queue.rkt

#lang racket
(require compatibility/mlist)

(define (make-queue) (mcons '() '()))

(define (empty-queue? queue) (null? (front-ptr queue)))

(define (front-queue queue)
  (if (empty-queue? queue)
      (error "FRONT called with an empty queue" queue)
      (mcar (front-ptr queue))))

(define (insert-queue! queue item)
  (let ((new-pair (mcons item '())))
    (cond ((empty-queue? queue)
           (set-front-ptr! queue new-pair)
           (set-rear-ptr! queue new-pair)
           'ok)
          (else
           (set-mcdr! (rear-ptr queue) new-pair)
           (set-rear-ptr! queue new-pair)
           'ok))))

(define (delete-queue! queue)
  (cond ((empty-queue? queue)
         (error "DELETE! called with an empty queue" queue))
        (else
         (set-front-ptr! queue (mcdr (front-ptr queue)))
         'ok)))

(define (print-queue queue)
  (mlist->list (front-ptr queue)))

(define (front-ptr queue) (mcar queue))
(define (rear-ptr queue) (mcdr queue))
(define (set-front-ptr! queue item) (set-mcar! queue item))
(define (set-rear-ptr! queue item) (set-mcdr! queue item))

(define q (make-queue))
(insert-queue! q 'a)
(insert-queue! q 'b)
(insert-queue! q 'c)