Add implementation of infinite rule

Author: ramsayleung

chapter3/infinite-stream.rkt

@@ -0,0 +1,32 @@
+#lang racket
+(require "stream.rkt")
+
+(define (integers-starting-from n)
+  (cons-stream n (integers-starting-from (+ n 1))))
+
+(define (divisible? n x)
+  (zero? (remainder n x)))
+
+;;; https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
+(define (sieve stream)
+  (cons-stream
+   (stream-car stream)
+   (sieve (stream-filter
+           (lambda (x)
+             (not (divisible? x (stream-car stream))))
+           (stream-cdr stream)))))
+
+(module+ test
+  (require rackunit)
+
+  (test-case "Test for divisible?"
+             (check-true (divisible? 12 2))
+             (check-true (divisible? 12 6))
+             (check-false (divisible? 3 4))
+             )
+
+  (test-case "Test for sieve"
+             (define primes (sieve (integers-starting-from 2)))
+             (check-equal? (stream-ref primes 50) 233)
+             )
+  )