Add implementation of exercise 3-76

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: 69/82
+  - 章节三: 70/82
   - 章节四: TODO
   - 章节五: TODO
 * 运行

chapter3/exercise3-74.rkt

@@ -18,7 +18,7 @@
 (provide sign-change-detector make-zero-crossings)
 (define sense-data (list-to-stream '(1 2 1.5 1 0.5 -0.1 -2 -3 -2 -0.5 0.2 3 4)))
 
-(define zero-crossings (map-stream sign-change-detector (stream-cdr sense-data) sense-data))
+(define zero-crossings (map-stream sign-change-detector sense-data (cons-stream 0 sense-data)))
 
 (module+ test
   (require rackunit)
@@ -28,6 +28,6 @@
              (check-equal? (stream-take-n zero-crossings 13) '(0 0 0 0 0 -1 0 0 0 0 1 0 0))
              )
   (test-case "Test for zero-crossings"
-             (check-equal? (stream-take-n zero-crossings 11) '(0 0 0 0 -1 0 0 0 0 1 0))
+             (check-equal? (stream-take-n zero-crossings 13) '(0 0 0 0 0 -1 0 0 0 0 1 0 0))
              )
   )

chapter3/exercise3-76.rkt

@@ -0,0 +1,34 @@
+#lang racket
+(require "stream.rkt")
+(require "exercise3-74.rkt")
+(require "exercise3-50.rkt")
+
+(define (smooth input-s)
+  (if (or (stream-null? input-s)
+          (stream-null? (stream-cdr input-s)))
+      the-empty-stream
+      (let ((prev (stream-car input-s))
+            (cur (stream-car (stream-cdr input-s))))
+        (cons-stream
+         (/ (+ cur prev) 2)
+         (smooth (stream-cdr input-s))))))
+
+
+(define (make-zero-crossings-smooth input-s)
+  (let ((smooth-stream (smooth input-s)))
+    (map-stream sign-change-detector smooth-stream (cons-stream 0 smooth-stream))))
+
+
+(module+ test
+  (require rackunit)
+
+  (test-case "Test for smooth"
+             (define s1 (list-to-stream '(1 3 5 9 13 17)))
+             (check-equal? (stream-take-n (smooth s1) 6) '(2 4 7 11 15))
+             )
+
+  (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))
+             (check-equal? (stream-take-n crossings 6) '(0 0 -1 1 0)))
+  )