47 lines
1.2 KiB
Racket
47 lines
1.2 KiB
Racket
#lang sicp
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(define (make-queue)
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(let ((front-ptr '())
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(rear-ptr '()))
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(define (front)
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(cond ((empty?)
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(error "FRONT called with empty queue"))
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(else
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(car front-ptr))))
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(define (insert! item)
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(let ((new-pair (cons item '())))
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(cond ((empty?)
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(set! front-ptr new-pair)
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(set! rear-ptr new-pair))
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(else
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(set-cdr! rear-ptr new-pair)
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(set! rear-ptr new-pair)))))
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(define (delete!)
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(cond ((empty?)
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(error "DELETE! called with empty queue"))
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(else
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(set! front-ptr (cdr front-ptr)))))
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(define (dispatch m)
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(cond ((eq? m 'empty?)
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(null? front-ptr))
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((eq? m 'front) (front))
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((eq? m 'insert!) insert!)
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((eq? m 'delete!) (delete!))
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((eq? m 'print)
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(display front-ptr)
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(newline))
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(else
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(error "Unknown method: " m))))
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dispatch))
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(define q1 (make-queue))
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(q1 'print)
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((q1 'insert!) 'a)
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(q1 'print)
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((q1 'insert!) 'b)
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(q1 'print)
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(q1 'front)
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(q1 'print)
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(q1 'delete!)
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(q1 'print)
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