34 lines
1023 B
Scheme
34 lines
1023 B
Scheme
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; Queue implementation from 3.21
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(define (front-ptr queue) (car queue))
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(define (rear-ptr queue) (cdr queue))
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(define (set-front-ptr! queue item) (set-car! queue item))
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(define (set-rear-ptr! queue item) (set-cdr! queue item))
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(define (empty-queue? queue) (null? (front-ptr queue)))
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(define (make-queue) (cons '() '()))
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(define (front-queue queue)
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(if (empty-queue? queue)
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(error "FRONT called with an empty queue" queue)
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(car (front-ptr queue))))
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(define (insert-queue! queue item)
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(let ((new-pair (cons item '())))
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(cond ((empty-queue? queue)
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(set-front-ptr! queue new-pair)
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(set-rear-ptr! queue new-pair)
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queue)
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(else
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(set-cdr! (rear-ptr queue) new-pair)
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(set-rear-ptr! queue new-pair)
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queue))))
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(define (delete-queue! queue)
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(cond ((empty-queue? queue)
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(error "DELETE! called with an empty queue" queue))
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(else
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(set-front-ptr! queue (cdr (front-ptr queue)))
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queue)))
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'queue-loaded
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