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#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)))
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