tidy.nvim/lua/tidy/init.lua

local M = {}

local function list_to_set(list)
  local set = {}

  for _, item in ipairs(list) do
    set[item] = true
  end

  return set
end

local function is_excluded_ft(opts)
  if not opts.filetype_exclude then
    return false
  end

  local ft = vim.api.nvim_buf_get_option(0, "filetype")
  local ft_set = list_to_set(opts.filetype_exclude)

  return ft_set[ft]
end

local function reset_cursor_pos(pos)
  local num_rows = vim.api.nvim_buf_line_count(0)

  --[[
        if the row value in the original cursor
        position tuple is greater than the
        line count after empty line deletion
        (meaning that the cursor was inside of
        the group of empty lines at the end of
        the file when they were deleted), set
        the cursor row to the last line.
    ]]
  if pos[1] > num_rows then
    pos[1] = num_rows
  end

  vim.api.nvim_win_set_cursor(0, pos)
end

function M.setup(opts)

  local defaults = {
    filetype_exclude = {},
    newline_at_eof = false
  }

  opts = vim.tbl_extend("force", defaults, opts or {})

  local tidy_grp = vim.api.nvim_create_augroup("tidy", { clear = true })

  vim.api.nvim_create_autocmd("BufWritePre", {
    group = tidy_grp,
    callback = function()
      if is_excluded_ft(opts) then
        return false
      end

      local cursor_pos = vim.api.nvim_win_get_cursor(0)

      -- delete trailing whitespace
      vim.cmd([[:keepjumps keeppatterns %s/\s\+$//e]])

      -- delete lines @ eof
      vim.cmd([[:keepjumps keeppatterns silent! 0;/^\%(\n*.\)\@!/,$d]])

      reset_cursor_pos(cursor_pos)
    end,
  })
end

return M