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comparison lisp/undo-stack.el @ 209:41ff10fd062f r20-4b3

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date Mon, 13 Aug 2007 10:04:58 +0200
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1 ;;; undo-stack.el --- An "undoable stack" object.
2
3 ;; Copyright (C) 1997 Free Software Foundation, Inc.
4 ;; Copyright (C) 1996 Ben Wing.
5
6 ;; Maintainer: XEmacs Development Team
7 ;; Keywords: extensions, dumped
8
9 ;; This file is part of XEmacs.
10
11 ;; XEmacs is free software; you can redistribute it and/or modify it
12 ;; under the terms of the GNU General Public License as published by
13 ;; the Free Software Foundation; either version 2, or (at your option)
14 ;; any later version.
15
16 ;; XEmacs is distributed in the hope that it will be useful, but
17 ;; WITHOUT ANY WARRANTY; without even the implied warranty of
18 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
19 ;; General Public License for more details.
20
21 ;; You should have received a copy of the GNU General Public License
22 ;; along with XEmacs; see the file COPYING. If not, write to the
23 ;; Free Software Foundation, 59 Temple Place - Suite 330,
24 ;; Boston, MA 02111-1307, USA.
25
26 ;;; Synched up with: Not in FSF.
27
28 ;;; Commentary:
29
30 ;; This file is dumped with XEmacs.
31
32 ;; An "undoable stack" is an object that can be used to implement
33 ;; a history of positions, with undo and redo. Conceptually, it
34 ;; is the kind of data structure used to keep track of (e.g.)
35 ;; visited Web pages, so that the "Back" and "Forward" operations
36 ;; in the browser work. Basically, I can successively visit a
37 ;; number of Web pages through links, and then hit "Back" a
38 ;; few times to go to previous positions, and then "Forward" a
39 ;; few times to reverse this process. This is similar to an
40 ;; "undo" and "redo" mechanism.
41
42 ;; Note that Emacs does not standardly contain structures like
43 ;; this. Instead, it implements history using either a ring
44 ;; (the kill ring, the mark ring), or something like the undo
45 ;; stack, where successive "undo" operations get recorded as
46 ;; normal modifications, so that if you do a bunch of successive
47 ;; undo's, then something else, then start undoing, you will
48 ;; be redoing all your undo's back to the point before you did
49 ;; the undo's, and then further undo's will act like the previous
50 ;; round of undo's. I think that both of these paradigms are
51 ;; inferior to the "undoable-stack" paradigm because they're
52 ;; confusing and difficult to keep track of.
53
54 ;; Conceptually, imagine a position history like this:
55
56 ;; 1 -> 2 -> 3 -> 4 -> 5 -> 6
57 ;; ^^
58
59 ;; where the arrow indicates where you currently are. "Going back"
60 ;; and "going forward" just amount to moving the arrow. However,
61 ;; what happens if the history state is this:
62
63 ;; 1 -> 2 -> 3 -> 4 -> 5 -> 6
64 ;; ^^
65
66 ;; and then I visit new positions (7) and (8)? In the most general
67 ;; implementation, you've just caused a new branch like this:
68
69 ;; 1 -> 2 -> 3 -> 4 -> 5 -> 6
70 ;; |
71 ;; |
72 ;; 7 -> 8
73 ;; ^^
74
75 ;; But then you can end up with a whole big tree, and you need
76 ;; more sophisticated ways of navigating ("Forward" might involve
77 ;; a choice of paths to follow) and managing its size (if you don't
78 ;; want to keep unlimited history, you have to truncate at some point,
79 ;; and how do you truncate a tree?)
80
81 ;; My solution to this is just to insert the new positions like
82 ;; this:
83
84 ;; 1 -> 2 -> 3 -> 4 -> 7 -> 8 -> 5 -> 6
85 ;; ^^
86
87 ;; (Netscape, I think, would just truncate 5 and 6 completely,
88 ;; but that seems a bit drastic. In the Emacs-standard "ring"
89 ;; structure, this problem is avoided by simply moving 5 and 6
90 ;; to the beginning of the ring. However, it doesn't seem
91 ;; logical to me to have "going back past 1" get you to 6.)
92
93 ;; Now what if we have a "maximum" size of (say) 7 elements?
94 ;; When we add 8, we could truncate either 1 or 6. Since 5 and
95 ;; 6 are "undone" positions, we should presumably truncate
96 ;; them before 1. So, adding 8 truncates 6, adding 9 truncates
97 ;; 5, and adding 10 truncates 1 because there is nothing more
98 ;; that is forward of the insertion point.
99
100 ;; Interestingly, this method of truncation is almost like
101 ;; how a ring would truncate. A ring would move 5 and 6
102 ;; around to the back, like this:
103
104 ;; 5 -> 6 -> 1 -> 2 -> 3 -> 4 -> 7 -> 8
105 ;; ^^
106
107 ;; However, when 8 is added, the ring truncates 5 instead of
108 ;; 6, which is less than optimal.
109
110 ;; Conceptually, we can implement the "undoable stack" using
111 ;; two stacks of a sort called "truncatable stack", which are
112 ;; just simple stacks, but where you can truncate elements
113 ;; off of the bottom of the stack. Then, the undoable stack
114
115 ;; 1 -> 2 -> 3 -> 4 -> 5 -> 6
116 ;; ^^
117
118 ;; is equivalent to two truncatable stacks:
119
120 ;; 4 <- 3 <- 2 <- 1
121 ;; 5 <- 6
122
123 ;; where I reversed the direction to accord with the probable
124 ;; implementation of a standard list. To do another undo,
125 ;; I pop 4 off of the first stack and move it to the top of
126 ;; the second stack. A redo operation does the opposite.
127 ;; To truncate to the proper size, first chop off 6, then 5,
128 ;; then 1 -- in all cases, truncating off the bottom.
129
130 ;;; Code:
131
132 (define-error 'trunc-stack-bottom "Bottom of stack reached.")
133
134 (defsubst trunc-stack-stack (stack)
135 ;; return the list representing the trunc-stack's elements.
136 ;; the head of the list is the most recent element.
137 (aref stack 1))
138
139 (defsubst trunc-stack-length (stack)
140 ;; return the number of elements in the trunc-stack.
141 (aref stack 2))
142
143 (defsubst set-trunc-stack-stack (stack new)
144 ;; set the list representing the trunc-stack's elements.
145 (aset stack 1 new))
146
147 (defsubst set-trunc-stack-length (stack new)
148 ;; set the length of the trunc-stack.
149 (aset stack 2 new))
150
151 ;; public functions:
152
153 (defun make-trunc-stack ()
154 ;; make an empty trunc-stack.
155 (vector 'trunc-stack nil 0))
156
157 (defun trunc-stack-push (stack el)
158 ;; push a new element onto the head of the trunc-stack.
159 (set-trunc-stack-stack stack (cons el (trunc-stack-stack stack)))
160 (set-trunc-stack-length stack (1+ (trunc-stack-length stack))))
161
162 (defun trunc-stack-top (stack &optional n)
163 ;; return the nth topmost element from the trunc-stack.
164 ;; signal an error if the stack doesn't have that many elements.
165 (or n (setq n 0))
166 (if (>= n (trunc-stack-length stack))
167 (signal-error 'trunc-stack-bottom (list stack))
168 (nth n (trunc-stack-stack stack))))
169
170 (defun trunc-stack-pop (stack)
171 ;; pop and return the topmost element from the stack.
172 (prog1 (trunc-stack-top stack)
173 (set-trunc-stack-stack stack (cdr (trunc-stack-stack stack)))
174 (set-trunc-stack-length stack (1- (trunc-stack-length stack)))))
175
176 (defun trunc-stack-truncate (stack &optional n)
177 ;; truncate N items off the bottom of the stack. If the stack is
178 ;; not that big, it just becomes empty.
179 (or n (setq n 1))
180 (if (> n 0)
181 (let ((len (trunc-stack-length stack)))
182 (if (>= n len)
183 (progn
184 (set-trunc-stack-length stack 0)
185 (set-trunc-stack-stack stack nil))
186 (setcdr (nthcdr (1- (- len n)) (trunc-stack-stack stack)) nil)
187 (set-trunc-stack-length stack (- len n))))))
188
189 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
190
191 ;;; FMH! FMH! FMH! This object-oriented stuff doesn't really work
192 ;;; properly without built-in structures (vectors suck) and without
193 ;;; public and private functions and fields. Bogons descend on
194 ;;; RMS for not believing in any of this.
195
196 (defsubst undoable-stack-max (stack)
197 (aref stack 1))
198
199 (defsubst undoable-stack-a (stack)
200 (aref stack 2))
201
202 (defsubst undoable-stack-b (stack)
203 (aref stack 3))
204
205 ;; public functions:
206
207 (defun make-undoable-stack (max)
208 ;; make an empty undoable stack of max size MAX.
209 (vector 'undoable-stack max (make-trunc-stack) (make-trunc-stack)))
210
211 (defsubst set-undoable-stack-max (stack new)
212 ;; change the max size of an undoable stack.
213 (aset stack 1 new))
214
215 (defun undoable-stack-a-top (stack)
216 ;; return the topmost element off the "A" stack of an undoable stack.
217 ;; this is the most recent position pushed on the undoable stack.
218 (trunc-stack-top (undoable-stack-a stack)))
219
220 (defun undoable-stack-a-length (stack)
221 (trunc-stack-length (undoable-stack-a stack)))
222
223 (defun undoable-stack-b-top (stack)
224 ;; return the topmost element off the "B" stack of an undoable stack.
225 ;; this is the position that will become the most recent position,
226 ;; after a redo operation.
227 (trunc-stack-top (undoable-stack-b stack)))
228
229 (defun undoable-stack-b-length (stack)
230 (trunc-stack-length (undoable-stack-b stack)))
231
232 (defun undoable-stack-push (stack el)
233 ;; push an element onto the stack.
234 (let*
235 ((lena (trunc-stack-length (undoable-stack-a stack)))
236 (lenb (trunc-stack-length (undoable-stack-b stack)))
237 (max (undoable-stack-max stack))
238 (len (+ lena lenb)))
239 ;; maybe truncate some elements. We have to deal with the
240 ;; possibility that we have more elements than our max
241 ;; (someone might have reduced the max).
242 (if (>= len max)
243 (let ((must-nuke (1+ (- len max))))
244 ;; chop off must-nuke elements from the B stack.
245 (trunc-stack-truncate (undoable-stack-b stack) must-nuke)
246 ;; but if there weren't that many elements to chop,
247 ;; take the rest off the A stack.
248 (if (< lenb must-nuke)
249 (trunc-stack-truncate (undoable-stack-a stack)
250 (- must-nuke lenb)))))
251 (trunc-stack-push (undoable-stack-a stack) el)))
252
253 (defun undoable-stack-pop (stack)
254 ;; pop an element off the stack.
255 (trunc-stack-pop (undoable-stack-a stack)))
256
257 (defun undoable-stack-undo (stack)
258 ;; transfer an element from the top of A to the top of B.
259 ;; return value is undefined.
260 (trunc-stack-push (undoable-stack-b stack)
261 (trunc-stack-pop (undoable-stack-a stack))))
262
263 (defun undoable-stack-redo (stack)
264 ;; transfer an element from the top of B to the top of A.
265 ;; return value is undefined.
266 (trunc-stack-push (undoable-stack-a stack)
267 (trunc-stack-pop (undoable-stack-b stack))))
268
269
270 ;;; undo-stack.el ends here