Re: Rationale for add-to-ordered-list

[Stefan Monnier]

> A general solution is non-trivial, as there is a potential risk of memory
> leaks, as associating ordering information with arbitrary lisp objects
> means that we must store a pointer to such objects somewhere, and thus
> may leave references to otherwise unused data.

How 'bout the patch below?  Look 'ma, no leak!


        Stefan


--- subr.el     14 jun 2005 13:16:55 -0400      1.464
+++ subr.el     14 jun 2005 13:23:20 -0400      
@@ -974,25 +974,21 @@
 `list-order' property.
 
 The return value is the new value of LIST-VAR."
-  (let* ((ordering (get list-var 'list-order))
-        (cur (and (symbolp element) (assq element ordering))))
+  (let ((ordering (get list-var 'list-order)))
+    (unless ordering
+      (put list-var 'list-order
+           (setq ordering (make-hash-table :weakness 'key :test 'eq))))
     (when order
-      (unless (symbolp element)
-       (error "cannot specify order for non-symbols"))
-      (if cur
-         (setcdr cur order)
-       (setq cur (cons element order))
-       (setq ordering (cons cur ordering))
-       (put list-var 'list-order ordering)))
+      (puthash element order ordering))
     (add-to-list list-var element)
     (set list-var (sort (symbol-value list-var)
                        (lambda (a b)
-                         (let ((oa (and (symbolp a) (assq a ordering)))
-                               (ob (and (symbolp b) (assq b ordering))))
+                         (let ((oa (gethash a ordering))
+                               (ob (gethash b ordering)))
                            (cond
                             ((not oa) nil)
                             ((not ob) t)
-                            (t (< (cdr oa) (cdr ob))))))))))
+                            (t (< oa ob)))))))))
 
 
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