refactor(diff): improve line matching and replacement logic (#1329)

Refactors diff application logic to use a new utils.split_lines function for
consistent line splitting. Applies diffs from bottom to top to preserve line
numbering and ensures correct replacement of lines in buffers. This improves
accuracy when applying multiple diffs to the same file and enhances code
readability.

Author: deathbeam

lua/CopilotChat/config/mappings.lua

@@ -51,7 +51,8 @@ local function get_diff(block)
 
     -- If we found a valid buffer, get the reference content
     if bufnr and utils.buf_valid(bufnr) then
-      reference = table.concat(vim.api.nvim_buf_get_lines(bufnr, start_line - 1, end_line, false), '\n')
+      local lines = vim.api.nvim_buf_get_lines(bufnr, start_line - 1, end_line, false)
+      reference = table.concat(lines, '\n')
       filetype = vim.bo[bufnr].filetype
     end
   end
@@ -212,7 +213,7 @@ return {
         return
       end
 
-      local lines = vim.split(message.content, '\n')
+      local lines = utils.split_lines(message.content)
       local new_lines = {}
       local changed = false
 
@@ -241,9 +242,9 @@ return {
         return
       end
 
-      local lines = vim.split(diff.change, '\n', { trimempty = false })
+      local lines = utils.split_lines(diff.change)
       vim.api.nvim_buf_set_lines(diff.bufnr, diff.start_line - 1, diff.end_line, false, lines)
-      copilot.set_selection(diff.bufnr, diff.start_line, diff.start_line + #lines - 1)
+      copilot.set_selection(diff.bufnr, diff.start_line, diff.end_line)
     end,
   },
 
@@ -352,10 +353,9 @@ return {
       }
 
       if copilot.config.mappings.show_diff.full_diff then
-        local modified = utils.buf_valid(diff.bufnr) and vim.api.nvim_buf_get_lines(diff.bufnr, 0, -1, false) or {}
+        local original = utils.buf_valid(diff.bufnr) and vim.api.nvim_buf_get_lines(diff.bufnr, 0, -1, false) or {}
 
-        -- Apply all diffs from same file
-        if #modified > 0 then
+        if #original > 0 then
           -- Find all diffs from the same file in this section
           local message = copilot.chat:get_message(constants.ROLE.ASSISTANT, true)
           local section = message and message.section
@@ -367,30 +367,38 @@ return {
                 table.insert(same_file_diffs, block_diff)
               end
             end
+          end
 
-            -- Sort diffs bottom to top to preserve line numbering
-            table.sort(same_file_diffs, function(a, b)
-              return a.start_line > b.start_line
-            end)
+          -- Ensure we at least apply the current diff
+          if #same_file_diffs == 0 then
+            table.insert(same_file_diffs, diff)
           end
 
-          for _, file_diff in ipairs(same_file_diffs) do
-            local start_idx = file_diff.start_line
-            local end_idx = file_diff.end_line
-            for _ = start_idx, end_idx do
-              table.remove(modified, start_idx)
+          -- Sort diffs by start_line in descending order (apply from bottom to top)
+          table.sort(same_file_diffs, function(a, b)
+            return a.start_line > b.start_line
+          end)
+
+          local result = vim.deepcopy(original)
+
+          -- Apply diffs from bottom to top so line numbers remain valid
+          for _, d in ipairs(same_file_diffs) do
+            local change_lines = utils.split_lines(d.change)
+
+            -- Remove original lines (from end to start to avoid index shifting)
+            for i = d.end_line, d.start_line, -1 do
+              if result[i] then
+                table.remove(result, i)
+              end
             end
-            local change_lines = vim.split(file_diff.change, '\n')
-            for i, line in ipairs(change_lines) do
-              table.insert(modified, start_idx + i, line)
+
+            -- Insert replacement lines at start_line
+            for i = #change_lines, 1, -1 do
+              table.insert(result, d.start_line, change_lines[i])
             end
           end
 
-          modified = vim.tbl_filter(function(line)
-            return line ~= nil
-          end, modified)
-
-          opts.text = table.concat(modified, '\n')
+          opts.text = table.concat(result, '\n')
         else
           opts.text = diff.change
         end

lua/CopilotChat/utils.lua

@@ -772,6 +772,17 @@ function M.empty(v)
   return false
 end
 
+--- Split text into lines
+---@param text string The text to split
+---@return string[] A table of lines
+function M.split_lines(text)
+  if not text or text == '' then
+    return {}
+  end
+
+  return vim.split(text, '\r?\n', { trimempty = false })
+end
+
 --- Convert glob pattern to regex pattern
 --- https://github.com/davidm/lua-glob-pattern/blob/master/lua/globtopattern.lua
 ---@param g string The glob pattern