Add implementation of exercise 3-75

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: 68/82
+  - 章节三: 69/82
   - 章节四: TODO
   - 章节五: TODO
 * 运行

chapter3/exercise3-75.rkt

@@ -0,0 +1,38 @@
+#lang racket
+(require "stream.rkt")
+(require "exercise3-74.rkt")
+
+(define (make-zero-crossings
+         input-stream last-value)
+  (let ((avpt
+         (/ (+ (stream-car input-stream)
+               last-value)
+            2)))
+    (cons-stream
+     (sign-change-detector avpt last-value)
+     (make-zero-crossings
+      ;; last-value 参数传入的是平均值，而非前一个值
+      (stream-cdr input-stream) avpt))))
+
+(define (make-zero-crossings-smooth input-stream last-value last-avg)
+  (if (stream-null? input-stream)
+      the-empty-stream
+      (let ((avpt
+             (/ (+ (stream-car input-stream)
+                   last-value)
+                2)))
+        (cons-stream
+         (sign-change-detector avpt last-avg)
+         (make-zero-crossings-smooth
+          (stream-cdr input-stream)
+          (stream-car input-stream)
+          avpt)))))
+
+(module+ test
+  (require rackunit)
+
+  (test-case "Test for make-zero-crossings-smooth"
+             (define sense-data (list-to-stream '(10 2 -2 -10 10 -1)))
+             (define crossings (make-zero-crossings-smooth sense-data 0 0))
+             (check-equal? (stream-take-n crossings 6) '(0 0 0 -1 1 0)))
+  )