gistfile1.el

(defun ruby-mode-set-encoding ()
  (save-excursion
    (widen)
    (goto-char (point-min))
    (when (re-search-forward "[^\0-\177]" nil t)
      (goto-char (point-min))
      (let ((coding-system
             (coding-system-to-mime-charset
              (or coding-system-for-write
                  buffer-file-coding-system))))
        (setq coding-system
              (if coding-system
                  (symbol-name
                   (or (and ruby-use-encoding-map
                            (cdr (assq coding-system ruby-encoding-map)))
                       coding-system))
                "ascii-8bit"))
        (if (looking-at "^#![^\n]*ruby") (beginning-of-line 2))
        (cond ((looking-at "\\s *#.*-\*-\\s *\\(en\\)?coding\\s *:\\s *\\([-a-z0-9_]+\\)")
               (unless (string= (match-string 2) coding-system)
                 (goto-char (match-beginning 2))
                 (delete-region (point) (match-end 2))
                 (insert coding-system)))
              ((looking-at "\\s *#.*coding\\s *[:=]"))
              (t (insert "# -*- coding: " coding-system " -*-\n"))
              )))))