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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!)
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