indent ifdef

(defun my-indent-ifdef ()
  (let ((extra-indent 0)
        (ifdef "^[ \t]*#if\\(def\\)?\\b")
        (endif "^[ \t]*#endif\\b"))
    (save-excursion
      (beginning-of-line)
      (if (not (bobp))
          (progn
            (c-backward-comments)
            (if (/= (line-number-at-pos) 1)
                (progn
                  (beginning-of-line)
                  (if (looking-at ifdef)
                      (setq extra-indent tab-width)
                    (if (looking-at endif)
                        (setq extra-indent (- extra-indent tab-width)))))))))
    (save-excursion
      (beginning-of-line)
      (if (looking-at endif)
          (setq extra-indent (- extra-indent tab-width))))
    (if (/= extra-indent 0)
        (let ((new-indent (+ (current-indentation) extra-indent)))
          (if (>= new-indent 0)
              (if (eq (current-column) (current-indentation))
                  (indent-line-to new-indent)
                (save-excursion
                  (indent-line-to new-indent))))))))
(add-hook 'c-mode-common-hook
  (lambda () (add-hook 'c-special-indent-hook 'my-indent-ifdef)))