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26 ;;; compute-class-precedence-list
28 ;;; Knuth section 2.2.3 has some interesting notes on this.
30 ;;; What appears here is basically the algorithm presented there.
32 ;;; The key idea is that we use class-precedence-description (CPD) structures
33 ;;; to store the precedence information as we proceed. The CPD structure for
34 ;;; a class stores two critical pieces of information:
36 ;;; - a count of the number of "reasons" why the class can't go
37 ;;; into the class precedence list yet.
39 ;;; - a list of the "reasons" this class prevents others from
40 ;;; going in until after it
42 ;;; A "reason" is essentially a single local precedence constraint. If a
43 ;;; constraint between two classes arises more than once it generates more
44 ;;; than one reason. This makes things simpler, linear, and isn't a problem
45 ;;; as long as we make sure to keep track of each instance of a "reason".
47 ;;; This code is divided into three phases.
49 ;;; - the first phase simply generates the CPD's for each of the class
50 ;;; and its superclasses. The remainder of the code will manipulate
51 ;;; these CPDs rather than the class objects themselves. At the end
52 ;;; of this pass, the CPD-SUPERS field of a CPD is a list of the CPDs
53 ;;; of the direct superclasses of the class.
55 ;;; - the second phase folds all the local constraints into the CPD
56 ;;; structure. The CPD-COUNT of each CPD is built up, and the
57 ;;; CPD-AFTER fields are augmented to include precedence constraints
58 ;;; from the CPD-SUPERS field and from the order of classes in other
59 ;;; CPD-SUPERS fields.
61 ;;; After this phase, the CPD-AFTER field of a class includes all the
62 ;;; direct superclasses of the class plus any class that immediately
63 ;;; follows the class in the direct superclasses of another. There
64 ;;; can be duplicates in this list. The CPD-COUNT field is equal to
65 ;;; the number of times this class appears in the CPD-AFTER field of
66 ;;; all the other CPDs.
68 ;;; - In the third phase, classes are put into the precedence list one
69 ;;; at a time, with only those classes with a CPD-COUNT of 0 being
70 ;;; candidates for insertion. When a class is inserted , every CPD
71 ;;; in its CPD-AFTER field has its count decremented.
73 ;;; In the usual case, there is only one candidate for insertion at
74 ;;; any point. If there is more than one, the specified tiebreaker
75 ;;; rule is used to choose among them.
77 (defmethod compute-class-precedence-list ((root slot-class))
78 (compute-std-cpl root (class-direct-superclasses root)))
80 (defstruct (class-precedence-description
82 (:print-object (lambda (obj str)
83 (print-unreadable-object (obj str :type t)
84 (format str "~D" (cpd-count obj)))))
85 (:constructor make-cpd ()))
91 (defun compute-std-cpl (class supers)
92 (cond ((null supers) ;First two branches of COND
93 (list class)) ;are implementing the single
94 ((null (cdr supers)) ;inheritance optimization.
96 (compute-std-cpl (car supers)
97 (class-direct-superclasses (car supers)))))
99 (multiple-value-bind (all-cpds nclasses)
100 (compute-std-cpl-phase-1 class supers)
101 (compute-std-cpl-phase-2 all-cpds)
102 (compute-std-cpl-phase-3 class all-cpds nclasses)))))
104 (defvar *compute-std-cpl-class->entry-table-size* 60)
106 (defun compute-std-cpl-phase-1 (class supers)
109 (table (make-hash-table :size *compute-std-cpl-class->entry-table-size*
111 (declare (fixnum nclasses))
112 (labels ((get-cpd (c)
113 (or (gethash c table)
114 (setf (gethash c table) (make-cpd))))
116 (if (forward-referenced-class-p c)
117 (cpl-forward-referenced-class-error class c)
118 (let ((cpd (get-cpd c)))
119 (unless (cpd-class cpd) ;If we have already done this
120 ;class before, we can quit.
121 (setf (cpd-class cpd) c)
124 (setf (cpd-supers cpd) (mapcar #'get-cpd supers))
125 (dolist (super supers)
126 (walk super (class-direct-superclasses super))))))))
128 (values all-cpds nclasses))))
130 (defun compute-std-cpl-phase-2 (all-cpds)
131 (dolist (cpd all-cpds)
132 (let ((supers (cpd-supers cpd)))
134 (setf (cpd-after cpd) (nconc (cpd-after cpd) supers))
135 (incf (cpd-count (car supers)) 1)
137 (t2 (cdr t1) (cdr t1)))
139 (incf (cpd-count (car t2)) 2)
140 (push (car t2) (cpd-after (car t1))))))))
142 (defun compute-std-cpl-phase-3 (class all-cpds nclasses)
143 (let ((candidates ())
147 ;; We have to bootstrap the collection of those CPD's that
148 ;; have a zero count. Once we get going, we will maintain
149 ;; this list incrementally.
150 (dolist (cpd all-cpds)
151 (when (zerop (cpd-count cpd)) (push cpd candidates)))
154 (when (null candidates)
156 ;; If there are no candidates, and enough classes have been put
157 ;; into the precedence list, then we are all done. Otherwise
158 ;; it means there is a consistency problem.
160 (return (reverse rcpl))
161 (cpl-inconsistent-error class all-cpds)))
163 ;; Try to find the next class to put in from among the candidates.
164 ;; If there is only one, its easy, otherwise we have to use the
165 ;; famous RPG tiebreaker rule. There is some hair here to avoid
166 ;; having to call DELETE on the list of candidates. I dunno if
167 ;; its worth it but what the hell.
169 (if (null (cdr candidates))
170 (prog1 (car candidates)
171 (setq candidates ()))
174 (let ((supers (class-direct-superclasses c)))
175 (if (memq (cpd-class (car candidates)) supers)
176 (return-from tie-breaker (pop candidates))
177 (do ((loc candidates (cdr loc)))
179 (let ((cpd (cadr loc)))
180 (when (memq (cpd-class cpd) supers)
181 (setf (cdr loc) (cddr loc))
182 (return-from tie-breaker cpd))))))))))
184 (push (cpd-class next-cpd) rcpl)
185 (dolist (after (cpd-after next-cpd))
186 (when (zerop (decf (cpd-count after)))
187 (push after candidates))))))
189 ;;;; support code for signalling nice error messages
191 (defun cpl-error (class format-string &rest format-args)
192 (error "While computing the class precedence list of the class ~A.~%~A"
193 (if (class-name class)
194 (format nil "named ~S" (class-name class))
196 (apply #'format nil format-string format-args)))
198 (defun cpl-forward-referenced-class-error (class forward-class)
199 (flet ((class-or-name (class)
200 (if (class-name class)
201 (format nil "named ~S" (class-name class))
203 (let ((names (mapcar #'class-or-name
204 (cdr (find-superclass-chain class forward-class)))))
206 "The class ~A is a forward referenced class.~@
208 (class-or-name forward-class)
209 (class-or-name forward-class)
210 (if (null (cdr names))
212 "a direct superclass of the class ~A"
213 (class-or-name class))
215 "reached from the class ~A by following~@
216 the direct superclass chain through: ~A~
217 ~% ending at the class ~A"
218 (class-or-name class)
220 "~{~% the class ~A,~}"
222 (car (last names))))))))
224 (defun find-superclass-chain (bottom top)
225 (labels ((walk (c chain)
227 (return-from find-superclass-chain (nreverse chain))
228 (dolist (super (class-direct-superclasses c))
229 (walk super (cons super chain))))))
230 (walk bottom (list bottom))))
232 (defun cpl-inconsistent-error (class all-cpds)
233 (let ((reasons (find-cycle-reasons all-cpds)))
235 "It is not possible to compute the class precedence list because~@
236 there ~A in the local precedence relations.~@
237 ~A because:~{~% ~A~}."
238 (if (cdr reasons) "are circularities" "is a circularity")
239 (if (cdr reasons) "These arise" "This arises")
240 (format-cycle-reasons (apply #'append reasons)))))
242 (defun format-cycle-reasons (reasons)
243 (flet ((class-or-name (cpd)
244 (let ((class (cpd-class cpd)))
245 (if (class-name class)
246 (format nil "named ~S" (class-name class))
250 (ecase (caddr reason)
254 "The class ~A appears in the supers of the class ~A."
255 (class-or-name (cadr reason))
256 (class-or-name (car reason))))
260 "The class ~A follows the class ~A in the supers of the class ~A."
261 (class-or-name (cadr reason))
262 (class-or-name (car reason))
263 (class-or-name (cadddr reason))))))
266 (defun find-cycle-reasons (all-cpds)
267 (let ((been-here ()) ; list of classes we have visited
270 (labels ((chase (path)
271 (if (memq (car path) (cdr path))
272 (record-cycle (memq (car path) (nreverse path)))
273 (unless (memq (car path) been-here)
274 (push (car path) been-here)
275 (dolist (after (cpd-after (car path)))
276 (chase (cons after path))))))
277 (record-cycle (cycle)
280 (t2 (cdr t1) (cdr t1)))
284 (if (memq c2 (cpd-supers c1))
285 (push (list c1 c2 :super) reasons)
286 (dolist (cpd all-cpds)
287 (when (memq c2 (memq c1 (cpd-supers cpd)))
289 (push (list c1 c2 :in-supers cpd) reasons)))))))
290 (push (nreverse reasons) cycle-reasons))))
292 (dolist (cpd all-cpds)
293 (unless (zerop (cpd-count cpd))