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+;;; goto-chg.el --- goto last change
+;;--------------------------------------------------------------------
+;;
+;; Copyright (C) 2002-2008,2013 David Andersson
+;;
+;; This program 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 of
+;; the License, or (at your option) any later version.
+;;
+;; This program 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 this program; if not, write to the Free
+;; Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
+;; MA 02111-1307 USA
+;;
+;;-------------------------------------------------------------------
+;;
+;; Author: David Andersson <l.david.andersson(at)sverige.nu>
+;; Created: 16 May 2002
+;; Version: 1.6
+;; Keywords: convenience, matching
+;;
+;;; Commentary:
+;;
+;; Goto Last Change
+;;
+;; Goto the point of the most recent edit in the buffer.
+;; When repeated, goto the second most recent edit, etc.
+;; Negative argument, C-u -, for reverse direction.
+;; Works by looking into buffer-undo-list to find points of edit.
+;;
+;; You would probably like to bind this command to a key.
+;; For example in your ~/.emacs:
+;;
+;;   (require 'goto-chg)
+;;
+;;   (global-set-key [(control ?.)] 'goto-last-change)
+;;   (global-set-key [(control ?,)] 'goto-last-change-reverse)
+;;
+;; Works with emacs-19.29, 19.31, 20.3, 20.7, 21.1, 21.4, 22.1 and 23.1
+;; Works with XEmacs-20.4 and 21.4 (but see todo about `last-command' below)
+;;
+;;--------------------------------------------------------------------
+;; History
+;;
+;; Ver 1.6 2013-12-12 David Andersson
+;;    Add keywords; Cleanup comments
+;; Ver 1.5 2013-12-11 David Andersson
+;;    Autoload and document `goto-last-change-reverse'
+;; Ver 1.4 2008-09-20 David Andersson
+;;    Improved property change description; Update comments.
+;; Ver 1.3 2007-03-14 David Andersson
+;;    Added `goto-last-change-reverse'
+;; Ver 1.2 2003-04-06 David Andersson
+;;    Don't let repeating error depthen glc-probe-depth.
+;; Ver 1.1 2003-04-06 David Andersson
+;;    Zero arg describe changes. Negative arg go back.
+;;    Autoload. Remove message using nil in stead of an empty string.
+;; Ver 1.0 2002-05-18 David Andersson
+;;    Initial version
+;;
+;;--------------------------------------------------------------------
+;;
+;;todo: Rename "goto-chg.el" -> "gotochange.el" or "goto-chgs" ?
+;;todo: Rename function goto-last-change -> goto-last-edit ?
+;;todo: Rename adjective "-last-" -> "-latest-" or "-most-recent-" ?
+;;todo: There are some, maybe useful, funcs  for region undo 
+;;       in simple.el in emacs 20. Take a look.
+;;todo: Add functionality to visit changed point in text order, not only in
+;;        chronological order. (Naa, highlight-changes-mode does that).
+;;todo: Inverse indication that a change has been saved or not
+;;todo: Highlight the range of text involved in the last change?
+;;todo: See session-jump-to-last-change in session.el?
+;;todo: Unhide invisible text (e.g. outline mode) like isearch do.
+;;todo: XEmacs sets last-command to `t' after an error, so you cannot reverse
+;;        after "No furter change info". Should we bother?
+;;todo: Try distinguish "No further change info" (end of truncated undo list)
+;;        and "No further changes" (end of a complete undo list).
+;;
+;;--------------------------------------------------------------------
+
+;;; Code:
+
+(defvar glc-default-span 8 "*goto-last-change don't visit the same point twice. glc-default-span tells how far around a visited point not to visit again.")
+(defvar glc-current-span 8 "Internal for goto-last-change.\nA copy of glc-default-span or the ARG passed to goto-last-change.")
+(defvar glc-probe-depth 0 "Internal for goto-last-change.\nIt is non-zero between successive goto-last-change.")
+
+;;todo: Find begin and end of line, then use it somewhere
+
+(defun glc-center-ellipsis (str maxlen &optional ellipsis)
+  "Truncate STRING in the middle to length MAXLEN.
+If STRING is max MAXLEN just return the string.
+Optional third argument is the replacement, which defaults to \"...\"."
+  (if (<= (length str) maxlen)
+      str
+    ;; else
+    (let* ((lipsis (or ellipsis "..."))
+	   (i (/ (- maxlen (length lipsis)) 2)))
+      (concat (substring str 0 i)
+	      lipsis 
+	      (substring str (- i))))))
+
+(defun glc-adjust-pos2 (pos p1 p2 adj)
+  ;; Helper function to glc-adjust-pos
+  ;; p1, p2: interval where an edit occured
+  ;; adj: amount of text added (positive) or removed (negativ) by the edit
+  ;; Return pos if well before p1, or pos+adj if well after p2, or nil if too close
+  (cond ((<= pos (- p1 glc-current-span))
+	 pos)
+	((> pos (+ p2 glc-current-span))
+	 (+ pos adj))
+	((zerop glc-current-span)
+	 p1)
+	(t
+	 nil)))
+
+(defun glc-adjust-pos (pos e)
+  "Given POS, a buffer position before the edit E, compute and return
+the \"same\" buffer position after E happened.
+Exception: return nil if POS is closer than `glc-current-span' to the edit E.
+\nInsertion edits before POS returns a larger value.
+Deletion edits before POS returns a smaller value.
+\nThe edit E is an entry from the `buffer-undo-list'. See for details."
+  (cond ((atom e)			; nil==cmd boundary, or, num==changed pos
+	 pos)
+	((numberp (car e))		; (beg . end)==insertion
+	 (glc-adjust-pos2 pos (car e) (car e) (- (cdr e) (car e))))
+	((stringp (car e))		; (string . pos)==deletion
+	 (glc-adjust-pos2 pos (abs (cdr e)) (+ (abs (cdr e)) (length (car e))) (- (length (car e)))))
+	((null (car e))			; (nil prop val beg . end)==prop change
+	 (glc-adjust-pos2 pos (nth 3 e) (nthcdr 4 e) 0))
+	(t				; (marker . dist)==marker moved
+	 pos)))
+
+;; If recursive in stead of iterative (while), it tends to fill the call stack.
+;; (Isn't it tail optimized?)
+(defun glc-adjust-list (r)
+  "R is list of edit entries in chronological order.
+Pick the point of the first edit entry and update that point with
+the second, third, etc, edit entries. Return the final updated point,
+or nil if the point was closer than `glc-current-span' to some edit in R.
+\nR is basically a reversed slice from the buffer-undo-list."
+  (if r
+      ;; Get pos
+      (let ((pos (glc-get-pos (car r))))
+	(setq r (cdr r))
+	;; Walk back in reverse list
+	(while (and r pos)
+	  (setq pos (glc-adjust-pos pos (car r))
+		r (cdr r)))
+	pos)
+    ;; else
+    nil))
+
+(defun glc-get-pos (e)
+  "If E represents an edit, return a position value in E, the position
+where the edit took place. Return nil if E represents no real change.
+\nE is an entry in the buffer-undo-list."
+  (cond ((numberp e) e)			; num==changed position
+	((atom e) nil)			; nil==command boundary
+	((numberp (car e)) (cdr e))	; (beg . end)==insertion
+	((stringp (car e)) (abs (cdr e))) ; (string . pos)==deletion
+	((null (car e)) (nthcdr 4 e))	; (nil ...)==text property change
+	((atom (car e)) nil)		; (t ...)==file modification time
+	(t nil)))			; (marker ...)==marker moved
+
+(defun glc-get-descript (e &optional n)
+  "If E represents an edit, return a short string describing E.
+Return nil if E represents no real change.
+\nE is an entry in the buffer-undo-list."
+  (let ((nn (or (format "T-%d: " n) "")))
+    (cond ((numberp e) "New position")	; num==changed position
+	  ((atom e) nil)		; nil==command boundary
+	  ((numberp (car e))		; (beg . end)==insertion
+	   (if (and n (< n 2))
+	       (format "%sInserted %d chars \"%s\"" nn (- (cdr e) (car e)) 
+		       (glc-center-ellipsis (buffer-substring (car e) (cdr e)) 60))
+	     ;; else
+	     ;; An older insert. The inserted text cannot easily be computed.
+	     ;; Just show the char count.
+	     (format "%sInserted %d chars" nn (- (cdr e) (car e)))))
+	  ((stringp (car e))		; (string . pos)==deletion
+	   (format "%sDeleted \"%s\"" nn (glc-center-ellipsis (car e) 60)))
+	  ((null (car e))		; (nil ...)==text property change
+	   (format "%sProperty change" nn))
+	  ((atom (car e)) nil)		; (t ...)==file modification time
+	  (t nil))))			; (marker ...)==marker moved
+
+(defun glc-is-positionable (e)
+  "Return non-nil if E is an insertion, deletion or text property change.
+\nE is an entry in the buffer-undo-list."
+  (and (not (numberp e)) (glc-get-pos e)))
+
+(defun glc-is-filetime (e)
+  "Return t if E indicates a buffer became \"modified\",
+that is, it was previously saved or unchanged. Nil otherwise."
+  (and (listp e) (eq (car e) t)))
+
+;;;###autoload
+(defun goto-last-change (arg)
+"Go to the point where the last edit was made in the current buffer.
+Repeat the command to go to the second last edit, etc.
+\nTo go back to more recent edit, the reverse of this command, use \\[goto-last-change-reverse]
+or precede this command with \\[universal-argument] - (minus).
+\nIt does not go to the same point twice even if there has been many edits
+there. I call the minimal distance between distinguishable edits \"span\".
+Set variable `glc-default-span' to control how close is \"the same point\".
+Default span is 8.
+The span can be changed temporarily with \\[universal-argument] right before \\[goto-last-change]:
+\\[universal-argument] <NUMBER> set current span to that number,
+\\[universal-argument] (no number) multiplies span by 4, starting with default.
+The so set span remains until it is changed again with \\[universal-argument], or the consecutive
+repetition of this command is ended by any other command.
+\nWhen span is zero (i.e. \\[universal-argument] 0) subsequent \\[goto-last-change] visits each and
+every point of edit and a message shows what change was made there.
+In this case it may go to the same point twice.
+\nThis command uses undo information. If undo is disabled, so is this command.
+At times, when undo information becomes too large, the oldest information is
+discarded. See variable `undo-limit'."
+  (interactive "P")
+  (cond ((not (eq this-command last-command))
+	 ;; Start a glc sequence
+	 ;; Don't go to current point if last command was an obvious edit
+	 ;; (yank or self-insert, but not kill-region). Makes it easier to
+	 ;; jump back and forth when copying seleced lines.
+	 (setq glc-probe-depth (if (memq last-command '(yank self-insert-command)) 1 0)
+	       glc-direction 1
+	       glc-current-span glc-default-span)
+	 (if (< (prefix-numeric-value arg) 0)
+	     (error "Negative arg: Cannot reverse as the first operation"))))
+  (cond ((null buffer-undo-list)
+	 (error "Buffer has not been changed"))
+	((eq buffer-undo-list t)
+	 (error "No change info (undo is disabled)")))
+  (cond ((numberp arg)			; Numeric arg sets span
+	 (setq glc-current-span (abs arg)))
+	((consp arg)			; C-u's multiply previous span by 4
+	 (setq glc-current-span (* (abs (car arg)) glc-default-span))
+	 (message "Current span is %d chars" glc-current-span))) ;todo: keep message with "waiting" and "is saved"
+  (cond ((< (prefix-numeric-value arg) 0)
+	 (setq glc-direction -1))
+	(t
+	 (setq glc-direction 1)))
+  (let (rev				; Reversed (and filtered) undo list
+	pos				; The pos we look for, nil until found
+	(n 0)				; Steps in undo list (length of 'rev')
+	(l buffer-undo-list) 
+	(passed-save-entry (not (buffer-modified-p)))
+	(new-probe-depth glc-probe-depth))
+    ;; Walk back and forth in the buffer-undo-list, each time one step deeper,
+    ;; until we can walk back the whole list with a 'pos' that is not coming
+    ;; too close to another edit.
+    (while (null pos)
+      (setq new-probe-depth (+ new-probe-depth glc-direction))
+      (if (< glc-direction 0)
+	  (setq rev ()
+		n 0
+		l buffer-undo-list
+		passed-save-entry (not (buffer-modified-p))))
+      (if (< new-probe-depth 1)
+	  (error "No later change info"))
+      (if (> n 150)
+	  (message "working..."))
+      ;; Walk forward in buffer-undo-list, glc-probe-depth steps.
+      ;; Build reverse list along the way
+      (while (< n new-probe-depth)
+	(cond ((null l)
+	       ;(setq this-command t)	; Disrupt repeat sequence
+	       (error "No further change info"))
+	      ((glc-is-positionable (car l))
+	       (setq n (1+ n)
+		     rev (cons (car l) rev)))
+	      ((or passed-save-entry (glc-is-filetime (car l)))
+	       (setq passed-save-entry t)))
+	(setq l (cdr l)))
+      ;; Walk back in reverse list, from older to newer edits.
+      ;; Adjusting pos along the way.
+      (setq pos (glc-adjust-list rev)))
+    ;; Found a place not previously visited, in 'pos'.
+    ;; (An error have been issued if nothing (more) found.)
+    (if (> n 150)
+	(message nil))			; remove message "working..."
+    (if (and (= glc-current-span 0) (glc-get-descript (car rev) n))
+	(message "%s" (glc-get-descript (car rev) n))
+      ;; else
+      (if passed-save-entry
+	  (message "(This change is saved)")))
+    (setq glc-probe-depth new-probe-depth)
+    (goto-char pos)))
+
+;;;###autoload
+(defun goto-last-change-reverse (arg)
+  "Go back to more recent changes after \\[goto-last-change] have been used.
+See `goto-last-change' for use of prefix argument."
+  (interactive "P")
+  ;; Negate arg, all kinds
+  (cond ((eq arg nil)  (setq arg '-))
+	((eq arg '-)   (setq arg nil))
+	((listp arg)   (setq arg (list (- (car arg)))))
+	(t (setq arg   (- arg))))
+  ;; Make 'goto-last-change-reverse' look like 'goto-last-change'
+  (cond ((eq last-command this-command)
+	 (setq last-command 'goto-last-change)))
+  (setq this-command 'goto-last-change)
+  ;; Call 'goto-last-change' to do the job
+  (goto-last-change arg))
+
+(provide 'goto-chg)
+
+;;; goto-chg.el ends here