
+        table.sort(new_filtered_items, function(a_inf, b_inf)
+          local a = formatted_strings[a_inf.orig_idx]
+          local b = formatted_strings[b_inf.orig_idx]
+          if a == b then return false end
+
+          local a_match = matches[a_inf.orig_idx]
+          local b_match = matches[b_inf.orig_idx]
+          return #a_match < #b_match
+        end)
+      end
+
+      s_top_offset:set(0)
+      s_cursor_index:set(1)
+      return new_filtered_items
+    end),
+    'e:(filter_text=>filtered_items)'
+  )
+
+  -- Visible items, are _just_ the items that fit into the current viewport.
+  -- This is an optimization so that we are not rendering thousands of lines of
+  -- items on each state-change.
+  local s_visible_items = tracker.create_memo(
+    safe_wrap(function()
+      return vim
+        .iter(s_filtered_items:get())
+        :enumerate()
+        :skip(s_top_offset:get())
+        :take(s_w_list_coords:get().height)
+        :map(
+          function(i, inf)
+            return {
+              filtered_idx = i,
+              orig_idx = inf.orig_idx,
+              item = inf.item,
+              formatted = inf.formatted,
+            }
+          end
+        )
+        :totable()
+    end),
+    'm:visible_items'
+  )
+
+  -- Track selection information:
+  local s_selection_info = tracker.create_memo(
+    safe_wrap(function()
+      local items = s_items:get()
+      local selected_indices = s_selected_indices:get()
+      --- @type { orig_idx: number; item: T }[]
+      local filtered_items = s_filtered_items:get()
+      local cursor_index = s_cursor_index:get()
+      local indices = shallow_copy_arr(selected_indices)
+      if #indices == 0 and #filtered_items > 0 then
+        indices = { filtered_items[cursor_index].orig_idx }
+      end
+      return {
+        items = vim.iter(indices):map(function(i) return items[i] end):totable(),
+        indices = indices,
+      }
+    end),
+    'm:selection_info'
+  )
+
+  --- When it is time to close the picker, this is the main cleanup routine
+  --- that runs in all cases:
+  ---
+  --- @param esc? boolean Whether the user pressed <Esc> or not
+  --- @param err? any Any error that occurred
+  function H.finish(esc, err)
+    -- s_editor_dimensions is the only signal that is cloned from a global,
+    -- one. It is therefore the only one that needs to be manually disposed.
+    -- The other ones should get cleaned up by the GC
+    s_editor_dimensions:dispose()
+    -- If we happen to have any async state-changes coming down the pipeline,
+    -- we can say right now that we are done rendering new UI (to avoid
+    -- "invalid window ID" errors):
+    H.unsubscribe_render_effect()
+    -- buftype=prompt buffers are not "temporary", so delete the buffer manually:
+    vim.api.nvim_buf_delete(w_input_buf.bufnr, { force = true })
+    -- The following is not needed, since the buffer is deleted above:
+    -- vim.api.nvim_win_close(w_input, false)
+    vim.api.nvim_win_close(w_list, false)
+    if stopinsert then vim.cmd.stopinsert() end
+    local inf = s_selection_info:get()
+    if not err and opts.on_finish then
+      -- If on_finish opens another picker, the closing of this one can happen
+      -- in _too_ quick succession, so put a small delay in there.
+      --
+      -- TODO: figure out _why_ this is actually happening, and then a better
+      -- way to handle this.
+      vim.defer_fn(function()
+        if esc then
+          opts.on_finish({}, {})
+        else
+          opts.on_finish(inf.items, inf.indices)
+        end
+      end, 100)
+    end
+  end
+
+  -- On selection info changed:
+  tracker.create_effect(
+    safe_wrap(function()
+      local inf = s_selection_info:get()
+      if opts.on_selection_changed then opts.on_selection_changed(inf.items, inf.indices) end
+    end),
+    'e:selection_changed'
+  )
+
+  --
+  -- Public API (i.e., `controller`):
+  -- We will fill in the methods further down, but we need this variable in scope so that it can be
+  -- closed over by some of the event handlers:
+  --
+  local controller = {}
+
+  --
+  -- Events
+  --
+  vim.keymap.set('i', '<Esc>', function() H.finish(true) end, { buffer = w_input_buf.bufnr })
+
+  vim.keymap.set('i', '<CR>', function() H.finish() end, { buffer = w_input_buf.bufnr })
+
+  local function action_next_line()
+    local max_line = #s_filtered_items:get()
+    local next_cursor_index = clamp(1, s_cursor_index:get() + 1, max_line)
+    if next_cursor_index - s_top_offset:get() > s_w_list_coords:get().height then
+      s_top_offset:set(s_top_offset:get() + 1)
+    end
+    s_cursor_index:set(next_cursor_index)
+  end
+  vim.keymap.set(
+    'i',
+    '<C-n>',
+    safe_wrap(action_next_line),
+    { buffer = w_input_buf.bufnr, desc = 'Picker: next' }
+  )
+  vim.keymap.set(
+    'i',
+    '<Down>',
+    safe_wrap(action_next_line),
+    { buffer = w_input_buf.bufnr, desc = 'Picker: next' }
+  )
+
+  local function action_prev_line()
+    local max_line = #s_filtered_items:get()
+    local next_cursor_index = clamp(1, s_cursor_index:get() - 1, max_line)
+    if next_cursor_index - s_top_offset:get() < 1 then s_top_offset:set(s_top_offset:get() - 1) end
+    s_cursor_index:set(next_cursor_index)
+  end
+  vim.keymap.set(
+    'i',
+    '<C-p>',
+    safe_wrap(action_prev_line),
+    { buffer = w_input_buf.bufnr, desc = 'Picker: previous' }
+  )
+  vim.keymap.set(
+    'i',
+    '<Up>',
+    safe_wrap(action_prev_line),
+    { buffer = w_input_buf.bufnr, desc = 'Picker: previous' }
+  )
+
+  vim.keymap.set(
+    'i',
+    '<Tab>',
+    safe_wrap(function()
+      if not opts.multi then return end
+
+      local index = s_filtered_items:get()[s_cursor_index:get()].orig_idx
+      if vim.tbl_contains(s_selected_indices:get(), index) then
+        s_selected_indices:set(
+          vim.iter(s_selected_indices:get()):filter(function(i) return i ~= index end):totable()
+        )
+      else
+        local new_selected_indices = shallow_copy_arr(s_selected_indices:get())
+        table.insert(new_selected_indices, index)
+        s_selected_indices:set(new_selected_indices)
+      end
+      action_next_line()
+    end),
+    { buffer = w_input_buf.bufnr }
+  )
+
+  for key, fn in pairs(opts.mappings or {}) do
+    vim.keymap.set(
+      'i',
+      key,
+      safe_wrap(function() return fn(controller) end),
+      { buffer = w_input_buf.bufnr }
+    )
+  end
+
+  -- Render:
+  H.unsubscribe_render_effect = tracker.create_effect(
+    safe_wrap(function()
+      local selected_indices = s_selected_indices:get()
+      local top_offset = s_top_offset:get()
+      local cursor_index = s_cursor_index:get()
+      --- @type { filtered_idx: number; orig_idx: number; item: T; formatted: string }[]
+      local visible_items = s_visible_items:get()
+
+      -- The above has to run in the execution context for the signaling to work, but
+      -- the following cannot run in a NeoVim loop-callback:
+      vim.schedule(function()
+        w_list_buf:render(TreeBuilder.new()
+          :nest(function(tb)
+            for loop_idx, inf in ipairs(visible_items) do
+              local is_cur_line = inf.filtered_idx == cursor_index
+              local is_selected = vim.tbl_contains(selected_indices, inf.orig_idx)
+
+              tb:put(loop_idx > 1 and '\n')
+              tb:put(is_cur_line and h('text', { hl = 'Structure' }, '❯') or ' ')
+              tb:put(is_selected and h('text', { hl = 'Comment' }, '* ') or '  ')
+              tb:put(inf.formatted)
+            end
+          end)
+          :tree())
+
+        -- set the window viewport to have the first line in view:
+        pcall(vim.api.nvim_win_call, w_list, function() vim.fn.winrestview { topline = 1 } end)
+        pcall(vim.api.nvim_win_set_cursor, w_list, { cursor_index - top_offset, 0 })
+      end)
+    end),
+    'e:render'
+  )
+
+  --
+  -- Populate the public API:
+  --
+  function controller.get_items()
+    return safe_run(function() return s_items_raw:get() end)
+  end
+
+  --- @param items T[]
+  function controller.set_items(items)
+    return safe_run(function() s_items_raw:set(items) end)
+  end
+
+  function controller.set_filter_text(filter_text)
+    return safe_run(function()
+      vim.api.nvim_win_call(w_input, function() vim.api.nvim_set_current_line(filter_text) end)
+    end)
+  end
+
+  function controller.get_selected_indices()
+    return safe_run(function() return s_selection_info:get().indices end)
+  end
+
+  function controller.get_selected_items()
+    return safe_run(function() return s_selection_info:get().items end)
+  end
+
+  --- @param indicies number[]
+  --- @param ephemeral? boolean
+  function controller.set_selected_indices(indicies, ephemeral)
+    return safe_run(function()
+      if ephemeral == nil then ephemeral = false end
+
+      if ephemeral and #indicies == 1 then
+        local matching_filtered_item_idx, _ = vim.iter(s_filtered_items:get()):enumerate():find(
+          function(_idx, inf) return inf.orig_idx == indicies[1] end
+        )
+        if matching_filtered_item_idx ~= nil then s_cursor_index:set(indicies[1]) end
+      else
+        if not opts.multi then
+          local err = 'Cannot set multiple selected indices on a single-select picker'
+          H.finish(true, err)
+          error(err)
+        end
+        s_selected_indices:set(indicies)
+      end
+    end)
+  end
+
+  function controller.close()
+    return safe_run(function() H.finish(true) end)
+  end
+
+  return controller --[[@as SelectController]]
+end -- }}}
+
+--------------------------------------------------------------------------------
+-- END create_picker
+--------------------------------------------------------------------------------
+
+--------------------------------------------------------------------------------
+-- vim.ui.select override
+--------------------------------------------------------------------------------
+
+local ORIGINAL_UI_SELECT
+function M.register_ui_select()
+  ORIGINAL_UI_SELECT = vim.ui.select
+  --- @generic T
+  --- @param items `T`[]
+  --- @param opts { prompt?: string, kind?: any, format_item?: fun(item: T):string }
+  --- @param cb fun(item: T|nil):any
+  --- @diagnostic disable-next-line: duplicate-set-field
+  function vim.ui.select(items, opts, cb)
+    M.create_picker {
+      items = items,
+      format_item = function(item)
+        local s = opts.format_item and opts.format_item(item) or tostring(item)
+        s = s:gsub('<', '&lt;')
+        return s
+      end,
+      on_finish = function(sel_items)
+        if #sel_items == 0 then cb(nil) end
+        cb(sel_items[#sel_items])
+      end,
+    }
+  end
+end
+function M.unregister_ui_select()
+  if not ORIGINAL_UI_SELECT then return end
+
+  vim.ui.select = ORIGINAL_UI_SELECT
+  ORIGINAL_UI_SELECT = nil
+end
+
+--------------------------------------------------------------------------------
+-- Built-in pickers
+-- 1. files
+-- 2. buffers
+-- 3. code-symbols
+--------------------------------------------------------------------------------
+
+--- @param opts? { limit?: number }
+function M.files(opts) -- {{{
+  opts = opts or {}
+  opts.limit = opts.limit or 10000
+
+  local cmd = {}
+  if vim.fn.executable 'rg' then
+    cmd = {
+      'rg',
+      '--color=never',
+      '--files',
+      '--hidden',
+      '--follow',
+      '-g',
+      '!.git',
+      '-g',
+      '!node_modules',
+      '-g',
+      '!target',
+    }
+  elseif vim.fn.executable 'fd' then
+    cmd = {
+      'fd',
+      '--color=never',
+      '--type',
+      'f',
+      '--hidden',
+      '--follow',
+      '--exclude',
+      '.git',
+      '--exclude',
+      'node_modules',
+      '--exclude',
+      'target',
+    }
+  elseif vim.fn.executable 'find' then
+    cmd = {
+      'find',
+      '-type',
+      'f',
+      '-not',
+      '-path',
+      "'*/.git/*'",
+      '-not',
+      '-path',
+      "'*/node_modules/*'",
+      '-not',
+      '-path',
+      "'*/target/*'",
+      '-printf',
+      "'%P\n'",
+    }
+  end
+
+  if #cmd == 0 then
+    vim.notify('rg/fd/find executable not found: cannot list files', vim.log.levels.ERROR)
+    return
+  end
+
+  -- Keep track of the job that will list files independent from the picker. We
+  -- will stream lines from this process to the picker as they come in:
+  local job_inf = { id = 0, proc_lines = {}, notified_over_limit = false }
+
+  -- Initially, create the picker with no items:
+  local picker = M.create_picker {
+    multi = true,
+    items = {},
+
+    --- @params items string[]
+    on_finish = function(items)
+      pcall(vim.fn.jobstop, job_inf.id)
+
+      if #items == 0 then return end
+      if #items == 1 then
+        vim.cmd.edit(items[1])
+      else
+        -- populate quickfix:
+        vim.fn.setqflist(vim
+          .iter(items)
+          :map(
+            function(item)
+              return {
+                filename = item,
+                lnum = 1,
+                col = 1,
+              }
+            end
+          )
+          :totable())
+        vim.cmd.copen()
+      end
+    end,
+
+    mappings = {
+      ['<C-t>'] = function(sel)
+        sel.close()
+        --- @type string[]
+        local items = sel.get_selected_items()
+
+        -- open in new tab:
+        for _, item in ipairs(items) do
+          vim.cmd.tabnew(item)
+        end
+      end,
+
+      ['<C-v>'] = function(sel)
+        sel.close()
+        --- @type string[]
+        local items = sel.get_selected_items()
+
+        -- open in vertical split:
+        for _, item in ipairs(items) do
+          vim.cmd.vsplit(item)
+        end
+      end,
+
+      ['<C-s>'] = function(sel)
+        sel.close()
+        --- @type string[]
+        local items = sel.get_selected_items()
+
+        -- open in horizontal split:
+        for _, item in ipairs(items) do
+          vim.cmd.split(item)
+        end
+      end,
+    },
+  }
+
+  -- Kick off the process that lists the files. As lines come in, send them to
+  -- the picker:
+  job_inf.id = vim.fn.jobstart(cmd, {
+    --- @param data string[]
+    on_stdout = vim.schedule_wrap(function(_chanid, data, _name)
+      local lines = job_inf.proc_lines
+      local function set_lines_as_items_state()
+        picker.set_items(vim
+          .iter(lines)
+          :enumerate()
+          :filter(function(idx, item)
+            -- Filter out an incomplete last line:
+            local is_last_line = idx == #lines
+            if is_last_line and item == '' then return false end
+            return true
+          end)
+          :map(function(_, item) return item end)
+          :totable())
+      end
+
+      -- It's just not a good idea to process large lists with Lua. The default
+      -- limit is 10,000 items, and even crunching through this is iffy on a
+      -- fast laptop. Show a warning and truncate the list in this case.
+      if #lines >= opts.limit then
+        if not job_inf.notified_over_limit then
+          vim.notify(
+            'Picker list is too large (truncating list to ' .. opts.limit .. ' items)',
+            vim.log.levels.WARN
+          )
+          pcall(vim.fn.jobstop, job_inf.id)
+          job_inf.notified_over_limit = true
+        end
+        return
+      end
+
+      -- :help channel-lines
+
+      local eof = #data == 1 and data[1] == ''
+      if eof then set_lines_as_items_state() end
+
+      -- Complete the previous line:
+      if #lines > 0 then lines[#lines] = lines[#lines] .. table.remove(data, 1) end
+
+      for _, l in ipairs(data) do
+        table.insert(lines, l)
+      end
+
+      set_lines_as_items_state()
+    end),
+  })
+end -- }}}
+
+function M.buffers() -- {{{
+  local cwd = vim.fn.getcwd()
+  -- ensure that `cwd` ends with a trailing slash:
+  if cwd[#cwd] ~= '/' then cwd = cwd .. '/' end
+
+  --- @type { name: string; changed: number; bufnr: number }[]
+  local bufs = vim.fn.getbufinfo { buflisted = 1 }
+
+  M.create_picker {
+    multi = true,
+    items = bufs,
+
+    --- @param item { name: string; changed: number; bufnr: number }
+    format_item = function(item)
+      local item_name = item.name
+      if item_name == '' then item_name = '[No Name]' end
+      -- trim leading `cwd` from the buffer name:
+      if item_name:sub(1, #cwd) == cwd then item_name = item_name:sub(#cwd + 1) end
+
+      return TreeBuilder.new():put(item.changed == 1 and '[+] ' or '    '):put(item_name):tree()
+    end,
+
+    --- @params items { bufnr: number }[]
+    on_finish = function(items)
+      if #items == 0 then return end
+      if #items == 1 then
+        vim.cmd.buffer(items[1].bufnr)
+      else
+        -- populate quickfix:
+        vim.fn.setqflist(vim
+          .iter(items)
+          :map(
+            function(item)
+              return {
+                bufnr = item.bufnr,
+                filename = item.name,
+                lnum = 1,
+                col = 1,
+              }
+            end
+          )
+          :totable())
+        vim.cmd.copen()
+      end
+    end,
+
+    mappings = {
+      ['<C-t>'] = function(sel)
+        sel.close()
+        --- @type { bufnr: number }[]
+        local items = sel.get_selected_items()
+
+        -- open in new tab:
+        for _, item in ipairs(items) do
+          vim.cmd.tabnew()
+          vim.cmd.buffer(item.bufnr)
+        end
+      end,
+
+      ['<C-v>'] = function(sel)
+        sel.close()
+        --- @type { bufnr: number }[]
+        local items = sel.get_selected_items()
+
+        -- open in new vertial split:
+        for _, item in ipairs(items) do
+          vim.cmd.vsplit()
+          vim.cmd.buffer(item.bufnr)
+        end
+      end,
+
+      ['<C-s>'] = function(sel)
+        sel.close()
+        --- @type { bufnr: number }[]
+        local items = sel.get_selected_items()
+
+        -- open in horizontal split:
+        for _, item in ipairs(items) do
+          vim.cmd.split()
+          vim.cmd.buffer(item.bufnr)
+        end
+      end,
+
+      ['<C-x>'] = function(sel)
+        local selected_items = sel.get_selected_items()
+        for _, item in ipairs(selected_items) do
+          -- delete the buffer
+          vim.cmd.bdelete(item.bufnr)
+        end
+
+        sel.set_selected_indices {}
+        sel.set_items(
+          vim
+            .iter(sel.get_items())
+            :filter(function(item) return not vim.tbl_contains(selected_items, item) end)
+            :totable()
+        )
+      end,
+    },
+  }
+end -- }}}
+
+local IS_CODE_SYMBOL_RUNNING = false
+function M.lsp_code_symbols() -- {{{
+  if IS_CODE_SYMBOL_RUNNING then return end
+  IS_CODE_SYMBOL_RUNNING = true
+
+  -- Avoid callback-hell with a wizard-based "steps"-system. Define each "step"
+  -- sequentially in the code, and wire up the callbacks to call the next step:
+  -- a simple, yet powerful, and easy to understand pattern/approach.
+  local STEPS = {}
+
+  --- @param info vim.lsp.LocationOpts.OnList
+  function STEPS._1_on_symbols(info)
+    M.create_picker {
+      items = info.items,
+      --- @param item { text: string }
+      format_item = function(item)
+        local s = item.text:gsub('<', '&lt;')
+        return s
+      end,
+      on_finish = STEPS._2_on_symbol_picked,
+    }
+  end
+
+  --- @param items { filename: string, lnum: integer, col: integer }[]
+  function STEPS._2_on_symbol_picked(items)
+    if #items == 0 then return STEPS._finally() end
+
+    local item = items[1]
+
+    -- Jump to the file/buffer:
+    local buf = vim
+      .iter(vim.fn.getbufinfo { buflisted = 1 })
+      :find(function(b) return b.name == item.filename end)
+    if buf ~= nil then
+      vim.api.nvim_win_set_buf(0, buf.bufnr)
+    else
+      vim.cmd.edit(item.filename)
+    end
+
+    -- Jump to the specific location:
+    vim.api.nvim_win_set_cursor(0, { item.lnum, item.col - 1 })
+    vim.cmd.normal 'zz'
+
+    STEPS._finally()
+  end
+
+  function STEPS._finally() IS_CODE_SYMBOL_RUNNING = false end
+
+  -- Kick off the async operation:
+  vim.lsp.buf.document_symbol { on_list = STEPS._1_on_symbols }
+end -- }}}
+
+function M.setup()
+  utils.ucmd('Files', M.files)
+  utils.ucmd('Buffers', M.buffers)
+  utils.ucmd('Lspcodesymbols', M.lsp_code_symbols)
+end
+
+return M

examples/splitjoin.lua

@@ -4,7 +4,7 @@ local Range = require 'u.range'
 
 local M = {}
 
----@param bracket_range Range
+--- @param bracket_range u.Range
 --- @param left string
 --- @param right string
 local function split(bracket_range, left, right)
@@ -52,17 +52,21 @@ local function split(bracket_range, left, right)
   bracket_range:replace(code.lines)
 end
 
----@param bracket_range Range
+--- @param bracket_range u.Range
 --- @param left string
 --- @param right string
 local function join(bracket_range, left, right)
-  local inner_range = Range.new(bracket_range.start:must_next(), bracket_range.stop:must_next(-1), bracket_range.mode)
+  local inner_range = bracket_range:shrink(1)
+  if inner_range then
     local newline = vim
       .iter(inner_range:lines())
       :map(function(l) return vim.trim(l) end)
       :filter(function(l) return l ~= '' end)
       :join ' '
     bracket_range:replace { left .. newline .. right }
+  else
+    bracket_range:replace { left .. right }
+  end
 end
 
 local function splitjoin()
@@ -80,7 +84,7 @@ local function splitjoin()
 end
 
 function M.setup()
-  vim.keymap.set('n', 'gS', function() vim_repeat.run(splitjoin) end)
+  vim.keymap.set('n', 'gS', function() vim_repeat.run_repeatable(splitjoin) end)
 end
 
 return M

examples/surround.lua

@@ -1,12 +1,12 @@
 local vim_repeat = require 'u.repeat'
-local opkeymap = require 'u.opkeymap'
-local Pos = require 'u.pos'
 local Range = require 'u.range'
 local Buffer = require 'u.buffer'
 local CodeWriter = require 'u.codewriter'
 
 local M = {}
 
+local ESC = vim.api.nvim_replace_termcodes('<Esc>', true, false, true)
+
 local surrounds = {
   [')'] = { left = '(', right = ')' },
   ['('] = { left = '( ', right = ' )' },
@@ -21,8 +21,17 @@ local surrounds = {
   ['`'] = { left = '`', right = '`' },
 }
 
+--- @type { left: string; right: string } | nil
+local CACHED_BOUNDS = nil
+
 --- @return { left: string; right: string }|nil
 local function prompt_for_bounds()
+  if vim_repeat.is_repeating() then
+    -- If we are repeating, we don't want to prompt for bounds, because
+    -- we want to reuse the last bounds:
+    return CACHED_BOUNDS
+  end
+
   local cn = vim.fn.getchar()
   -- Check for non-printable characters:
   if type(cn) ~= 'number' or cn < 32 or cn > 126 then return end
@@ -36,14 +45,16 @@ local function prompt_for_bounds()
     vim.keymap.del('c', '>')
     local endtag = '</' .. tag:sub(2):match '[^ >]*' .. '>'
     -- selene: allow(global_usage)
-    return { left = tag, right = endtag }
+    CACHED_BOUNDS = { left = tag, right = endtag }
+    return CACHED_BOUNDS
   else
     -- Default surround:
-    return (surrounds)[c] or { left = c, right = c }
+    CACHED_BOUNDS = (surrounds)[c] or { left = c, right = c }
+    return CACHED_BOUNDS
   end
 end
 
----@param range Range
+--- @param range u.Range
 --- @param bounds { left: string; right: string }
 local function do_surround(range, bounds)
   local left = bounds.left
@@ -59,7 +70,7 @@ local function do_surround(range, bounds)
     range:replace(left .. range:text() .. right)
   elseif range.mode == 'V' then
     local buf = Buffer.current()
-    local cw = CodeWriter.from_line(buf:line0(range.start.lnum):text(), buf.buf)
+    local cw = CodeWriter.from_line(range.start:line(), buf.bufnr)
 
     -- write the left bound at the current indent level:
     cw:write(left)
@@ -86,31 +97,51 @@ local function do_surround(range, bounds)
   range.start:save_to_pos '.'
 end
 
+-- Add surround:
+--- @param ty 'line' | 'char' | 'block'
+function _G.MySurroundOpFunc(ty)
+  if ty == 'block' then
+    -- We won't handle block-selection:
+    return
+  end
+
+  local range = Range.from_op_func(ty)
+  local hl
+  if not vim_repeat.is_repeating() then hl = range:highlight('IncSearch', { priority = 999 }) end
+
+  local bounds = prompt_for_bounds()
+  if hl then hl.clear() end
+  if bounds == nil then return end
+
+  do_surround(range, bounds)
+end
+
 function M.setup()
   require('u.repeat').setup()
 
   -- Visual
-  vim.keymap.set('v', 'S', function()
-    local c = vim.fn.getcharstr()
+  vim.keymap.set('x', 'S', function()
     local range = Range.from_vtext()
-    local bounds = surrounds[c] or { left = c, right = c }
-    vim_repeat.run(function()
+    local bounds = prompt_for_bounds()
+    if bounds == nil then return end
+
     do_surround(range, bounds)
     -- this is a visual mapping: end in normal mode:
-      vim.cmd { cmd = 'normal', args = { '' }, bang = true }
-    end)
+    vim.cmd.normal(ESC)
   end, { noremap = true, silent = true })
 
   -- Change
   vim.keymap.set('n', 'cs', function()
-    local from_cn = vim.fn.getchar()
+    local from_cn = vim.fn.getchar() --[[@as number]]
     -- Check for non-printable characters:
     if from_cn < 32 or from_cn > 126 then return end
+
+    vim_repeat.run_repeatable(function()
       local from_c = vim.fn.nr2char(from_cn)
       local from = surrounds[from_c] or { left = from_c, right = from_c }
       local function get_fresh_arange()
-      local arange = Range.from_text_object('a' .. from_c, { user_defined = true })
-      if arange == nil then return nil end
+        local arange = Range.from_motion('a' .. from_c, { user_defined = true })
+        if arange == nil then return end
         if from_c == 'q' then
           from.left = arange.start:char()
           from.right = arange.stop:char()
@@ -119,10 +150,12 @@ function M.setup()
       end
 
       local arange = get_fresh_arange()
-    if arange == nil then return nil end
+      if arange == nil then return end
 
-    local hl_info1 = Range.new(arange.start, arange.start, 'v'):highlight('IncSearch', { priority = 999 })
-    local hl_info2 = Range.new(arange.stop, arange.stop, 'v'):highlight('IncSearch', { priority = 999 })
+      local hl_info1 = vim_repeat.is_repeating() and nil
+        or Range.new(arange.start, arange.start, 'v'):highlight('IncSearch', { priority = 999 })
+      local hl_info2 = vim_repeat.is_repeating() and nil
+        or Range.new(arange.stop, arange.stop, 'v'):highlight('IncSearch', { priority = 999 })
       local hl_clear = function()
         if hl_info1 then hl_info1.clear() end
         if hl_info2 then hl_info2.clear() end
@@ -132,51 +165,39 @@ function M.setup()
       hl_clear()
       if to == nil then return end
 
-    vim_repeat.run(function()
-      -- Re-fetch the arange, just in case this action is being repeated:
-      arange = get_fresh_arange()
-      if arange == nil then return nil end
-
       if from_c == 't' then
         -- For tags, we want to replace the inner text, not the tag:
-        local irange = Range.from_text_object('i' .. from_c, { user_defined = true })
-        if arange == nil or irange == nil then return nil end
+        local irange = Range.from_motion('i' .. from_c, { user_defined = true })
+        if arange == nil or irange == nil then return end
 
-        local lrange = Range.new(arange.start, irange.start:must_next(-1))
-        local rrange = Range.new(irange.stop:must_next(1), arange.stop)
+        local lrange, rrange = arange:difference(irange)
+        if not lrange or not rrange then return end
 
         rrange:replace(to.right)
         lrange:replace(to.left)
       else
         -- replace `from.right` with `to.right`:
-        local last_line = arange:line0(-1).text() --[[@as string]]
-        local from_right_match = last_line:match(vim.pesc(from.right) .. '$')
-        if from_right_match then
-          local match_start = arange.stop:clone()
-          match_start.col = match_start.col - #from_right_match + 1
-          Range.new(match_start, arange.stop):replace(to.right)
-        end
+        local right_text = arange:sub(-1, -#from.right)
+        right_text:replace(to.right)
 
         -- replace `from.left` with `to.left`:
-        local first_line = arange:line0(0).text() --[[@as string]]
-        local from_left_match = first_line:match('^' .. vim.pesc(from.left))
-        if from_left_match then
-          local match_end = arange.start:clone()
-          match_end.col = match_end.col + #from_left_match - 1
-          Range.new(arange.start, match_end):replace(to.left)
-        end
+        local left_text = arange:sub(1, #from.left)
+        left_text:replace(to.left)
       end
     end)
   end, { noremap = true, silent = true })
 
   -- Delete
+  local CACHED_DELETE_FROM = nil
   vim.keymap.set('n', 'ds', function()
-    local txt_obj = vim.fn.getcharstr()
-    vim_repeat.run(function()
+    vim_repeat.run_repeatable(function()
+      local txt_obj = vim_repeat.is_repeating() and CACHED_DELETE_FROM or vim.fn.getcharstr()
+      CACHED_DELETE_FROM = txt_obj
+
       local buf = Buffer.current()
-      local irange = Range.from_text_object('i' .. txt_obj)
-      local arange = Range.from_text_object('a' .. txt_obj)
-      if arange == nil or irange == nil then return nil end
+      local irange = Range.from_motion('i' .. txt_obj)
+      local arange = Range.from_motion('a' .. txt_obj)
+      if arange == nil or irange == nil then return end
       local starting_cursor_pos = arange.start:clone()
 
       -- Now, replace `arange` with the content of `irange`.  If `arange` was multiple lines,
@@ -187,28 +208,19 @@ function M.setup()
         -- Dedenting moves the cursor, so we need to set the cursor to a consistent starting spot:
         arange.start:save_to_pos '.'
         -- Dedenting also changed the inner text, so re-acquire it:
-        arange = Range.from_text_object('a' .. txt_obj)
-        irange = Range.from_text_object('i' .. txt_obj)
+        arange = Range.from_motion('a' .. txt_obj)
+        irange = Range.from_motion('i' .. txt_obj)
         if arange == nil or irange == nil then return end -- should never be true
         arange:replace(irange:lines())
-
-        local final_range = Range.new(
-          arange.start,
-          Pos.new(
-            arange.stop.buf,
-            irange.start.lnum + (arange.stop.lnum + arange.start.lnum),
-            arange.stop.col,
-            arange.stop.off
-          ),
-          irange.mode
-        )
+        -- `arange:replace(..)` updates its own `stop` position, so we will use
+        -- `arange` as the final resulting range that holds the modified text
 
         -- delete last line, if it is empty:
-        local last = buf:line0(final_range.stop.lnum)
+        local last = buf:line(arange.stop.lnum)
         if last:text():match '^%s*$' then last:replace(nil) end
 
         -- delete first line, if it is empty:
-        local first = buf:line0(final_range.start.lnum)
+        local first = buf:line(arange.start.lnum)
         if first:text():match '^%s*$' then first:replace(nil) end
       else
         -- trim start:
@@ -220,35 +232,10 @@ function M.setup()
     end)
   end, { noremap = true, silent = true })
 
-  opkeymap('n', 'ys', function(range)
-    local hl_info = range:highlight('IncSearch', { priority = 999 })
-
-    ---@type { left: string; right: string }
-    local bounds
-    -- selene: allow(global_usage)
-    if _G.my_surround_bounds ~= nil then
-      -- This command was repeated with `.`, we don't need
-      -- to prompt for the bounds:
-      -- selene: allow(global_usage)
-      bounds = _G.my_surround_bounds
-    else
-      local prompted_bounds = prompt_for_bounds()
-      if prompted_bounds == nil and hl_info then return hl_info.clear() end
-      if prompted_bounds then bounds = prompted_bounds end
-    end
-
-    if hl_info then hl_info.clear() end
-    do_surround(range, bounds)
-    -- selene: allow(global_usage)
-    _G.my_surround_bounds = nil
-
-    -- return repeatable injection
-    return function()
-      -- on_repeat, we "stage" the bounds that we were originally called with:
-      -- selene: allow(global_usage)
-      _G.my_surround_bounds = bounds
-    end
-  end)
+  vim.keymap.set('n', 'ys', function()
+    vim.o.operatorfunc = 'v:lua.MySurroundOpFunc'
+    return 'g@'
+  end, { expr = true })
 end
 
 return M

examples/text-objects.lua

@@ -1,4 +1,4 @@
-local utils = require 'u.utils'
+local txtobj = require 'u.txtobj'
 local Pos = require 'u.pos'
 local Range = require 'u.range'
 local Buffer = require 'u.buffer'
@@ -7,17 +7,14 @@ local M = {}
 
 function M.setup()
   -- Select whole file:
-  utils.define_text_object('ag', function() return Buffer.current():all() end)
+  txtobj.define('ag', function() return Buffer.current():all() end)
 
   -- Select current line:
-  utils.define_text_object('a.', function()
-    local lnum = Pos.from_pos('.').lnum
-    return Buffer.current():line0(lnum)
-  end)
+  txtobj.define('a.', function() return Buffer.current():line(Pos.from_pos('.').lnum) end)
 
   -- Select the nearest quote:
-  utils.define_text_object('aq', function() return Range.find_nearest_quotes() end)
-  utils.define_text_object('iq', function()
+  txtobj.define('aq', function() return Range.find_nearest_quotes() end)
+  txtobj.define('iq', function()
     local range = Range.find_nearest_quotes()
     if range == nil then return end
     return range:shrink(1)
@@ -26,33 +23,17 @@ function M.setup()
   ---Selects the next quote object (searches forward)
   --- @param q string
   local function define_quote_obj(q)
-    local function select_around()
-      -- Operator mappings are effectively running in visual mode, the way
-      -- `define_text_object` is implemented, so feed the keys `a${q}` to vim
-      -- to select the appropriate text-object
-      vim.cmd { cmd = 'normal', args = { 'a' .. q }, bang = true }
+    local function select_around() return Range.from_motion('a' .. q) end
 
-      -- Now check on the visually selected text:
-      local range = Range.from_vtext()
-      if range:is_empty() then return range.start end
-      range.start = range.start:find_next(1, q) or range.start
-      range.stop = range.stop:find_next(-1, q) or range.stop
-      return range
-    end
+    txtobj.define('a' .. q, function() return select_around() end)
+    txtobj.define('i' .. q, function()
+      local range = select_around()
+      if range == nil or range:is_empty() then return range end
 
-    utils.define_text_object('a' .. q, function() return select_around() end)
-    utils.define_text_object('i' .. q, function()
-      local range_or_pos = select_around()
-      if Range.is(range_or_pos) then
-        local start_next = range_or_pos.start:next(1)
-        local stop_prev = range_or_pos.stop:next(-1)
-        if start_next > stop_prev then return start_next end
-
-        local range = range_or_pos:shrink(1)
-        return range
-      else
-        return range_or_pos
-      end
+      local start_next = range.start:next(1) or range.start
+      local stop_prev = range.stop:next(-1)
+      if start_next > stop_prev then return Range.new(start_next) end
+      return range:shrink(1) or range
     end)
   end
   define_quote_obj [["]]
@@ -66,30 +47,20 @@ function M.setup()
       local curr = Pos.from_pos('.'):find_next(-1, q)
       if not curr then return end
       -- Reset visual selection to current context:
-      Range.new(curr, curr):set_visual_selection()
-      vim.cmd.normal('a' .. q)
-      local range = Range.from_vtext()
-      if range:is_empty() then return range.start end
-      range.start = range.start:find_next(1, q) or range.start
-      range.stop = range.stop:find_next(-1, q) or range.stop
-      return range
+      curr:save_to_pos '.'
+      return Range.from_motion('a' .. q)
     end
 
-    utils.define_text_object('al' .. q, function() return select_around() end)
-    utils.define_text_object('il' .. q, function()
-      local range_or_pos = select_around()
-      if range_or_pos == nil then return end
+    txtobj.define('al' .. q, function() return select_around() end)
+    txtobj.define('il' .. q, function()
+      local range = select_around()
+      if range == nil or range:is_empty() then return range end
 
-      if Range.is(range_or_pos) then
-        local start_next = range_or_pos.start:next(1)
-        local stop_prev = range_or_pos.stop:next(-1)
-        if start_next > stop_prev then return start_next end
+      local start_next = range.start:next(1) or range.start
+      local stop_prev = range.stop:next(-1)
+      if start_next > stop_prev then return Range.new(start_next) end
 
-        local range = range_or_pos:shrink(1)
-        return range
-      else
-        return range_or_pos
-      end
+      return range:shrink(1) or range
     end)
   end
   define_last_quote_obj [["]]
@@ -111,8 +82,8 @@ function M.setup()
     local keybinds = { ... }
     table.insert(keybinds, b)
     for _, k in ipairs(keybinds) do
-      utils.define_text_object('al' .. k, function() return select_around() end)
-      utils.define_text_object('il' .. k, function()
+      txtobj.define('al' .. k, function() return select_around() end)
+      txtobj.define('il' .. k, function()
         local range = select_around()
         return range and range:shrink(1)
       end)

lua/u/.luarc.json

@@ -0,0 +1,5 @@
+{
+  "$schema": "https://raw.githubusercontent.com/LuaLS/vscode-lua/master/setting/schema.json",
+  "diagnostics.globals": ["assert", "vim"],
+  "runtime.version": "LuaJIT"
+}

lua/u/buffer.lua

@@ -1,72 +1,128 @@
 local Range = require 'u.range'
+local Renderer = require('u.renderer').Renderer
 
----@class Buffer
----@field buf number
+--- @class u.Buffer
+--- @field bufnr number
+--- @field b vim.var_accessor
+--- @field bo vim.bo
+--- @field private renderer u.Renderer
 local Buffer = {}
+Buffer.__index = Buffer
 
----@param buf? number
----@return Buffer
-function Buffer.from_nr(buf)
-  if buf == nil or buf == 0 then buf = vim.api.nvim_get_current_buf() end
-  local b = { buf = buf }
-  setmetatable(b, { __index = Buffer })
-  return b
+--- @param bufnr? number
+--- @return u.Buffer
+function Buffer.from_nr(bufnr)
+  if bufnr == nil or bufnr == 0 then bufnr = vim.api.nvim_get_current_buf() end
+  local renderer = Renderer.new(bufnr)
+  return setmetatable({
+    bufnr = bufnr,
+    b = vim.b[bufnr],
+    bo = vim.bo[bufnr],
+    renderer = renderer,
+  }, Buffer)
 end
 
----@return Buffer
+--- @return u.Buffer
 function Buffer.current() return Buffer.from_nr(0) end
 
 --- @param listed boolean
 --- @param scratch boolean
----@return Buffer
-function Buffer.create(listed, scratch) return Buffer.from_nr(vim.api.nvim_create_buf(listed, scratch)) end
-
-function Buffer:set_tmp_options()
-  self:set_option('bufhidden', 'delete')
-  self:set_option('buflisted', false)
-  self:set_option('buftype', 'nowrite')
+--- @return u.Buffer
+function Buffer.create(listed, scratch)
+  return Buffer.from_nr(vim.api.nvim_create_buf(listed, scratch))
 end
 
----@param nm string
-function Buffer:get_option(nm) return vim.api.nvim_get_option_value(nm, { buf = self.buf }) end
+function Buffer:set_tmp_options()
+  self.bo.bufhidden = 'delete'
+  self.bo.buflisted = false
+  self.bo.buftype = 'nowrite'
+end
 
----@param nm string
-function Buffer:set_option(nm, val) return vim.api.nvim_set_option_value(nm, val, { buf = self.buf }) end
+function Buffer:line_count() return vim.api.nvim_buf_line_count(self.bufnr) end
 
----@param nm string
-function Buffer:get_var(nm) return vim.api.nvim_buf_get_var(self.buf, nm) end
+function Buffer:all() return Range.from_buf_text(self.bufnr) end
 
----@param nm string
-function Buffer:set_var(nm, val) return vim.api.nvim_buf_set_var(self.buf, nm, val) end
-
-function Buffer:line_count() return vim.api.nvim_buf_line_count(self.buf) end
-
-function Buffer:all() return Range.from_buf_text(self.buf) end
-
-function Buffer:is_empty() return self:line_count() == 1 and self:line0(0):text() == '' end
+function Buffer:is_empty() return self:line_count() == 1 and self:line(1):text() == '' end
 
 --- @param line string
 function Buffer:append_line(line)
   local start = -1
   if self:is_empty() then start = -2 end
-  vim.api.nvim_buf_set_lines(self.buf, start, -1, false, { line })
+  vim.api.nvim_buf_set_lines(self.bufnr, start, -1, false, { line })
 end
 
----@param num number 0-based line index
-function Buffer:line0(num)
-  if num < 0 then return self:line0(self:line_count() + num) end
-  return Range.from_line(self.buf, num)
+--- @param num number 1-based line index
+function Buffer:line(num)
+  if num < 0 then num = self:line_count() + num + 1 end
+  return Range.from_line(self.bufnr, num)
 end
 
----@param start number 0-based line index
----@param stop number 0-based line index
-function Buffer:lines(start, stop) return Range.from_lines(self.buf, start, stop) end
+--- @param start number 1-based line index
+--- @param stop number 1-based line index
+function Buffer:lines(start, stop) return Range.from_lines(self.bufnr, start, stop) end
 
----@param txt_obj string
----@param opts? { contains_cursor?: boolean; pos?: Pos }
-function Buffer:text_object(txt_obj, opts)
-  opts = vim.tbl_extend('force', opts or {}, { buf = self.buf })
-  return Range.from_text_object(txt_obj, opts)
+--- @param motion string
+--- @param opts? { contains_cursor?: boolean; pos?: u.Pos }
+function Buffer:motion(motion, opts)
+  opts = vim.tbl_extend('force', opts or {}, { bufnr = self.bufnr })
+  return Range.from_motion(motion, opts)
+end
+
+--- @param event string|string[]
+--- @diagnostic disable-next-line: undefined-doc-name
+--- @param opts vim.api.keyset.create_autocmd
+function Buffer:autocmd(event, opts)
+  vim.api.nvim_create_autocmd(event, vim.tbl_extend('force', opts, { buffer = self.bufnr }))
+end
+
+--- @param tree u.renderer.Tree
+function Buffer:render(tree) return self.renderer:render(tree) end
+
+--- Filter buffer content through an external command (like Vim's :%!)
+--- @param cmd string[] Command to run (with arguments)
+--- @param opts? {cwd?: string, preserve_cursor?: boolean}
+--- @return nil
+--- @throws string Error message if command fails
+--- @note Special placeholders in cmd:
+---   - $FILE: replaced with the buffer's filename (if any)
+---   - $DIR: replaced with the buffer's directory (if any)
+function Buffer:filter_cmd(cmd, opts)
+  opts = opts or {}
+  local cwd = opts.cwd or vim.uv.cwd()
+  local old_lines = self:all():lines()
+  -- Save cursor position if needed, defaulting to true
+  local save_pos = opts.preserve_cursor ~= false and vim.fn.winsaveview()
+
+  -- Run the command
+  local result = vim
+    .system(
+      -- Replace special placeholders in `cmd` with their values:
+      vim
+        .iter(cmd)
+        :map(function(x)
+          if x == '$FILE' then return vim.api.nvim_buf_get_name(self.bufnr) end
+          if x == '$DIR' then return vim.fs.dirname(vim.api.nvim_buf_get_name(self.bufnr)) end
+          return x
+        end)
+        :totable(),
+      {
+        cwd = cwd,
+        stdin = old_lines,
+        text = true,
+      }
+    )
+    :wait()
+
+  -- Check for command failure
+  if result.code ~= 0 then error('Command failed: ' .. (result.stderr or '')) end
+
+  -- Process and apply the result
+  local new_lines = vim.split(result.stdout, '\n')
+  if new_lines[#new_lines] == '' then table.remove(new_lines) end
+  Renderer.patch_lines(self.bufnr, old_lines, new_lines)
+
+  -- Restore cursor position if saved
+  if save_pos then vim.fn.winrestview(save_pos) end
 end
 
 return Buffer

lua/u/codewriter.lua

@@ -1,14 +1,15 @@
 local Buffer = require 'u.buffer'
 
----@class CodeWriter
+--- @class u.CodeWriter
 --- @field lines string[]
 --- @field indent_level number
 --- @field indent_str string
 local CodeWriter = {}
+CodeWriter.__index = CodeWriter
 
 --- @param indent_level? number
 --- @param indent_str? string
----@return CodeWriter
+--- @return u.CodeWriter
 function CodeWriter.new(indent_level, indent_str)
   if indent_level == nil then indent_level = 0 end
   if indent_str == nil then indent_str = '  ' end
@@ -18,26 +19,27 @@ function CodeWriter.new(indent_level, indent_str)
     indent_level = indent_level,
     indent_str = indent_str,
   }
-  setmetatable(cw, { __index = CodeWriter })
+  setmetatable(cw, CodeWriter)
   return cw
 end
 
----@param p Pos
+--- @param p u.Pos
 function CodeWriter.from_pos(p)
-  local line = Buffer.from_nr(p.buf):line0(p.lnum):text()
-  return CodeWriter.from_line(line, p.buf)
+  local line = Buffer.from_nr(p.bufnr):line(p.lnum):text()
+  return CodeWriter.from_line(line, p.bufnr)
 end
 
 --- @param line string
----@param buf? number
-function CodeWriter.from_line(line, buf)
-  if buf == nil then buf = vim.api.nvim_get_current_buf() end
+--- @param bufnr? number
+function CodeWriter.from_line(line, bufnr)
+  if bufnr == nil then bufnr = vim.api.nvim_get_current_buf() end
 
   local ws = line:match '^%s*'
-  local expandtab = vim.api.nvim_get_option_value('expandtab', { buf = buf })
-  local shiftwidth = vim.api.nvim_get_option_value('shiftwidth', { buf = buf })
+  local expandtab = vim.api.nvim_get_option_value('expandtab', { buf = bufnr })
+  local shiftwidth = vim.api.nvim_get_option_value('shiftwidth', { buf = bufnr })
 
-  local indent_level = 0
+  --- @type number
+  local indent_level
   local indent_str = ''
   if expandtab then
     while #indent_str < shiftwidth do
@@ -61,7 +63,7 @@ end
 --- @param line string
 function CodeWriter:write(line) self:write_raw(self.indent_str:rep(self.indent_level) .. line) end
 
----@param f? fun(cw: CodeWriter):any
+--- @param f? fun(cw: u.CodeWriter):any
 function CodeWriter:indent(f)
   local cw = {
     lines = self.lines,

lua/u/logger.lua

@@ -0,0 +1,69 @@
+local M = {}
+
+--- @params name string
+function M.file_for_name(name)
+  return vim.fs.joinpath(vim.fn.stdpath 'cache', 'u.log', name .. '.log.jsonl')
+end
+
+--------------------------------------------------------------------------------
+-- Logger class
+--------------------------------------------------------------------------------
+
+--- @class u.Logger
+--- @field name string
+--- @field private fd number
+local Logger = {}
+Logger.__index = Logger
+M.Logger = Logger
+
+--- @param name string
+function Logger.new(name)
+  local file_path = M.file_for_name(name)
+  vim.fn.mkdir(vim.fs.dirname(file_path), 'p')
+  local self = setmetatable({
+    name = name,
+    fd = (vim.uv or vim.loop).fs_open(file_path, 'a', tonumber('644', 8)),
+  }, Logger)
+  return self
+end
+
+--- @private
+--- @param level string
+function Logger:write(level, ...)
+  local data = { ... }
+  if #data == 1 then data = data[1] end
+  (vim.uv or vim.loop).fs_write(
+    self.fd,
+    vim.json.encode { ts = os.date(), level = level, data = data } .. '\n'
+  )
+end
+
+function Logger:trace(...) self:write('INFO', ...) end
+function Logger:debug(...) self:write('DEBUG', ...) end
+function Logger:info(...) self:write('INFO', ...) end
+function Logger:warn(...) self:write('WARN', ...) end
+function Logger:error(...) self:write('ERROR', ...) end
+
+function M.setup()
+  vim.api.nvim_create_user_command('Logfollow', function(args)
+    if #args.fargs == 0 then
+      vim.print 'expected log name'
+      return
+    end
+
+    local log_file_path = M.file_for_name(args.fargs[1])
+    vim.fn.mkdir(vim.fs.dirname(log_file_path), 'p')
+    vim.system({ 'touch', log_file_path }):wait()
+
+    vim.cmd.new()
+
+    local winnr = vim.api.nvim_get_current_win()
+    vim.wo[winnr][0].number = false
+    vim.wo[winnr][0].relativenumber = false
+
+    vim.cmd.terminal('tail -f "' .. log_file_path .. '"')
+    vim.cmd.startinsert()
+  end, { nargs = '*' })
+end
+
+return M

lua/u/opkeymap.lua

@@ -1,24 +1,17 @@
 local Range = require 'u.range'
-local vim_repeat = require 'u.repeat'
 
----@type fun(range: Range): nil|(fun():any)
+--- @type fun(range: u.Range): nil|(fun():any)
 local __U__OpKeymapOpFunc_rhs = nil
 
 --- This is the global utility function used for operatorfunc
 --- in opkeymap
----@type nil|fun(range: Range): fun():any|nil
+--- @type nil|fun(range: u.Range): fun():any|nil
 --- @param ty 'line'|'char'|'block'
 -- selene: allow(unused_variable)
-function __U__OpKeymapOpFunc(ty)
+function _G.__U__OpKeymapOpFunc(ty)
   if __U__OpKeymapOpFunc_rhs ~= nil then
     local range = Range.from_op_func(ty)
-    local repeat_inject = __U__OpKeymapOpFunc_rhs(range)
-
-    vim_repeat.set(function()
-      vim.o.operatorfunc = 'v:lua.__U__OpKeymapOpFunc'
-      if repeat_inject ~= nil and type(repeat_inject) == 'function' then repeat_inject() end
-      vim_repeat.native_repeat()
-    end)
+    __U__OpKeymapOpFunc_rhs(range)
   end
 end
 
@@ -30,10 +23,15 @@ end
 ---    then calls the original passed callback with said range.
 --- @param mode string|string[]
 --- @param lhs string
----@param rhs fun(range: Range): nil|(fun():any) This function may return another function, which is called whenever the operator is repeated
+--- @param rhs fun(range: u.Range): nil
+--- @diagnostic disable-next-line: undefined-doc-name
 --- @param opts? vim.keymap.set.Opts
 local function opkeymap(mode, lhs, rhs, opts)
   vim.keymap.set(mode, lhs, function()
+    -- We don't need to wrap the operation in a repeat, because expr mappings are
+    -- repeated seamlessly by Vim anyway. In addition, the u.repeat:`.` mapping will
+    -- set IS_REPEATING to true, so that callbacks can check if they should used cached
+    -- values.
     __U__OpKeymapOpFunc_rhs = rhs
     vim.o.operatorfunc = 'v:lua.__U__OpKeymapOpFunc'
     return 'g@'

lua/u/pos.lua

@@ -1,149 +1,157 @@
 local MAX_COL = vim.v.maxcol
 
----@param buf number
----@param lnum number
-local function line_text(buf, lnum) return vim.api.nvim_buf_get_lines(buf, lnum, lnum + 1, false)[1] end
+--- @param bufnr number
+--- @param lnum number 1-based
+local function line_text(bufnr, lnum)
+  return vim.api.nvim_buf_get_lines(bufnr, lnum - 1, lnum, false)[1]
+end
 
----@class Pos
----@field buf number buffer number
+--- @class u.Pos
+--- @field bufnr number buffer number
 --- @field lnum number 1-based line index
 --- @field col number 1-based column index
 --- @field off number
 local Pos = {}
+Pos.__index = Pos
 Pos.MAX_COL = MAX_COL
 
----@param buf? number
----@param lnum number
----@param col number
+function Pos.__tostring(self)
+  if self.off ~= 0 then
+    return string.format('Pos(%d:%d){bufnr=%d, off=%d}', self.lnum, self.col, self.bufnr, self.off)
+  else
+    return string.format('Pos(%d:%d){bufnr=%d}', self.lnum, self.col, self.bufnr)
+  end
+end
+
+--- @param bufnr? number
+--- @param lnum number 1-based
+--- @param col number 1-based
 --- @param off? number
----@return Pos
-function Pos.new(buf, lnum, col, off)
-  if buf == nil or buf == 0 then buf = vim.api.nvim_get_current_buf() end
+--- @return u.Pos
+function Pos.new(bufnr, lnum, col, off)
+  if bufnr == nil or bufnr == 0 then bufnr = vim.api.nvim_get_current_buf() end
   if off == nil then off = 0 end
   local pos = {
-    buf = buf,
+    bufnr = bufnr,
     lnum = lnum,
     col = col,
     off = off,
   }
-
-  local function str()
-    if pos.off ~= 0 then
-      return string.format('Pos(%d:%d){buf=%d, off=%d}', pos.lnum, pos.col, pos.buf, pos.off)
-    else
-      return string.format('Pos(%d:%d){buf=%d}', pos.lnum, pos.col, pos.buf)
-    end
-  end
-  setmetatable(pos, {
-    __index = Pos,
-    __tostring = str,
-    __lt = Pos.__lt,
-    __le = Pos.__le,
-    __eq = Pos.__eq,
-  })
+  setmetatable(pos, Pos)
   return pos
 end
 
-function Pos.is(x)
-  local mt = getmetatable(x)
-  return mt and mt.__index == Pos
-end
+function Pos.invalid() return Pos.new(0, 0, 0, 0) end
 
 function Pos.__lt(a, b) return a.lnum < b.lnum or (a.lnum == b.lnum and a.col < b.col) end
 function Pos.__le(a, b) return a < b or a == b end
-function Pos.__eq(a, b) return a.lnum == b.lnum and a.col == b.col end
-
----@param name string
----@return Pos
-function Pos.from_pos(name)
-  local p = vim.fn.getpos(name)
-  local col = p[3]
-  if col ~= MAX_COL then col = col - 1 end
-  return Pos.new(p[1], p[2] - 1, col, p[4])
+function Pos.__eq(a, b)
+  return getmetatable(a) == Pos
+    and getmetatable(b) == Pos
+    and a.bufnr == b.bufnr
+    and a.lnum == b.lnum
+    and a.col == b.col
+end
+function Pos.__add(x, y)
+  if type(x) == 'number' then
+    x, y = y, x
+  end
+  if getmetatable(x) ~= Pos or type(y) ~= 'number' then return nil end
+  return x:next(y)
+end
+function Pos.__sub(x, y)
+  if type(x) == 'number' then
+    x, y = y, x
+  end
+  if getmetatable(x) ~= Pos or type(y) ~= 'number' then return nil end
+  return x:next(-y)
 end
 
-function Pos:clone() return Pos.new(self.buf, self.lnum, self.col, self.off) end
+--- @param name string
+--- @return u.Pos
+function Pos.from_pos(name)
+  local p = vim.fn.getpos(name)
+  return Pos.new(p[1], p[2], p[3], p[4])
+end
+
+function Pos:is_invalid() return self.lnum == 0 and self.col == 0 and self.off == 0 end
+
+function Pos:clone() return Pos.new(self.bufnr, self.lnum, self.col, self.off) end
 
 --- @return boolean
 function Pos:is_col_max() return self.col == MAX_COL end
 
----@return number[]
-function Pos:as_vim() return { self.buf, self.lnum, self.col, self.off } end
-
 --- Normalize the position to a real position (take into account vim.v.maxcol).
 function Pos:as_real()
+  local maxlen = #line_text(self.bufnr, self.lnum)
   local col = self.col
-  if self:is_col_max() then
+  if col > maxlen then
     -- We could use utilities in this file to get the given line, but
     -- since this is a low-level function, we are going to optimize and
     -- use the API directly:
-    col = #line_text(self.buf, self.lnum) - 1
+    col = maxlen
   end
-  return Pos.new(self.buf, self.lnum, col, self.off)
+  return Pos.new(self.bufnr, self.lnum, col, self.off)
 end
 
+function Pos:as_vim() return { self.bufnr, self.lnum, self.col, self.off } end
+
 --- @param pos string
-function Pos:save_to_pos(pos)
-  if pos == '.' then
-    vim.api.nvim_win_set_cursor(0, { self.lnum + 1, self.col })
-    return
-  end
-
-  local p = self:as_real()
-  vim.fn.setpos(pos, { p.buf, p.lnum + 1, p.col + 1, p.off })
-end
+function Pos:save_to_pos(pos) vim.fn.setpos(pos, { self.bufnr, self.lnum, self.col, self.off }) end
 
 --- @param mark string
 function Pos:save_to_mark(mark)
   local p = self:as_real()
-  vim.api.nvim_buf_set_mark(p.buf, mark, p.lnum + 1, p.col, {})
+  vim.api.nvim_buf_set_mark(p.bufnr, mark, p.lnum, p.col - 1, {})
 end
 
 --- @return string
 function Pos:char()
-  local line = line_text(self.buf, self.lnum)
+  local line = line_text(self.bufnr, self.lnum)
   if line == nil then return '' end
-  return line:sub(self.col + 1, self.col + 1)
+  return line:sub(self.col, self.col)
 end
 
+function Pos:line() return line_text(self.bufnr, self.lnum) end
+
 --- @param dir? -1|1
 --- @param must? boolean
----@return Pos|nil
+--- @return u.Pos|nil
 function Pos:next(dir, must)
   if must == nil then must = false end
 
   if dir == nil or dir == 1 then
     -- Next:
-    local num_lines = vim.api.nvim_buf_line_count(self.buf)
-    local last_line = line_text(self.buf, num_lines - 1) -- buf:line0(-1)
-    if self.lnum == num_lines - 1 and self.col == (#last_line - 1) then
+    local num_lines = vim.api.nvim_buf_line_count(self.bufnr)
+    local last_line = line_text(self.bufnr, num_lines)
+    if self.lnum == num_lines and self.col == #last_line then
       if must then error 'error in Pos:next(): Pos:next() returned nil' end
       return nil
     end
 
     local col = self.col + 1
     local line = self.lnum
-    local line_max_col = #line_text(self.buf, self.lnum) - 1
+    local line_max_col = #line_text(self.bufnr, self.lnum)
     if col > line_max_col then
-      col = 0
+      col = 1
       line = line + 1
     end
-    return Pos.new(self.buf, line, col, self.off)
+    return Pos.new(self.bufnr, line, col, self.off)
   else
     -- Previous:
-    if self.col == 0 and self.lnum == 0 then
+    if self.col == 1 and self.lnum == 1 then
       if must then error 'error in Pos:next(): Pos:next() returned nil' end
       return nil
     end
 
     local col = self.col - 1
     local line = self.lnum
-    local prev_line_max_col = #(line_text(self.buf, self.lnum - 1) or '') - 1
-    if col < 0 then
-      col = math.max(prev_line_max_col, 0)
+    local prev_line_max_col = #(line_text(self.bufnr, self.lnum - 1) or '')
+    if col < 1 then
+      col = math.max(prev_line_max_col, 1)
       line = line - 1
     end
-    return Pos.new(self.buf, line, col, self.off)
+    return Pos.new(self.bufnr, line, col, self.off)
   end
 end
 
@@ -155,7 +163,7 @@ function Pos:must_next(dir)
 end
 
 --- @param dir -1|1
----@param predicate fun(p: Pos): boolean
+--- @param predicate fun(p: u.Pos): boolean
 --- @param test_current? boolean
 function Pos:next_while(dir, predicate, test_current)
   if test_current and not predicate(self) then return end
@@ -169,14 +177,14 @@ function Pos:next_while(dir, predicate, test_current)
 end
 
 --- @param dir -1|1
----@param predicate string|fun(p: Pos): boolean
+--- @param predicate string|fun(p: u.Pos): boolean
 function Pos:find_next(dir, predicate)
   if type(predicate) == 'string' then
     local s = predicate
     predicate = function(p) return s == p:char() end
   end
 
-  ---@type Pos|nil
+  --- @type u.Pos|nil
   local curr = self
   while curr ~= nil do
     if predicate(curr) then return curr end
@@ -187,11 +195,13 @@ end
 
 --- Finds the matching bracket/paren for the current position.
 --- @param max_chars? number|nil
----@param invocations? Pos[]
----@return Pos|nil
+--- @param invocations? u.Pos[]
+--- @return u.Pos|nil
 function Pos:find_match(max_chars, invocations)
   if invocations == nil then invocations = {} end
-  if vim.tbl_contains(invocations, function(p) return self == p end, { predicate = true }) then return nil end
+  if vim.tbl_contains(invocations, function(p) return self == p end, { predicate = true }) then
+    return nil
+  end
   table.insert(invocations, self)
 
   local openers = { '{', '[', '(', '<' }
@@ -201,12 +211,20 @@ function Pos:find_match(max_chars, invocations)
   local is_closer = vim.tbl_contains(closers, c)
   if not is_opener and not is_closer then return nil end
 
-  local i, _ = vim.iter(is_opener and openers or closers):enumerate():find(function(_, c2) return c == c2 end)
+  local i, _ = vim
+    .iter(is_opener and openers or closers)
+    :enumerate()
+    :find(function(_, c2) return c == c2 end)
+  -- Store the character we will be looking for:
   local c_match = (is_opener and closers or openers)[i]
 
-  ---@type Pos|nil
+  --- @type u.Pos|nil
   local cur = self
-  ---@return Pos|nil
+  --- `adv` is a helper that moves the current position backward or forward,
+  --- depending on whether we are looking for an opener or a closer. It returns
+  --- nil if 1) the watch-dog `max_chars` falls bellow 0, or 2) if we have gone
+  --- beyond the beginning/end of the file.
+  --- @return u.Pos|nil
   local function adv()
     if cur == nil then return nil end
 
@@ -218,7 +236,7 @@ function Pos:find_match(max_chars, invocations)
     return cur:next(is_opener and 1 or -1)
   end
 
-  -- scan until we find a match:
+  -- scan until we find `c_match`:
   cur = adv()
   while cur ~= nil and cur:char() ~= c_match do
     cur = adv()
@@ -236,4 +254,17 @@ function Pos:find_match(max_chars, invocations)
   return cur
 end
 
+--- @param lines string|string[]
+function Pos:insert_before(lines)
+  if type(lines) == 'string' then lines = vim.split(lines, '\n') end
+  vim.api.nvim_buf_set_text(
+    self.bufnr,
+    self.lnum - 1,
+    self.col - 1,
+    self.lnum - 1,
+    self.col - 1,
+    lines
+  )
+end
+
 return Pos

lua/u/range.lua

@@ -1,31 +1,23 @@
 local Pos = require 'u.pos'
-local State = require 'u.state'
 
-local orig_on_yank = vim.highlight.on_yank
-local on_yank_enabled = true;
-(vim.highlight --[[@as any]]).on_yank = function(opts)
+-- Certain functions in the Range class yank text. In order to prevent unwanted
+-- highlighting, we intercept and discard some calls to the `on_yank` callback.
+local orig_on_yank = vim.hl.on_yank
+local on_yank_enabled = true
+--- @diagnostic disable-next-line: duplicate-set-field
+function vim.hl.on_yank(opts)
   if not on_yank_enabled then return end
   return orig_on_yank(opts)
 end
 
----@class Range
----@field start Pos
----@field stop Pos|nil
+--- @class u.Range
+--- @field start u.Pos
+--- @field stop u.Pos|nil
 --- @field mode 'v'|'V'
 local Range = {}
-
----@param start Pos
----@param stop Pos|nil
----@param mode? 'v'|'V'
----@return Range
-function Range.new(start, stop, mode)
-  if stop ~= nil and stop < start then
-    start, stop = stop, start
-  end
-
-  local r = { start = start, stop = stop, mode = mode or 'v' }
-  local function str()
-    ---@param p Pos
+Range.__index = Range
+function Range.__tostring(self)
+  --- @param p u.Pos
   local function posstr(p)
     if p == nil then
       return 'nil'
@@ -36,22 +28,54 @@ function Range.new(start, stop, mode)
     end
   end
 
-    local _1 = posstr(r.start)
-    local _2 = posstr(r.stop)
-    return string.format('Range{buf=%d, mode=%s, start=%s, stop=%s}', r.start.buf, r.mode, _1, _2)
+  local _1 = posstr(self.start)
+  local _2 = posstr(self.stop)
+  return string.format(
+    'Range{bufnr=%d, mode=%s, start=%s, stop=%s}',
+    self.start.bufnr,
+    self.mode,
+    _1,
+    _2
+  )
 end
-  setmetatable(r, { __index = Range, __tostring = str })
+
+--------------------------------------------------------------------------------
+-- Range constructors:
+--------------------------------------------------------------------------------
+
+--- @param start u.Pos
+--- @param stop u.Pos|nil
+--- @param mode? 'v'|'V'
+--- @return u.Range
+function Range.new(start, stop, mode)
+  if stop ~= nil and stop < start then
+    start, stop = stop, start
+  end
+
+  local r = { start = start, stop = stop, mode = mode or 'v' }
+
+  setmetatable(r, Range)
   return r
 end
 
-function Range.is(x)
-  local mt = getmetatable(x)
-  return mt and mt.__index == Range
+--- @param ranges (u.Range|nil)[]
+function Range.smallest(ranges)
+  --- @type u.Range[]
+  ranges = vim.iter(ranges):filter(function(r) return r ~= nil and not r:is_empty() end):totable()
+  if #ranges == 0 then return nil end
+
+  -- find smallest match
+  local smallest = ranges[1]
+  for _, r in ipairs(ranges) do
+    local start, stop = r.start, r.stop
+    if start > smallest.start and stop < smallest.stop then smallest = r end
+  end
+  return smallest
 end
 
 --- @param lpos string
 --- @param rpos string
----@return Range
+--- @return u.Range
 function Range.from_marks(lpos, rpos)
   local start = Pos.from_pos(lpos)
   local stop = Pos.from_pos(rpos)
@@ -67,98 +91,109 @@ function Range.from_marks(lpos, rpos)
   return Range.new(start, stop, mode)
 end
 
----@param buf? number
-function Range.from_buf_text(buf)
-  if buf == nil or buf == 0 then buf = vim.api.nvim_get_current_buf() end
-  local num_lines = vim.api.nvim_buf_line_count(buf)
+--- @param bufnr? number
+function Range.from_buf_text(bufnr)
+  if bufnr == nil or bufnr == 0 then bufnr = vim.api.nvim_get_current_buf() end
+  local num_lines = vim.api.nvim_buf_line_count(bufnr)
 
-  local start = Pos.new(buf, 0, 0)
-  local stop = Pos.new(buf, num_lines - 1, Pos.MAX_COL)
+  local start = Pos.new(bufnr, 1, 1)
+  local stop = Pos.new(bufnr, num_lines, Pos.MAX_COL)
   return Range.new(start, stop, 'V')
 end
 
----@param buf? number
----@param line number 0-based line index
-function Range.from_line(buf, line) return Range.from_lines(buf, line, line) end
+--- @param bufnr? number
+--- @param line number 1-based line index
+function Range.from_line(bufnr, line) return Range.from_lines(bufnr, line, line) end
 
----@param buf? number
----@param start_line number 0-based line index
----@param stop_line number 0-based line index
-function Range.from_lines(buf, start_line, stop_line)
-  if buf == nil or buf == 0 then buf = vim.api.nvim_get_current_buf() end
+--- @param bufnr? number
+--- @param start_line number based line index
+--- @param stop_line number based line index
+function Range.from_lines(bufnr, start_line, stop_line)
+  if bufnr == nil or bufnr == 0 then bufnr = vim.api.nvim_get_current_buf() end
   if stop_line < 0 then
-    local num_lines = vim.api.nvim_buf_line_count(buf)
-    stop_line = num_lines + stop_line
+    local num_lines = vim.api.nvim_buf_line_count(bufnr)
+    stop_line = num_lines + stop_line + 1
   end
-  return Range.new(Pos.new(buf, start_line, 0), Pos.new(buf, stop_line, Pos.MAX_COL), 'V')
+  return Range.new(Pos.new(bufnr, start_line, 1), Pos.new(bufnr, stop_line, Pos.MAX_COL), 'V')
 end
 
----@param text_obj string
----@param opts? { buf?: number; contains_cursor?: boolean; pos?: Pos, user_defined?: boolean }
----@return Range|nil
-function Range.from_text_object(text_obj, opts)
+--- @param motion string
+--- @param opts? { bufnr?: number; contains_cursor?: boolean; pos?: u.Pos, user_defined?: boolean }
+--- @return u.Range|nil
+function Range.from_motion(motion, opts)
+  -- Options handling:
   opts = opts or {}
-  if opts.buf == nil then opts.buf = vim.api.nvim_get_current_buf() end
+  if opts.bufnr == nil then opts.bufnr = vim.api.nvim_get_current_buf() end
   if opts.contains_cursor == nil then opts.contains_cursor = false end
   if opts.user_defined == nil then opts.user_defined = false end
 
-  ---@type "a" | "i"
-  local selection_type = text_obj:sub(1, 1)
-  local obj_type = text_obj:sub(#text_obj, #text_obj)
-  local is_quote = vim.tbl_contains({ "'", '"', '`' }, obj_type)
-  local cursor = Pos.from_pos '.'
-
-  -- Yank, then read '[ and '] to know the bounds:
-  ---@type { start: Pos; stop: Pos }
-  local positions
-  vim.api.nvim_buf_call(opts.buf, function()
-    positions = State.run(0, function(s)
-      s:track_winview()
-      s:track_register '"'
-      s:track_pos '.'
-      s:track_pos "'["
-      s:track_pos "']"
+  -- Extract some information from the motion:
+  --- @type 'a'|'i', string
+  local scope, motion_rest = motion:sub(1, 1), motion:sub(2)
+  local is_txtobj = scope == 'a' or scope == 'i'
+  local is_quote_txtobj = is_txtobj and vim.tbl_contains({ "'", '"', '`' }, motion_rest)
 
+  --- @type u.Pos
+  local start
+  --- @type u.Pos
+  local stop
+  -- Capture the original state of the buffer for restoration later.
+  local original_state = {
+    winview = vim.fn.winsaveview(),
+    regquote = vim.fn.getreg '"',
+    cursor = vim.fn.getpos '.',
+    pos_lbrack = vim.fn.getpos "'[",
+    pos_rbrack = vim.fn.getpos "']",
+  }
+  vim.api.nvim_buf_call(opts.bufnr, function()
     if opts.pos ~= nil then opts.pos:save_to_pos '.' end
 
-      local null_pos = Pos.new(0, 0, 0, 0)
-      null_pos:save_to_pos "'["
-      null_pos:save_to_pos "']"
+    Pos.invalid():save_to_pos "'["
+    Pos.invalid():save_to_pos "']"
 
     local prev_on_yank_enabled = on_yank_enabled
     on_yank_enabled = false
     vim.cmd {
       cmd = 'normal',
       bang = not opts.user_defined,
-        args = { '""y' .. text_obj },
+      args = { '""y' .. motion },
       mods = { silent = true },
     }
     on_yank_enabled = prev_on_yank_enabled
 
-      local start = Pos.from_pos "'["
-      local stop = Pos.from_pos "']"
+    start = Pos.from_pos "'["
+    stop = Pos.from_pos "']"
+  end)
+  -- Restore original state:
+  vim.fn.winrestview(original_state.winview)
+  vim.fn.setreg('"', original_state.regquote)
+  vim.fn.setpos('.', original_state.cursor)
+  vim.fn.setpos("'[", original_state.pos_lbrack)
+  vim.fn.setpos("']", original_state.pos_rbrack)
 
+  if start == stop and start:is_invalid() then return nil end
+
+  -- Fixup the bounds:
   if
     -- I have no idea why, but when yanking `i"`, the stop-mark is
     -- placed on the ending quote. For other text-objects, the stop-
     -- mark is placed before the closing character.
-        (is_quote and selection_type == 'i' and stop:char() == obj_type)
+    (is_quote_txtobj and scope == 'i' and stop:char() == motion_rest)
     -- *Sigh*, this also sometimes happens for `it` as well.
-        or (text_obj == 'it' and stop:char() == '<')
+    or (motion == 'it' and stop:char() == '<')
   then
     stop = stop:next(-1) or stop
   end
-      return { start = start, stop = stop }
-    end)
-  end)
-  local start = positions.start
-  local stop = positions.stop
-  if start == stop and start.lnum == 0 and start.col == 0 and start.off == 0 then return nil end
-  if opts.contains_cursor and not Range.new(start, stop):contains(cursor) then return nil end
+  if is_quote_txtobj and scope == 'a' then
+    start = start:find_next(1, motion_rest) or start
+    stop = stop:find_next(-1, motion_rest) or stop
+  end
 
-  if is_quote and selection_type == 'a' then
-    start = start:find_next(1, obj_type) or start
-    stop = stop:find_next(-1, obj_type) or stop
+  if
+    opts.contains_cursor
+    and not Range.new(start, stop):contains(Pos.new(unpack(original_state.cursor)))
+  then
+    return nil
   end
 
   return Range.new(start, stop)
@@ -189,11 +224,11 @@ end
 
 --- Get range information from command arguments.
 --- @param args unknown
----@return Range|nil
+--- @return u.Range|nil
 function Range.from_cmd_args(args)
   --- @type 'v'|'V'
   local mode
-  ---@type nil|Pos
+  --- @type nil|u.Pos
   local start
   local stop
   if args.range == 0 then
@@ -201,235 +236,84 @@ function Range.from_cmd_args(args)
   else
     start = Pos.from_pos "'<"
     stop = Pos.from_pos "'>"
-    if stop:is_col_max() then
-      mode = 'V'
-    else
-      mode = 'v'
-    end
+    mode = stop:is_col_max() and 'V' or 'v'
   end
   return Range.new(start, stop, mode)
 end
 
----
 function Range.find_nearest_brackets()
-  local a = Range.from_text_object('a<', { contains_cursor = true })
-  local b = Range.from_text_object('a[', { contains_cursor = true })
-  local c = Range.from_text_object('a(', { contains_cursor = true })
-  local d = Range.from_text_object('a{', { contains_cursor = true })
-  return Range.smallest { a, b, c, d }
+  return Range.smallest {
+    Range.from_motion('a<', { contains_cursor = true }),
+    Range.from_motion('a[', { contains_cursor = true }),
+    Range.from_motion('a(', { contains_cursor = true }),
+    Range.from_motion('a{', { contains_cursor = true }),
+  }
 end
 
 function Range.find_nearest_quotes()
-  local a = Range.from_text_object([[a']], { contains_cursor = true })
-  if a ~= nil and a:is_empty() then a = nil end
-  local b = Range.from_text_object([[a"]], { contains_cursor = true })
-  if b ~= nil and b:is_empty() then b = nil end
-  local c = Range.from_text_object([[a`]], { contains_cursor = true })
-  if c ~= nil and c:is_empty() then c = nil end
-  return Range.smallest { a, b, c }
+  return Range.smallest {
+    Range.from_motion([[a']], { contains_cursor = true }),
+    Range.from_motion([[a"]], { contains_cursor = true }),
+    Range.from_motion([[a`]], { contains_cursor = true }),
+  }
 end
 
----@param ranges (Range|nil)[]
-function Range.smallest(ranges)
-  ---@type Range[]
-  local new_ranges = {}
-  for _, r in pairs(ranges) do
-    if r ~= nil then table.insert(new_ranges, r) end
-  end
-  ranges = new_ranges
-  if #ranges == 0 then return nil end
+--------------------------------------------------------------------------------
+-- Structural utilities:
+--------------------------------------------------------------------------------
 
-  -- find smallest match
-  local max_start = ranges[1].start
-  local min_stop = ranges[1].stop
-  local result = ranges[1]
-
-  for _, r in ipairs(ranges) do
-    local start, stop = r.start, r.stop
-    if start > max_start and stop < min_stop then
-      max_start = start
-      min_stop = stop
-      result = r
-    end
+function Range:clone()
+  return Range.new(self.start:clone(), self.stop ~= nil and self.stop:clone() or nil, self.mode)
 end
 
-  return result
-end
-
-function Range:clone() return Range.new(self.start:clone(), self.stop ~= nil and self.stop:clone() or nil, self.mode) end
-function Range:line_count()
-  if self:is_empty() then return 0 end
-  return self.stop.lnum - self.start.lnum + 1
-end
+function Range:is_empty() return self.stop == nil end
 
 function Range:to_linewise()
   local r = self:clone()
 
   r.mode = 'V'
-  r.start.col = 0
+  r.start.col = 1
   if r.stop ~= nil then r.stop.col = Pos.MAX_COL end
 
   return r
 end
 
-function Range:is_empty() return self.stop == nil end
-
-function Range:trim_start()
-  if self:is_empty() then return end
-
-  local r = self:clone()
-  while r.start:char():match '%s' do
-    local next = r.start:next(1)
-    if next == nil then break end
-    r.start = next
+--- @param x u.Pos | u.Range
+function Range:contains(x)
+  if getmetatable(x) == Pos then
+    return not self:is_empty() and x >= self.start and x <= self.stop
+  elseif getmetatable(x) == Range then
+    return self:contains(x.start) and self:contains(x.stop)
   end
-  return r
+  return false
 end
 
-function Range:trim_stop()
-  if self:is_empty() then return end
-
-  local r = self:clone()
-  while r.stop:char():match '%s' do
-    local next = r.stop:next(-1)
-    if next == nil then break end
-    r.stop = next
-  end
-  return r
+--- @param other u.Range
+--- @return u.Range|nil, u.Range|nil
+function Range:difference(other)
+  local outer, inner = self, other
+  if not outer:contains(inner) then
+    outer, inner = inner, outer
   end
+  if not outer:contains(inner) then return nil, nil end
 
----@param p Pos
-function Range:contains(p) return not self:is_empty() and p >= self.start and p <= self.stop end
-
----@return string[]
-function Range:lines()
-  if self:is_empty() then return {} end
-
-  local lines = {}
-  for i = 0, self.stop.lnum - self.start.lnum do
-    local line = self:line0(i)
-    if line ~= nil then table.insert(lines, line.text()) end
-  end
-  return lines
-end
-
----@return string
-function Range:text() return vim.fn.join(self:lines(), '\n') end
-
----@param i number 1-based
----@param j? number 1-based
-function Range:sub(i, j) return self:text():sub(i, j) end
-
----@param l number
----@return { line: string; idx0: { start: number; stop: number; }; lnum: number; range: fun():Range; text: fun():string }|nil
-function Range:line0(l)
-  if l < 0 then return self:line0(self:line_count() + l) end
-  if l > self:line_count() then return end
-
-  local line = vim.api.nvim_buf_get_lines(self.start.buf, self.start.lnum + l, self.start.lnum + l + 1, false)[1]
-  if line == nil then return end
-
-  local start = 0
-  local stop = #line - 1
-  if l == 0 then start = self.start.col end
-  if l == self.stop.lnum - self.start.lnum then stop = self.stop.col end
-  if stop == Pos.MAX_COL then stop = #line - 1 end
-  local lnum = self.start.lnum + l
-
-  return {
-    line = line,
-    idx0 = { start = start, stop = stop },
-    lnum = lnum,
-    range = function()
-      return Range.new(
-        Pos.new(self.start.buf, lnum, start, self.start.off),
-        Pos.new(self.start.buf, lnum, stop, self.stop.off),
-        'v'
-      )
-    end,
-    text = function() return line:sub(start + 1, stop + 1) end,
-  }
-end
-
----@param replacement nil|string|string[]
-function Range:replace(replacement)
-  if replacement == nil then replacement = {} end
-  if type(replacement) == 'string' then replacement = vim.fn.split(replacement, '\n') end
-
-  local buf = self.start.buf
-  -- convert to start-inclusive, stop-exclusive coordinates:
-  local start_lnum, stop_lnum = self.start.lnum, (self.stop and self.stop.lnum or self.start.lnum) + 1
-  local start_col, stop_col = self.start.col, (self.stop and self.stop.col or self.start.col) + 1
-
-  local replace_type = (self:is_empty() and 'insert') or (self.mode == 'v' and 'region') or 'lines'
-
-  ---@param alnum number
-  ---@param acol number
-  ---@param blnum number
-  ---@param bcol number
-  local function set_text(alnum, acol, blnum, bcol, repl)
-    -- row indices are end-inclusive, and column indices are end-exclusive.
-    vim.api.nvim_buf_set_text(buf, alnum, acol, blnum, bcol, repl)
-
-    local new_last_line_num = self.start.lnum + #replacement - 1
-    local new_last_col = #(replacement[#replacement] or '')
-    if new_last_line_num == start_lnum then new_last_col = new_last_col + start_col - 1 end
-
-    self.stop = Pos.new(buf, new_last_line_num, new_last_col)
-  end
-
-  ---@param alnum number
-  ---@param blnum number
-  local function set_lines(alnum, blnum, repl)
-    -- indexing is zero-based, end-exclusive
-    vim.api.nvim_buf_set_lines(buf, alnum, blnum, false, repl)
-
-    if #repl == 0 then
-      self.stop = nil
+  local left
+  if outer.start ~= inner.start then
+    local stop = inner.start:clone() - 1
+    left = Range.new(outer.start, stop)
   else
-      local new_last_line_num = start_lnum + #replacement - 1
-      self.stop = Pos.new(self.start.buf, new_last_line_num, Pos.MAX_COL, self.stop.off)
-    end
-    self.mode = 'v'
+    left = Range.new(outer.start) -- empty range
   end
 
-  if replace_type == 'insert' then
-    set_text(start_lnum, start_col, start_lnum, start_col, replacement)
-  elseif replace_type == 'region' then
-    -- Fixup the bounds:
-    local last_line = vim.api.nvim_buf_get_lines(buf, stop_lnum - 1, stop_lnum, false)[1] or ''
-    local max_col = #last_line
-    set_text(start_lnum, start_col, stop_lnum - 1, math.min(stop_col, max_col), replacement)
-  elseif replace_type == 'lines' then
-    set_lines(start_lnum, stop_lnum, replacement)
+  local right
+  if inner.stop ~= outer.stop then
+    local start = inner.stop:clone() + 1
+    right = Range.new(start, outer.stop)
   else
-    error 'unreachable'
-  end
+    right = Range.new(inner.stop) -- empty range
   end
 
----@param amount number
-function Range:shrink(amount)
-  local start = self.start
-  local stop = self.stop
-  if stop == nil then return self:clone() end
-
-  for _ = 1, amount do
-    local next_start = start:next(1)
-    local next_stop = stop:next(-1)
-    if next_start == nil or next_stop == nil then return end
-    start = next_start
-    stop = next_stop
-    if next_start > next_stop then break end
-  end
-  if start > stop then stop = nil end
-  return Range.new(start, stop, self.mode)
-end
-
----@param amount number
-function Range:must_shrink(amount)
-  local shrunk = self:shrink(amount)
-  if shrunk == nil or shrunk:is_empty() then error 'error in Range:must_shrink: Range:shrink() returned nil' end
-  return shrunk
+  return left, right
 end
 
 --- @param left string
@@ -458,24 +342,225 @@ end
 
 function Range:set_visual_selection()
   if self:is_empty() then return end
+  if vim.api.nvim_get_current_buf() ~= self.start.bufnr then
+    error 'Range:set_visual_selection() called on a buffer other than the current buffer'
+  end
 
-  if vim.api.nvim_get_current_buf() ~= self.start.buf then vim.api.nvim_set_current_buf(self.start.buf) end
+  local curr_mode = vim.fn.mode()
+  if curr_mode ~= self.mode then vim.cmd.normal { args = { self.mode }, bang = true } end
 
-  State.run(self.start.buf, function(s)
-    s:track_mark 'a'
-    s:track_mark 'b'
+  self.start:save_to_pos '.'
+  vim.cmd.normal { args = { 'o' }, bang = true }
+  self.stop:save_to_pos '.'
+end
 
-    self.start:save_to_mark 'a'
-    self.stop:save_to_mark 'b'
-    local mode = self.mode
+--------------------------------------------------------------------------------
+-- Range.from_* functions:
+--------------------------------------------------------------------------------
 
-    local normal_cmd_args = ''
-    if vim.api.nvim_get_mode().mode == 'n' then normal_cmd_args = normal_cmd_args .. mode end
-    normal_cmd_args = normal_cmd_args .. '`ao`b'
-    vim.cmd { cmd = 'normal', args = { normal_cmd_args }, bang = true }
+--------------------------------------------------------------------------------
+-- Text access/manipulation utilities:
+--------------------------------------------------------------------------------
 
-    return nil
-  end)
+function Range:length()
+  if self:is_empty() then return 0 end
+
+  local line_positions =
+    vim.fn.getregionpos(self.start:as_real():as_vim(), self.stop:as_real():as_vim(), { type = 'v' })
+
+  local len = 0
+  for linenr, line in ipairs(line_positions) do
+    if linenr > 1 then len = len + 1 end -- each newline is counted as a char
+    local line_start_col = line[1][3]
+    local line_stop_col = line[2][3]
+    local line_len = line_stop_col - line_start_col + 1
+    len = len + line_len
+  end
+  return len
+end
+
+function Range:line_count()
+  if self:is_empty() then return 0 end
+  return self.stop.lnum - self.start.lnum + 1
+end
+
+function Range:trim_start()
+  if self:is_empty() then return end
+
+  local r = self:clone()
+  while r.start:char():match '%s' do
+    local next = r.start:next(1)
+    if next == nil then break end
+    r.start = next
+  end
+  return r
+end
+
+function Range:trim_stop()
+  if self:is_empty() then return end
+
+  local r = self:clone()
+  while r.stop:char():match '%s' do
+    local next = r.stop:next(-1)
+    if next == nil then break end
+    r.stop = next
+  end
+  return r
+end
+
+--- @param i number 1-based
+--- @param j? number 1-based
+function Range:sub(i, j)
+  local line_positions =
+    vim.fn.getregionpos(self.start:as_real():as_vim(), self.stop:as_real():as_vim(), { type = 'v' })
+
+  --- @param idx number 1-based
+  --- @return u.Pos|nil
+  local function get_pos(idx)
+    if idx < 0 then return get_pos(self:length() + idx + 1) end
+
+    -- find the position of the first line that contains the i-th character:
+    local curr_len = 0
+    for linenr, line in ipairs(line_positions) do
+      if linenr > 1 then curr_len = curr_len + 1 end -- each newline is counted as a char
+      local line_start_col = line[1][3]
+      local line_stop_col = line[2][3]
+      local line_len = line_stop_col - line_start_col + 1
+
+      if curr_len + line_len >= idx then
+        return Pos.new(self.start.bufnr, line[1][2], line_start_col + (idx - curr_len) - 1)
+      end
+      curr_len = curr_len + line_len
+    end
+  end
+
+  local start = get_pos(i)
+  if not start then
+    -- start is inalid, so return an empty range:
+    return Range.new(self.start, nil, self.mode)
+  end
+
+  local stop
+  if j then stop = get_pos(j) end
+  if not stop then
+    -- stop is inalid, so return an empty range:
+    return Range.new(start, nil, self.mode)
+  end
+  return Range.new(start, stop, 'v')
+end
+
+--- @return string[]
+function Range:lines()
+  if self:is_empty() then return {} end
+  return vim.fn.getregion(self.start:as_vim(), self.stop:as_vim(), { type = self.mode })
+end
+
+--- @return string
+function Range:text() return vim.fn.join(self:lines(), '\n') end
+
+--- @param l number
+-- luacheck: ignore
+--- @return { line: string; idx0: { start: number; stop: number; }; lnum: number; range: fun():u.Range; text: fun():string }|nil
+function Range:line(l)
+  if l < 0 then l = self:line_count() + l + 1 end
+  if l > self:line_count() then return end
+
+  local line_indices =
+    vim.fn.getregionpos(self.start:as_vim(), self.stop:as_vim(), { type = self.mode })
+  local line_bounds = line_indices[l]
+
+  local start = Pos.new(unpack(line_bounds[1]))
+  local stop = Pos.new(unpack(line_bounds[2]))
+  return Range.new(start, stop)
+end
+
+--- @param replacement nil|string|string[]
+function Range:replace(replacement)
+  if replacement == nil then replacement = {} end
+  if type(replacement) == 'string' then replacement = vim.fn.split(replacement, '\n') end
+
+  local bufnr = self.start.bufnr
+  local replace_type = (self:is_empty() and 'insert') or (self.mode == 'v' and 'region') or 'lines'
+
+  local function update_stop_non_linewise()
+    local new_last_line_num = self.start.lnum + #replacement - 1
+    local new_last_col = #(replacement[#replacement] or '')
+    if new_last_line_num == self.start.lnum then
+      new_last_col = new_last_col + self.start.col - 1
+    end
+    self.stop = Pos.new(bufnr, new_last_line_num, new_last_col)
+  end
+  local function update_stop_linewise()
+    if #replacement == 0 then
+      self.stop = nil
+    else
+      local new_last_line_num = self.start.lnum - 1 + #replacement - 1
+      self.stop = Pos.new(bufnr, new_last_line_num + 1, Pos.MAX_COL, self.stop.off)
+    end
+    self.mode = 'v'
+  end
+
+  if replace_type == 'insert' then
+    -- To insert text at a given `(row, column)` location, use `start_row =
+    -- end_row = row` and `start_col = end_col = col`.
+    vim.api.nvim_buf_set_text(
+      bufnr,
+      self.start.lnum - 1,
+      self.start.col - 1,
+      self.start.lnum - 1,
+      self.start.col - 1,
+      replacement
+    )
+    update_stop_non_linewise()
+  elseif replace_type == 'region' then
+    -- Fixup the bounds:
+    local max_col = #self.stop:line()
+
+    -- Indexing is zero-based. Row indices are end-inclusive, and column indices
+    -- are end-exclusive.
+    vim.api.nvim_buf_set_text(
+      bufnr,
+      self.start.lnum - 1,
+      self.start.col - 1,
+      self.stop.lnum - 1,
+      math.min(self.stop.col, max_col),
+      replacement
+    )
+    update_stop_non_linewise()
+  elseif replace_type == 'lines' then
+    -- Indexing is zero-based, end-exclusive.
+    vim.api.nvim_buf_set_lines(bufnr, self.start.lnum - 1, self.stop.lnum, true, replacement)
+    update_stop_linewise()
+  else
+    error 'unreachable'
+  end
+end
+
+--- @param amount number
+function Range:shrink(amount)
+  local start = self.start
+  local stop = self.stop
+  if stop == nil then return self:clone() end
+
+  for _ = 1, amount do
+    local next_start = start:next(1)
+    local next_stop = stop:next(-1)
+    if next_start == nil or next_stop == nil then return end
+    start = next_start
+    stop = next_stop
+    if next_start > next_stop then break end
+  end
+  if start > stop then stop = nil end
+  return Range.new(start, stop, self.mode)
+end
+
+--- @param amount number
+function Range:must_shrink(amount)
+  local shrunk = self:shrink(amount)
+  if shrunk == nil or shrunk:is_empty() then
+    error 'error in Range:must_shrink: Range:shrink() returned nil'
+  end
+  return shrunk
 end
 
 --- @param group string
@@ -490,33 +575,31 @@ function Range:highlight(group, opts)
   if not opts.on_macro and in_macro then return { clear = function() end } end
 
   local ns = vim.api.nvim_create_namespace ''
-  State.run(self.start.buf, function(s)
-    if not in_macro then s:track_winview() end
 
-    vim.highlight.range(
-      self.start.buf,
+  local winview = vim.fn.winsaveview()
+  vim.hl.range(
+    self.start.bufnr,
     ns,
     group,
-      { self.start.lnum, self.start.col },
-      { self.stop.lnum, self.stop.col },
+    { self.start.lnum - 1, self.start.col - 1 },
+    { self.stop.lnum - 1, self.stop.col - 1 },
     {
       inclusive = true,
       priority = opts.priority,
+      timeout = opts.timeout,
       regtype = self.mode,
     }
   )
-
-    return nil
-  end)
+  if not in_macro then vim.fn.winrestview(winview) end
   vim.cmd.redraw()
 
-  local function clear()
-    vim.api.nvim_buf_clear_namespace(self.start.buf, ns, self.start.lnum, self.stop.lnum + 1)
+  return {
+    ns = ns,
+    clear = function()
+      vim.api.nvim_buf_clear_namespace(self.start.bufnr, ns, self.start.lnum - 1, self.stop.lnum)
       vim.cmd.redraw()
-  end
-  if opts.timeout ~= nil then vim.defer_fn(clear, opts.timeout) end
-
-  return { ns = ns, clear = clear }
+    end,
+  }
 end
 
 return Range

lua/u/renderer.lua

@@ -0,0 +1,570 @@
+local M = {}
+local H = {}
+
+--- @alias u.renderer.Tag { kind: 'tag'; name: string, attributes: table<string, unknown>, children: u.renderer.Tree }
+--- @alias u.renderer.Node nil | boolean | string | u.renderer.Tag
+--- @alias u.renderer.Tree u.renderer.Node | u.renderer.Node[]
+
+-- luacheck: ignore
+--- @type table<string, fun(attributes: table<string, any>, children: u.renderer.Tree): u.renderer.Tag> & fun(name: string, attributes: table<string, any>, children: u.renderer.Tree): u.renderer.Tag>
+M.h = setmetatable({}, {
+  __call = function(_, name, attributes, children)
+    return {
+      kind = 'tag',
+      name = name,
+      attributes = attributes or {},
+      children = children,
+    }
+  end,
+  __index = function(_, name)
+    -- vim.print('dynamic hl ' .. name)
+    return function(attributes, children)
+      return M.h('text', vim.tbl_deep_extend('force', { hl = name }, attributes), children)
+    end
+  end,
+})
+
+-- Renderer {{{
+--- @alias RendererExtmark { id?: number; start: [number, number]; stop: [number, number]; opts: any; tag: any }
+
+--- @class u.Renderer
+--- @field bufnr number
+--- @field ns number
+--- @field changedtick number
+--- @field old { lines: string[]; extmarks: RendererExtmark[] }
+--- @field curr { lines: string[]; extmarks: RendererExtmark[] }
+local Renderer = {}
+Renderer.__index = Renderer
+M.Renderer = Renderer
+
+--- @private
+--- @param x any
+--- @return boolean
+function Renderer.is_tag(x) return type(x) == 'table' and x.kind == 'tag' end
+
+--- @private
+--- @param x any
+--- @return boolean
+function Renderer.is_tag_arr(x)
+  if type(x) ~= 'table' then return false end
+  return #x == 0 or not Renderer.is_tag(x)
+end
+--- @param bufnr number|nil
+function Renderer.new(bufnr) -- {{{
+  if bufnr == nil then bufnr = vim.api.nvim_get_current_buf() end
+
+  if vim.b[bufnr]._renderer_ns == nil then
+    vim.b[bufnr]._renderer_ns = vim.api.nvim_create_namespace('my.renderer:' .. tostring(bufnr))
+  end
+
+  local self = setmetatable({
+    bufnr = bufnr,
+    ns = vim.b[bufnr]._renderer_ns,
+    changedtick = 0,
+    old = { lines = {}, extmarks = {} },
+    curr = { lines = {}, extmarks = {} },
+  }, Renderer)
+  return self
+end -- }}}
+
+--- @param opts {
+---   tree: u.renderer.Tree;
+---   on_tag?: fun(tag: u.renderer.Tag, start0: [number, number], stop0: [number, number]): any;
+--- }
+function Renderer.markup_to_lines(opts) -- {{{
+  --- @type string[]
+  local lines = {}
+
+  local curr_line1 = 1
+  local curr_col1 = 1 -- exclusive: sits one position **beyond** the last inserted text
+  --- @param s string
+  local function put(s)
+    lines[curr_line1] = (lines[curr_line1] or '') .. s
+    curr_col1 = #lines[curr_line1] + 1
+  end
+  local function put_line()
+    table.insert(lines, '')
+    curr_line1 = curr_line1 + 1
+    curr_col1 = 1
+  end
+
+  --- @param node u.renderer.Node
+  local function visit(node) -- {{{
+    if node == nil or type(node) == 'boolean' then return end
+
+    if type(node) == 'string' then
+      local node_lines = vim.split(node, '\n')
+      for lnum, s in ipairs(node_lines) do
+        if lnum > 1 then put_line() end
+        put(s)
+      end
+    elseif Renderer.is_tag(node) then
+      local start0 = { curr_line1 - 1, curr_col1 - 1 }
+
+      -- visit the children:
+      if Renderer.is_tag_arr(node.children) then
+        for _, child in
+          ipairs(node.children --[[@as u.renderer.Node[] ]])
+        do
+          -- newlines are not controlled by array entries, do NOT output a line here:
+          visit(child)
+        end
+      else -- luacheck: ignore
+        visit(node.children)
+      end
+
+      local stop0 = { curr_line1 - 1, curr_col1 - 1 }
+      if opts.on_tag then opts.on_tag(node, start0, stop0) end
+    elseif Renderer.is_tag_arr(node) then
+      for _, child in ipairs(node) do
+        -- newlines are not controlled by array entries, do NOT output a line here:
+        visit(child)
+      end
+    end
+  end -- }}}
+  visit(opts.tree)
+
+  return lines
+end -- }}}
+
+--- @param opts {
+---   tree: string;
+---   format_tag?: fun(tag: u.renderer.Tag): string;
+--- }
+function Renderer.markup_to_string(opts) return table.concat(Renderer.markup_to_lines(opts), '\n') end
+
+--- @param bufnr number
+--- @param old_lines string[] | nil
+--- @param new_lines string[]
+function Renderer.patch_lines(bufnr, old_lines, new_lines)
+  --
+  -- Helpers:
+  --
+
+  --- @param start integer
+  --- @param end_ integer
+  --- @param strict_indexing boolean
+  --- @param replacement string[]
+  local function _set_lines(start, end_, strict_indexing, replacement)
+    vim.api.nvim_buf_set_lines(bufnr, start, end_, strict_indexing, replacement)
+  end
+
+  --- @param start_row integer
+  --- @param start_col integer
+  --- @param end_row integer
+  --- @param end_col integer
+  --- @param replacement string[]
+  local function _set_text(start_row, start_col, end_row, end_col, replacement)
+    vim.api.nvim_buf_set_text(bufnr, start_row, start_col, end_row, end_col, replacement)
+  end
+
+  -- Morph the text to the desired state:
+  local line_changes =
+    H.levenshtein(old_lines or vim.api.nvim_buf_get_lines(bufnr, 0, -1, false), new_lines)
+  for _, line_change in ipairs(line_changes) do
+    local lnum0 = line_change.index - 1
+
+    if line_change.kind == 'add' then
+      _set_lines(lnum0, lnum0, true, { line_change.item })
+    elseif line_change.kind == 'change' then
+      -- Compute inter-line diff, and apply:
+      local col_changes =
+        H.levenshtein(vim.split(line_change.from, ''), vim.split(line_change.to, ''))
+
+      for _, col_change in ipairs(col_changes) do
+        local cnum0 = col_change.index - 1
+        if col_change.kind == 'add' then
+          _set_text(lnum0, cnum0, lnum0, cnum0, { col_change.item })
+        elseif col_change.kind == 'change' then
+          _set_text(lnum0, cnum0, lnum0, cnum0 + 1, { col_change.to })
+        elseif col_change.kind == 'delete' then
+          _set_text(lnum0, cnum0, lnum0, cnum0 + 1, {})
+        else -- luacheck: ignore
+          -- No change
+        end
+      end
+    elseif line_change.kind == 'delete' then
+      _set_lines(lnum0, lnum0 + 1, true, {})
+    else -- luacheck: ignore
+      -- No change
+    end
+  end
+end
+
+--- @param tree u.renderer.Tree
+function Renderer:render(tree) -- {{{
+  local changedtick = vim.b[self.bufnr].changedtick
+  if changedtick ~= self.changedtick then
+    self.curr = { lines = vim.api.nvim_buf_get_lines(self.bufnr, 0, -1, false) }
+    self.changedtick = changedtick
+  end
+
+  --- @type RendererExtmark[]
+  local extmarks = {}
+
+  --- @type string[]
+  local lines = Renderer.markup_to_lines {
+    tree = tree,
+
+    on_tag = function(tag, start0, stop0) -- {{{
+      if tag.name == 'text' then
+        local hl = tag.attributes.hl
+        if type(hl) == 'string' then
+          tag.attributes.extmark = tag.attributes.extmark or {}
+          tag.attributes.extmark.hl_group = tag.attributes.extmark.hl_group or hl
+        end
+
+        local extmark = tag.attributes.extmark
+
+        -- Set any necessary keymaps:
+        for _, mode in ipairs { 'i', 'n', 'v', 'x', 'o' } do
+          for lhs, _ in pairs(tag.attributes[mode .. 'map'] or {}) do
+            -- Force creating an extmark if there are key handlers. To accurately
+            -- sense the bounds of the text, we need an extmark:
+            extmark = extmark or {}
+            vim.keymap.set(
+              'n',
+              lhs,
+              function() return self:_expr_map_callback('n', lhs) end,
+              { buffer = self.bufnr, expr = true, replace_keycodes = true }
+            )
+          end
+        end
+
+        if extmark then
+          table.insert(extmarks, {
+            start = start0,
+            stop = stop0,
+            opts = extmark,
+            tag = tag,
+          })
+        end
+      end
+    end, -- }}}
+  }
+
+  self.old = self.curr
+  self.curr = { lines = lines, extmarks = extmarks }
+  self:_reconcile()
+end -- }}}
+
+--- @private
+--- @param start integer
+--- @param end_ integer
+--- @param strict_indexing boolean
+--- @param replacement string[]
+function Renderer:_set_lines(start, end_, strict_indexing, replacement)
+  vim.api.nvim_buf_set_lines(self.bufnr, start, end_, strict_indexing, replacement)
+end
+
+--- @private
+--- @param start_row integer
+--- @param start_col integer
+--- @param end_row integer
+--- @param end_col integer
+--- @param replacement string[]
+function Renderer:_set_text(start_row, start_col, end_row, end_col, replacement)
+  vim.api.nvim_buf_set_text(self.bufnr, start_row, start_col, end_row, end_col, replacement)
+end
+
+--- @private
+function Renderer:_reconcile() -- {{{
+  --
+  -- Step 1: morph the text to the desired state:
+  --
+  Renderer.patch_lines(self.bufnr, self.old.lines, self.curr.lines)
+  self.changedtick = vim.b[self.bufnr].changedtick
+
+  --
+  -- Step 2: reconcile extmarks:
+  --
+
+  -- Clear current extmarks:
+  vim.api.nvim_buf_clear_namespace(self.bufnr, self.ns, 0, -1)
+  -- Set current extmarks:
+  for _, extmark in ipairs(self.curr.extmarks) do
+    extmark.id = vim.api.nvim_buf_set_extmark(
+      self.bufnr,
+      self.ns,
+      extmark.start[1],
+      extmark.start[2],
+      vim.tbl_extend('force', {
+        id = extmark.id,
+        end_row = extmark.stop[1],
+        end_col = extmark.stop[2],
+      }, extmark.opts)
+    )
+  end
+
+  self.old = self.curr
+end -- }}}
+
+--- @private
+--- @param mode string
+--- @param lhs string
+function Renderer:_expr_map_callback(mode, lhs) -- {{{
+  -- find the tag with the smallest intersection that contains the cursor:
+  local pos0 = vim.api.nvim_win_get_cursor(0)
+  pos0[1] = pos0[1] - 1 -- make it actually 0-based
+  local pos_infos = self:get_pos_infos(pos0)
+
+  if #pos_infos == 0 then return lhs end
+
+  -- Find the first tag that is listening for this event:
+  local cancel = false
+  for _, pos_info in ipairs(pos_infos) do
+    local tag = pos_info.tag
+
+    -- is the tag listening?
+    local f = vim.tbl_get(tag.attributes, mode .. 'map', lhs)
+    if type(f) == 'function' then
+      local result = f()
+      if result == '' then
+        -- bubble-up to the next tag, but set cancel to true, in case there are
+        -- no more tags to bubble up to:
+        cancel = true
+      else
+        return result
+      end
+    end
+  end
+
+  -- Resort to default behavior:
+  return cancel and '' or lhs
+end -- }}}
+
+--- Returns pairs of extmarks and tags associate with said extmarks. The
+--- returned tags/extmarks are sorted smallest (innermost) to largest
+--- (outermost).
+---
+--- @private (private for now)
+--- @param pos0 [number; number]
+--- @return { extmark: RendererExtmark; tag: u.renderer.Tag; }[]
+function Renderer:get_pos_infos(pos0) -- {{{
+  local cursor_line0, cursor_col0 = pos0[1], pos0[2]
+
+  -- The cursor (block) occupies **two** extmark spaces: one for it's left
+  -- edge, and one for it's right. We need to do our own intersection test,
+  -- because the NeoVim API is over-inclusive in what it returns:
+  --- @type RendererExtmark[]
+  local intersecting_extmarks = vim
+    .iter(
+      vim.api.nvim_buf_get_extmarks(
+        self.bufnr,
+        self.ns,
+        pos0,
+        pos0,
+        { details = true, overlap = true }
+      )
+    )
+    --- @return RendererExtmark
+    :map(function(ext)
+      --- @type number, number, number, { end_row?: number; end_col?: number }|nil
+      local id, line0, col0, details = unpack(ext)
+      local start = { line0, col0 }
+      local stop = { line0, col0 }
+      if details and details.end_row ~= nil and details.end_col ~= nil then
+        stop = { details.end_row, details.end_col }
+      end
+      return { id = id, start = start, stop = stop, opts = details }
+    end)
+    --- @param ext RendererExtmark
+    :filter(function(ext)
+      if ext.stop[1] ~= nil and ext.stop[2] ~= nil then
+        return cursor_line0 >= ext.start[1]
+          and cursor_col0 >= ext.start[2]
+          and cursor_line0 <= ext.stop[1]
+          and cursor_col0 < ext.stop[2]
+      else
+        return true
+      end
+    end)
+    :totable()
+
+  -- Sort the tags into smallest (inner) to largest (outer):
+  table.sort(
+    intersecting_extmarks,
+    --- @param x1 RendererExtmark
+    --- @param x2 RendererExtmark
+    function(x1, x2)
+      if
+        x1.start[1] == x2.start[1]
+        and x1.start[2] == x2.start[2]
+        and x1.stop[1] == x2.stop[1]
+        and x1.stop[2] == x2.stop[2]
+      then
+        return x1.id < x2.id
+      end
+
+      return x1.start[1] >= x2.start[1]
+        and x1.start[2] >= x2.start[2]
+        and x1.stop[1] <= x2.stop[1]
+        and x1.stop[2] <= x2.stop[2]
+    end
+  )
+
+  -- When we set the extmarks in the step above, we captured the IDs of the
+  -- created extmarks in self.curr.extmarks, which also has which tag each
+  -- extmark is associated with. Cross-reference with that list to get a list
+  -- of tags that we need to fire events for:
+  --- @type { extmark: RendererExtmark; tag: u.renderer.Tag }[]
+  local matching_tags = vim
+    .iter(intersecting_extmarks)
+    --- @param ext RendererExtmark
+    :map(function(ext)
+      for _, extmark_cache in ipairs(self.curr.extmarks) do
+        if extmark_cache.id == ext.id then return { extmark = ext, tag = extmark_cache.tag } end
+      end
+    end)
+    :totable()
+
+  return matching_tags
+end -- }}}
+-- }}}
+
+-- TreeBuilder {{{
+--- @class u.TreeBuilder
+--- @field private nodes u.renderer.Node[]
+local TreeBuilder = {}
+TreeBuilder.__index = TreeBuilder
+M.TreeBuilder = TreeBuilder
+
+function TreeBuilder.new()
+  local self = setmetatable({ nodes = {} }, TreeBuilder)
+  return self
+end
+
+--- @param nodes u.renderer.Tree
+--- @return u.TreeBuilder
+function TreeBuilder:put(nodes)
+  table.insert(self.nodes, nodes)
+  return self
+end
+
+--- @param name string
+--- @param attributes? table<string, any>
+--- @param children? u.renderer.Node | u.renderer.Node[]
+--- @return u.TreeBuilder
+function TreeBuilder:put_h(name, attributes, children)
+  local tag = M.h(name, attributes, children)
+  table.insert(self.nodes, tag)
+  return self
+end
+
+--- @param fn fun(TreeBuilder): any
+--- @return u.TreeBuilder
+function TreeBuilder:nest(fn)
+  local nested_writer = TreeBuilder.new()
+  fn(nested_writer)
+  table.insert(self.nodes, nested_writer.nodes)
+  return self
+end
+
+--- @return u.renderer.Tree
+function TreeBuilder:tree() return self.nodes end
+-- }}}
+
+-- Levenshtein utility {{{
+-- luacheck: ignore
+--- @alias LevenshteinChange<T> ({ kind: 'add'; item: T; index: number; } | { kind: 'delete'; item: T; index: number; } | { kind: 'change'; from: T; to: T; index: number; })
+--- @private
+--- @generic T
+--- @param x `T`[]
+--- @param y T[]
+--- @param cost? { of_delete?: fun(x: T): number; of_add?: fun(x: T): number; of_change?: fun(x: T, y: T): number; }
+--- @return LevenshteinChange<T>[] The changes, from last (greatest index) to first (smallest index).
+function H.levenshtein(x, y, cost)
+  -- At the moment, this whole `cost` plumbing is not used. Deletes have the
+  -- same cost as Adds or Changes. I can imagine a future, however, where
+  -- fudging with the costs of operations produces a more optimized change-set
+  -- that is tailored to working better with how NeoVim manipulates text. I've
+  -- done no further investigation in this area, however, so it's impossible to
+  -- tell if such tuning would produce real benefit. For now, I'm leaving this
+  -- in here even though it's not actively used. Hopefully having this
+  -- callback-based plumbing does not cause too much of a performance hit to
+  -- the renderer.
+  cost = cost or {}
+  local cost_of_delete_f = cost.of_delete or function() return 1 end
+  local cost_of_add_f = cost.of_add or function() return 1 end
+  local cost_of_change_f = cost.of_change or function() return 1 end
+
+  local m, n = #x, #y
+  -- Initialize the distance matrix
+  local dp = {}
+  for i = 0, m do
+    dp[i] = {}
+    for j = 0, n do
+      dp[i][j] = 0
+    end
+  end
+
+  -- Fill the base cases
+  for i = 0, m do
+    dp[i][0] = i
+  end
+  for j = 0, n do
+    dp[0][j] = j
+  end
+
+  -- Compute the Levenshtein distance dynamically
+  for i = 1, m do
+    for j = 1, n do
+      if x[i] == y[j] then
+        dp[i][j] = dp[i - 1][j - 1] -- no cost if items are the same
+      else
+        local cost_delete = dp[i - 1][j] + cost_of_delete_f(x[i])
+        local cost_add = dp[i][j - 1] + cost_of_add_f(y[j])
+        local cost_change = dp[i - 1][j - 1] + cost_of_change_f(x[i], y[j])
+        dp[i][j] = math.min(cost_delete, cost_add, cost_change)
+      end
+    end
+  end
+
+  -- Backtrack to find the changes
+  local i = m
+  local j = n
+  --- @type LevenshteinChange[]
+  local changes = {}
+
+  while i > 0 or j > 0 do
+    local default_cost = dp[i][j]
+    local cost_of_change = (i > 0 and j > 0) and dp[i - 1][j - 1] or default_cost
+    local cost_of_add = j > 0 and dp[i][j - 1] or default_cost
+    local cost_of_delete = i > 0 and dp[i - 1][j] or default_cost
+
+    --- @param u number
+    --- @param v number
+    --- @param w number
+    local function is_first_min(u, v, w) return u <= v and u <= w end
+
+    if is_first_min(cost_of_change, cost_of_add, cost_of_delete) then
+      -- potential change
+      if x[i] ~= y[j] then
+        --- @type LevenshteinChange
+        local change = { kind = 'change', from = x[i], index = i, to = y[j] }
+        table.insert(changes, change)
+      end
+      i = i - 1
+      j = j - 1
+    elseif is_first_min(cost_of_add, cost_of_change, cost_of_delete) then
+      -- addition
+      --- @type LevenshteinChange
+      local change = { kind = 'add', item = y[j], index = i + 1 }
+      table.insert(changes, change)
+      j = j - 1
+    elseif is_first_min(cost_of_delete, cost_of_change, cost_of_add) then
+      -- deletion
+      --- @type LevenshteinChange
+      local change = { kind = 'delete', item = x[i], index = i }
+      table.insert(changes, change)
+      i = i - 1
+    else
+      error 'unreachable'
+    end
+  end
+
+  return changes
+end
+-- }}}
+
+return M

lua/u/repeat.lua

@@ -1,61 +1,39 @@
 local M = {}
 
-local function _normal(cmd) vim.cmd { cmd = 'normal', args = { cmd }, bang = true } end
+local IS_REPEATING = false
+--- @type function
+local REPEAT_ACTION = nil
 
-M.native_repeat = function() _normal '.' end
-M.native_undo = function() _normal 'u' end
+local function is_repeatable_last_mutator() return vim.b.changedtick <= (vim.b.my_changedtick or 0) end
 
-local function update_ts() vim.b.tt_changedtick = vim.b.changedtick end
-
----@param cmd? string|fun():unknown
-function M.set(cmd)
-  update_ts()
-  if cmd ~= nil then vim.b.tt_repeatcmd = cmd end
+--- @param f fun()
+function M.run_repeatable(f)
+  REPEAT_ACTION = f
+  REPEAT_ACTION()
+  vim.b.my_changedtick = vim.b.changedtick
 end
 
-local function tt_was_last_repeatable()
-  local ts, tt_ts = vim.b.changedtick, vim.b.tt_changedtick
-  return tt_ts ~= nil and ts <= tt_ts
-end
-
----@generic T
----@param cmd string|fun():T
----@return T
-function M.run(cmd)
-  M.set(cmd)
-  local result = cmd()
-  update_ts()
-  return result
-end
-
-function M.do_repeat()
-  local tt_cmd = vim.b.tt_repeatcmd
-  if not tt_was_last_repeatable() or (type(tt_cmd) ~= 'function' and type(tt_cmd) ~= 'string') then
-    return M.native_repeat()
-  end
-
-  -- execute the cached command:
-  local count = vim.api.nvim_get_vvar 'count1'
-  if type(tt_cmd) == 'string' then
-    _normal(count .. tt_cmd --[[@as string]])
-  else
-    local last_return
-    for _ = 1, count do
-      last_return = M.run(tt_cmd --[[@as fun():any]])
-    end
-    return last_return
-  end
-end
-
-function M.undo()
-  local tt_was_last_repeatable_before_undo = tt_was_last_repeatable()
-  M.native_undo()
-  if tt_was_last_repeatable_before_undo then update_ts() end
-end
+function M.is_repeating() return IS_REPEATING end
 
 function M.setup()
-  vim.keymap.set('n', '.', M.do_repeat)
-  vim.keymap.set('n', 'u', M.undo)
+  vim.keymap.set('n', '.', function()
+    IS_REPEATING = true
+    for _ = 1, vim.v.count1 do
+      if is_repeatable_last_mutator() and type(REPEAT_ACTION) == 'function' then
+        M.run_repeatable(REPEAT_ACTION)
+      else
+        vim.cmd { cmd = 'normal', args = { '.' }, bang = true }
+      end
+    end
+    IS_REPEATING = false
+  end)
+  vim.keymap.set('n', 'u', function()
+    local was_repeatable_last_mutator = is_repeatable_last_mutator()
+    for _ = 1, vim.v.count1 do
+      vim.cmd { cmd = 'normal', args = { 'u' }, bang = true }
+    end
+    if was_repeatable_last_mutator then vim.b.my_changedtick = vim.b.changedtick end
+  end)
 end
 
 return M

lua/u/state.lua

@@ -1,90 +0,0 @@
----@class State
----@field buf number
----@field registers table
----@field marks table
----@field positions table
----@field keymaps { mode: string; lhs: any, rhs: any, buffer?: number }[]
----@field global_options table<string, any>
----@field win_view vim.fn.winsaveview.ret|nil
-local State = {}
-
----@param buf number
----@return State
-function State.new(buf)
-  if buf == 0 then buf = vim.api.nvim_get_current_buf() end
-  local s = { buf = buf, registers = {}, marks = {}, positions = {}, keymaps = {}, global_options = {} }
-  setmetatable(s, { __index = State })
-  return s
-end
-
----@generic T
----@param buf number
----@param f fun(s: State):T
----@return T
-function State.run(buf, f)
-  local s = State.new(buf)
-  local ok, result = pcall(f, s)
-  s:restore()
-  if not ok then error(result) end
-  return result
-end
-
----@param buf number
----@param f fun(s: State, callback: fun(): any):any
----@param callback fun():any
-function State.run_async(buf, f, callback)
-  local s = State.new(buf)
-  f(s, function()
-    s:restore()
-    callback()
-  end)
-end
-
-function State:track_keymap(mode, lhs)
-  local old =
-    -- Look up the mapping in buffer-local maps:
-    vim.iter(vim.api.nvim_buf_get_keymap(self.buf, mode)):find(function(map) return map.lhs == lhs end)
-    -- Look up the mapping in global maps:
-    or vim.iter(vim.api.nvim_get_keymap(mode)):find(function(map) return map.lhs == lhs end)
-
-  -- Did we find a mapping?
-  if old == nil then return end
-
-  -- Track it:
-  table.insert(self.keymaps, { mode = mode, lhs = lhs, rhs = old.rhs or old.callback, buffer = old.buffer })
-end
-
----@param reg string
-function State:track_register(reg) self.registers[reg] = vim.fn.getreg(reg) end
-
----@param mark string
-function State:track_mark(mark) self.marks[mark] = vim.api.nvim_buf_get_mark(self.buf, mark) end
-
----@param pos string
-function State:track_pos(pos) self.positions[pos] = vim.fn.getpos(pos) end
-
----@param nm string
-function State:track_global_option(nm) self.global_options[nm] = vim.go[nm] end
-
-function State:track_winview() self.win_view = vim.fn.winsaveview() end
-
-function State:restore()
-  for reg, val in pairs(self.registers) do
-    vim.fn.setreg(reg, val)
-  end
-  for mark, val in pairs(self.marks) do
-    vim.api.nvim_buf_set_mark(self.buf, mark, val[1], val[2], {})
-  end
-  for pos, val in pairs(self.positions) do
-    vim.fn.setpos(pos, val)
-  end
-  for _, map in ipairs(self.keymaps) do
-    vim.keymap.set(map.mode, map.lhs, map.rhs, { buffer = map.buffer })
-  end
-  for nm, val in pairs(self.global_options) do
-    vim.go[nm] = val
-  end
-  if self.win_view ~= nil then vim.fn.winrestview(self.win_view) end
-end
-
-return State

lua/u/tracker.lua

@@ -0,0 +1,307 @@
+local M = {}
+
+M.debug = false
+
+--------------------------------------------------------------------------------
+-- class Signal
+--------------------------------------------------------------------------------
+
+--- @class u.Signal
+--- @field name? string
+--- @field private changing boolean
+--- @field private value any
+--- @field private subscribers table<function, boolean>
+--- @field private on_dispose_callbacks function[]
+local Signal = {}
+M.Signal = Signal
+Signal.__index = Signal
+
+--- @param value any
+--- @param name? string
+--- @return u.Signal
+function Signal:new(value, name)
+  local obj = setmetatable({
+    name = name,
+    changing = false,
+    value = value,
+    subscribers = {},
+    on_dispose_callbacks = {},
+  }, self)
+  return obj
+end
+
+--- @param value any
+function Signal:set(value)
+  self.value = value
+
+  -- We don't handle cyclic updates:
+  if self.changing then
+    if M.debug then
+      vim.notify(
+        'circular dependency detected' .. (self.name and (' in ' .. self.name) or ''),
+        vim.log.levels.WARN
+      )
+    end
+    return
+  end
+
+  local prev_changing = self.changing
+  self.changing = true
+  local ok = true
+  local err = nil
+  for _, cb in ipairs(self.subscribers) do
+    local ok2, err2 = pcall(cb, value)
+    if not ok2 then
+      ok = false
+      err = err or err2
+    end
+  end
+  self.changing = prev_changing
+
+  if not ok then
+    vim.notify(
+      'error notifying' .. (self.name and (' in ' .. self.name) or '') .. ': ' .. tostring(err),
+      vim.log.levels.WARN
+    )
+    error(err)
+  end
+end
+
+function Signal:schedule_set(value)
+  vim.schedule(function() self:set(value) end)
+end
+
+--- @return any
+function Signal:get()
+  local ctx = M.ExecutionContext.current()
+  if ctx then ctx:track(self) end
+  return self.value
+end
+
+--- @param fn function
+function Signal:update(fn) self:set(fn(self.value)) end
+
+--- @param fn function
+function Signal:schedule_update(fn) self:schedule_set(fn(self.value)) end
+
+--- @generic U
+--- @param fn fun(value: T): U
+--- @return u.Signal --<U>
+function Signal:map(fn)
+  local mapped_signal = M.create_memo(function()
+    local value = self:get()
+    return fn(value)
+  end, self.name and self.name .. ':mapped' or nil)
+  return mapped_signal
+end
+
+--- @return u.Signal
+function Signal:clone()
+  return self:map(function(x) return x end)
+end
+
+--- @param fn fun(value: T): boolean
+--- @return u.Signal -- <T>
+function Signal:filter(fn)
+  local filtered_signal = M.create_signal(nil, self.name and self.name .. ':filtered' or nil)
+  local unsubscribe_from_self = self:subscribe(function(value)
+    if fn(value) then filtered_signal:set(value) end
+  end)
+  filtered_signal:on_dispose(unsubscribe_from_self)
+  return filtered_signal
+end
+
+--- @param ms number
+--- @return u.Signal -- <T>
+function Signal:debounce(ms)
+  local function set_timeout(timeout, callback)
+    local timer = (vim.uv or vim.loop).new_timer()
+    timer:start(timeout, 0, function()
+      timer:stop()
+      timer:close()
+      callback()
+    end)
+    return timer
+  end
+
+  local filtered = M.create_signal(self.value, self.name and self.name .. ':debounced' or nil)
+
+  --- @diagnostic disable-next-line: undefined-doc-name
+  --- @type { queued: { value: T, ts: number }[]; timer?: uv_timer_t; }
+  local state = { queued = {}, timer = nil }
+  local function clear_timeout()
+    if state.timer == nil then return end
+    pcall(function()
+      --- @diagnostic disable-next-line: undefined-field
+      state.timer:stop()
+      --- @diagnostic disable-next-line: undefined-field
+      state.timer:close()
+    end)
+    state.timer = nil
+  end
+
+  local unsubscribe_from_self = self:subscribe(function(value)
+    -- Stop any previously running timer:
+    if state.timer then clear_timeout() end
+    local now_ms = (vim.uv or vim.loop).hrtime() / 1e6
+
+    -- If there is anything older than `ms` in our queue, emit it:
+    local older_than_ms = vim
+      .iter(state.queued)
+      :filter(function(item) return now_ms - item.ts > ms end)
+      :totable()
+    local last_older_than_ms = older_than_ms[#older_than_ms]
+    if last_older_than_ms then
+      filtered:set(last_older_than_ms.value)
+      state.queued = {}
+    end
+
+    -- overwrite anything young enough
+    table.insert(state.queued, { value = value, ts = now_ms })
+    state.timer = set_timeout(ms, function()
+      vim.schedule(function() filtered:set(value) end)
+      -- If a timer was allowed to run to completion, that means that no other
+      -- item has been queued, since the timer is reset every time a new item
+      -- comes in. This means we can reset the queue
+      clear_timeout()
+      state.queued = {}
+    end)
+  end)
+  filtered:on_dispose(unsubscribe_from_self)
+
+  return filtered
+end
+
+--- @param callback function
+function Signal:subscribe(callback)
+  table.insert(self.subscribers, callback)
+  return function() self:unsubscribe(callback) end
+end
+
+--- @param callback function
+function Signal:on_dispose(callback) table.insert(self.on_dispose_callbacks, callback) end
+
+--- @param callback function
+function Signal:unsubscribe(callback)
+  for i, cb in ipairs(self.subscribers) do
+    if cb == callback then
+      table.remove(self.subscribers, i)
+      break
+    end
+  end
+end
+
+function Signal:dispose()
+  self.subscribers = {}
+  for _, callback in ipairs(self.on_dispose_callbacks) do
+    callback()
+  end
+end
+
+--------------------------------------------------------------------------------
+-- class ExecutionContext
+--------------------------------------------------------------------------------
+
+local CURRENT_CONTEXT = nil
+
+--- @class u.ExecutionContext
+--- @field signals table<u.Signal, boolean>
+local ExecutionContext = {}
+M.ExecutionContext = ExecutionContext
+ExecutionContext.__index = ExecutionContext
+
+--- @return u.ExecutionContext
+function ExecutionContext.new()
+  return setmetatable({
+    signals = {},
+    subscribers = {},
+  }, ExecutionContext)
+end
+
+function ExecutionContext.current() return CURRENT_CONTEXT end
+
+--- @param fn function
+--- @param ctx u.ExecutionContext
+function ExecutionContext.run(fn, ctx)
+  local oldCtx = CURRENT_CONTEXT
+  CURRENT_CONTEXT = ctx
+  local result
+  local success, err = pcall(function() result = fn() end)
+
+  CURRENT_CONTEXT = oldCtx
+
+  if not success then error(err) end
+
+  return result
+end
+
+function ExecutionContext:track(signal) self.signals[signal] = true end
+
+--- @param callback function
+function ExecutionContext:subscribe(callback)
+  local wrapped_callback = function() callback() end
+  for signal in pairs(self.signals) do
+    signal:subscribe(wrapped_callback)
+  end
+
+  return function()
+    for signal in pairs(self.signals) do
+      signal:unsubscribe(wrapped_callback)
+    end
+  end
+end
+
+function ExecutionContext:dispose()
+  for signal, _ in pairs(self.signals) do
+    signal:dispose()
+  end
+  self.signals = {}
+end
+
+--------------------------------------------------------------------------------
+-- Helpers
+--------------------------------------------------------------------------------
+
+--- @param value any
+--- @param name? string
+--- @return u.Signal
+function M.create_signal(value, name) return Signal:new(value, name) end
+
+--- @param fn function
+--- @param name? string
+--- @return u.Signal
+function M.create_memo(fn, name)
+  --- @type u.Signal
+  local result
+  local unsubscribe = M.create_effect(function()
+    local value = fn()
+    if name and M.debug then vim.notify(name) end
+    if result then
+      result:set(value)
+    else
+      result = M.create_signal(value, name and ('m.s:' .. name) or nil)
+    end
+  end, name)
+  result:on_dispose(unsubscribe)
+  return result
+end
+
+--- @param fn function
+--- @param name? string
+function M.create_effect(fn, name)
+  local ctx = M.ExecutionContext.new()
+  M.ExecutionContext.run(fn, ctx)
+  return ctx:subscribe(function()
+    if name and M.debug then
+      local deps = vim
+        .iter(vim.tbl_keys(ctx.signals))
+        :map(function(s) return s.name end)
+        :filter(function(nm) return nm ~= nil end)
+        :join ','
+      vim.notify(name .. '(deps=' .. deps .. ')')
+    end
+    fn()
+  end)
+end
+
+return M

lua/u/txtobj.lua

@@ -0,0 +1,45 @@
+local Range = require 'u.range'
+
+local M = {}
+
+local ESC = vim.api.nvim_replace_termcodes('<Esc>', true, false, true)
+
+--- @param key_seq string
+--- @param fn fun(key_seq: string):u.Range|nil
+--- @param opts? { buffer: number|nil }
+function M.define(key_seq, fn, opts)
+  if opts ~= nil and opts.buffer == 0 then opts.buffer = vim.api.nvim_get_current_buf() end
+
+  local function handle_visual()
+    local range = fn(key_seq)
+    if range == nil or range:is_empty() then
+      vim.cmd.normal(ESC)
+      return
+    end
+    range:set_visual_selection()
+  end
+  vim.keymap.set({ 'x' }, key_seq, handle_visual, opts and { buffer = opts.buffer } or nil)
+
+  local function handle_normal()
+    local range = fn(key_seq)
+    if range == nil then return end
+
+    if not range:is_empty() then
+      range:set_visual_selection()
+    else
+      local original_eventignore = vim.go.eventignore
+      vim.go.eventignore = 'all'
+
+      -- insert a single space, so we can select it:
+      local p = range.start
+      p:insert_before ' '
+      vim.go.eventignore = original_eventignore
+
+      -- select the space:
+      Range.new(p, p, 'v'):set_visual_selection()
+    end
+  end
+  vim.keymap.set({ 'o' }, key_seq, handle_normal, opts and { buffer = opts.buffer } or nil)
+end
+
+return M

lua/u/utils.lua

@@ -6,7 +6,20 @@ local M = {}
 
 --- @alias QfItem { col: number, filename: string, kind: string, lnum: number, text: string }
 --- @alias KeyMaps table<string, fun(): any | string> }
----@alias CmdArgs { args: string; bang: boolean; count: number; fargs: string[]; line1: number; line2: number; mods: string; name: string; range: 0|1|2; reg: string; smods: any; info: Range|nil }
+-- luacheck: ignore
+--- @alias CmdArgs { args: string; bang: boolean; count: number; fargs: string[]; line1: number; line2: number; mods: string; name: string; range: 0|1|2; reg: string; smods: any; info: u.Range|nil }
+
+--- @generic T
+--- @param x `T`
+--- @param message? string
+--- @return T
+function M.dbg(x, message)
+  local t = {}
+  if message ~= nil then table.insert(t, message) end
+  table.insert(t, x)
+  vim.print(t)
+  return x
+end
 
 --- A utility for creating user commands that also pre-computes useful information
 --- and attaches it to the arguments.
@@ -22,6 +35,7 @@ local M = {}
 --- ```
 --- @param name string
 --- @param cmd string | fun(args: CmdArgs): any
+-- luacheck: ignore
 --- @param opts? { nargs?: 0|1|'*'|'?'|'+'; range?: boolean|'%'|number; count?: boolean|number, addr?: string; completion?: string }
 function M.ucmd(name, cmd, opts)
   local Range = require 'u.range'
@@ -37,98 +51,6 @@ function M.ucmd(name, cmd, opts)
   vim.api.nvim_create_user_command(name, cmd2, opts or {})
 end
 
----@param key_seq string
----@param fn fun(key_seq: string):Range|Pos|nil
----@param opts? { buffer: number|nil }
-function M.define_text_object(key_seq, fn, opts)
-  local Range = require 'u.range'
-  local Pos = require 'u.pos'
-
-  if opts ~= nil and opts.buffer == 0 then opts.buffer = vim.api.nvim_get_current_buf() end
-
-  local function handle_visual()
-    local range_or_pos = fn(key_seq)
-    if range_or_pos == nil then return end
-    if Range.is(range_or_pos) and range_or_pos:is_empty() then range_or_pos = range_or_pos.start end
-
-    if Range.is(range_or_pos) then
-      local range = range_or_pos --[[@as Range]]
-      range:set_visual_selection()
-    else
-      vim.cmd { cmd = 'normal', args = { '<Esc>' }, bang = true }
-    end
-  end
-  vim.keymap.set({ 'x' }, key_seq, handle_visual, opts and { buffer = opts.buffer } or nil)
-
-  local function handle_normal()
-    local State = require 'u.state'
-
-    -- enter visual mode:
-    vim.cmd { cmd = 'normal', args = { 'v' }, bang = true }
-
-    local range_or_pos = fn(key_seq)
-    if range_or_pos == nil then return end
-    if Range.is(range_or_pos) and range_or_pos:is_empty() then range_or_pos = range_or_pos.start end
-
-    if Range.is(range_or_pos) then
-      range_or_pos:set_visual_selection()
-    elseif Pos.is(range_or_pos) then
-      local p = range_or_pos --[[@as Pos]]
-      State.run(0, function(s)
-        s:track_global_option 'eventignore'
-        vim.go.eventignore = 'all'
-
-        -- insert a single space, so we can select it:
-        vim.api.nvim_buf_set_text(0, p.lnum, p.col, p.lnum, p.col, { ' ' })
-        -- select the space:
-        Range.new(p, p, 'v'):set_visual_selection()
-      end)
-    end
-  end
-  vim.keymap.set({ 'o' }, key_seq, handle_normal, opts and { buffer = opts.buffer } or nil)
-end
-
----@type fun(): nil|(fun():any)
-local __U__RepeatableOpFunc_rhs = nil
-
---- This is the global utility function used for operatorfunc
---- in repeatablemap
----@type nil|fun(range: Range): fun():any|nil
--- selene: allow(unused_variable)
-function __U__RepeatableOpFunc()
-  if __U__RepeatableOpFunc_rhs ~= nil then __U__RepeatableOpFunc_rhs() end
-end
-
-function M.repeatablemap(mode, lhs, rhs, opts)
-  vim.keymap.set(mode, lhs, function()
-    __U__RepeatableOpFunc_rhs = rhs
-    vim.o.operatorfunc = 'v:lua.__U__RepeatableOpFunc'
-    return 'g@ '
-  end, vim.tbl_extend('force', opts or {}, { expr = true }))
-end
-
-function M.get_editor_dimensions()
-  local w = 0
-  local h = 0
-  local tabnr = vim.api.nvim_get_current_tabpage()
-  for _, winid in ipairs(vim.api.nvim_list_wins()) do
-    local tabpage = vim.api.nvim_win_get_tabpage(winid)
-    if tabpage == tabnr then
-      local pos = vim.api.nvim_win_get_position(winid)
-      local r, c = pos[1], pos[2]
-      local win_w = vim.api.nvim_win_get_width(winid)
-      local win_h = vim.api.nvim_win_get_height(winid)
-      local right = c + win_w
-      local bottom = r + win_h
-      if right > w then w = right end
-      if bottom > h then h = bottom end
-    end
-  end
-  if w == 0 or h == 0 then
-    w = vim.api.nvim_win_get_width(0)
-    h = vim.api.nvim_win_get_height(0)
-  end
-  return { width = w, height = h }
-end
+function M.get_editor_dimensions() return { width = vim.go.columns, height = vim.go.lines } end
 
 return M

lux.toml

@@ -0,0 +1,19 @@
+package = "u.nvim"
+version = "0.2.0"
+lua = ">=5.1"
+
+[description]
+summary = ""
+maintainer = "jrop"
+labels = [ "library", "neovim", "neovim-plugin", "range", "utility" ]
+
+
+[dependencies]
+# Add your dependencies here
+# `busted = ">=2.0"`
+
+[run]
+args = [ "src/main.lua" ]
+
+[build]
+type = "builtin"

spec/buffer_spec.lua

@@ -6,7 +6,7 @@ describe('Buffer', function()
     withbuf({}, function()
       local buf = Buffer.from_nr()
       buf:all():replace 'bleh'
-      local actual_lines = vim.api.nvim_buf_get_lines(buf.buf, 0, -1, false)
+      local actual_lines = vim.api.nvim_buf_get_lines(buf.bufnr, 0, -1, false)
       assert.are.same({ 'bleh' }, actual_lines)
     end)
   end)
@@ -18,8 +18,8 @@ describe('Buffer', function()
       'three',
     }, function()
       local buf = Buffer.from_nr()
-      buf:lines(1, -2):replace 'too'
-      local actual_lines = vim.api.nvim_buf_get_lines(buf.buf, 0, -1, false)
+      buf:lines(2, -2):replace 'too'
+      local actual_lines = vim.api.nvim_buf_get_lines(buf.bufnr, 0, -1, false)
       assert.are.same({
         'one',
         'too',

spec/pos_spec.lua

@@ -4,12 +4,12 @@ local withbuf = loadfile './spec/withbuf.lua'()
 describe('Pos', function()
   it('get a char from a given position', function()
     withbuf({ 'asdf', 'bleh', 'a', '', 'goo' }, function()
-      assert.are.same('a', Pos.new(nil, 0, 0):char())
-      assert.are.same('d', Pos.new(nil, 0, 2):char())
-      assert.are.same('f', Pos.new(nil, 0, 3):char())
-      assert.are.same('a', Pos.new(nil, 2, 0):char())
-      assert.are.same('', Pos.new(nil, 3, 0):char())
-      assert.are.same('o', Pos.new(nil, 4, 2):char())
+      assert.are.same('a', Pos.new(nil, 1, 1):char())
+      assert.are.same('d', Pos.new(nil, 1, 3):char())
+      assert.are.same('f', Pos.new(nil, 1, 4):char())
+      assert.are.same('a', Pos.new(nil, 3, 1):char())
+      assert.are.same('', Pos.new(nil, 4, 1):char())
+      assert.are.same('o', Pos.new(nil, 5, 3):char())
     end)
   end)
 
@@ -23,47 +23,47 @@ describe('Pos', function()
   it('get the next position', function()
     withbuf({ 'asdf', 'bleh', 'a', '', 'goo' }, function()
       -- line 1: a => s
-      assert.are.same(Pos.new(nil, 0, 1), Pos.new(nil, 0, 0):next())
+      assert.are.same(Pos.new(nil, 1, 2), Pos.new(nil, 1, 1):next())
       -- line 1: d => f
-      assert.are.same(Pos.new(nil, 0, 3), Pos.new(nil, 0, 2):next())
+      assert.are.same(Pos.new(nil, 1, 4), Pos.new(nil, 1, 3):next())
       -- line 1 => 2
-      assert.are.same(Pos.new(nil, 1, 0), Pos.new(nil, 0, 3):next())
+      assert.are.same(Pos.new(nil, 2, 1), Pos.new(nil, 1, 4):next())
       -- line 3 => 4
-      assert.are.same(Pos.new(nil, 3, 0), Pos.new(nil, 2, 0):next())
+      assert.are.same(Pos.new(nil, 4, 1), Pos.new(nil, 3, 1):next())
       -- line 4 => 5
-      assert.are.same(Pos.new(nil, 4, 0), Pos.new(nil, 3, 0):next())
+      assert.are.same(Pos.new(nil, 5, 1), Pos.new(nil, 4, 1):next())
       -- end returns nil
-      assert.are.same(nil, Pos.new(nil, 4, 2):next())
+      assert.are.same(nil, Pos.new(nil, 5, 3):next())
     end)
   end)
 
   it('get the previous position', function()
     withbuf({ 'asdf', 'bleh', 'a', '', 'goo' }, function()
       -- line 1: s => a
-      assert.are.same(Pos.new(nil, 0, 0), Pos.new(nil, 0, 1):next(-1))
+      assert.are.same(Pos.new(nil, 1, 1), Pos.new(nil, 1, 2):next(-1))
       -- line 1: f => d
-      assert.are.same(Pos.new(nil, 0, 2), Pos.new(nil, 0, 3):next(-1))
+      assert.are.same(Pos.new(nil, 1, 3), Pos.new(nil, 1, 4):next(-1))
       -- line 2 => 1
-      assert.are.same(Pos.new(nil, 0, 3), Pos.new(nil, 1, 0):next(-1))
+      assert.are.same(Pos.new(nil, 1, 4), Pos.new(nil, 2, 1):next(-1))
       -- line 4 => 3
-      assert.are.same(Pos.new(nil, 2, 0), Pos.new(nil, 3, 0):next(-1))
+      assert.are.same(Pos.new(nil, 3, 1), Pos.new(nil, 4, 1):next(-1))
       -- line 5 => 4
-      assert.are.same(Pos.new(nil, 3, 0), Pos.new(nil, 4, 0):next(-1))
+      assert.are.same(Pos.new(nil, 4, 1), Pos.new(nil, 5, 1):next(-1))
       -- beginning returns nil
-      assert.are.same(nil, Pos.new(nil, 0, 0):next(-1))
+      assert.are.same(nil, Pos.new(nil, 1, 1):next(-1))
     end)
   end)
 
   it('find matching brackets', function()
     withbuf({ 'asdf ({} def <[{}]>) ;lkj' }, function()
       -- outer parens are matched:
-      assert.are.same(Pos.new(nil, 0, 19), Pos.new(nil, 0, 5):find_match())
+      assert.are.same(Pos.new(nil, 1, 20), Pos.new(nil, 1, 6):find_match())
       -- outer parens are matched (backward):
-      assert.are.same(Pos.new(nil, 0, 5), Pos.new(nil, 0, 19):find_match())
+      assert.are.same(Pos.new(nil, 1, 6), Pos.new(nil, 1, 20):find_match())
       -- no potential match returns nil
-      assert.are.same(nil, Pos.new(nil, 0, 0):find_match())
+      assert.are.same(nil, Pos.new(nil, 1, 1):find_match())
       -- watchdog expires before an otherwise valid match is found:
-      assert.are.same(nil, Pos.new(nil, 0, 5):find_match(2))
+      assert.are.same(nil, Pos.new(nil, 1, 6):find_match(2))
     end)
   end)
 end)

spec/range_spec.lua

@@ -28,7 +28,7 @@ describe('Range', function()
 
   it('get from positions: v in single line', function()
     withbuf({ 'line one', 'and line two' }, function()
-      local range = Range.new(Pos.new(nil, 0, 1), Pos.new(nil, 0, 3), 'v')
+      local range = Range.new(Pos.new(nil, 1, 2), Pos.new(nil, 1, 4), 'v')
       local lines = range:lines()
       assert.are.same({ 'ine' }, lines)
 
@@ -39,7 +39,7 @@ describe('Range', function()
 
   it('get from positions: v across multiple lines', function()
     withbuf({ 'pre line', 'the quick brown fox', 'jumps over a lazy dog', 'post line' }, function()
-      local range = Range.new(Pos.new(nil, 1, 4), Pos.new(nil, 2, 4), 'v')
+      local range = Range.new(Pos.new(nil, 2, 5), Pos.new(nil, 3, 5), 'v')
       local lines = range:lines()
       assert.are.same({ 'quick brown fox', 'jumps' }, lines)
     end)
@@ -47,7 +47,7 @@ describe('Range', function()
 
   it('get from positions: V', function()
     withbuf({ 'line one', 'and line two' }, function()
-      local range = Range.new(Pos.new(nil, 0, 0), Pos.new(nil, 0, Pos.MAX_COL), 'V')
+      local range = Range.new(Pos.new(nil, 1, 1), Pos.new(nil, 1, Pos.MAX_COL), 'V')
       local lines = range:lines()
       assert.are.same({ 'line one' }, lines)
 
@@ -58,7 +58,7 @@ describe('Range', function()
 
   it('get from positions: V across multiple lines', function()
     withbuf({ 'pre line', 'the quick brown fox', 'jumps over a lazy dog', 'post line' }, function()
-      local range = Range.new(Pos.new(nil, 1, 0), Pos.new(nil, 2, Pos.MAX_COL), 'V')
+      local range = Range.new(Pos.new(nil, 2, 1), Pos.new(nil, 3, Pos.MAX_COL), 'V')
       local lines = range:lines()
       assert.are.same({ 'the quick brown fox', 'jumps over a lazy dog' }, lines)
     end)
@@ -66,7 +66,7 @@ describe('Range', function()
 
   it('get from line', function()
     withbuf({ 'line one', 'and line two' }, function()
-      local range = Range.from_line(nil, 0)
+      local range = Range.from_line(nil, 1)
       local lines = range:lines()
       assert.are.same({ 'line one' }, lines)
 
@@ -77,7 +77,7 @@ describe('Range', function()
 
   it('get from lines', function()
     withbuf({ 'line one', 'and line two', 'and line 3' }, function()
-      local range = Range.from_lines(nil, 0, 1)
+      local range = Range.from_lines(nil, 1, 2)
       local lines = range:lines()
       assert.are.same({ 'line one', 'and line two' }, lines)
 
@@ -88,35 +88,35 @@ describe('Range', function()
 
   it('replace within line', function()
     withbuf({ 'pre line', 'the quick brown fox', 'jumps over a lazy dog', 'post line' }, function()
-      local range = Range.new(Pos.new(nil, 1, 4), Pos.new(nil, 1, 8), 'v')
+      local range = Range.new(Pos.new(nil, 2, 5), Pos.new(nil, 2, 9), 'v')
       range:replace 'quack'
 
-      local text = Range.from_line(nil, 1):text()
+      local text = Range.from_line(nil, 2):text()
       assert.are.same('the quack brown fox', text)
     end)
   end)
 
   it('delete within line', function()
     withbuf({ 'pre line', 'the quick brown fox', 'jumps over a lazy dog', 'post line' }, function()
-      local range = Range.new(Pos.new(nil, 1, 4), Pos.new(nil, 1, 9), 'v')
+      local range = Range.new(Pos.new(nil, 2, 5), Pos.new(nil, 2, 10), 'v')
       range:replace ''
 
-      local text = Range.from_line(nil, 1):text()
+      local text = Range.from_line(nil, 2):text()
       assert.are.same('the brown fox', text)
     end)
 
     withbuf({ 'pre line', 'the quick brown fox', 'jumps over a lazy dog', 'post line' }, function()
-      local range = Range.new(Pos.new(nil, 1, 4), Pos.new(nil, 1, 9), 'v')
+      local range = Range.new(Pos.new(nil, 2, 5), Pos.new(nil, 2, 10), 'v')
       range:replace(nil)
 
-      local text = Range.from_line(nil, 1):text()
+      local text = Range.from_line(nil, 2):text()
       assert.are.same('the brown fox', text)
     end)
   end)
 
   it('replace across multiple lines: v', function()
     withbuf({ 'pre line', 'the quick brown fox', 'jumps over a lazy dog', 'post line' }, function()
-      local range = Range.new(Pos.new(nil, 1, 4), Pos.new(nil, 2, 4), 'v')
+      local range = Range.new(Pos.new(nil, 2, 5), Pos.new(nil, 3, 5), 'v')
       range:replace 'plane flew'
 
       local lines = Range.from_buf_text():lines()
@@ -130,7 +130,7 @@ describe('Range', function()
 
   it('replace a line', function()
     withbuf({ 'pre line', 'the quick brown fox', 'jumps over a lazy dog', 'post line' }, function()
-      local range = Range.from_line(nil, 1)
+      local range = Range.from_line(nil, 2)
       range:replace 'the rabbit'
 
       local lines = Range.from_buf_text():lines()
@@ -145,7 +145,7 @@ describe('Range', function()
 
   it('replace multiple lines', function()
     withbuf({ 'pre line', 'the quick brown fox', 'jumps over a lazy dog', 'post line' }, function()
-      local range = Range.from_lines(nil, 1, 2)
+      local range = Range.from_lines(nil, 2, 3)
       range:replace 'the rabbit'
 
       local lines = Range.from_buf_text():lines()
@@ -159,7 +159,7 @@ describe('Range', function()
 
   it('delete single line', function()
     withbuf({ 'pre line', 'the quick brown fox', 'jumps over a lazy dog', 'post line' }, function()
-      local range = Range.from_line(nil, 1)
+      local range = Range.from_line(nil, 2)
       range:replace(nil) -- delete lines
 
       local lines = Range.from_buf_text():lines()
@@ -173,7 +173,7 @@ describe('Range', function()
 
   it('delete multiple lines', function()
     withbuf({ 'pre line', 'the quick brown fox', 'jumps over a lazy dog', 'post line' }, function()
-      local range = Range.from_lines(nil, 1, 2)
+      local range = Range.from_lines(nil, 2, 3)
       range:replace(nil) -- delete lines
 
       local lines = Range.from_buf_text():lines()
@@ -187,53 +187,53 @@ describe('Range', function()
   it('text object: word', function()
     withbuf({ 'the quick brown fox' }, function()
       vim.fn.setpos('.', { 0, 1, 5, 0 })
-      assert.are.same('quick ', Range.from_text_object('aw'):text())
+      assert.are.same('quick ', Range.from_motion('aw'):text())
 
       vim.fn.setpos('.', { 0, 1, 5, 0 })
-      assert.are.same('quick', Range.from_text_object('iw'):text())
+      assert.are.same('quick', Range.from_motion('iw'):text())
     end)
   end)
 
   it('text object: quote', function()
     withbuf({ [[the "quick" brown fox]] }, function()
       vim.fn.setpos('.', { 0, 1, 5, 0 })
-      assert.are.same('"quick"', Range.from_text_object('a"'):text())
+      assert.are.same('"quick"', Range.from_motion('a"'):text())
 
       vim.fn.setpos('.', { 0, 1, 6, 0 })
-      assert.are.same('quick', Range.from_text_object('i"'):text())
+      assert.are.same('quick', Range.from_motion('i"'):text())
     end)
 
     withbuf({ [[the 'quick' brown fox]] }, function()
       vim.fn.setpos('.', { 0, 1, 5, 0 })
-      assert.are.same("'quick'", Range.from_text_object([[a']]):text())
+      assert.are.same("'quick'", Range.from_motion([[a']]):text())
 
       vim.fn.setpos('.', { 0, 1, 6, 0 })
-      assert.are.same('quick', Range.from_text_object([[i']]):text())
+      assert.are.same('quick', Range.from_motion([[i']]):text())
     end)
 
     withbuf({ [[the `quick` brown fox]] }, function()
       vim.fn.setpos('.', { 0, 1, 5, 0 })
-      assert.are.same('`quick`', Range.from_text_object([[a`]]):text())
+      assert.are.same('`quick`', Range.from_motion([[a`]]):text())
 
       vim.fn.setpos('.', { 0, 1, 6, 0 })
-      assert.are.same('quick', Range.from_text_object([[i`]]):text())
+      assert.are.same('quick', Range.from_motion([[i`]]):text())
     end)
   end)
 
   it('text object: block', function()
     withbuf({ 'this is a {', 'block', '} here' }, function()
       vim.fn.setpos('.', { 0, 2, 1, 0 })
-      assert.are.same('{\nblock\n}', Range.from_text_object('a{'):text())
+      assert.are.same('{\nblock\n}', Range.from_motion('a{'):text())
 
       vim.fn.setpos('.', { 0, 2, 1, 0 })
-      assert.are.same('block', Range.from_text_object('i{'):text())
+      assert.are.same('block', Range.from_motion('i{'):text())
     end)
   end)
 
   it('text object: restores cursor position', function()
     withbuf({ 'this is a {block} here' }, function()
       vim.fn.setpos('.', { 0, 1, 13, 0 })
-      assert.are.same('{block}', Range.from_text_object('a{'):text())
+      assert.are.same('{block}', Range.from_motion('a{'):text())
       assert.are.same(vim.api.nvim_win_get_cursor(0), { 1, 12 })
     end)
   end)
@@ -258,22 +258,18 @@ describe('Range', function()
     end)
   end)
 
-  it('line0', function()
+  it('line', function()
     withbuf({
       'this is a {',
       'block',
       '} here',
     }, function()
-      local range = Range.new(Pos.new(0, 0, 5), Pos.new(0, 1, 4), 'v')
-      local lfirst = range:line0(0)
-      assert.are.same(5, lfirst.idx0.start)
-      assert.are.same(10, lfirst.idx0.stop)
-      assert.are.same(0, lfirst.lnum)
-      assert.are.same('is a {', lfirst.text())
-      assert.are.same('is a {', lfirst.range():text())
-      assert.are.same(Pos.new(0, 0, 5), lfirst.range().start)
-      assert.are.same(Pos.new(0, 0, 10), lfirst.range().stop)
-      assert.are.same('block', range:line0(1).text())
+      local range = Range.new(Pos.new(0, 1, 6), Pos.new(0, 2, 5), 'v')
+      local lfirst = assert(range:line(1), 'lfirst null')
+      assert.are.same('is a {', lfirst:text())
+      assert.are.same(Pos.new(0, 1, 6), lfirst.start)
+      assert.are.same(Pos.new(0, 1, 11), lfirst.stop)
+      assert.are.same('block', range:line(2):text())
     end)
   end)
 
@@ -297,16 +293,16 @@ describe('Range', function()
       vim.cmd.normal 'v' -- enter visual mode
       vim.cmd.normal 'l' -- select one character to the right
       local range = Range.from_vtext()
-      assert.are.same(range.start, Pos.new(nil, 0, 2))
-      assert.are.same(range.stop, Pos.new(nil, 0, 3))
+      assert.are.same(range.start, Pos.new(nil, 1, 3))
+      assert.are.same(range.stop, Pos.new(nil, 1, 4))
       assert.are.same(range.mode, 'v')
     end)
   end)
 
   it('from_op_func', function()
     withbuf({ 'line one', 'and line two' }, function()
-      local a = Pos.new(nil, 0, 0)
-      local b = Pos.new(nil, 1, 1)
+      local a = Pos.new(nil, 1, 1)
+      local b = Pos.new(nil, 2, 2)
       a:save_to_pos "'["
       b:save_to_pos "']"
 
@@ -317,7 +313,7 @@ describe('Range', function()
 
       range = Range.from_op_func 'line'
       assert.are.same(range.start, a)
-      assert.are.same(range.stop, Pos.new(nil, 1, Pos.MAX_COL))
+      assert.are.same(range.stop, Pos.new(nil, 2, Pos.MAX_COL))
       assert.are.same(range.mode, 'V')
     end)
   end)
@@ -325,8 +321,8 @@ describe('Range', function()
   it('from_cmd_args', function()
     local args = { range = 1 }
     withbuf({ 'line one', 'and line two' }, function()
-      local a = Pos.new(nil, 0, 0)
-      local b = Pos.new(nil, 1, 1)
+      local a = Pos.new(nil, 1, 1)
+      local b = Pos.new(nil, 2, 2)
       a:save_to_pos "'<"
       b:save_to_pos "'>"
 
@@ -341,25 +337,25 @@ describe('Range', function()
     withbuf({ [[the "quick" brown fox]] }, function()
       vim.fn.setpos('.', { 0, 1, 5, 0 })
       local range = Range.find_nearest_quotes()
-      assert.are.same(range.start, Pos.new(nil, 0, 4))
-      assert.are.same(range.stop, Pos.new(nil, 0, 10))
+      assert.are.same(range.start, Pos.new(nil, 1, 5))
+      assert.are.same(range.stop, Pos.new(nil, 1, 11))
     end)
 
     withbuf({ [[the 'quick' brown fox]] }, function()
       vim.fn.setpos('.', { 0, 1, 5, 0 })
       local range = Range.find_nearest_quotes()
-      assert.are.same(range.start, Pos.new(nil, 0, 4))
-      assert.are.same(range.stop, Pos.new(nil, 0, 10))
+      assert.are.same(range.start, Pos.new(nil, 1, 5))
+      assert.are.same(range.stop, Pos.new(nil, 1, 11))
     end)
   end)
 
   it('smallest', function()
-    local r1 = Range.new(Pos.new(nil, 0, 1), Pos.new(nil, 0, 3), 'v')
-    local r2 = Range.new(Pos.new(nil, 0, 2), Pos.new(nil, 0, 4), 'v')
-    local r3 = Range.new(Pos.new(nil, 0, 0), Pos.new(nil, 0, 5), 'v')
+    local r1 = Range.new(Pos.new(nil, 1, 2), Pos.new(nil, 1, 4), 'v')
+    local r2 = Range.new(Pos.new(nil, 1, 3), Pos.new(nil, 1, 5), 'v')
+    local r3 = Range.new(Pos.new(nil, 1, 1), Pos.new(nil, 1, 6), 'v')
     local smallest = Range.smallest { r1, r2, r3 }
-    assert.are.same(smallest.start, Pos.new(nil, 0, 1))
-    assert.are.same(smallest.stop, Pos.new(nil, 0, 3))
+    assert.are.same(smallest.start, Pos.new(nil, 1, 2))
+    assert.are.same(smallest.stop, Pos.new(nil, 1, 4))
   end)
 
   it('clone', function()
@@ -381,9 +377,9 @@ describe('Range', function()
 
   it('to_linewise()', function()
     withbuf({ 'line one', 'and line two' }, function()
-      local range = Range.new(Pos.new(nil, 0, 1), Pos.new(nil, 1, 3), 'v')
+      local range = Range.new(Pos.new(nil, 1, 2), Pos.new(nil, 2, 4), 'v')
       local linewise_range = range:to_linewise()
-      assert.are.same(linewise_range.start.col, 0)
+      assert.are.same(linewise_range.start.col, 1)
       assert.are.same(linewise_range.stop.col, Pos.MAX_COL)
       assert.are.same(linewise_range.mode, 'V')
     end)
@@ -391,82 +387,133 @@ describe('Range', function()
 
   it('is_empty', function()
     withbuf({ 'line one', 'and line two' }, function()
-      local range = Range.new(Pos.new(nil, 0, 0), nil, 'v')
+      local range = Range.new(Pos.new(nil, 1, 1), nil, 'v')
       assert.is_true(range:is_empty())
 
-      local range2 = Range.new(Pos.new(nil, 0, 0), Pos.new(nil, 0, 1), 'v')
+      local range2 = Range.new(Pos.new(nil, 1, 1), Pos.new(nil, 1, 2), 'v')
       assert.is_false(range2:is_empty())
     end)
   end)
 
   it('trim_start', function()
     withbuf({ '   line one', 'line two' }, function()
-      local range = Range.new(Pos.new(nil, 0, 0), Pos.new(nil, 0, 9), 'v')
+      local range = Range.new(Pos.new(nil, 1, 1), Pos.new(nil, 1, 10), 'v')
       local trimmed = range:trim_start()
-      assert.are.same(trimmed.start, Pos.new(nil, 0, 3)) -- should be after the spaces
+      assert.are.same(trimmed.start, Pos.new(nil, 1, 4)) -- should be after the spaces
     end)
   end)
 
   it('trim_stop', function()
     withbuf({ 'line one   ', 'line two' }, function()
-      local range = Range.new(Pos.new(nil, 0, 0), Pos.new(nil, 0, 9), 'v')
+      local range = Range.new(Pos.new(nil, 1, 1), Pos.new(nil, 1, 10), 'v')
       local trimmed = range:trim_stop()
-      assert.are.same(trimmed.stop, Pos.new(nil, 0, 7)) -- should be before the spaces
+      assert.are.same(trimmed.stop, Pos.new(nil, 1, 8)) -- should be before the spaces
     end)
   end)
 
   it('contains', function()
     withbuf({ 'line one', 'and line two' }, function()
-      local range = Range.new(Pos.new(nil, 0, 1), Pos.new(nil, 0, 3), 'v')
-      local pos = Pos.new(nil, 0, 2)
+      local range = Range.new(Pos.new(nil, 1, 2), Pos.new(nil, 1, 4), 'v')
+      local pos = Pos.new(nil, 1, 3)
       assert.is_true(range:contains(pos))
 
-      pos = Pos.new(nil, 0, 4) -- outside of range
+      pos = Pos.new(nil, 1, 5) -- outside of range
       assert.is_false(range:contains(pos))
     end)
   end)
 
+  it('difference', function()
+    withbuf({ 'line one', 'and line two' }, function()
+      local range_outer = Range.new(Pos.new(nil, 2, 1), Pos.new(nil, 2, 12), 'v')
+      local range_inner = Range.new(Pos.new(nil, 2, 5), Pos.new(nil, 2, 8), 'v')
+
+      assert.are.same(range_outer:text(), 'and line two')
+      assert.are.same(range_inner:text(), 'line')
+
+      local left, right = range_outer:difference(range_inner)
+      assert.are.same(left:text(), 'and ')
+      assert.are.same(right:text(), ' two')
+
+      left, right = range_inner:difference(range_outer)
+      assert.are.same(left:text(), 'and ')
+      assert.are.same(right:text(), ' two')
+
+      left, right = range_outer:difference(range_outer)
+      assert.are.same(left:is_empty(), true)
+      assert.are.same(left:text(), '')
+      assert.are.same(right:is_empty(), true)
+      assert.are.same(right:text(), '')
+    end)
+  end)
+
+  it('length', function()
+    withbuf({ 'line one', 'and line two' }, function()
+      local range = Range.new(Pos.new(nil, 2, 4), Pos.new(nil, 2, 9), 'v')
+      assert.are.same(range:length(), #range:text())
+
+      range = Range.new(Pos.new(nil, 1, 4), Pos.new(nil, 2, 9), 'v')
+      assert.are.same(range:length(), #range:text())
+    end)
+  end)
+
+  it('sub', function()
+    withbuf({ 'line one', 'and line two' }, function()
+      local range = Range.new(Pos.new(nil, 2, 4), Pos.new(nil, 2, 9), 'v')
+      assert.are.same(range:text(), ' line ')
+      assert.are.same(range:sub(1, -1):text(), ' line ')
+      assert.are.same(range:sub(2, -2):text(), 'line')
+      assert.are.same(range:sub(1, 5):text(), ' line')
+      assert.are.same(range:sub(2, 5):text(), 'line')
+      assert.are.same(range:sub(20, 25):text(), '')
+    end)
+  end)
+
   it('shrink', function()
     withbuf({ 'line one', 'and line two' }, function()
-      local range = Range.new(Pos.new(nil, 0, 1), Pos.new(nil, 1, 3), 'v')
+      local range = Range.new(Pos.new(nil, 2, 3), Pos.new(nil, 3, 5), 'v')
       local shrunk = range:shrink(1)
-      assert.are.same(shrunk.start, Pos.new(nil, 0, 2))
-      assert.are.same(shrunk.stop, Pos.new(nil, 1, 2))
+      assert.are.same(shrunk.start, Pos.new(nil, 2, 4))
+      assert.are.same(shrunk.stop, Pos.new(nil, 3, 4))
     end)
   end)
 
   it('must_shrink', function()
     withbuf({ 'line one', 'and line two' }, function()
-      local range = Range.new(Pos.new(nil, 0, 1), Pos.new(nil, 1, 3), 'v')
+      local range = Range.new(Pos.new(nil, 2, 3), Pos.new(nil, 3, 5), 'v')
       local shrunk = range:must_shrink(1)
-      assert.are.same(shrunk.start, Pos.new(nil, 0, 2))
-      assert.are.same(shrunk.stop, Pos.new(nil, 1, 2))
+      assert.are.same(shrunk.start, Pos.new(nil, 2, 4))
+      assert.are.same(shrunk.stop, Pos.new(nil, 3, 4))
 
-      assert.has.error(function() range:must_shrink(100) end, 'error in Range:must_shrink: Range:shrink() returned nil')
+      assert.has.error(
+        function() range:must_shrink(100) end,
+        'error in Range:must_shrink: Range:shrink() returned nil'
+      )
     end)
   end)
 
   it('set_visual_selection', function()
     withbuf({ 'line one', 'and line two' }, function()
-      local range = Range.from_lines(nil, 0, 1)
+      local range = Range.from_lines(nil, 1, 2)
       range:set_visual_selection()
 
-      assert.are.same(Pos.from_pos 'v', Pos.new(nil, 0, 0))
-      assert.are.same(Pos.from_pos '.', Pos.new(nil, 1, 11))
+      assert.are.same(Pos.from_pos 'v', Pos.new(nil, 1, 1))
+      -- Since the selection is 'V' (instead of 'v'), the end
+      -- selects one character past the end:
+      assert.are.same(Pos.from_pos '.', Pos.new(nil, 2, 13))
     end)
   end)
 
   it('selections set to past the EOL should not error', function()
     withbuf({ 'Rg SET NAMES' }, function()
       local b = vim.api.nvim_get_current_buf()
-      local r = Range.new(Pos.new(b, 0, 3), Pos.new(b, 0, 12), 'v')
+      local r = Range.new(Pos.new(b, 1, 4), Pos.new(b, 1, 13), 'v')
       r:replace 'bleh'
       assert.are.same({ 'Rg bleh' }, vim.api.nvim_buf_get_lines(b, 0, -1, false))
     end)
 
     withbuf({ 'Rg SET NAMES' }, function()
       local b = vim.api.nvim_get_current_buf()
-      local r = Range.new(Pos.new(b, 0, 3), Pos.new(b, 0, 11), 'v')
+      local r = Range.new(Pos.new(b, 1, 4), Pos.new(b, 1, 12), 'v')
       r:replace 'bleh'
       assert.are.same({ 'Rg bleh' }, vim.api.nvim_buf_get_lines(b, 0, -1, false))
     end)
@@ -475,13 +522,19 @@ describe('Range', function()
   it('replace updates Range.stop: same line', function()
     withbuf({ 'The quick brown fox jumps over the lazy dog' }, function()
       local b = vim.api.nvim_get_current_buf()
-      local r = Range.new(Pos.new(b, 0, 4), Pos.new(b, 0, 8), 'v')
+      local r = Range.new(Pos.new(b, 1, 5), Pos.new(b, 1, 9), 'v')
 
       r:replace 'bleh1'
-      assert.are.same({ 'The bleh1 brown fox jumps over the lazy dog' }, vim.api.nvim_buf_get_lines(b, 0, -1, false))
+      assert.are.same(
+        { 'The bleh1 brown fox jumps over the lazy dog' },
+        vim.api.nvim_buf_get_lines(b, 0, -1, false)
+      )
 
       r:replace 'bleh2'
-      assert.are.same({ 'The bleh2 brown fox jumps over the lazy dog' }, vim.api.nvim_buf_get_lines(b, 0, -1, false))
+      assert.are.same(
+        { 'The bleh2 brown fox jumps over the lazy dog' },
+        vim.api.nvim_buf_get_lines(b, 0, -1, false)
+      )
     end)
   end)
 
@@ -491,14 +544,21 @@ describe('Range', function()
       'over the lazy dog',
     }, function()
       local b = vim.api.nvim_get_current_buf()
-      local r = Range.new(Pos.new(b, 0, 20), Pos.new(b, 1, 3), 'v')
+      local r = Range.new(Pos.new(b, 1, 21), Pos.new(b, 2, 4), 'v')
       assert.are.same({ 'jumps', 'over' }, r:lines())
 
       r:replace 'bleh1'
-      assert.are.same({ 'The quick brown fox bleh1 the lazy dog' }, vim.api.nvim_buf_get_lines(b, 0, -1, false))
+      assert.are.same(
+        { 'The quick brown fox bleh1 the lazy dog' },
+        vim.api.nvim_buf_get_lines(b, 0, -1, false)
+      )
+      assert.are.same({ 'bleh1' }, r:lines())
 
       r:replace 'blehGoo2'
-      assert.are.same({ 'The quick brown fox blehGoo2 the lazy dog' }, vim.api.nvim_buf_get_lines(b, 0, -1, false))
+      assert.are.same(
+        { 'The quick brown fox blehGoo2 the lazy dog' },
+        vim.api.nvim_buf_get_lines(b, 0, -1, false)
+      )
     end)
   end)
 
@@ -511,7 +571,7 @@ describe('Range', function()
       'the lazy dog',
     }, function()
       local b = vim.api.nvim_get_current_buf()
-      local r = Range.new(Pos.new(b, 1, 0), Pos.new(b, 3, Pos.MAX_COL), 'V')
+      local r = Range.new(Pos.new(b, 2, 1), Pos.new(b, 4, Pos.MAX_COL), 'V')
       assert.are.same({ 'fox', 'jumps', 'over' }, r:lines())
 
       r:replace { 'bleh1', 'bleh2' }
@@ -540,7 +600,7 @@ describe('Range', function()
       'the lazy dog',
     }, function()
       local b = vim.api.nvim_get_current_buf()
-      local r = Range.new(Pos.new(b, 1, 0), Pos.new(b, 3, Pos.MAX_COL), 'V')
+      local r = Range.new(Pos.new(b, 2, 1), Pos.new(b, 4, Pos.MAX_COL), 'V')
       assert.are.same({ 'fox', 'jumps', 'over' }, r:lines())
 
       r:replace(nil)

spec/renderer_spec.lua

@@ -0,0 +1,135 @@
+local R = require 'u.renderer'
+local withbuf = loadfile './spec/withbuf.lua'()
+
+local function getlines() return vim.api.nvim_buf_get_lines(0, 0, -1, true) end
+
+describe('Renderer', function()
+  it('should render text in an empty buffer', function()
+    withbuf({}, function()
+      local r = R.Renderer.new(0)
+      r:render { 'hello', ' ', 'world' }
+      assert.are.same(getlines(), { 'hello world' })
+    end)
+  end)
+
+  it('should result in the correct text after repeated renders', function()
+    withbuf({}, function()
+      local r = R.Renderer.new(0)
+      r:render { 'hello', ' ', 'world' }
+      assert.are.same(getlines(), { 'hello world' })
+
+      r:render { 'goodbye', ' ', 'world' }
+      assert.are.same(getlines(), { 'goodbye world' })
+
+      r:render { 'hello', ' ', 'universe' }
+      assert.are.same(getlines(), { 'hello universe' })
+    end)
+  end)
+
+  it('should handle tags correctly', function()
+    withbuf({}, function()
+      local r = R.Renderer.new(0)
+      r:render {
+        R.h('text', { hl = 'HighlightGroup' }, 'hello '),
+        R.h('text', { hl = 'HighlightGroup' }, 'world'),
+      }
+      assert.are.same(getlines(), { 'hello world' })
+    end)
+  end)
+
+  it('should reconcile added lines', function()
+    withbuf({}, function()
+      local r = R.Renderer.new(0)
+      r:render { 'line 1', '\n', 'line 2' }
+      assert.are.same(getlines(), { 'line 1', 'line 2' })
+
+      -- Add a new line:
+      r:render { 'line 1', '\n', 'line 2\n', 'line 3' }
+      assert.are.same(getlines(), { 'line 1', 'line 2', 'line 3' })
+    end)
+  end)
+
+  it('should reconcile deleted lines', function()
+    withbuf({}, function()
+      local r = R.Renderer.new(0)
+      r:render { 'line 1', '\nline 2', '\nline 3' }
+      assert.are.same(getlines(), { 'line 1', 'line 2', 'line 3' })
+
+      -- Remove a line:
+      r:render { 'line 1', '\nline 3' }
+      assert.are.same(getlines(), { 'line 1', 'line 3' })
+    end)
+  end)
+
+  it('should handle multiple nested elements', function()
+    withbuf({}, function()
+      local r = R.Renderer.new(0)
+      r:render {
+        R.h('text', {}, {
+          'first line',
+        }),
+        '\n',
+        R.h('text', {}, 'second line'),
+      }
+      assert.are.same(getlines(), { 'first line', 'second line' })
+
+      r:render {
+        R.h('text', {}, 'updated first line'),
+        '\n',
+        R.h('text', {}, 'third line'),
+      }
+      assert.are.same(getlines(), { 'updated first line', 'third line' })
+    end)
+  end)
+
+  --
+  -- get_pos_infos
+  --
+
+  it('should return no extmarks for an empty buffer', function()
+    withbuf({}, function()
+      local r = R.Renderer.new(0)
+      local pos_infos = r:get_pos_infos { 0, 0 }
+      assert.are.same(pos_infos, {})
+    end)
+  end)
+
+  it('should return correct extmark for a given position', function()
+    withbuf({}, function()
+      local r = R.Renderer.new(0)
+      r:render {
+        R.h('text', { hl = 'HighlightGroup1' }, 'Hello'),
+        R.h('text', { hl = 'HighlightGroup2' }, ' World'),
+      }
+
+      local pos_infos = r:get_pos_infos { 0, 2 }
+
+      assert.are.same(#pos_infos, 1)
+      assert.are.same(pos_infos[1].tag.attributes.hl, 'HighlightGroup1')
+      assert.are.same(pos_infos[1].extmark.start, { 0, 0 })
+      assert.are.same(pos_infos[1].extmark.stop, { 0, 5 })
+    end)
+  end)
+
+  it('should return multiple extmarks for overlapping text', function()
+    withbuf({}, function()
+      local r = R.Renderer.new(0)
+      r:render {
+        R.h('text', { hl = 'HighlightGroup1' }, {
+          'Hello',
+          R.h(
+            'text',
+            { hl = 'HighlightGroup2', extmark = { hl_group = 'HighlightGroup2' } },
+            ' World'
+          ),
+        }),
+      }
+
+      local pos_infos = r:get_pos_infos { 0, 5 }
+
+      assert.are.same(#pos_infos, 2)
+      assert.are.same(pos_infos[1].tag.attributes.hl, 'HighlightGroup2')
+      assert.are.same(pos_infos[2].tag.attributes.hl, 'HighlightGroup1')
+    end)
+  end)
+end)

spec/tracker_spec.lua

@@ -0,0 +1,206 @@
+local tracker = require 'u.tracker'
+local Signal = tracker.Signal
+local ExecutionContext = tracker.ExecutionContext
+
+describe('Signal', function()
+  local signal
+
+  before_each(function() signal = Signal:new(0, 'testSignal') end)
+
+  it('should initialize with correct parameters', function()
+    assert.is.equal(signal.value, 0)
+    assert.is.equal(signal.name, 'testSignal')
+    assert.is.not_nil(signal.subscribers)
+    assert.is.equal(#signal.subscribers, 0)
+    assert.is.equal(signal.changing, false)
+  end)
+
+  it('should set new value and notify subscribers', function()
+    local called = false
+    signal:subscribe(function(value)
+      called = true
+      assert.is.equal(value, 42)
+    end)
+
+    signal:set(42)
+    assert.is.equal(called, true)
+  end)
+
+  it('should not notify subscribers during circular dependency', function()
+    signal.changing = true
+    local notified = false
+
+    signal:subscribe(function() notified = true end)
+
+    signal:set(42)
+    assert.is.equal(notified, false) -- No notification should occur
+  end)
+
+  it('should get current value', function()
+    signal:set(100)
+    assert.is.equal(signal:get(), 100)
+  end)
+
+  it('should update value with function', function()
+    signal:set(10)
+    signal:update(function(value) return value * 2 end)
+    assert.is.equal(signal:get(), 20)
+  end)
+
+  it('should dispose subscribers', function()
+    local called = false
+    local unsubscribe = signal:subscribe(function() called = true end)
+
+    unsubscribe()
+    signal:set(10)
+    assert.is.equal(called, false) -- Should not be notified
+  end)
+
+  describe('Signal:map', function()
+    it('should transform the signal value', function()
+      local test_signal = Signal:new(5)
+      local mapped_signal = test_signal:map(function(value) return value * 2 end)
+
+      assert.is.equal(mapped_signal:get(), 10) -- Initial transformation
+      test_signal:set(10)
+      assert.is.equal(mapped_signal:get(), 20) -- Updated transformation
+    end)
+
+    it('should handle empty transformations', function()
+      local test_signal = Signal:new(nil)
+      local mapped_signal = test_signal:map(function(value) return value or 'default' end)
+
+      assert.is.equal(mapped_signal:get(), 'default') -- Return default
+      test_signal:set 'new value'
+      assert.is.equal(mapped_signal:get(), 'new value') -- Return new value
+    end)
+  end)
+
+  describe('Signal:filter', function()
+    it('should only emit values that pass the filter', function()
+      local test_signal = Signal:new(5)
+      local filtered_signal = test_signal:filter(function(value) return value > 10 end)
+
+      assert.is.equal(filtered_signal:get(), nil) -- Initial value should not pass
+      test_signal:set(15)
+      assert.is.equal(filtered_signal:get(), 15) -- Now filtered
+      test_signal:set(8)
+      assert.is.equal(filtered_signal:get(), 15) -- Does not pass the filter
+    end)
+
+    it('should handle empty initial values', function()
+      local test_signal = Signal:new(nil)
+      local filtered_signal = test_signal:filter(function(value) return value ~= nil end)
+
+      assert.is.equal(filtered_signal:get(), nil) -- Should be nil
+      test_signal:set(10)
+      assert.is.equal(filtered_signal:get(), 10) -- Should pass now
+    end)
+  end)
+
+  describe('create_memo', function()
+    it('should compute a derived value and update when dependencies change', function()
+      local test_signal = Signal:new(2)
+      local memoized_signal = tracker.create_memo(function() return test_signal:get() * 2 end)
+
+      assert.is.equal(memoized_signal:get(), 4) -- Initially compute 2 * 2
+
+      test_signal:set(3)
+      assert.is.equal(memoized_signal:get(), 6) -- Update to 3 * 2 = 6
+
+      test_signal:set(5)
+      assert.is.equal(memoized_signal:get(), 10) -- Update to 5 * 2 = 10
+    end)
+
+    it('should not recompute if the dependencies do not change', function()
+      local call_count = 0
+      local test_signal = Signal:new(10)
+      local memoized_signal = tracker.create_memo(function()
+        call_count = call_count + 1
+        return test_signal:get() + 1
+      end)
+
+      assert.is.equal(memoized_signal:get(), 11) -- Compute first value
+      assert.is.equal(call_count, 1) -- Should compute once
+
+      memoized_signal:get() -- Call again, should use memoized value
+      assert.is.equal(call_count, 1) -- Still should only be one call
+
+      test_signal:set(10) -- Set the same value
+      assert.is.equal(memoized_signal:get(), 11)
+      assert.is.equal(call_count, 2)
+
+      test_signal:set(20)
+      assert.is.equal(memoized_signal:get(), 21)
+      assert.is.equal(call_count, 3)
+    end)
+  end)
+
+  describe('create_effect', function()
+    it('should track changes and execute callback', function()
+      local test_signal = Signal:new(5)
+      local call_count = 0
+
+      tracker.create_effect(function()
+        test_signal:get() -- track as a dependency
+        call_count = call_count + 1
+      end)
+
+      assert.is.equal(call_count, 1)
+      test_signal:set(10)
+      assert.is.equal(call_count, 2)
+    end)
+
+    it('should clean up signals and not call after dispose', function()
+      local test_signal = Signal:new(5)
+      local call_count = 0
+
+      local unsubscribe = tracker.create_effect(function()
+        call_count = call_count + 1
+        return test_signal:get() * 2
+      end)
+
+      assert.is.equal(call_count, 1) -- Initially calls
+      unsubscribe() -- Unsubscribe the effect
+      test_signal:set(10) -- Update signal value
+      assert.is.equal(call_count, 1) -- Callback should not be called again
+    end)
+  end)
+end)
+
+describe('ExecutionContext', function()
+  local context
+
+  before_each(function() context = ExecutionContext:new() end)
+
+  it('should initialize a new context', function()
+    assert.is.table(context.signals)
+    assert.is.table(context.subscribers)
+  end)
+
+  it('should track signals', function()
+    local signal = Signal:new(0)
+    context:track(signal)
+
+    assert.is.equal(next(context.signals), signal) -- Check if signal is tracked
+  end)
+
+  it('should subscribe to signals', function()
+    local signal = Signal:new(0)
+    local callback_called = false
+
+    context:track(signal)
+    context:subscribe(function() callback_called = true end)
+
+    signal:set(100)
+    assert.is.equal(callback_called, true) -- Callback should be called
+  end)
+
+  it('should dispose tracked signals', function()
+    local signal = Signal:new(0)
+    context:track(signal)
+
+    context:dispose()
+    assert.is.falsy(next(context.signals)) -- Should not have any tracked signals
+  end)
+end)

stylua.toml

@@ -1,6 +1,6 @@
 call_parentheses = "None"
 collapse_simple_statement = "Always"
-column_width = 120
+column_width = 100
 indent_type = "Spaces"
 indent_width = 2
 quote_style = "AutoPreferSingle"