refactor: use default settings in place of conditionals

- instead of checking for the existence of
      an option whenever that option may be
      accessed, initialize default settings and
      merge them with user-provided settings.

    - instead of looping through all excluded
      filetypes and comparing them to the current
      filetype, create a set from excluded
      filetypes and check if the current filetype
      is present.

lua/tidy/init.lua

@@ -1,19 +1,24 @@
 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 or not opts.filetype_exclude then
+  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)
 
-    for _, entry in ipairs(opts.filetype_exclude) do
-        if entry == ft then
-            return true
-        end
-    end
-
-    return false
+  return ft_set[ft]
 end
 
 local function reset_cursor_pos(pos)
@@ -36,6 +41,14 @@ local function reset_cursor_pos(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", {
@@ -47,9 +60,10 @@ function M.setup(opts)
 
       local cursor_pos = vim.api.nvim_win_get_cursor(0)
 
+      -- delete trailing whitespace
       vim.cmd([[:keepjumps keeppatterns %s/\s\+$//e]])
 
-            -- delete all lines at end of buffer, see source 2
+      -- delete lines @ eof
       vim.cmd([[:keepjumps keeppatterns silent! 0;/^\%(\n*.\)\@!/,$d]])
 
       reset_cursor_pos(cursor_pos)