#lang racket

(require compatibility/mlist)
;; queue methods
(define (front-ptr q) (mcar q))
(define (rear-ptr q) (mcdr q))
(define (set-front-ptr! q v) (set-mcar! q v))
(define (set-rear-ptr! q v) (set-mcdr! q v))
(define (empty-queue? q) (null? (front-ptr q)))
(define (make-queue) (mcons '() '()))
(define (front-queue q)
  (if (empty-queue? q)
    (error "FRONT called on empty queue" q)
    (mcar (front-ptr q))))
(define (insert-queue! q item)
  (let ((new-item (mlist item)))
    (cond ((empty-queue? q)
           (set-front-ptr! q new-item)
           (set-rear-ptr! q new-item)
           q)
          (else
            (set-mcdr! (rear-ptr q) new-item)
            (set-rear-ptr! q new-item)
            q))))
(define (delete-queue! q)
  (cond ((empty-queue? q)
         (error "DELETE! called with an empty queue" q))
        (else
          (set-front-ptr! q (mcdr (front-ptr q)))
          q)))