Finished 3.22

Author: xudon9

3.22.scm

@@ -0,0 +1,37 @@
+(define (make-queue)
+  (let ((front-ptr '())
+        (rear-ptr  '()))
+    (define (dispatch m)
+      (cond ((eq? m 'front-ptr) front-ptr)
+            ((eq? m 'rear-ptr)  rear-ptr)
+            ((eq? m 'set-front-ptr!) (lambda (v) (set! front-ptr v)))
+            ((eq? m 'set-rear-ptr!)  (lambda (v) (set!  rear-ptr v)))
+            ((eq? m 'empty-queue?) (null? front-ptr))
+            ((eq? m 'front-queue) (if (null? front-ptr)
+                                    (error "FRONT called with an empty queue")
+                                    (car front-ptr)))
+            ((eq? m 'insert-queue!) (lambda (item)
+                                      (let ((new-pair (cons item '())))
+                                       (cond ((null? front-ptr)
+                                              (set! front-ptr new-pair)
+                                              (set! rear-ptr new-pair)
+                                              front-ptr)
+                                             (else
+                                               (set-cdr! rear-ptr new-pair)
+                                               (set! rear-ptr new-pair)
+                                               front-ptr)))))
+            ((eq? m 'delete-queue!) (cond ((null? front-ptr)
+                                           (error "DELETE! called with empty queue"))
+                                          (else
+                                            (set! front-ptr (cdr front-ptr))
+                                            front-ptr)))))
+    dispatch))
+
+(define (front-ptr queue) (queue 'front-ptr))
+(define (rear-ptr queue) (queue 'rear-ptr))
+(define (set-front-ptr! queue v) ((queue 'set-front-ptr!) v))
+(define (set-rear-ptr! queue v) ((queue 'set-rear-ptr!) v))
+(define (empty-queue? queue) (queue 'empty-queue?))
+(define (front-queue queue) (queue 'front-queue))
+(define (insert-queue! queue item) ((queue 'insert-queue!) item))
+(define (delete-queue! queue) (queue 'delete-queue!))