Add implementation of exercise 3-72

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: 65/82
+  - 章节三: 66/82
   - 章节四: TODO
   - 章节五: TODO
 * 运行

chapter3/exercise3-72.rkt

@@ -0,0 +1,42 @@
+#lang racket
+(require "stream.rkt")
+(require "infinite-stream.rkt")
+(require "exercise3-70.rkt")
+
+(define square-weight (lambda (pair) (let ((i (car pair))
+                                           (j (cadr pair)))
+                                       (+ (* i i)
+                                          (* j j)))))
+(define square-pairs (pairs-weighted integers integers square-weight))
+
+(define (find-square-sum-of-3ways s)
+  (if (or (stream-null? s)
+          (stream-null? (stream-cdr s))
+          (stream-null? (stream-cdr (stream-cdr s))))
+      (error "The given stream contains less than 3 elements")
+      (let* ((first (stream-car s))
+             (second (stream-car (stream-cdr s)))
+             (third (stream-car (stream-cdr (stream-cdr s))))
+             (first-w (square-weight first))
+             (second-w (square-weight second))
+             (third-w (square-weight third)))
+        (if (and (= first-w second-w) (= second-w third-w))
+            ;; 找到两个平方数之和的三种表达方式
+            (cons-stream
+             (list first-w first second third)
+             ;; 将找到的有相同权重的三个序对排除
+             (find-square-sum-of-3ways
+              (stream-cdr (stream-cdr (stream-cdr s)))))
+            ;; 没找到，递归寻找
+            (find-square-sum-of-3ways (stream-cdr s) )))))
+
+(define 3ways-square-sum-stream (find-square-sum-of-3ways square-pairs))
+
+(module+ test
+  (require rackunit)
+  (test-case "Test for square sum of 3ways"
+             (check-equal? (stream-take-n 3ways-square-sum-stream 3) '((325 (1 18) (6 17) (10 15))
+                                                                       (425 (5 20) (8 19) (13 16))
+                                                                       (650 (5 25) (11 23) (17 19))))
+             )
+  )