--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/lisp/undo-stack.el	Mon Aug 13 10:04:58 2007 +0200
@@ -0,0 +1,270 @@
+;;; undo-stack.el --- An "undoable stack" object.
+
+;; Copyright (C) 1997 Free Software Foundation, Inc.
+;; Copyright (C) 1996 Ben Wing.
+
+;; Maintainer: XEmacs Development Team
+;; Keywords: extensions, dumped
+
+;; This file is part of XEmacs.
+
+;; XEmacs 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.
+
+;; XEmacs 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 XEmacs; see the file COPYING.  If not, write to the 
+;; Free Software Foundation, 59 Temple Place - Suite 330,
+;; Boston, MA 02111-1307, USA.
+
+;;; Synched up with: Not in FSF.
+
+;;; Commentary:
+
+;; This file is dumped with XEmacs.
+
+;; An "undoable stack" is an object that can be used to implement
+;; a history of positions, with undo and redo.  Conceptually, it
+;; is the kind of data structure used to keep track of (e.g.)
+;; visited Web pages, so that the "Back" and "Forward" operations
+;; in the browser work.  Basically, I can successively visit a
+;; number of Web pages through links, and then hit "Back" a
+;; few times to go to previous positions, and then "Forward" a
+;; few times to reverse this process.  This is similar to an
+;; "undo" and "redo" mechanism.
+
+;; Note that Emacs does not standardly contain structures like
+;; this.  Instead, it implements history using either a ring
+;; (the kill ring, the mark ring), or something like the undo
+;; stack, where successive "undo" operations get recorded as
+;; normal modifications, so that if you do a bunch of successive
+;; undo's, then something else, then start undoing, you will
+;; be redoing all your undo's back to the point before you did
+;; the undo's, and then further undo's will act like the previous
+;; round of undo's.  I think that both of these paradigms are
+;; inferior to the "undoable-stack" paradigm because they're
+;; confusing and difficult to keep track of.
+
+;; Conceptually, imagine a position history like this:
+
+;;   1 -> 2 -> 3 -> 4 -> 5 -> 6
+;;                            ^^
+
+;; where the arrow indicates where you currently are.  "Going back"
+;; and "going forward" just amount to moving the arrow.  However,
+;; what happens if the history state is this:
+
+;;   1 -> 2 -> 3 -> 4 -> 5 -> 6
+;;                  ^^
+
+;; and then I visit new positions (7) and (8)?  In the most general
+;; implementation, you've just caused a new branch like this:
+
+;;   1 -> 2 -> 3 -> 4 -> 5 -> 6
+;;                  |
+;;                  |
+;;                  7 -> 8
+;;                       ^^
+
+;; But then you can end up with a whole big tree, and you need
+;; more sophisticated ways of navigating ("Forward" might involve
+;; a choice of paths to follow) and managing its size (if you don't
+;; want to keep unlimited history, you have to truncate at some point,
+;; and how do you truncate a tree?)
+
+;; My solution to this is just to insert the new positions like
+;; this:
+
+;;   1 -> 2 -> 3 -> 4 -> 7 -> 8 -> 5 -> 6
+;;                            ^^
+
+;; (Netscape, I think, would just truncate 5 and 6 completely,
+;; but that seems a bit drastic.  In the Emacs-standard "ring"
+;; structure, this problem is avoided by simply moving 5 and 6
+;; to the beginning of the ring.  However, it doesn't seem
+;; logical to me to have "going back past 1" get you to 6.)
+
+;; Now what if we have a "maximum" size of (say) 7 elements?
+;; When we add 8, we could truncate either 1 or 6.  Since 5 and
+;; 6 are "undone" positions, we should presumably truncate
+;; them before 1.  So, adding 8 truncates 6, adding 9 truncates
+;; 5, and adding 10 truncates 1 because there is nothing more
+;; that is forward of the insertion point.
+
+;; Interestingly, this method of truncation is almost like
+;; how a ring would truncate.  A ring would move 5 and 6
+;; around to the back, like this:
+
+;;   5 -> 6 -> 1 -> 2 -> 3 -> 4 -> 7 -> 8
+;;                                      ^^
+
+;; However, when 8 is added, the ring truncates 5 instead of
+;; 6, which is less than optimal.
+
+;; Conceptually, we can implement the "undoable stack" using
+;; two stacks of a sort called "truncatable stack", which are
+;; just simple stacks, but where you can truncate elements
+;; off of the bottom of the stack.  Then, the undoable stack
+
+;;   1 -> 2 -> 3 -> 4 -> 5 -> 6
+;;                  ^^
+
+;; is equivalent to two truncatable stacks:
+
+;;   4 <- 3 <- 2 <- 1
+;;   5 <- 6
+
+;; where I reversed the direction to accord with the probable
+;; implementation of a standard list.  To do another undo,
+;; I pop 4 off of the first stack and move it to the top of
+;; the second stack.  A redo operation does the opposite.
+;; To truncate to the proper size, first chop off 6, then 5,
+;; then 1 -- in all cases, truncating off the bottom.
+
+;;; Code:
+
+(define-error 'trunc-stack-bottom "Bottom of stack reached.")
+
+(defsubst trunc-stack-stack (stack)
+  ;; return the list representing the trunc-stack's elements.
+  ;; the head of the list is the most recent element.
+  (aref stack 1))
+
+(defsubst trunc-stack-length (stack)
+  ;; return the number of elements in the trunc-stack.
+  (aref stack 2))
+
+(defsubst set-trunc-stack-stack (stack new)
+  ;; set the list representing the trunc-stack's elements.
+  (aset stack 1 new))
+
+(defsubst set-trunc-stack-length (stack new)
+  ;; set the length of the trunc-stack.
+  (aset stack 2 new))
+
+;; public functions:
+
+(defun make-trunc-stack ()
+  ;; make an empty trunc-stack.
+  (vector 'trunc-stack nil 0))
+
+(defun trunc-stack-push (stack el)
+  ;; push a new element onto the head of the trunc-stack.
+  (set-trunc-stack-stack stack (cons el (trunc-stack-stack stack)))
+  (set-trunc-stack-length stack (1+ (trunc-stack-length stack))))
+
+(defun trunc-stack-top (stack &optional n)
+  ;; return the nth topmost element from the trunc-stack.
+  ;; signal an error if the stack doesn't have that many elements.
+  (or n (setq n 0))
+  (if (>= n (trunc-stack-length stack))
+      (signal-error 'trunc-stack-bottom (list stack))
+    (nth n (trunc-stack-stack stack))))
+
+(defun trunc-stack-pop (stack)
+  ;; pop and return the topmost element from the stack.
+  (prog1 (trunc-stack-top stack)
+    (set-trunc-stack-stack stack (cdr (trunc-stack-stack stack)))
+    (set-trunc-stack-length stack (1- (trunc-stack-length stack)))))
+
+(defun trunc-stack-truncate (stack &optional n)
+  ;; truncate N items off the bottom of the stack.  If the stack is
+  ;; not that big, it just becomes empty.
+  (or n (setq n 1))
+  (if (> n 0)
+      (let ((len (trunc-stack-length stack)))
+	(if (>= n len)
+	    (progn
+	      (set-trunc-stack-length stack 0)
+	      (set-trunc-stack-stack stack nil))
+	  (setcdr (nthcdr (1- (- len n)) (trunc-stack-stack stack)) nil)
+	  (set-trunc-stack-length stack (- len n))))))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;;; FMH! FMH! FMH!  This object-oriented stuff doesn't really work
+;;; properly without built-in structures (vectors suck) and without
+;;; public and private functions and fields.  Bogons descend on
+;;; RMS for not believing in any of this.
+
+(defsubst undoable-stack-max (stack)
+  (aref stack 1))
+
+(defsubst undoable-stack-a (stack)
+  (aref stack 2))
+
+(defsubst undoable-stack-b (stack)
+  (aref stack 3))
+
+;; public functions:
+
+(defun make-undoable-stack (max)
+  ;; make an empty undoable stack of max size MAX.
+  (vector 'undoable-stack max (make-trunc-stack) (make-trunc-stack)))
+
+(defsubst set-undoable-stack-max (stack new)
+  ;; change the max size of an undoable stack.
+  (aset stack 1 new))
+
+(defun undoable-stack-a-top (stack)
+  ;; return the topmost element off the "A" stack of an undoable stack.
+  ;; this is the most recent position pushed on the undoable stack.
+  (trunc-stack-top (undoable-stack-a stack)))
+
+(defun undoable-stack-a-length (stack)
+  (trunc-stack-length (undoable-stack-a stack)))
+
+(defun undoable-stack-b-top (stack)
+  ;; return the topmost element off the "B" stack of an undoable stack.
+  ;; this is the position that will become the most recent position,
+  ;; after a redo operation.
+  (trunc-stack-top (undoable-stack-b stack)))
+
+(defun undoable-stack-b-length (stack)
+  (trunc-stack-length (undoable-stack-b stack)))
+
+(defun undoable-stack-push (stack el)
+  ;; push an element onto the stack.
+  (let*
+      ((lena (trunc-stack-length (undoable-stack-a stack)))
+       (lenb (trunc-stack-length (undoable-stack-b stack)))
+       (max (undoable-stack-max stack))
+       (len (+ lena lenb)))
+    ;; maybe truncate some elements.  We have to deal with the
+    ;; possibility that we have more elements than our max
+    ;; (someone might have reduced the max).
+    (if (>= len max)
+	(let ((must-nuke (1+ (- len max))))
+	  ;; chop off must-nuke elements from the B stack.
+	  (trunc-stack-truncate (undoable-stack-b stack) must-nuke)
+	  ;; but if there weren't that many elements to chop,
+	  ;; take the rest off the A stack.
+	  (if (< lenb must-nuke)
+	      (trunc-stack-truncate (undoable-stack-a stack)
+				    (- must-nuke lenb)))))
+    (trunc-stack-push (undoable-stack-a stack) el)))
+
+(defun undoable-stack-pop (stack)
+  ;; pop an element off the stack.
+  (trunc-stack-pop (undoable-stack-a stack)))
+
+(defun undoable-stack-undo (stack)
+  ;; transfer an element from the top of A to the top of B.
+  ;; return value is undefined.
+  (trunc-stack-push (undoable-stack-b stack)
+		    (trunc-stack-pop (undoable-stack-a stack))))
+
+(defun undoable-stack-redo (stack)
+  ;; transfer an element from the top of B to the top of A.
+  ;; return value is undefined.
+  (trunc-stack-push (undoable-stack-a stack)
+		    (trunc-stack-pop (undoable-stack-b stack))))
+
+
+;;; undo-stack.el ends here