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Uses a simpler single-linked list, about half of the meat code, and much

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faster due to the removed overhead of dealing with the redundant
pointers.
This commit is contained in:
Eli Barzilay 2010-06-23 15:48:23 -04:00
parent 8a5bceece1
commit d986fed725
2 changed files with 29 additions and 46 deletions
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2
collects/tests/unstable/queue.rkt
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@ -1,4 +1,4 @@
#lang racket
#lang racket/base
(require rackunit rackunit/text-ui unstable/queue "helpers.rkt")

73
collects/unstable/queue.rkt
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@ -1,56 +1,39 @@
#lang racket
;; ELI: why is it using the `racket' language?
#lang racket/base
;; A Queue is a circularly linked list of queue structures.
;; The head of the circle is identified by the distinguished head value.
;; The top of the queue (front of the line) is to the right of the head.
;; The bottom of the queue (back of the line) is to the left of the head.
;; ELI: need to say that this is not thread-safe.
(define-struct queue (value left right) #:mutable)
;; ELI: why is `value' mutable?
;; A Queue contains a linked list with mutable cdrs, hoding two pointers
;; to the head and the tail -- where items are pulled from the head and
;; pushed on the tail. It is not thread safe: mutating a queue from
;; different threads can break it.
(struct queue (head tail) #:mutable)
;; (Note: uses #f for `head' to mark an empty queue, but in those cases
;; the tail will be set to #f too, to avoid holding on to values that
;; should be collected.)
(struct link (value [tail #:mutable]))
;; ELI: why is this needed, if nobody can get a hold of anything other
;; than the head?
(define head (gensym 'queue-head))
(define (make-queue) (queue #f #f))
;; ELI: if it's a single direction queue, then why is it maintaining
;; bidirectional links?
(define (empty-queue)
(let* ([q (make-queue head #f #f)])
(set-queue-left! q q)
(set-queue-right! q q)
q))
(define (queue-empty? q) (not (queue-head q)))
(define (queue-head? q)
(eq? (queue-value q) head))
(define (head-queue? v) ; ELI: `queue-head?' and `head-queue?' --??
(and (queue? v) (queue-head? v)))
(define (queue-empty? q)
(and (queue-head? q) (queue-head? (queue-right q))))
(define (nonempty-queue? v)
(and (queue? v) ; \ ELI: ...and this is `head-queue?'
(queue-head? v) ; /
(queue? (queue-right v)) ; ELI: can this ever be false?
(not (queue-head? (queue-right v)))))
(define (nonempty-queue? v) (and (queue? v) (queue-head v) #t))
(define (enqueue! q v)
(let* ([bot (queue-left q)]
[new (make-queue v bot q)])
(set-queue-left! q new)
(set-queue-right! bot new)))
(let ([new (link v #f)])
(if (queue-head q)
(set-link-tail! (queue-tail q) new)
(set-queue-head! q new))
(set-queue-tail! q new)))
(define (dequeue! q)
(let* ([old (queue-right q)]
[top (queue-right old)])
(set-queue-right! q top)
(set-queue-left! top q)
(queue-value old)))
(let ([old (queue-head q)])
(set-queue-head! q (link-tail old))
(link-value old)))
;; --- contracts ---
(require racket/contract)
(define queue/c
(flat-named-contract "queue" head-queue?))
(flat-named-contract "queue" queue?))
(define nonempty-queue/c
(flat-named-contract "nonempty-queue" nonempty-queue?))
@ -58,8 +41,8 @@
(provide/contract
[queue/c flat-contract?]
[nonempty-queue/c flat-contract?]
[rename head-queue? queue? (-> any/c boolean?)]
[rename empty-queue make-queue (-> queue/c)]
[queue? (-> any/c boolean?)]
[make-queue (-> queue/c)]
[queue-empty? (-> queue/c boolean?)]
[enqueue! (-> queue/c any/c void?)]
[dequeue! (-> nonempty-queue/c any/c)])