1 665c255d 2023-08-04 jrmu (define (front-ptr queue) (car queue))
 2 665c255d 2023-08-04 jrmu (define (rear-ptr queue) (cdr queue))
 3 665c255d 2023-08-04 jrmu (define (set-front-ptr! queue item) (set-car! queue item))
 4 665c255d 2023-08-04 jrmu (define (set-rear-ptr! queue item) (set-cdr! queue item))
 5 665c255d 2023-08-04 jrmu 
 6 665c255d 2023-08-04 jrmu (define (empty-queue? queue) (null? (front-ptr queue)))
 7 665c255d 2023-08-04 jrmu (define (make-queue) (cons '() '()))
 8 665c255d 2023-08-04 jrmu (define (front-queue queue)
 9 665c255d 2023-08-04 jrmu   (if (empty-queue? queue)
10 665c255d 2023-08-04 jrmu       (error "FRONT called with an empty queue" queue)
11 665c255d 2023-08-04 jrmu       (car (front-ptr queue))))
12 665c255d 2023-08-04 jrmu 
13 665c255d 2023-08-04 jrmu (define (insert-queue! queue item)
14 665c255d 2023-08-04 jrmu   (let ((new-pair (cons item '())))
15 665c255d 2023-08-04 jrmu     (cond ((empty-queue? queue)
16 665c255d 2023-08-04 jrmu            (set-front-ptr! queue new-pair)
17 665c255d 2023-08-04 jrmu            (set-rear-ptr! queue new-pair)
18 665c255d 2023-08-04 jrmu            queue)
19 665c255d 2023-08-04 jrmu           (else
20 665c255d 2023-08-04 jrmu            (set-cdr! (rear-ptr queue) new-pair)
21 665c255d 2023-08-04 jrmu            (set-rear-ptr! queue new-pair)
22 665c255d 2023-08-04 jrmu            queue)))) 
23 665c255d 2023-08-04 jrmu (define (delete-queue! queue)
24 665c255d 2023-08-04 jrmu   (cond ((empty-queue? queue)
25 665c255d 2023-08-04 jrmu          (error "DELETE! called with an empty queue" queue))
26 665c255d 2023-08-04 jrmu         (else
27 665c255d 2023-08-04 jrmu          (set-front-ptr! queue (cdr (front-ptr queue)))
28 665c255d 2023-08-04 jrmu          queue)))