Add implementation of exercise 3-71

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: 64/82
+  - 章节三: 65/82
   - 章节四: TODO
   - 章节五: TODO
 * 运行

chapter3/exercise3-70.rkt

@@ -26,6 +26,7 @@
                     (stream-cdr t))
         (pairs-weighted (stream-cdr s) (stream-cdr t) weight)
         weight))))
+(provide pairs-weighted merge-weighted)
 
 (module+ test
   (require rackunit)

chapter3/exercise3-71.rkt

@@ -0,0 +1,42 @@
+#lang racket
+(require "stream.rkt")
+(require "infinite-stream.rkt")
+(require "exercise3-70.rkt")
+
+(define cube-weight (lambda (pair) (let ((i (car pair))
+                                         (j (cadr pair)))
+                                     (+ (* i i i)
+                                        (* j j j)))))
+(define cube-sum-stream (pairs-weighted integers integers cube-weight))
+
+(define (find-ramanujan-numbers s)
+  (if (or (stream-null? s)
+          (stream-null? (stream-cdr s)))
+      (error "The given stream is empty")
+      (let* ((first (stream-car s))
+             (second (stream-car (stream-cdr s)))
+             (first-w (cube-weight first))
+             (second-w (cube-weight second)))
+        (if (= first-w second-w)
+            ;; 找到权重相同的前后相邻两个序对
+            (cons-stream
+             (list first-w first second)
+             ;; 将有后面有相同权重的序对排除
+             (find-ramanujan-numbers (stream-filter (lambda (x) (not (= (cube-weight x) first-w)))
+                                                    (stream-cdr s)))
+             )
+            ;; 没找到，递归寻找
+            (find-ramanujan-numbers (stream-cdr s) )))))
+
+(define ramanujan-stream (find-ramanujan-numbers cube-sum-stream))
+
+(module+ test
+  (require rackunit)
+
+  (test-case "Test for ramanujan-numbers"
+             (check-equal? (stream-take-n ramanujan-stream 5) '((1729 (1 12) (9 10))
+                                                                (4104 (2 16) (9 15))
+                                                                (13832 (2 24) (18 20))
+                                                                (20683 (10 27) (19 24))
+                                                                (32832 (4 32) (18 30)))))
+  )