;;; changes-mode.el --- Basic tools for highlighting changes in a buffer.

;; Copyright (C) 1995 Niels Möller

;; Author: Niels Möller <nisse@lysator.liu.se>
;; Created: 11 Dec 1995
;; Version: 1.0
;; Keywords: C lisp tools editing

;; This file is part of GNU Emacs.

;; GNU Emacs is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation; either version 2, or (at your option)
;; any later version.

;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs; see the file COPYING.  If not, write to
;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.

;;; Commentary:

;; CHANGES-MODE turns the minor mode on or off.
;; CHANGES-CLEAR forgets which parts of the buffer have been modified
;; CHANGES-MODE-PROPERTIES is an alist of properties and values that
;; should be set for text that is changed.

;;; Code:

;;; User parameters
(defvar changes-mode-properties '(modified t face bold)
  "Alist of properties and values to be set on modified characters.")


;;; Minor mode definitions
(defvar changes-mode nil
  "Non-nil if changes-mode is active.")
(make-variable-buffer-local 'changes-mode)

(or (assq 'changes-mode minor-mode-alist)
    (setq minor-mode-alist
	  (cons '(changes-mode " Changes") minor-mode-alist)))


;;; For internal use only
;; Bound to non-nil locally in when modifying text properties,
;; to stop infinite recursion
(defconst changes-mode-busy nil)


;; Hook to be called when buffer changes.
(defun changes-hook-function (start end length)
  (and changes-mode
       (not changes-mode-busy)
       (let ((changes-mode-busy 'busy))
	 (add-text-properties start end changes-mode-properties))))


(defun changes-clear ()
  "Removes text properties added by changes-mode."
  (interactive)
  (let ((changes-mode-busy 'busy))
    (remove-text-properties (point-min) (point-max)
			    changes-mode-properties)))


;; Turn on or off the changes minor mode
(defun changes-mode (arg)
  "Adds the text properties in CHANGES-MODE-PROPERTIES to characters in
the buffer as they are modified."
  (interactive "P")
  (setq changes-mode
	(if (null arg) (not changes-mode)
	  (> (prefix-numeric-value arg) 0)))
  (if changes-mode
      (progn
	(make-local-variable 'after-change-function)
	(add-hook 'after-change-functions 'changes-hook-function ))))

(provide 'changes-mode)

;;; changes-mode.el ends here