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1 ;;; undo-stack.el --- An "undoable stack" object.
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2 ;; Keywords: extensions
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3
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4 ;; Copyright (C) 1996 Ben Wing.
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5
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6 ;; This file is part of XEmacs.
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7
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8 ;; XEmacs is free software; you can redistribute it and/or modify it
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9 ;; under the terms of the GNU General Public License as published by
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10 ;; the Free Software Foundation; either version 2, or (at your option)
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11 ;; any later version.
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12
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13 ;; XEmacs is distributed in the hope that it will be useful, but
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14 ;; WITHOUT ANY WARRANTY; without even the implied warranty of
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15 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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16 ;; General Public License for more details.
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17
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18 ;; You should have received a copy of the GNU General Public License
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19 ;; along with XEmacs; see the file COPYING. If not, write to the Free
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20 ;; Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
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21
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22 ;;; Synched up with: Not in FSF.
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23
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24 ;;; Commentary:
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25
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26 ;;; An "undoable stack" is an object that can be used to implement
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27 ;;; a history of positions, with undo and redo. Conceptually, it
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28 ;;; is the kind of data structure used to keep track of (e.g.)
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29 ;;; visited Web pages, so that the "Back" and "Forward" operations
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30 ;;; in the browser work. Basically, I can successively visit a
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31 ;;; number of Web pages through links, and then hit "Back" a
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32 ;;; few times to go to previous positions, and then "Forward" a
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33 ;;; few times to reverse this process. This is similar to an
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34 ;;; "undo" and "redo" mechanism.
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35 ;;;
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36 ;;; Note that Emacs does not standardly contain structures like
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37 ;;; this. Instead, it implements history using either a ring
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38 ;;; (the kill ring, the mark ring), or something like the undo
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39 ;;; stack, where successive "undo" operations get recorded as
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40 ;;; normal modifications, so that if you do a bunch of successive
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41 ;;; undo's, then something else, then start undoing, you will
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42 ;;; be redoing all your undo's back to the point before you did
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43 ;;; the undo's, and then further undo's will act like the previous
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44 ;;; round of undo's. I think that both of these paradigms are
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45 ;;; inferior to the "undoable-stack" paradigm because they're
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46 ;;; confusing and difficult to keep track of.
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47 ;;;
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48 ;;; Conceptually, imagine a position history like this:
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49 ;;;
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50 ;;; 1 -> 2 -> 3 -> 4 -> 5 -> 6
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51 ;;; ^^
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52 ;;;
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53 ;;; where the arrow indicates where you currently are. "Going back"
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54 ;;; and "going forward" just amount to moving the arrow. However,
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55 ;;; what happens if the history state is this:
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56 ;;;
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57 ;;; 1 -> 2 -> 3 -> 4 -> 5 -> 6
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58 ;;; ^^
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59 ;;;
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60 ;;; and then I visit new positions (7) and (8)? In the most general
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61 ;;; implementation, you've just caused a new branch like this:
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62 ;;;
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63 ;;; 1 -> 2 -> 3 -> 4 -> 5 -> 6
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64 ;;; |
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65 ;;; |
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66 ;;; 7 -> 8
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67 ;;; ^^
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68 ;;;
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69 ;;; But then you can end up with a whole big tree, and you need
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70 ;;; more sophisticated ways of navigating ("Forward" might involve
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71 ;;; a choice of paths to follow) and managing its size (if you don't
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72 ;;; want to keep unlimited history, you have to truncate at some point,
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73 ;;; and how do you truncate a tree?)
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74 ;;;
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75 ;;; My solution to this is just to insert the new positions like
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76 ;;; this:
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77 ;;;
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78 ;;; 1 -> 2 -> 3 -> 4 -> 7 -> 8 -> 5 -> 6
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79 ;;; ^^
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80 ;;;
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81 ;;; (Netscape, I think, would just truncate 5 and 6 completely,
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82 ;;; but that seems a bit drastic. In the Emacs-standard "ring"
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83 ;;; structure, this problem is avoided by simply moving 5 and 6
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84 ;;; to the beginning of the ring. However, it doesn't seem
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85 ;;; logical to me to have "going back past 1" get you to 6.)
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86 ;;;
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87 ;;; Now what if we have a "maximum" size of (say) 7 elements?
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88 ;;; When we add 8, we could truncate either 1 or 6. Since 5 and
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89 ;;; 6 are "undone" positions, we should presumably truncate
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90 ;;; them before 1. So, adding 8 truncates 6, adding 9 truncates
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91 ;;; 5, and adding 10 truncates 1 because there is nothing more
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92 ;;; that is forward of the insertion point.
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93 ;;;
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94 ;;; Interestingly, this method of truncation is almost like
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95 ;;; how a ring would truncate. A ring would move 5 and 6
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96 ;;; around to the back, like this:
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97 ;;;
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98 ;;; 5 -> 6 -> 1 -> 2 -> 3 -> 4 -> 7 -> 8
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99 ;;; ^^
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100 ;;;
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101 ;;; However, when 8 is added, the ring truncates 5 instead of
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102 ;;; 6, which is less than optimal.
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103 ;;;
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104 ;;; Conceptually, we can implement the "undoable stack" using
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105 ;;; two stacks of a sort called "truncatable stack", which are
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106 ;;; just simple stacks, but where you can truncate elements
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2
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107 ;;; off of the bottom of the stack. Then, the undoable stack
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0
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108 ;;;
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109 ;;; 1 -> 2 -> 3 -> 4 -> 5 -> 6
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110 ;;; ^^
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111 ;;;
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112 ;;; is equivalent to two truncatable stacks:
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113 ;;;
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114 ;;; 4 <- 3 <- 2 <- 1
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115 ;;; 5 <- 6
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116 ;;;
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117 ;;; where I reversed the direction to accord with the probable
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118 ;;; implementation of a standard list. To do another undo,
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119 ;;; I pop 4 off of the first stack and move it to the top of
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120 ;;; the second stack. A redo operation does the opposite.
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121 ;;; To truncate to the proper size, first chop off 6, then 5,
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122 ;;; then 1 -- in all cases, truncating off the bottom.
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123
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124 (define-error 'trunc-stack-bottom "Bottom of stack reached.")
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125
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126 (defsubst trunc-stack-stack (stack)
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127 ;; return the list representing the trunc-stack's elements.
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128 ;; the head of the list is the most recent element.
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129 (aref stack 1))
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130
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131 (defsubst trunc-stack-length (stack)
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132 ;; return the number of elements in the trunc-stack.
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133 (aref stack 2))
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134
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135 (defsubst set-trunc-stack-stack (stack new)
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136 ;; set the list representing the trunc-stack's elements.
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137 (aset stack 1 new))
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138
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139 (defsubst set-trunc-stack-length (stack new)
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140 ;; set the length of the trunc-stack.
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141 (aset stack 2 new))
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142
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143 ;; public functions:
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144
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145 (defun make-trunc-stack ()
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146 ;; make an empty trunc-stack.
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147 (vector 'trunc-stack nil 0))
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148
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149 (defun trunc-stack-push (stack el)
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150 ;; push a new element onto the head of the trunc-stack.
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151 (set-trunc-stack-stack stack (cons el (trunc-stack-stack stack)))
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152 (set-trunc-stack-length stack (1+ (trunc-stack-length stack))))
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153
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154 (defun trunc-stack-top (stack &optional n)
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155 ;; return the nth topmost element from the trunc-stack.
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156 ;; signal an error if the stack doesn't have that many elements.
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157 (or n (setq n 0))
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158 (if (>= n (trunc-stack-length stack))
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159 (signal-error 'trunc-stack-bottom (list stack))
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160 (nth n (trunc-stack-stack stack))))
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161
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162 (defun trunc-stack-pop (stack)
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163 ;; pop and return the topmost element from the stack.
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164 (prog1 (trunc-stack-top stack)
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165 (set-trunc-stack-stack stack (cdr (trunc-stack-stack stack)))
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166 (set-trunc-stack-length stack (1- (trunc-stack-length stack)))))
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167
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168 (defun trunc-stack-truncate (stack &optional n)
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169 ;; truncate N items off the bottom of the stack. If the stack is
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170 ;; not that big, it just becomes empty.
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171 (or n (setq n 1))
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172 (if (> n 0)
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173 (let ((len (trunc-stack-length stack)))
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174 (if (>= n len)
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175 (progn
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176 (set-trunc-stack-length stack 0)
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177 (set-trunc-stack-stack stack nil))
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178 (setcdr (nthcdr (1- (- len n)) (trunc-stack-stack stack)) nil)
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179 (set-trunc-stack-length stack (- len n))))))
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180
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181 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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182
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183 ;;; FMH! FMH! FMH! This object-oriented stuff doesn't really work
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184 ;;; properly without built-in structures (vectors suck) and without
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185 ;;; public and private functions and fields. Bogons descend on
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186 ;;; RMS for not believing in any of this.
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187
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188 (defsubst undoable-stack-max (stack)
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189 (aref stack 1))
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190
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191 (defsubst undoable-stack-a (stack)
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192 (aref stack 2))
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193
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194 (defsubst undoable-stack-b (stack)
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195 (aref stack 3))
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196
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197 ;; public functions:
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198
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199 (defun make-undoable-stack (max)
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200 ;; make an empty undoable stack of max size MAX.
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201 (vector 'undoable-stack max (make-trunc-stack) (make-trunc-stack)))
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202
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203 (defsubst set-undoable-stack-max (stack new)
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204 ;; change the max size of an undoable stack.
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205 (aset stack 1 new))
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206
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207 (defun undoable-stack-a-top (stack)
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208 ;; return the topmost element off the "A" stack of an undoable stack.
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209 ;; this is the most recent position pushed on the undoable stack.
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210 (trunc-stack-top (undoable-stack-a stack)))
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211
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212 (defun undoable-stack-a-length (stack)
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213 (trunc-stack-length (undoable-stack-a stack)))
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214
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215 (defun undoable-stack-b-top (stack)
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216 ;; return the topmost element off the "B" stack of an undoable stack.
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217 ;; this is the position that will become the most recent position,
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218 ;; after a redo operation.
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219 (trunc-stack-top (undoable-stack-b stack)))
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220
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221 (defun undoable-stack-b-length (stack)
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222 (trunc-stack-length (undoable-stack-b stack)))
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223
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224 (defun undoable-stack-push (stack el)
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225 ;; push an element onto the stack.
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226 (let*
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227 ((lena (trunc-stack-length (undoable-stack-a stack)))
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228 (lenb (trunc-stack-length (undoable-stack-b stack)))
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229 (max (undoable-stack-max stack))
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230 (len (+ lena lenb)))
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231 ;; maybe truncate some elements. We have to deal with the
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232 ;; possibility that we have more elements than our max
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233 ;; (someone might have reduced the max).
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234 (if (>= len max)
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235 (let ((must-nuke (1+ (- len max))))
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236 ;; chop off must-nuke elements from the B stack.
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237 (trunc-stack-truncate (undoable-stack-b stack) must-nuke)
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238 ;; but if there weren't that many elements to chop,
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239 ;; take the rest off the A stack.
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240 (if (< lenb must-nuke)
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241 (trunc-stack-truncate (undoable-stack-a stack)
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242 (- must-nuke lenb)))))
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243 (trunc-stack-push (undoable-stack-a stack) el)))
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244
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245 (defun undoable-stack-pop (stack)
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246 ;; pop an element off the stack.
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247 (trunc-stack-pop (undoable-stack-a stack)))
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248
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249 (defun undoable-stack-undo (stack)
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250 ;; transfer an element from the top of A to the top of B.
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251 ;; return value is undefined.
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252 (trunc-stack-push (undoable-stack-b stack)
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253 (trunc-stack-pop (undoable-stack-a stack))))
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254
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255 (defun undoable-stack-redo (stack)
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256 ;; transfer an element from the top of B to the top of A.
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257 ;; return value is undefined.
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258 (trunc-stack-push (undoable-stack-a stack)
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259 (trunc-stack-pop (undoable-stack-b stack))))
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260
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261
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262
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