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

faster due to the removed overhead of dealing with the redundant
pointers.

collects/tests/unstable/queue.rkt

@@ -1,4 +1,4 @@
-#lang racket
+#lang racket/base
 
 (require rackunit rackunit/text-ui unstable/queue "helpers.rkt")
 

collects/unstable/queue.rkt

@@ -1,56 +1,39 @@
-#lang racket
+#lang racket/base
-;; ELI: why is it using the `racket' language?
 
-;; A Queue is a circularly linked list of queue structures.
+;; A Queue contains a linked list with mutable cdrs, hoding two pointers
-;; The head of the circle is identified by the distinguished head value.
+;; to the head and the tail -- where items are pulled from the head and
-;; The top of the queue (front of the line) is to the right of the head.
+;; pushed on the tail.  It is not thread safe: mutating a queue from
-;; The bottom of the queue (back of the line) is to the left of the head.
+;; different threads can break it.
-;; ELI: need to say that this is not thread-safe.
+(struct queue (head tail) #:mutable)
-(define-struct queue (value left right) #:mutable)
+;; (Note: uses #f for `head' to mark an empty queue, but in those cases
-;; ELI: why is `value' mutable?
+;; 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
+(define (make-queue) (queue #f #f))
-;; than the head?
-(define head (gensym 'queue-head))
 
-;; ELI: if it's a single direction queue, then why is it maintaining
+(define (queue-empty? q) (not (queue-head q)))
-;; 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-head? q)
+(define (nonempty-queue? v) (and (queue? v) (queue-head v) #t))
-  (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 (enqueue! q v)
-  (let* ([bot (queue-left q)]
+  (let ([new (link v #f)])
-         [new (make-queue v bot q)])
+    (if (queue-head q)
-    (set-queue-left! q new)
+      (set-link-tail! (queue-tail q) new)
-    (set-queue-right! bot new)))
+      (set-queue-head! q new))
+    (set-queue-tail! q new)))
 
 (define (dequeue! q)
-  (let* ([old (queue-right q)]
+  (let ([old (queue-head q)])
-         [top (queue-right old)])
+    (set-queue-head! q (link-tail old))
-    (set-queue-right! q top)
+    (link-value old)))
-    (set-queue-left! top q)
+
-    (queue-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?)]
+ [queue? (-> any/c boolean?)]
- [rename empty-queue make-queue (-> queue/c)]
+ [make-queue (-> queue/c)]
  [queue-empty? (-> queue/c boolean?)]
  [enqueue! (-> queue/c any/c void?)]
  [dequeue! (-> nonempty-queue/c any/c)])