feat(lcs): add LCS & minimal edit support

Introduce new support for LCS-based diff generation and minimal text edits.
• Add lcs.lua to compute diffs based on longest common subsequence.
• Add mimimal_edits.lua to reduce and optimize text edits.
• Update nes/init.lua to apply minimal edits when a single edit is returned.

Author: tris203

lua/copilot-lsp/lcs.lua

@@ -0,0 +1,179 @@
+local M = {}
+
+---@class copilotlsp.lcs.Edit
+---@field kind copilotlsp.lcs.EditKind
+---@field text string
+
+---@enum copilotlsp.lcs.EditKind
+M.edit_kind = {
+    addition = "addition",
+    removal = "removal",
+    unchanged = "unchanged",
+}
+
+--- Computes the Long Common Subequence table.
+--- Reference: [https://en.wikipedia.org/wiki/Longest_common_subsequence#Computing_the_length_of_the_LCS]
+---@param source string
+---@param target string
+function M.generate_table(source, target)
+    local n = #source + 1
+    local m = #target + 1
+
+    ---@type integer[][]
+    local lcs = {}
+    for i = 1, n do
+        lcs[i] = {}
+        for j = 1, m do
+            lcs[i][j] = 0
+        end
+    end
+
+    for i = 2, n do
+        for j = 2, m do
+            if source:byte(i - 1) == target:byte(j - 1) then
+                lcs[i][j] = 1 + lcs[i - 1][j - 1]
+            else
+                lcs[i][j] = math.max(lcs[i - 1][j], lcs[i][j - 1])
+            end
+        end
+    end
+    return lcs
+end
+
+---@generic T
+---@param tbl T[]
+---@return T[]
+local function reverse_table(tbl)
+    local ret = {}
+    for i = #tbl, 1, -1 do
+        table.insert(ret, tbl[i])
+    end
+    return ret
+end
+
+--- Calculates a diff between two strings using LCS
+---@param source string
+---@param target string
+---@return copilotlsp.lcs.Edit[]
+function M.diff(source, target)
+    local src_idx, trt_idx = #source + 1, #target + 1
+    local lcs
+
+    ---@type copilotlsp.lcs.Edit[]
+    local edits = {}
+    local edit_idx = 1
+
+    local edit_kind = M.edit_kind
+
+    while src_idx > 1 or trt_idx > 1 do
+        if src_idx == 1 then
+            trt_idx = trt_idx - 1
+            edits[edit_idx] = {
+                kind = edit_kind.addition,
+                text = string.char(string.byte(target, trt_idx)),
+            }
+        elseif trt_idx == 1 then
+            src_idx = src_idx - 1
+            edits[edit_idx] = {
+                kind = edit_kind.removal,
+                text = string.char(string.byte(source, src_idx)),
+            }
+        else
+            local src_char = string.byte(source, src_idx - 1)
+            local trt_char = string.byte(target, trt_idx - 1)
+
+            if src_char == trt_char then
+                src_idx, trt_idx = src_idx - 1, trt_idx - 1
+                edits[edit_idx] = {
+                    kind = edit_kind.unchanged,
+                    text = string.char(src_char),
+                }
+            else
+                lcs = lcs or M.generate_table(source, target)
+                if lcs[src_idx - 1][trt_idx] <= lcs[src_idx][trt_idx - 1] then
+                    trt_idx = trt_idx - 1
+                    edits[edit_idx] = {
+                        kind = edit_kind.addition,
+                        text = string.char(trt_char),
+                    }
+                else
+                    src_idx = src_idx - 1
+                    edits[edit_idx] = {
+                        kind = edit_kind.removal,
+                        text = string.char(src_char),
+                    }
+                end
+            end
+        end
+        edit_idx = edit_idx + 1
+    end
+
+    return reverse_table(edits)
+end
+
+---@param edits copilotlsp.lcs.Edit[]
+---@param line integer
+---@param character integer
+---@return lsp.TextEdit[]
+function M.to_lsp_edits(edits, line, character)
+    local function advance_cursor(edit)
+        if edit.text == "\n" then
+            line = line + 1
+            character = 0
+        else
+            character = character + 1
+        end
+    end
+
+    ---@type lsp.TextEdit[]
+    local lsp_edits = {}
+    local i = 1
+    while i < #edits do
+        -- Skip all unchanged edits and advance cursor
+        while i < #edits and edits[i].kind == M.edit_kind.unchanged do
+            advance_cursor(edits[i])
+            i = i + 1
+        end
+
+        -- No more edits to compute
+        if i >= #edits then
+            break
+        end
+
+        local new_text = ""
+        local start_line, start_character = line, character
+
+        -- Collect consecutive additions and removals
+        while i < #edits and edits[i].kind ~= M.edit_kind.unchanged do
+            if edits[i].kind == M.edit_kind.addition then
+                new_text = new_text .. edits[i].text
+            elseif edits[i].kind == M.edit_kind.removal then
+                advance_cursor(edits[i])
+            else
+                error("unexcepted edit kind " .. edits[i].kind)
+            end
+            i = i + 1
+        end
+
+        ---@type lsp.TextEdit
+        local lsp_edit = {
+            newText = new_text,
+            range = {
+                start = {
+                    line = start_line,
+                    character = start_character,
+                },
+                ["end"] = {
+                    line = line,
+                    character = character,
+                },
+            },
+        }
+
+        table.insert(lsp_edits, lsp_edit)
+    end
+
+    return lsp_edits
+end
+
+return M

lua/copilot-lsp/mimimal_edits.lua

@@ -0,0 +1,88 @@
+local lcs = require("copilot-lsp.lcs")
+local M = {}
+
+---@param lines string[]
+---@param range lsp.Range
+local function extract_lines_from_range(lines, range)
+    local start_row = range.start.line + 1
+    local start_col = range.start.character + 1
+    local end_row = range["end"].line + 1
+    local end_col = range["end"].character + 1
+
+    local source_lines = {}
+    -- Loop through the zero-indexed range [source_start_row, source_end_row)
+    for i = start_row, end_row do
+        local line = lines[i]
+
+        if i == start_row then
+            line = line:sub(start_col, -1)
+        elseif i == end_row then
+            line = line:sub(1, end_col - 1)
+        end
+
+        -- strip CR characters when neovim fails to identify the correct file format
+        if vim.endswith(line, "\r") then
+            table.insert(source_lines, line:sub(1, -2))
+        else
+            table.insert(source_lines, line)
+        end
+    end
+
+    return source_lines
+end
+
+---@param source_buf string[]
+---@param target_edit lsp.TextEdit
+---@return lsp.TextEdit[]
+function M.compute_minimal_edits(source_buf, target_edit)
+    local source_lines = extract_lines_from_range(source_buf, target_edit.range)
+    local target_lines = vim.split(target_edit.newText, "\r?\n")
+
+    local source_text = table.concat(source_lines, "\n")
+    local target_text = table.concat(target_lines, "\n")
+
+    local indices = vim.diff(source_text, target_text, {
+        algorithm = "histogram",
+        result_type = "indices",
+    })
+    assert(type(indices) == "table")
+
+    ---@type lsp.TextEdit[]
+    local edits = {}
+
+    for _, idx in ipairs(indices) do
+        local source_line_start, source_line_count, target_line_start, target_line_count = unpack(idx)
+        local source_line_end = source_line_start + source_line_count - 1
+        local target_line_end = target_line_start + target_line_count - 1
+
+        local source = table.concat(source_lines, "\n", source_line_start, source_line_end)
+        local target = table.concat(target_lines, "\n", target_line_start, target_line_end)
+
+        local text_edits = lcs.to_lsp_edits(
+            lcs.diff(source, target),
+            source_line_start + target_edit.range.start.line - 1,
+            target_edit.range.start.character
+        )
+
+        vim.list_extend(edits, text_edits)
+    end
+
+    local contains_non_whitespace_edit = vim.iter(edits):any(function(edit)
+        return edit.newText:find("%S") ~= nil
+    end)
+
+    -- Diff the whole text if we encounter a non whitespace character in the edit.
+    -- This might happen when the formatted document deletes many lines
+    -- and `vim.diff` split those deletions into multiple hunks.
+    if contains_non_whitespace_edit then
+        edits = lcs.to_lsp_edits(
+            lcs.diff(source_text, target_text),
+            target_edit.range.start.line,
+            target_edit.range.start.character
+        )
+    end
+
+    return edits
+end
+
+return M

lua/copilot-lsp/nes/init.lua

@@ -1,6 +1,7 @@
 local errs = require("copilot-lsp.errors")
 local nes_ui = require("copilot-lsp.nes.ui")
 local utils = require("copilot-lsp.util")
+local me = require("copilot-lsp.mimimal_edits")
 
 local M = {}
 
@@ -18,6 +19,25 @@ local function handle_nes_response(err, result, ctx)
         --- Convert to textEdit fields
         edit.newText = edit.text
     end
+    if #result.edits == 1 then
+        ---@type copilotlsp.InlineEdit[]
+        local min_edits = {}
+
+        -- minimise the edits
+        local source_lines = vim.api.nvim_buf_get_lines(ctx.bufnr, 0, -1, false)
+        local min_te = me.compute_minimal_edits(source_lines, result.edits[1])
+        for _, edit in ipairs(min_te) do
+            local new_edit = {
+                range = edit.range,
+                newText = edit.newText,
+                command = result.edits[1].command,
+                textDocument = result.edits[1].textDocument,
+            }
+            table.insert(min_edits, new_edit)
+        end
+
+        result.edits = min_edits
+    end
     nes_ui._display_next_suggestion(ctx.bufnr, nes_ns, result.edits)
 end