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+; "queue.scm"  Queues/Stacks for Scheme
+; Written by Andrew Wilcox (awilcox@astro.psu.edu) on April 1, 1992.
+;
+; This code is in the public domain.
+
+(require 'record)
+
+; Elements in a queue are stored in a list.  The last pair in the list
+; is stored in the queue type so that datums can be added in constant
+; time.
+
+(define queue:record-type
+  (make-record-type "queue" '(first-pair last-pair)))
+(define make-queue
+  (let ((construct-queue (record-constructor queue:record-type)))
+    (lambda ()
+      (construct-queue '() '()))))
+
+(define queue? (record-predicate queue:record-type))
+
+(define queue:first-pair (record-accessor queue:record-type
+					  'first-pair))
+(define queue:set-first-pair! (record-modifier queue:record-type
+					       'first-pair))
+(define queue:last-pair (record-accessor queue:record-type
+					 'last-pair))
+(define queue:set-last-pair! (record-modifier queue:record-type
+					      'last-pair))
+
+(define (queue-empty? q)
+  (null? (queue:first-pair q)))
+
+(define (queue-front q)
+  (let ((first-pair (queue:first-pair q)))
+    (if (null? first-pair)
+	(slib:error "queue is empty" q))
+    (car first-pair)))
+
+(define (queue-rear q)
+  (let ((last-pair (queue:last-pair q)))
+    (if (null? last-pair)
+	(slib:error "queue is empty" q))
+    (car last-pair)))
+
+(define (queue-push! q datum)
+  (let* ((old-first-pair (queue:first-pair q))
+	 (new-first-pair (cons datum old-first-pair)))
+    (queue:set-first-pair! q new-first-pair)
+    (if (null? old-first-pair)
+	(queue:set-last-pair! q new-first-pair)))
+  q)
+    
+(define (enqueue! q datum)
+  (let ((new-pair (cons datum '())))
+    (cond ((null? (queue:first-pair q))
+	   (queue:set-first-pair! q new-pair))
+	  (else
+	   (set-cdr! (queue:last-pair q) new-pair)))
+    (queue:set-last-pair! q new-pair))
+  q)
+
+(define (dequeue! q)
+  (let ((first-pair (queue:first-pair q)))
+    (if (null? first-pair)
+	(slib:error "queue is empty" q))
+    (let ((first-cdr (cdr first-pair)))
+      (queue:set-first-pair! q first-cdr)
+      (if (null? first-cdr)
+	  (queue:set-last-pair! q '()))
+      (car first-pair))))
+
+(define queue-pop! dequeue!)