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1 | ;;; -*-lisp-*- |
2 | ;;; | |
3 | ;;; Maintenance and recalculation of dependent values | |
4 | ;;; | |
5 | ;;; (c) 2008 Mark Wooding | |
6 | ;;; | |
7 | ||
8 | ;;;----- Licensing notice --------------------------------------------------- | |
9 | ;;; | |
10 | ;;; This program is free software; you can redistribute it and/or modify | |
11 | ;;; it under the terms of the GNU General Public License as published by | |
12 | ;;; the Free Software Foundation; either version 2 of the License, or | |
13 | ;;; (at your option) any later version. | |
14 | ;;; | |
15 | ;;; This program is distributed in the hope that it will be useful, | |
16 | ;;; but WITHOUT ANY WARRANTY; without even the implied warranty of | |
17 | ;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | |
18 | ;;; GNU General Public License for more details. | |
19 | ;;; | |
20 | ;;; You should have received a copy of the GNU General Public License | |
21 | ;;; along with this program; if not, write to the Free Software Foundation, | |
22 | ;;; Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. | |
23 | ||
24 | (defpackage #:dep | |
25 | (:use #:common-lisp #:queue) | |
26 | (:export #:dep #:depp #:make-dep #:dep-goodp | |
23bc61bd | 27 | #:delay-recomputing-deps |
2626af66 MW |
28 | #:install-dep-syntax |
29 | #:dep-value #:dep-make-bad #:dep-bad #:dep-try | |
30 | #:dep-add-listener)) | |
31 | (in-package #:dep) | |
32 | ||
33 | ;;;-------------------------------------------------------------------------- | |
34 | ;;; Dependencies. | |
35 | ||
23bc61bd MW |
36 | (defvar *generation* (list '*generation*) |
37 | "Generation marker, used to remember when we last updated a particular dep. | |
38 | Essentially, if the dep's generation matches *GENERATION* then it doesn't | |
39 | need updating again.") | |
40 | ||
41 | (defvar *evaluating-dep* nil | |
42 | "The dep currently being evaluated. This is bound only during the call of | |
43 | a value-func, and is used to track the dependencies implied during the | |
44 | function's evaluation.") | |
45 | ||
46 | (defvar *pending-deps* nil | |
47 | "A queue of deps pending recomputation. This is bound to a queue during | |
48 | recomputation and restored afterwards, so it can also be used as a flag to | |
49 | detect whether recomputation is happening.") | |
50 | ||
2626af66 MW |
51 | (defstruct (dep (:predicate depp) |
52 | (:constructor %make-dep)) | |
53 | "There are two kinds of `dep', though we use the same object type for both. | |
54 | A leaf dep has no dependencies, and its value is set explicitly by the | |
55 | programmer. A non-leaf dep has a value /function/, which computes the | |
56 | dep's value as a function of other deps' values. The dependencies don't | |
57 | need to be declared in advance, or remain constant over time. | |
58 | ||
59 | When not during a recomputation phase (i.e., when `stable'), a dep is | |
60 | either `good' (i.e., it has a value) or `bad'. An attempt to read the | |
61 | value of a bad dep results in a throw of `bad-dep'. Badness propagates | |
62 | automatically during recomputation phases." | |
63 | (%value nil :type t) | |
64 | (value-func nil :type (or function null)) | |
65 | (value-predicate #'eql :type function) | |
66 | (goodp nil :type boolean) | |
67 | (state :pending :type (member :stable :pending :recomputing)) | |
23bc61bd | 68 | (generation *generation* :type list) |
2626af66 MW |
69 | (listeners nil :type list) |
70 | (dependents nil :type list)) | |
71 | ||
2626af66 MW |
72 | (defun kick-dep (dep) |
73 | "Call when DEP's value (or good/bad state) has changed. Marks the | |
74 | dependents of DEP as :pending, if they're currently :stable, and then | |
75 | clears the dependent list. Also invokes DEP's listener functions." | |
23bc61bd | 76 | (setf (dep-generation dep) *generation*) |
2626af66 MW |
77 | (dolist (d (dep-dependents dep)) |
78 | (when (eq (dep-state d) :stable) | |
79 | (enqueue d *pending-deps*) | |
80 | (setf (dep-state d) :pending))) | |
81 | (setf (dep-dependents dep) nil) | |
82 | (dolist (l (dep-listeners dep)) | |
83 | (funcall l))) | |
84 | ||
85 | (defun update-dep (dep value &optional (goodp t)) | |
86 | "Modify the value of DEP. If GOODP is t, then mark it as good and store | |
87 | VALUE is its new value; otherwise mark it bad. If DEP's value is now | |
88 | different (according to its value-predicate) then return true; otherwise | |
89 | return false." | |
90 | (setf (dep-state dep) :stable) | |
91 | (cond ((not goodp) | |
92 | (if (dep-goodp dep) | |
93 | (progn (setf (dep-goodp dep) nil) t) | |
94 | nil)) | |
95 | ((and (dep-goodp dep) | |
96 | (funcall (dep-value-predicate dep) value (dep-%value dep))) | |
97 | nil) | |
98 | (t | |
99 | (setf (dep-goodp dep) t | |
100 | (dep-%value dep) value) | |
101 | t))) | |
102 | ||
103 | (defun recompute-dep (dep) | |
104 | "Recompute the value of DEP. This function is careful to trap nonlocal | |
105 | transfers from the value-func." | |
23bc61bd MW |
106 | (unless (eq (dep-generation dep) *generation*) |
107 | (let ((winning nil)) | |
108 | (unwind-protect | |
109 | (catch 'dep-bad | |
110 | (setf (dep-state dep) :recomputing) | |
111 | (when (update-dep dep (let ((*evaluating-dep* dep)) | |
112 | (funcall (dep-value-func dep)))) | |
113 | (kick-dep dep)) | |
114 | (setf winning t)) | |
115 | (unless winning | |
116 | (when (update-dep dep nil nil) | |
117 | (kick-dep dep))))))) | |
2626af66 MW |
118 | |
119 | (defun recompute-deps () | |
120 | "Recompute all the pending deps, and any others that depend on them." | |
121 | (unwind-protect | |
122 | (loop (when (queue-emptyp *pending-deps*) | |
123 | (return)) | |
124 | (let ((dep (dequeue *pending-deps*))) | |
125 | (when (eq (dep-state dep) :pending) | |
126 | (recompute-dep dep)))) | |
127 | (loop (when (queue-emptyp *pending-deps*) | |
128 | (return)) | |
129 | (let ((d (dequeue *pending-deps*))) | |
130 | (setf (dep-state d) :stable | |
131 | (dep-goodp d) nil))))) | |
132 | ||
133 | (defun ensure-dep-has-value (dep) | |
134 | "Ensure that DEP has a stable value. If DEP is currently computing, | |
135 | signals an error." | |
136 | (ecase (dep-state dep) | |
137 | (:stable) | |
138 | (:pending | |
139 | (recompute-dep dep)) | |
140 | (:recomputing | |
141 | (error "Ouch! Cyclic dependency.")))) | |
142 | ||
143 | (defun pulse-dep (dep) | |
144 | "Notifies DEP of a change in its value. If a recomputation phase is | |
145 | currently under way, queue the dependents and leave fixing things up to | |
146 | the outer loop; otherwise start up a recomputation phase." | |
23bc61bd MW |
147 | (setf *generation* (list '*generation*)) |
148 | (flet ((kick (dep) | |
149 | (kick-dep dep) | |
150 | (when (dep-value-func dep) | |
151 | (catch 'dep-bad | |
152 | (let ((*evaluating-dep* dep)) | |
153 | (funcall (dep-value-func dep))))))) | |
154 | (if *pending-deps* | |
155 | (kick dep) | |
156 | (let ((*pending-deps* (make-queue))) | |
157 | (kick dep) | |
158 | (recompute-deps))))) | |
2626af66 MW |
159 | |
160 | (defun (setf dep-value) (value dep) | |
161 | "Set DEP's value to be VALUE (and mark it as being good)." | |
23bc61bd | 162 | (when (update-dep dep value) (pulse-dep dep)) |
2626af66 MW |
163 | value) |
164 | ||
165 | (defun dep-make-bad (dep) | |
166 | "Mark DEP as being bad." | |
23bc61bd | 167 | (when (update-dep dep nil nil) (pulse-dep dep))) |
2626af66 MW |
168 | |
169 | (defun dep-add-listener (dep func) | |
170 | "Add a listener function FUNC to the DEP. The FUNC is called each time the | |
171 | DEP's value (or good/bad state) changes. It is called with no arguments, | |
172 | and its return value is ignored." | |
173 | (push func (dep-listeners dep))) | |
174 | ||
175 | (defun dep-value (dep) | |
176 | "Retrieve the current value from DEP." | |
177 | (when *evaluating-dep* | |
178 | (pushnew *evaluating-dep* (dep-dependents dep))) | |
179 | (ensure-dep-has-value dep) | |
180 | (if (dep-goodp dep) (dep-%value dep) (throw 'dep-bad nil))) | |
181 | ||
182 | (defun make-dep (&rest args) | |
183 | "Create a new DEP object. There are two basic argument forms: | |
184 | ||
23bc61bd | 185 | (:value &optional OBJECT) |
2626af66 | 186 | Return a leaf dep, whose value is OBJECT; if no OBJECT is given, the |
23bc61bd | 187 | dep is initially bad. The keyword :leaf is accepted as a synonym. |
2626af66 MW |
188 | |
189 | (:function FUNCTION) | |
190 | Return a non-leaf dep whose value is computed by FUNCTION. | |
191 | ||
23bc61bd MW |
192 | Additionally, if the first argument is something other than :VALUE or |
193 | :FUNCTION (ideally not a keyword, for forward compatibility), then the | |
2626af66 MW |
194 | first argument is inspected: if it's a function, then a function dep is |
195 | retuerned (as if you'd specified :function); otherwise a leaf dep is | |
196 | returned. | |
197 | ||
23bc61bd MW |
198 | Finally, it's possible to specify both :value and :function |
199 | simultaneously; this will set the initial values as requested, but | |
200 | recompute them as necessary. It is possible to establish dependency | |
201 | cycles, but you need to suppress recomputation in order to do this | |
202 | correctly -- see the DELAY-RECOMPUTING-DEPS macro. | |
203 | ||
2626af66 MW |
204 | If no arguments are given, a bad leaf dep is returned." |
205 | ||
23bc61bd MW |
206 | (flet ((arg () (if args (pop args) |
207 | (error "Not enough arguments to MAKE-DEP.")))) | |
2626af66 MW |
208 | |
209 | ;; Sort out the arguments. | |
23bc61bd MW |
210 | (let ((value nil) |
211 | (valuep nil) | |
212 | (function nil)) | |
213 | (do () ((endp args)) | |
214 | (let ((indicator (pop args))) | |
215 | (cond ((or (eq indicator :value) | |
216 | (eq indicator :leaf)) | |
217 | (if args | |
218 | (setf value (pop args) valuep t) | |
219 | (setf value nil valuep t))) | |
220 | ((eq indicator :function) | |
221 | (setf function (arg))) | |
222 | ((functionp indicator) | |
223 | (setf function indicator)) | |
224 | (t | |
225 | (setf value indicator valuep t))))) | |
2626af66 MW |
226 | |
227 | ;; Create the object appropriately. | |
23bc61bd MW |
228 | (let ((dep (%make-dep :value-func function |
229 | :%value value | |
230 | :state (if valuep :stable :pending) | |
231 | :generation (if function nil *generation*) | |
232 | :goodp valuep))) | |
233 | (cond ((not function) t) | |
234 | (valuep (pulse-dep dep)) | |
235 | (*pending-deps* | |
236 | (enqueue dep *pending-deps*)) | |
237 | (t | |
2626af66 MW |
238 | (let ((*pending-deps* (make-queue))) |
239 | (enqueue dep *pending-deps*) | |
23bc61bd MW |
240 | (recompute-deps)))) |
241 | dep)))) | |
2626af66 MW |
242 | |
243 | (defmacro dep-try (expr &body body) | |
244 | "Evaluate EXPR. If it throws dep-bad then evaluate BODY instead." | |
245 | (let ((block-name (gensym "TRY"))) | |
246 | `(block ,block-name | |
247 | (catch 'dep-bad | |
248 | (return-from ,block-name ,expr)) | |
249 | ,@body))) | |
250 | ||
251 | (defun dep-bad () | |
252 | "Call from a value-func: indicates that the dep should marked as bad." | |
253 | (throw 'dep-bad nil)) | |
254 | ||
255 | (defun delay-recomputing-deps* (thunk) | |
23bc61bd | 256 | "The guts of the DELAY-RECOMPUTING-DEPS macro. Evaluate THUNK without |
2626af66 MW |
257 | immediately updating dependencies until THUNK finishes. Returns the |
258 | value(s) of THUNK." | |
259 | (if *pending-deps* | |
260 | (funcall thunk) | |
261 | (let ((*pending-deps* (make-queue))) | |
23bc61bd | 262 | (setf *generation* (list '*generation*)) |
2626af66 MW |
263 | (multiple-value-prog1 |
264 | (funcall thunk) | |
265 | (recompute-deps))))) | |
266 | ||
267 | (defmacro delay-recomputing-deps (&body body) | |
268 | "Evaluate BODY, but delay recomputing any deps until the BODY completes | |
269 | execution. | |
270 | ||
271 | Note that deps can report incorrect values while delayed recomputation is | |
272 | in effect. In the current implementation, the direct dependents of a leaf | |
273 | dep whose value has changed will be correctly marked as pending (and | |
274 | recomputed as necessary); higher-level dependents won't be noticed until | |
275 | the direct dependents are recomputed. | |
276 | ||
23bc61bd MW |
277 | It can be used to apply a number of updates simultaneously to the system. |
278 | This is useful for two reasons: | |
279 | ||
280 | * Firstly, it avoids the computational overheads of propagating changes | |
281 | repeatedly, so it can be used as a simple optimization. | |
282 | ||
283 | * Secondly, and perhaps more interestingly, it allows the values of | |
284 | mutually-dependent deps to be set simultaneously, even though the | |
285 | values being set may not be compatible with the deps' value | |
286 | functions." | |
287 | `(delay-recomputing-deps* #'(lambda () ,@body))) | |
2626af66 MW |
288 | |
289 | (defun install-dep-syntax (&optional (readtable *readtable*)) | |
290 | "Installs into the given READTABLE some syntactic shortcuts: | |
291 | ||
292 | ?FORM -> (dep-value FORM) | |
293 | Extract (or modify, for a leaf dep) the value of the dep indicated by | |
294 | FORM. | |
295 | ||
296 | #[FORM ...] -> (make-dep :funcion (lambda () FORM ...)) | |
297 | Return a derived dep whose value function computes the given FORMs | |
298 | (as an implicit PROGN) | |
299 | ||
300 | Returns the READTABLE." | |
301 | (set-macro-character #\? | |
302 | (lambda (stream char) | |
303 | (declare (ignore char)) | |
304 | (list 'dep-value (read stream t nil t))) | |
305 | readtable) | |
306 | (set-syntax-from-char #\] #\) readtable readtable) | |
307 | (set-dispatch-macro-character #\# #\[ | |
308 | (lambda (stream arg char) | |
309 | (declare (ignore arg char)) | |
310 | `(make-dep :function | |
311 | (lambda () | |
312 | ,@(read-delimited-list #\] | |
313 | stream | |
314 | t)))) | |
315 | readtable) | |
316 | readtable) | |
317 | ||
318 | (defmethod print-object ((dep dep) stream) | |
319 | (print-unreadable-object (dep stream :type t :identity t) | |
320 | (cond ((not (eq (dep-state dep) :stable)) | |
321 | (format stream "~S" (dep-state dep))) | |
322 | ((dep-goodp dep) | |
323 | (format stream "~S ~W" :good (dep-%value dep))) | |
324 | (t | |
325 | (format stream "~S" :bad))))) | |
326 | ||
327 | #+ test | |
328 | (progn | |
329 | (defparameter x (make-leaf-dep 1)) | |
330 | (defparameter y (make-leaf-dep 2)) | |
331 | (defparameter z (make-dep (lambda () (+ (dep-value x) (dep-value y))))) | |
332 | (defparameter w (make-dep (lambda () (* (dep-value x) (dep-value z))))) | |
333 | (dep-add-listener x (lambda () (format t "x now ~A~%" x))) | |
334 | (dep-add-listener z (lambda () (format t "z now ~A~%" z))) | |
335 | (dep-add-listener w (lambda () (format t "w now ~A~%" w)))) | |
336 | ||
337 | ;;;----- That's all, folks -------------------------------------------------- |