Add solution for exercise 3-22

Author: ramsayleung

README.org

@@ -25,7 +25,7 @@
   习题完成情况:
   - 章节一: 43/46
   - 章节二: 88/97
-  - 章节三: 21/82
+  - 章节三: 22/82
   - 章节四: TODO
   - 章节五: TODO
 * 运行

chapter3/exercise3-22.rkt

@@ -0,0 +1,62 @@
+#lang racket
+(define (make-queue)
+  (let ((front-ptr '())
+        (rear-ptr '()))
+    (define (empty-queue?)
+      (null? front-ptr))
+
+    (define (insert-queue! item)
+      (let ((new-pair (mcons item '())))
+        (cond ((empty-queue?)
+               (set! front-ptr new-pair)
+               (set! rear-ptr new-pair))
+              (else
+               (set-mcdr! rear-ptr new-pair)
+               (set! rear-ptr new-pair)))))
+
+    (define (delete-queue!)
+      (cond ((empty-queue?)
+             (error "DELETE! called with an empty queue"))
+            (else
+             (set! front-ptr (mcdr front-ptr)))))
+
+    (define (front-queue)
+      (if (empty-queue?)
+          (error "FRONT called with an empty queue")
+          (mcar front-ptr)))
+
+    (define (print-queue)
+      (displayln front-ptr))
+
+    (define (dispatch m)
+      (cond ((eq? m 'empty?) empty-queue?)
+            ((eq? m 'front) front-queue)
+            ((eq? m 'insert!)insert-queue!)
+            ((eq? m 'delete!)delete-queue!)
+            ((eq? m 'print)print-queue)))
+    dispatch))
+
+(module+ test
+  (require rackunit)
+
+  (test-case "Test for make-queue success"
+             (define q (make-queue))
+             (check-true ((q 'empty?)))
+             ((q 'insert!) 'a)
+             (check-equal? (with-output-to-string (lambda () ((q 'print)))) "{a}\n")
+             ((q 'insert!) 'b)
+             (check-equal? (with-output-to-string (lambda () ((q 'print)))) "{a b}\n")
+             (check-equal? ((q 'front)) 'a)
+             ((q 'delete!))
+             (check-equal? (with-output-to-string (lambda () ((q 'print)))) "{b}\n")
+             (check-equal? ((q 'front)) 'b)
+             (check-false ((q 'empty?)))
+             )
+
+  (test-case "Test for make-queue unhappy path"
+             (define q (make-queue))
+             (check-exn (regexp "DELETE! called with an empty queue") (lambda () ((q 'delete!))))
+             (check-exn (regexp "FRONT called with an empty queue") (lambda () ((q 'front))))
+             )
+
+  )