93 lines
2.2 KiB
Scheme
93 lines
2.2 KiB
Scheme
; Copyright (c) 1993, 1994 Richard Kelsey and Jonathan Rees. See file COPYING.
|
|
|
|
; Queues
|
|
; Richard's code with Jonathan's names.
|
|
;
|
|
; Richard's names: Jonathan's names:
|
|
; make-empty-queue make-queue
|
|
; add-to-queue! enqueue
|
|
; remove-from-queue! dequeue
|
|
|
|
(define-record-type queue :queue
|
|
(really-make-queue uid head tail)
|
|
queue?
|
|
(uid queue-uid)
|
|
(head queue-head set-queue-head!)
|
|
(tail queue-tail set-queue-tail!))
|
|
|
|
(define *queue-uid* 0)
|
|
|
|
(define (make-queue)
|
|
(let ((uid *queue-uid*))
|
|
(set! *queue-uid* (+ uid 1)) ;potential synchronization screw
|
|
(really-make-queue uid '() '())))
|
|
|
|
|
|
; The procedures for manipulating queues.
|
|
|
|
(define (queue-empty? q)
|
|
(null? (queue-head q)))
|
|
|
|
(define (enqueue q v)
|
|
(let ((p (cons v '())))
|
|
(if (null? (queue-head q)) ;(queue-empty? q)
|
|
(set-queue-head! q p)
|
|
(set-cdr! (queue-tail q) p))
|
|
(set-queue-tail! q p)))
|
|
|
|
(define (queue-front q)
|
|
(if (queue-empty? q)
|
|
(error "queue is empty" q)
|
|
(car (queue-head q))))
|
|
|
|
(define (dequeue q)
|
|
(let ((pair (queue-head q)))
|
|
(cond ((null? pair) ;(queue-empty? q)
|
|
(error "empty queue" q))
|
|
(else
|
|
(let ((value (car pair))
|
|
(next (cdr pair)))
|
|
(set-queue-head! q next)
|
|
(if (null? next)
|
|
(set-queue-tail! q '())) ; don't retain pointers
|
|
value)))))
|
|
|
|
(define (on-queue? v q)
|
|
(memq v (queue-head q)))
|
|
|
|
; This removes the first occurrence of V from Q.
|
|
|
|
(define (delete-from-queue! q v)
|
|
(delete-from-queue-if! q (lambda (x) (eq? x v))))
|
|
|
|
(define (delete-from-queue-if! q pred)
|
|
(let ((list (queue-head q)))
|
|
(cond ((null? list)
|
|
#f)
|
|
((pred (car list))
|
|
(set-queue-head! q (cdr list))
|
|
(if (null? (cdr list))
|
|
(set-queue-tail! q '())) ; don't retain pointers
|
|
#t)
|
|
((null? (cdr list))
|
|
#f)
|
|
(else
|
|
(let loop ((list list))
|
|
(let ((tail (cdr list)))
|
|
(cond ((null? tail)
|
|
#f)
|
|
((pred (car tail))
|
|
(set-cdr! list (cdr tail))
|
|
(if (null? (cdr tail))
|
|
(set-queue-tail! q list))
|
|
#t)
|
|
(else
|
|
(loop tail)))))))))
|
|
|
|
(define (queue->list q) ;For debugging
|
|
(map (lambda (x) x)
|
|
(queue-head q)))
|
|
|
|
(define (queue-length q)
|
|
(length (queue-head q)))
|