29 lines
569 B
Scheme
29 lines
569 B
Scheme
|
; Copyright (c) 1993-1999 by Richard Kelsey and Jonathan Rees. See file COPYING.
|
||
|
|
|
||
|
|
; Queues
|
||
|
|
|
||
|
|
(define (make-queue)
|
||
|
|
(cons '() '()))
|
||
|
|
|
||
|
|
(define (queue-empty? q)
|
||
|
|
(and (null? (car q))
|
||
|
|
(null? (cdr q))))
|
||
|
|
|
||
|
|
(define (enqueue! q obj)
|
||
|
|
(set-car! q (cons obj (car q))))
|
||
|
|
|
||
|
|
(define (dequeue! q)
|
||
|
|
(normalize-queue! q)
|
||
|
|
(let ((head (car (cdr q))))
|
||
|
|
(set-cdr! q (cdr (cdr q)))
|
||
|
|
head))
|
||
|
|
|
||
|
|
(define (normalize-queue! q)
|
||
|
|
(if (null? (cdr q))
|
||
|
|
(begin (set-cdr! q (reverse (car q)))
|
||
|
|
(set-car! q '()))))
|
||
|
|
|
||
|
|
(define (queue-head q)
|
||
|
|
(normalize-queue! q)
|
||
|
|
(car (cdr q)))
|