;;
;; toggle-exec-on-current-file - does what it says it does
;;
;; Copyright (c) 2010, Dorian Taylor
;; All rights reserved.
;;
;; Redistribution and use in source and binary forms, with or without
;; modification, are permitted provided that the following conditions
;; are met:
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;;       copyright notice, this list of conditions and the following
;;       disclaimer.
;;     * Redistributions in binary form must reproduce the above
;;       copyright notice, this list of conditions and the following
;;       disclaimer in the documentation and/or other materials
;;       provided with the distribution.
;;     * Neither the name of Dorian Taylor nor the names of his
;;       contributors may be used to endorse or promote products
;;       derived from this software without specific prior written
;;       permission.
;;
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;; "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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;; DAMAGE.

; map to C-x !
(defun toggle-exec-on-current-file ()
  "Sets the executable bit on or off for the current file, based
on the current state of the read bits. Note this will clobber
executable bits where there are no read bits when setting the
toggle to 'off' (but then how often does it make sense to execute
a file you can't read?). Does nothing if the current buffer is
not mapped to an existing file."
  (interactive)
  ;; XXX: should this operate on a symlink or the real file?
  (let ((bfn (buffer-file-name)))
    (if (and (stringp bfn) (file-exists-p bfn))
        (let* ((mode (file-modes bfn))
               ;; right shifting by two bits gives whatever masked with 0111
               (read-bits (lsh (logand mode #o444) -2))
               (exec-bits (logand mode #o111)))
          (if (= exec-bits 0)
              (progn ; adds executable bits where the read bits already exist
                (set-file-modes bfn (logior mode read-bits))
                (message (format "%s is now executable" bfn)))
            (progn ; removes all executable bits
              (set-file-modes bfn (logand mode (lognot #o111)))
              (message (format "%s is no longer executable" bfn)))))
      (message "Current buffer is not a file."))))