feat(highlight): wire highlights.context into treesitter pipeline (#151)

## Problem

`highlights.context.enabled` and `highlights.context.lines` were
defined, validated, and range-checked but never read during
highlighting. Hunks inside incomplete constructs (e.g., a table literal
or function body whose opening is beyond the hunk's own context lines)
parsed incorrectly because treesitter had no surrounding code.

## Solution

`compute_hunk_context` in `init.lua` reads the working tree file using
the hunk's `@@ +start,count @@` line numbers to collect up to `lines`
(default 25) surrounding code lines in each direction. Files are read
once via `io.open` and cached across hunks in the same file.
`highlight_treesitter` in `highlight.lua` accepts an optional context
parameter that prepends/appends context lines to the parse string and
offsets capture rows by the prefix count, so extmarks only land on
actual hunk lines. Wired through `highlight_hunk` for the two
code-language treesitter calls (not headers, not `highlight_text`, not
vim syntax).

Closes #148.

doc/diffs.nvim.txt

@@ -225,16 +225,20 @@ Configuration is done via `vim.g.diffs`. Set this before the plugin loads:
                                                       *diffs.ContextConfig*
     Context config fields: ~
         {enabled}            (boolean, default: true)
-                             Read lines from disk before and after each hunk
-                             to provide surrounding syntax context. Improves
-                             accuracy at hunk boundaries where incomplete
-                             constructs (e.g., a function definition with no
-                             body) would otherwise confuse the parser.
+                             Read surrounding code from the working tree
+                             file and feed it into the treesitter string
+                             parser. Uses the hunk's `@@ +start,count @@`
+                             line numbers to read lines before and after
+                             the hunk from disk. Improves syntax accuracy
+                             when the hunk is inside an incomplete construct
+                             (e.g., a table literal or function body whose
+                             opening is not visible in the hunk's own
+                             context lines).
 
         {lines}              (integer, default: 25)
-                             Number of context lines to read in each
-                             direction. Lines are read with early exit —
-                             cost scales with this value, not file size.
+                             Max context lines to read in each direction.
+                             Files are read once per parse and cached across
+                             hunks in the same file.
 
                                                    *diffs.PrioritiesConfig*
     Priorities config fields: ~
@@ -695,10 +699,14 @@ KNOWN LIMITATIONS                                       *diffs-limitations*
 
 Incomplete Syntax Context ~
                                                       *diffs-syntax-context*
-Treesitter parses each diff hunk in isolation. Context lines within the hunk
-(lines with a ` ` prefix) provide syntactic context for the parser. In rare
-cases, hunks that start or end mid-expression may produce imperfect highlights
-due to treesitter error recovery.
+Treesitter parses each diff hunk in isolation. When `highlights.context` is
+enabled (the default), surrounding code is read from the working tree file
+and fed into the parser to improve accuracy at hunk boundaries. This helps
+when a hunk is inside a table, function body, or loop whose opening is
+beyond the hunk's own context lines. Requires `repo_root` and
+`file_new_start` to be available on the hunk (true for standard unified
+diffs). In rare cases, hunks that start or end mid-expression may still
+produce imperfect highlights due to treesitter error recovery.
 
 Syntax Highlighting Flash ~
                                                              *diffs-flash*

lua/diffs/highlight.lua

@@ -67,6 +67,10 @@ end
 ---@field defer_vim_syntax? boolean
 ---@field syntax_only? boolean
 
+---@class diffs.TSContext
+---@field before string[]?
+---@field after string[]?
+
 ---@param bufnr integer
 ---@param ns integer
 ---@param code_lines string[]
@@ -76,6 +80,7 @@ end
 ---@param covered_lines? table<integer, true>
 ---@param priorities diffs.PrioritiesConfig
 ---@param force_high_priority? boolean
+---@param context? diffs.TSContext
 ---@return integer
 local function highlight_treesitter(
   bufnr,
@@ -86,9 +91,34 @@ local function highlight_treesitter(
   col_offset,
   covered_lines,
   priorities,
-  force_high_priority
+  force_high_priority,
+  context
 )
-  local code = table.concat(code_lines, '\n')
+  local prefix_count = 0
+  local parse_lines = code_lines
+  if context then
+    local before = context.before
+    local after = context.after
+    if (before and #before > 0) or (after and #after > 0) then
+      parse_lines = {}
+      if before then
+        prefix_count = #before
+        for _, l in ipairs(before) do
+          parse_lines[#parse_lines + 1] = l
+        end
+      end
+      for _, l in ipairs(code_lines) do
+        parse_lines[#parse_lines + 1] = l
+      end
+      if after then
+        for _, l in ipairs(after) do
+          parse_lines[#parse_lines + 1] = l
+        end
+      end
+    end
+  end
+
+  local code = table.concat(parse_lines, '\n')
   if code == '' then
     return 0
   end
@@ -118,6 +148,8 @@ local function highlight_treesitter(
       if capture ~= 'spell' and capture ~= 'nospell' then
         local capture_name = '@' .. capture .. '.' .. tree_lang
         local sr, sc, er, ec = node:range()
+        sr = sr - prefix_count
+        er = er - prefix_count
 
         local buf_sr = line_map[sr]
         if buf_sr then
@@ -329,10 +361,36 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
       end
     end
 
-    extmark_count =
-      highlight_treesitter(bufnr, ns, new_code, hunk.lang, new_map, pw + qw, covered_lines, p)
+    local ts_context = nil
+    if opts.highlights.context.enabled and (hunk.context_before or hunk.context_after) then
+      ts_context = { before = hunk.context_before, after = hunk.context_after }
+    end
+
+    extmark_count = highlight_treesitter(
+      bufnr,
+      ns,
+      new_code,
+      hunk.lang,
+      new_map,
+      pw + qw,
+      covered_lines,
+      p,
+      nil,
+      ts_context
+    )
     extmark_count = extmark_count
-      + highlight_treesitter(bufnr, ns, old_code, hunk.lang, old_map, pw + qw, covered_lines, p)
+      + highlight_treesitter(
+        bufnr,
+        ns,
+        old_code,
+        hunk.lang,
+        old_map,
+        pw + qw,
+        covered_lines,
+        p,
+        nil,
+        ts_context
+      )
 
     if hunk.header_context and hunk.header_context_col then
       local header_extmarks = highlight_text(

lua/diffs/init.lua

@@ -297,6 +297,69 @@ local function carry_forward_highlighted(old_entry, new_hunks)
   return highlighted
 end
 
+---@param path string
+---@return string[]?
+local function read_file_lines(path)
+  local f = io.open(path, 'r')
+  if not f then
+    return nil
+  end
+  local lines = {}
+  for line in f:lines() do
+    lines[#lines + 1] = line
+  end
+  f:close()
+  return lines
+end
+
+---@param hunks diffs.Hunk[]
+---@param max_lines integer
+local function compute_hunk_context(hunks, max_lines)
+  ---@type table<string, string[]|false>
+  local file_cache = {}
+
+  for _, hunk in ipairs(hunks) do
+    if not hunk.repo_root or not hunk.filename or not hunk.file_new_start then
+      goto continue
+    end
+
+    local path = vim.fs.joinpath(hunk.repo_root, hunk.filename)
+    local file_lines = file_cache[path]
+    if file_lines == nil then
+      file_lines = read_file_lines(path) or false
+      file_cache[path] = file_lines
+    end
+    if not file_lines then
+      goto continue
+    end
+
+    local new_start = hunk.file_new_start
+    local new_count = hunk.file_new_count or 0
+    local total = #file_lines
+
+    local before_start = math.max(1, new_start - max_lines)
+    if before_start < new_start then
+      local before = {}
+      for i = before_start, new_start - 1 do
+        before[#before + 1] = file_lines[i]
+      end
+      hunk.context_before = before
+    end
+
+    local after_start = new_start + new_count
+    local after_end = math.min(total, after_start + max_lines - 1)
+    if after_start <= total then
+      local after = {}
+      for i = after_start, after_end do
+        after[#after + 1] = file_lines[i]
+      end
+      hunk.context_after = after
+    end
+
+    ::continue::
+  end
+end
+
 ---@param bufnr integer
 local function ensure_cache(bufnr)
   if not vim.api.nvim_buf_is_valid(bufnr) then
@@ -321,6 +384,9 @@ local function ensure_cache(bufnr)
   local lc = vim.api.nvim_buf_line_count(bufnr)
   local bc = vim.api.nvim_buf_get_offset(bufnr, lc)
   dbg('parsed %d hunks in buffer %d (tick %d)', #hunks, bufnr, tick)
+  if config.highlights.context.enabled then
+    compute_hunk_context(hunks, config.highlights.context.lines)
+  end
   local carried = entry and not entry.pending_clear and carry_forward_highlighted(entry, hunks)
   hunk_cache[bufnr] = {
     hunks = hunks,
@@ -941,6 +1007,7 @@ M._test = {
   hunks_eq = hunks_eq,
   process_pending_clear = process_pending_clear,
   ft_retry_pending = ft_retry_pending,
+  compute_hunk_context = compute_hunk_context,
 }
 
 return M

lua/diffs/parser.lua

@@ -15,6 +15,8 @@
 ---@field prefix_width integer
 ---@field quote_width integer
 ---@field repo_root string?
+---@field context_before string[]?
+---@field context_after string[]?
 
 local M = {}
 

spec/context_spec.lua

@@ -0,0 +1,382 @@
+require('spec.helpers')
+local diffs = require('diffs')
+local highlight = require('diffs.highlight')
+local compute_hunk_context = diffs._test.compute_hunk_context
+
+describe('context', function()
+  describe('compute_hunk_context', function()
+    local tmpdir
+
+    before_each(function()
+      tmpdir = vim.fn.tempname()
+      vim.fn.mkdir(tmpdir, 'p')
+    end)
+
+    after_each(function()
+      vim.fn.delete(tmpdir, 'rf')
+    end)
+
+    local function write_file(filename, lines)
+      local path = vim.fs.joinpath(tmpdir, filename)
+      local dir = vim.fn.fnamemodify(path, ':h')
+      if vim.fn.isdirectory(dir) == 0 then
+        vim.fn.mkdir(dir, 'p')
+      end
+      local f = io.open(path, 'w')
+      f:write(table.concat(lines, '\n') .. '\n')
+      f:close()
+    end
+
+    local function make_hunk(filename, opts)
+      return {
+        filename = filename,
+        ft = 'lua',
+        lang = 'lua',
+        start_line = opts.start_line or 1,
+        lines = opts.lines,
+        prefix_width = opts.prefix_width or 1,
+        quote_width = 0,
+        repo_root = tmpdir,
+        file_new_start = opts.file_new_start,
+        file_new_count = opts.file_new_count,
+      }
+    end
+
+    it('reads context_before from file lines preceding the hunk', function()
+      write_file('a.lua', {
+        'local M = {}',
+        'function M.foo()',
+        '  local x = 1',
+        '  local y = 2',
+        'end',
+        'return M',
+      })
+
+      local hunks = {
+        make_hunk('a.lua', {
+          file_new_start = 3,
+          file_new_count = 3,
+          lines = { ' local x = 1', '+local new = true', ' local y = 2' },
+        }),
+      }
+      compute_hunk_context(hunks, 25)
+
+      assert.same({ 'local M = {}', 'function M.foo()' }, hunks[1].context_before)
+    end)
+
+    it('reads context_after from file lines following the hunk', function()
+      write_file('a.lua', {
+        'local M = {}',
+        'function M.foo()',
+        '  local x = 1',
+        'end',
+        'return M',
+      })
+
+      local hunks = {
+        make_hunk('a.lua', {
+          file_new_start = 2,
+          file_new_count = 2,
+          lines = { ' function M.foo()', '+  local x = 1' },
+        }),
+      }
+      compute_hunk_context(hunks, 25)
+
+      assert.same({ 'end', 'return M' }, hunks[1].context_after)
+    end)
+
+    it('caps context_before to max_lines', function()
+      write_file('a.lua', {
+        'line1',
+        'line2',
+        'line3',
+        'line4',
+        'line5',
+        'target',
+      })
+
+      local hunks = {
+        make_hunk('a.lua', {
+          file_new_start = 6,
+          file_new_count = 1,
+          lines = { '+target' },
+        }),
+      }
+      compute_hunk_context(hunks, 2)
+
+      assert.same({ 'line4', 'line5' }, hunks[1].context_before)
+    end)
+
+    it('caps context_after to max_lines', function()
+      write_file('a.lua', {
+        'target',
+        'after1',
+        'after2',
+        'after3',
+        'after4',
+      })
+
+      local hunks = {
+        make_hunk('a.lua', {
+          file_new_start = 1,
+          file_new_count = 1,
+          lines = { '+target' },
+        }),
+      }
+      compute_hunk_context(hunks, 2)
+
+      assert.same({ 'after1', 'after2' }, hunks[1].context_after)
+    end)
+
+    it('skips hunks without file_new_start', function()
+      write_file('a.lua', { 'line1', 'line2' })
+
+      local hunks = {
+        make_hunk('a.lua', {
+          file_new_start = nil,
+          file_new_count = nil,
+          lines = { '+something' },
+        }),
+      }
+      compute_hunk_context(hunks, 25)
+
+      assert.is_nil(hunks[1].context_before)
+      assert.is_nil(hunks[1].context_after)
+    end)
+
+    it('skips hunks without repo_root', function()
+      local hunks = {
+        {
+          filename = 'a.lua',
+          ft = 'lua',
+          lang = 'lua',
+          start_line = 1,
+          lines = { '+x' },
+          prefix_width = 1,
+          quote_width = 0,
+          repo_root = nil,
+          file_new_start = 1,
+          file_new_count = 1,
+        },
+      }
+      compute_hunk_context(hunks, 25)
+
+      assert.is_nil(hunks[1].context_before)
+      assert.is_nil(hunks[1].context_after)
+    end)
+
+    it('skips when file does not exist on disk', function()
+      local hunks = {
+        make_hunk('nonexistent.lua', {
+          file_new_start = 1,
+          file_new_count = 1,
+          lines = { '+x' },
+        }),
+      }
+      compute_hunk_context(hunks, 25)
+
+      assert.is_nil(hunks[1].context_before)
+      assert.is_nil(hunks[1].context_after)
+    end)
+
+    it('returns nil context_before for hunk at line 1', function()
+      write_file('a.lua', { 'first', 'second' })
+
+      local hunks = {
+        make_hunk('a.lua', {
+          file_new_start = 1,
+          file_new_count = 1,
+          lines = { '+first' },
+        }),
+      }
+      compute_hunk_context(hunks, 25)
+
+      assert.is_nil(hunks[1].context_before)
+    end)
+
+    it('returns nil context_after for hunk at end of file', function()
+      write_file('a.lua', { 'first', 'last' })
+
+      local hunks = {
+        make_hunk('a.lua', {
+          file_new_start = 1,
+          file_new_count = 2,
+          lines = { ' first', '+last' },
+        }),
+      }
+      compute_hunk_context(hunks, 25)
+
+      assert.is_nil(hunks[1].context_after)
+    end)
+
+    it('reads file once for multiple hunks in same file', function()
+      write_file('a.lua', {
+        'local M = {}',
+        'function M.foo()',
+        '  return 1',
+        'end',
+        'function M.bar()',
+        '  return 2',
+        'end',
+        'return M',
+      })
+
+      local hunks = {
+        make_hunk('a.lua', {
+          file_new_start = 2,
+          file_new_count = 3,
+          lines = { ' function M.foo()', '+  return 1', ' end' },
+        }),
+        make_hunk('a.lua', {
+          file_new_start = 5,
+          file_new_count = 3,
+          lines = { ' function M.bar()', '+  return 2', ' end' },
+        }),
+      }
+      compute_hunk_context(hunks, 25)
+
+      assert.same({ 'local M = {}' }, hunks[1].context_before)
+      assert.same({ 'function M.bar()', '  return 2', 'end', 'return M' }, hunks[1].context_after)
+      assert.same({
+        'local M = {}',
+        'function M.foo()',
+        '  return 1',
+        'end',
+      }, hunks[2].context_before)
+      assert.same({ 'return M' }, hunks[2].context_after)
+    end)
+  end)
+
+  describe('highlight_treesitter with context', function()
+    local ns
+
+    before_each(function()
+      ns = vim.api.nvim_create_namespace('diffs_context_test')
+      local normal = vim.api.nvim_get_hl(0, { name = 'Normal' })
+      vim.api.nvim_set_hl(0, 'DiffsClear', { fg = normal.fg or 0xc0c0c0 })
+    end)
+
+    local function create_buffer(lines)
+      local bufnr = vim.api.nvim_create_buf(false, true)
+      vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, lines)
+      return bufnr
+    end
+
+    local function delete_buffer(bufnr)
+      if vim.api.nvim_buf_is_valid(bufnr) then
+        vim.api.nvim_buf_delete(bufnr, { force = true })
+      end
+    end
+
+    local function get_extmarks(bufnr)
+      return vim.api.nvim_buf_get_extmarks(bufnr, ns, 0, -1, { details = true })
+    end
+
+    local function default_opts(overrides)
+      local opts = {
+        hide_prefix = false,
+        highlights = {
+          background = false,
+          gutter = false,
+          context = { enabled = true, lines = 25 },
+          treesitter = { enabled = true, max_lines = 500 },
+          vim = { enabled = false, max_lines = 200 },
+          intra = { enabled = false, algorithm = 'default', max_lines = 500 },
+          priorities = { clear = 198, syntax = 199, line_bg = 200, char_bg = 201 },
+        },
+      }
+      if overrides then
+        if overrides.highlights then
+          opts.highlights = vim.tbl_deep_extend('force', opts.highlights, overrides.highlights)
+        end
+      end
+      return opts
+    end
+
+    it('applies extmarks only to hunk lines, not context lines', function()
+      local bufnr = create_buffer({
+        '@@ -1,2 +1,3 @@',
+        ' local x = 1',
+        ' local y = 2',
+        '+local z = 3',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        start_line = 2,
+        lines = { ' local x = 1', ' local y = 2', '+local z = 3' },
+        prefix_width = 1,
+        quote_width = 0,
+        context_before = { 'local function foo()' },
+        context_after = { 'end' },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      for _, mark in ipairs(extmarks) do
+        local row = mark[2]
+        assert.is_true(row >= 1 and row <= 3, 'extmark row ' .. row .. ' outside hunk range')
+      end
+      assert.is_true(#extmarks > 0)
+      delete_buffer(bufnr)
+    end)
+
+    it('does not pass context when context.enabled = false', function()
+      local bufnr = create_buffer({
+        '@@ -1,1 +1,2 @@',
+        ' local x = 1',
+        '+local y = 2',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        start_line = 2,
+        lines = { ' local x = 1', '+local y = 2' },
+        prefix_width = 1,
+        quote_width = 0,
+        context_before = { 'local function foo()' },
+        context_after = { 'end' },
+      }
+
+      local opts_enabled = default_opts({ highlights = { context = { enabled = true } } })
+      highlight.highlight_hunk(bufnr, ns, hunk, opts_enabled)
+      local extmarks_with = get_extmarks(bufnr)
+
+      vim.api.nvim_buf_clear_namespace(bufnr, ns, 0, -1)
+
+      local opts_disabled = default_opts({ highlights = { context = { enabled = false } } })
+      highlight.highlight_hunk(bufnr, ns, hunk, opts_disabled)
+      local extmarks_without = get_extmarks(bufnr)
+
+      assert.is_true(#extmarks_with > 0)
+      assert.is_true(#extmarks_without > 0)
+      delete_buffer(bufnr)
+    end)
+
+    it('skips context fields that are nil', function()
+      local bufnr = create_buffer({
+        '@@ -1,1 +1,2 @@',
+        ' local x = 1',
+        '+local y = 2',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        start_line = 2,
+        lines = { ' local x = 1', '+local y = 2' },
+        prefix_width = 1,
+        quote_width = 0,
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      assert.is_true(#extmarks > 0)
+      delete_buffer(bufnr)
+    end)
+  end)
+end)