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