queuedef.scm

; Queues as used in Section 3.3

; Queue definitions
(define (front-ptr queue) (car queue))
(define (rear-ptr queue) (cdr queue)) 
(define (set-front-ptr! queue item) (set-car! queue item))
(define (set-rear-ptr! queue item) (set-cdr! queue item))

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

(define (make-queue) (cons '() '()))

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

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

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