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;;; queue
;;; an abstract datatype
;;; operations:
;;; (queue) ;create a queue object
;;; if 'q' is a queue object:
;;; (q 'type?) ;return the type (queue), useful if there are other
;;; ;abstract datatypes floating around.
;;; (q 'empty?) ;returns true iff q is empty
;;; (q 'put val) ;adds val to end of q; returns val
;;; (q 'get) ;removes first element of q and returns it
;;; Examples
;;; (define! q (queue))
;;; (q 'type?) => queue
;;; (q 'empty?) => #!true
;;; (q 'put 3)
;;; (q 'put 4)
;;; (q 'put 5)
;;; (q 'empty?) => ()
;;; (q 'get) => 3
;;; (q 'get) => 4
;;; (q 'put 7)
;;; (q 'get) => 5
;;; (q 'get) => 7
;;; (q 'empty?) => #!true
(define queue
(lambda ()
(let ([head '()] [tail '()])
(lambda (request . args)
(case request
[type? 'queue]
[empty? (null? head)]
[put
(let ([v (car args)])
(if (null? head)
(let ([p (cons v '())])
(set! tail p)
(set! head p))
(let ([quebit (cons v '())])
(set-cdr! tail quebit)
(set! tail quebit)))
v)]
[get
(if (null? head)
(error 'queue "queue is empty")
(let ([v (car head)])
(set! head (cdr head))
(when (null? head) (set! tail '()))
v))]
[else
(error 'queue "~s is not a valid request" request)])))))
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