; "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!)