29 lines
566 B
Scheme
29 lines
566 B
Scheme
; Copyright (c) 1993, 1994 Richard Kelsey and Jonathan Rees. See file COPYING.
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; Queues
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(define (make-queue)
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(cons '() '()))
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(define (queue-empty? q)
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(and (null? (car q))
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(null? (cdr q))))
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(define (enqueue q obj)
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(set-car! q (cons obj (car q))))
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(define (dequeue q)
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(normalize-queue! q)
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(let ((head (car (cdr q))))
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(set-cdr! q (cdr (cdr q)))
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head))
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(define (normalize-queue! q)
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(if (null? (cdr q))
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(begin (set-cdr! q (reverse (car q)))
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(set-car! q '()))))
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(define (queue-head q)
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(normalize-queue! q)
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(car (cdr q)))
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