lualine.nvim/lua/lualine/components/filename.lua

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

local function count(base, pattern)
  return select(2, string.gsub(base, pattern, ''))
end

local function shorten_path(path, sep)
  -- ('([^/])[^/]+%/', '%1/', 1)
  return path:gsub(
             string.format('([^%s])[^%s]+%%%s', sep, sep, sep), '%1' .. sep, 1)
end

FileName.new = function(self, options, child)
  local new_instance = self._parent:new(options, child or FileName)
  local default_symbols = {modified = '[+]', readonly = '[-]'}
  new_instance.options.symbols = vim.tbl_extend('force', default_symbols,
                                                new_instance.options.symbols or
                                                    {})

  -- setting defaults
  if new_instance.options.file_status == nil then
    new_instance.options.file_status = true
  end
  if new_instance.options.path == nil then new_instance.options.path = 0 end
  if new_instance.options.full_path or new_instance.options.shorten then
    vim.schedule(function()
      vim.api.nvim_err_writeln(
        [[Lualine: filename component configuration changed, see :h lualine-component_options ]])
    end)
  end

  return new_instance
end

FileName.update_status = function(self)
  local data
  if self.options.path == 1 then
    -- relative path
    data = vim.fn.expand('%:~:.')
  elseif self.options.path == 2 then
    -- absolute path
    data = vim.fn.expand('%:p')
  else
    -- just filename
    data = vim.fn.expand('%:t')
  end

  if data == '' then data = '[No Name]' end

  local windwidth = vim.fn.winwidth(0)
  local estimated_space_available = 40
  local path_separator = package.config:sub(1, 1)
  for _ = 0, count(data, path_separator) do
    if windwidth <= 84 or #data > estimated_space_available then
      data = shorten_path(data, path_separator)
    end
  end

  if self.options.file_status then
    if vim.bo.modified then
      data = data .. self.options.symbols.modified
    elseif vim.bo.modifiable == false or vim.bo.readonly == true then
      data = data .. self.options.symbols.readonly
    end
  end
  return data
end

return FileName