lualine.nvim/lua/lualine/components/branch.lua

-- Copyright (c) 2020-2021 shadmansaleh
-- MIT license, see LICENSE for more details.
local Branch = require('lualine.component'):new()

-- vars
Branch.git_branch = ''
-- os specific path separator
Branch.sep = package.config:sub(1, 1)
-- event watcher to watch head file
Branch.file_changed = vim.loop.new_fs_event()

-- Initilizer
Branch.new = function(self, options, child)
  local new_branch = self._parent:new(options, child or Branch)
  if not new_branch.options.icon then
    new_branch.options.icon = '' -- e0a0
  end
  -- run watch head on load so branch is present when component is loaded
  Branch.update_branch()
  -- update branch state of BufEnter as different Buffer may be on different repos
  vim.cmd [[autocmd BufEnter * lua require'lualine.components.branch'.update_branch()]]
  return new_branch
end

Branch.update_status = function() return Branch.git_branch end

-- returns full path to git directory for current directory
function Branch.find_git_dir()
  -- get file dir so we can search from that dir
  local file_dir = vim.fn.expand('%:p:h') .. ';'
  -- find .git/ folder genaral case
  local git_dir = vim.fn.finddir('.git', file_dir)
  -- find .git file in case of submodules or any other case git dir is in
  -- any other place than .git/
  local git_file = vim.fn.findfile('.git', file_dir)
  -- for some weird reason findfile gives relative path so expand it to fullpath
  if #git_file > 0 then git_file = vim.fn.fnamemodify(git_file, ':p') end
  if #git_file > #git_dir then
    -- separate git-dir or submodule is used
    local file = io.open(git_file)
    git_dir = file:read()
    git_dir = git_dir:match('gitdir: (.+)$')
    file:close()
    -- submodule / relative file path
    if git_dir:sub(1, 1) ~= Branch.sep and not git_dir:match('^%a:.*$') then
      git_dir = git_file:match('(.*).git') .. git_dir
    end
  end
  return git_dir
end

-- sets git_branch veriable to branch name or commit hash if not on branch
function Branch.get_git_head(head_file)
  local f_head = io.open(head_file)
  if f_head then
    local HEAD = f_head:read()
    f_head:close()
    local branch = HEAD:match('ref: refs/heads/(.+)$')
    if branch then
      Branch.git_branch = branch
    else
      Branch.git_branch = HEAD:sub(1, 6)
    end
  end
  return nil
end

-- Update branch
function Branch.update_branch()
  Branch.file_changed:stop()
  local git_dir = Branch.find_git_dir()
  if #git_dir > 0 then
    local head_file = git_dir .. Branch.sep .. 'HEAD'
    Branch.get_git_head(head_file)
    Branch.file_changed:start(head_file, {}, vim.schedule_wrap(
                                  function()
          -- reset file-watch
          Branch.update_branch()
        end))
  else
    -- set to '' when git dir was not found
    Branch.git_branch = ''
  end
end

return Branch