factor out line-patching into a separate function

lua/u/renderer.lua

@@ -132,6 +132,64 @@ end -- }}}
 --- }
 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
+          -- No change
+        end
+      end
+    elseif line_change.kind == 'delete' then
+      _set_lines(lnum0, lnum0 + 1, true, {})
+    else
+      -- No change
+    end
+  end
+end
+
 --- @param tree u.renderer.Tree
 function Renderer:render(tree) -- {{{
   local changedtick = vim.b[self.bufnr].changedtick
@@ -210,45 +268,16 @@ end
 
 --- @private
 function Renderer:_reconcile() -- {{{
-  local line_changes = H.levenshtein(self.old.lines, self.curr.lines)
-  self.old = self.curr
-
   --
   -- Step 1: morph the text to the desired state:
   --
-  for _, line_change in ipairs(line_changes) do
-    local lnum0 = line_change.index - 1
-
-    if line_change.kind == 'add' then
-      self:_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
-          self:_set_text(lnum0, cnum0, lnum0, cnum0, { col_change.item })
-        elseif col_change.kind == 'change' then
-          self:_set_text(lnum0, cnum0, lnum0, cnum0 + 1, { col_change.to })
-        elseif col_change.kind == 'delete' then
-          self:_set_text(lnum0, cnum0, lnum0, cnum0 + 1, {})
-        else
-          -- No change
-        end
-      end
-    elseif line_change.kind == 'delete' then
-      self:_set_lines(lnum0, lnum0 + 1, true, {})
-    else
-      -- No change
-    end
-  end
+  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:
@@ -265,6 +294,8 @@ function Renderer:_reconcile() -- {{{
       }, extmark.opts)
     )
   end
+
+  self.old = self.curr
 end -- }}}
 
 --- @private