Change algorithm used to compute prime numbers to make it more

efficient.

diff --git a/v7/src/runtime/stream.scm b/v7/src/runtime/stream.scm
index ddf29d0885ca1bd8d99a158fced5e34c95f26536..4f65015eae66c85bd5a0a633633544bd5aa5e7ae 100644 (file)
--- a/v7/src/runtime/stream.scm
+++ b/v7/src/runtime/stream.scm
@@ -1,6 +1,6 @@
 #| -*-Scheme-*-
 
-$Header: /Users/cph/tmp/foo/mit-scheme/mit-scheme/v7/src/runtime/stream.scm,v 14.4 1989/08/15 13:20:25 cph Exp $
+$Header: /Users/cph/tmp/foo/mit-scheme/mit-scheme/v7/src/runtime/stream.scm,v 14.5 1989/09/20 15:06:47 cph Exp $
 
 Copyright (c) 1988, 1989 Massachusetts Institute of Technology
 
@@ -128,22 +128,25 @@ MIT in each case. |#
 (define prime-numbers-stream)
 
 (define (make-prime-numbers-stream)
-  (letrec ((primes
-           (cons-stream
-            (cons 2 4)
-            (let filter ((integer 3))
-              (if (let loop ((primes primes))
-                    (let ((prime (stream-car primes)))
-                      (or (> (cdr prime) integer)
-                          (and (not (zero? (remainder integer
-                                                      (car prime))))
-                               (loop (stream-cdr primes))))))
-                  (cons-stream (cons integer (* integer integer))
-                               (filter (1+ integer)))
-                  (filter (1+ integer)))))))
-    (let loop ((primes primes))
-      (cons-stream (car (stream-car primes))
-                  (loop (stream-cdr primes))))))
+  (cons-stream
+   2
+   (letrec ((primes
+            (cons-stream
+             (cons 3 9)
+             (let filter ((integer 5))
+               (let loop ((primes primes))
+                 (let ((prime (stream-car primes)))
+                   (cond ((< integer (cdr prime))
+                          (cons-stream (cons integer (* integer integer))
+                                       (filter (+ integer 2))))
+                         ((zero? (remainder integer (car prime)))
+                          (filter (+ integer 2)))
+                         (else
+                          (loop (stream-cdr primes))))))))))
+     (let loop ((primes primes))
+       (cons-stream (car (stream-car primes))
+                   (loop (stream-cdr primes)))))))
+
 (define (initialize-package!)
   (let ((reset-primes!
         (lambda ()