feat(fugitive): line position tracking for keymaps

When pressing `du`/`dU` from a hunk line in the fugitive status buffer
(after expanding with `=`), the unified diff now opens at the
corresponding line instead of line 1.

Implementation:
- `fugitive.get_hunk_position()` returns @@ header and offset when on a hunk line
- `commands.find_hunk_line()` finds matching @@ header in diff buffer
- `commands.gdiff_file()` accepts optional `hunk_position` and jumps after opening

Also updates @phanen's README credit for the previous two fixes.

Closes #65

README.md

@@ -69,4 +69,5 @@ luarocks install diffs.nvim
 - [`vim-fugitive`](https://github.com/tpope/vim-fugitive)
 - [`codediff.nvim`](https://github.com/esmuellert/codediff.nvim)
 - [`diffview.nvim`](https://github.com/sindrets/diffview.nvim)
-- [@phanen](https://github.com/phanen) - diff header highlighting
+- [@phanen](https://github.com/phanen) - diff header highlighting, unknown
+  filetype fix, shebang/modeline detection

lua/diffs/commands.lua

@@ -3,6 +3,18 @@ local M = {}
 local git = require('diffs.git')
 local dbg = require('diffs.log').dbg
 
+---@param diff_lines string[]
+---@param hunk_position { hunk_header: string, offset: integer }
+---@return integer?
+function M.find_hunk_line(diff_lines, hunk_position)
+  for i, line in ipairs(diff_lines) do
+    if line == hunk_position.hunk_header then
+      return i + hunk_position.offset
+    end
+  end
+  return nil
+end
+
 ---@param old_lines string[]
 ---@param new_lines string[]
 ---@param old_name string
@@ -98,6 +110,7 @@ end
 ---@field staged? boolean
 ---@field untracked? boolean
 ---@field old_filepath? string
+---@field hunk_position? { hunk_header: string, offset: integer }
 
 ---@param filepath string
 ---@param opts? diffs.GdiffFileOpts
@@ -175,6 +188,14 @@ function M.gdiff_file(filepath, opts)
   vim.cmd(opts.vertical and 'vsplit' or 'split')
   vim.api.nvim_win_set_buf(0, diff_buf)
 
+  if opts.hunk_position then
+    local target_line = M.find_hunk_line(diff_lines, opts.hunk_position)
+    if target_line then
+      vim.api.nvim_win_set_cursor(0, { target_line, 0 })
+      dbg('jumped to line %d for hunk', target_line)
+    end
+  end
+
   dbg('opened diff buffer %d for %s (%s)', diff_buf, rel_path, diff_label)
 
   vim.schedule(function()

lua/diffs/fugitive.lua

@@ -95,6 +95,42 @@ function M.get_file_at_line(bufnr, lnum)
   return nil, nil, false, nil
 end
 
+---@class diffs.HunkPosition
+---@field hunk_header string
+---@field offset integer
+
+---@param bufnr integer
+---@param lnum integer
+---@return diffs.HunkPosition?
+function M.get_hunk_position(bufnr, lnum)
+  local lines = vim.api.nvim_buf_get_lines(bufnr, 0, lnum, false)
+  local current = lines[lnum]
+
+  if not current then
+    return nil
+  end
+
+  local prefix = current:sub(1, 1)
+  if prefix ~= '+' and prefix ~= '-' and prefix ~= ' ' then
+    return nil
+  end
+
+  for i = lnum - 1, 1, -1 do
+    local line = lines[i]
+    if line:match('^@@.-@@') then
+      return {
+        hunk_header = line,
+        offset = lnum - i,
+      }
+    end
+    if line:match('^[MADRCU?!]%s') or line:match('^%w+ %(') then
+      break
+    end
+  end
+
+  return nil
+end
+
 ---@param bufnr integer
 ---@return string?
 local function get_repo_root_from_fugitive(bufnr)
@@ -142,12 +178,14 @@ function M.diff_file_under_cursor(vertical)
 
   local filepath = repo_root .. '/' .. filename
   local old_filepath = old_filename and (repo_root .. '/' .. old_filename) or nil
+  local hunk_position = M.get_hunk_position(bufnr, lnum)
 
   dbg(
-    'diff_file_under_cursor: %s (section: %s, old: %s)',
+    'diff_file_under_cursor: %s (section: %s, old: %s, hunk_offset: %s)',
     filename,
     section or 'unknown',
-    old_filename or 'none'
+    old_filename or 'none',
+    hunk_position and tostring(hunk_position.offset) or 'none'
   )
 
   commands.gdiff_file(filepath, {
@@ -155,6 +193,7 @@ function M.diff_file_under_cursor(vertical)
     staged = section == 'staged',
     untracked = section == 'untracked',
     old_filepath = old_filepath,
+    hunk_position = hunk_position,
   })
 end
 

spec/commands_spec.lua

@@ -1,13 +1,15 @@
 require('spec.helpers')
 
+local commands = require('diffs.commands')
+
 describe('commands', function()
   describe('setup', function()
     it('registers Gdiff, Gvdiff, and Ghdiff commands', function()
-      require('diffs.commands').setup()
-      local commands = vim.api.nvim_get_commands({})
-      assert.is_not_nil(commands.Gdiff)
-      assert.is_not_nil(commands.Gvdiff)
-      assert.is_not_nil(commands.Ghdiff)
+      commands.setup()
+      local cmds = vim.api.nvim_get_commands({})
+      assert.is_not_nil(cmds.Gdiff)
+      assert.is_not_nil(cmds.Gvdiff)
+      assert.is_not_nil(cmds.Ghdiff)
     end)
   end)
 
@@ -37,4 +39,60 @@ describe('commands', function()
       assert.are.equal('', diff_output)
     end)
   end)
+
+  describe('find_hunk_line', function()
+    it('finds matching @@ header and returns target line', function()
+      local diff_lines = {
+        'diff --git a/file.lua b/file.lua',
+        '--- a/file.lua',
+        '+++ b/file.lua',
+        '@@ -1,3 +1,4 @@',
+        ' local M = {}',
+        '+local new = true',
+        ' return M',
+      }
+      local hunk_position = {
+        hunk_header = '@@ -1,3 +1,4 @@',
+        offset = 2,
+      }
+      local target_line = commands.find_hunk_line(diff_lines, hunk_position)
+      assert.equals(6, target_line)
+    end)
+
+    it('returns nil when hunk header not found', function()
+      local diff_lines = {
+        'diff --git a/file.lua b/file.lua',
+        '@@ -1,3 +1,4 @@',
+        ' local M = {}',
+      }
+      local hunk_position = {
+        hunk_header = '@@ -99,3 +99,4 @@',
+        offset = 1,
+      }
+      local target_line = commands.find_hunk_line(diff_lines, hunk_position)
+      assert.is_nil(target_line)
+    end)
+
+    it('handles multiple hunks and finds correct one', function()
+      local diff_lines = {
+        'diff --git a/file.lua b/file.lua',
+        '--- a/file.lua',
+        '+++ b/file.lua',
+        '@@ -1,3 +1,4 @@',
+        ' local M = {}',
+        '+local x = 1',
+        ' ',
+        '@@ -10,3 +11,4 @@',
+        ' function M.foo()',
+        '+  print("hello")',
+        ' end',
+      }
+      local hunk_position = {
+        hunk_header = '@@ -10,3 +11,4 @@',
+        offset = 2,
+      }
+      local target_line = commands.find_hunk_line(diff_lines, hunk_position)
+      assert.equals(10, target_line)
+    end)
+  end)
 end)

spec/fugitive_spec.lua

@@ -357,4 +357,124 @@ describe('fugitive', function()
       vim.api.nvim_buf_delete(buf, { force = true })
     end)
   end)
+
+  describe('get_hunk_position', function()
+    local function create_status_buffer(lines)
+      local buf = vim.api.nvim_create_buf(false, true)
+      vim.api.nvim_buf_set_lines(buf, 0, -1, false, lines)
+      return buf
+    end
+
+    it('returns nil when on file header line', function()
+      local buf = create_status_buffer({
+        'Unstaged (1)',
+        'M  file.lua',
+        '@@ -1,3 +1,4 @@',
+        ' local M = {}',
+        '+local new = true',
+      })
+      local pos = fugitive.get_hunk_position(buf, 2)
+      assert.is_nil(pos)
+      vim.api.nvim_buf_delete(buf, { force = true })
+    end)
+
+    it('returns nil when on @@ header line', function()
+      local buf = create_status_buffer({
+        'Unstaged (1)',
+        'M  file.lua',
+        '@@ -1,3 +1,4 @@',
+        ' local M = {}',
+      })
+      local pos = fugitive.get_hunk_position(buf, 3)
+      assert.is_nil(pos)
+      vim.api.nvim_buf_delete(buf, { force = true })
+    end)
+
+    it('returns hunk header and offset for + line', function()
+      local buf = create_status_buffer({
+        'Unstaged (1)',
+        'M  file.lua',
+        '@@ -1,3 +1,4 @@',
+        ' local M = {}',
+        '+local new = true',
+        ' return M',
+      })
+      local pos = fugitive.get_hunk_position(buf, 5)
+      assert.is_not_nil(pos)
+      assert.equals('@@ -1,3 +1,4 @@', pos.hunk_header)
+      assert.equals(2, pos.offset)
+      vim.api.nvim_buf_delete(buf, { force = true })
+    end)
+
+    it('returns hunk header and offset for - line', function()
+      local buf = create_status_buffer({
+        'Unstaged (1)',
+        'M  file.lua',
+        '@@ -1,3 +1,3 @@',
+        ' local M = {}',
+        '-local old = false',
+        ' return M',
+      })
+      local pos = fugitive.get_hunk_position(buf, 5)
+      assert.is_not_nil(pos)
+      assert.equals('@@ -1,3 +1,3 @@', pos.hunk_header)
+      assert.equals(2, pos.offset)
+      vim.api.nvim_buf_delete(buf, { force = true })
+    end)
+
+    it('returns hunk header and offset for context line', function()
+      local buf = create_status_buffer({
+        'Unstaged (1)',
+        'M  file.lua',
+        '@@ -1,3 +1,4 @@',
+        ' local M = {}',
+        '+local new = true',
+        ' return M',
+      })
+      local pos = fugitive.get_hunk_position(buf, 6)
+      assert.is_not_nil(pos)
+      assert.equals('@@ -1,3 +1,4 @@', pos.hunk_header)
+      assert.equals(3, pos.offset)
+      vim.api.nvim_buf_delete(buf, { force = true })
+    end)
+
+    it('returns correct offset for first line after @@', function()
+      local buf = create_status_buffer({
+        'Unstaged (1)',
+        'M  file.lua',
+        '@@ -1,3 +1,4 @@',
+        ' local M = {}',
+      })
+      local pos = fugitive.get_hunk_position(buf, 4)
+      assert.is_not_nil(pos)
+      assert.equals(1, pos.offset)
+      vim.api.nvim_buf_delete(buf, { force = true })
+    end)
+
+    it('handles @@ header with context text', function()
+      local buf = create_status_buffer({
+        'Unstaged (1)',
+        'M  file.lua',
+        '@@ -10,3 +10,4 @@ function M.hello()',
+        '   print("hi")',
+        '+  print("world")',
+      })
+      local pos = fugitive.get_hunk_position(buf, 5)
+      assert.is_not_nil(pos)
+      assert.equals('@@ -10,3 +10,4 @@ function M.hello()', pos.hunk_header)
+      assert.equals(2, pos.offset)
+      vim.api.nvim_buf_delete(buf, { force = true })
+    end)
+
+    it('returns nil when section header interrupts search', function()
+      local buf = create_status_buffer({
+        'Unstaged (1)',
+        'M  file.lua',
+        ' some orphan line',
+      })
+      local pos = fugitive.get_hunk_position(buf, 3)
+      assert.is_nil(pos)
+      vim.api.nvim_buf_delete(buf, { force = true })
+    end)
+  end)
 end)