Re: [ID 20010309.004] my-variables lose values while goto'ing within a for(;;)-loop

Message-Id: <200103131736.MAA35615@raptor.research.att.com>

A more correct prime finder.

p4raw-id: //depot/perl@9132

diff --git a/lib/Tie/SubstrHash.pm b/lib/Tie/SubstrHash.pm
index afe5d8d..cd1125e 100644 (file)
--- a/lib/Tie/SubstrHash.pm
+++ b/lib/Tie/SubstrHash.pm
@@ -183,21 +183,29 @@ sub ceil {
     return $num;
 }
 
+# See:
+#
+# http://www-groups.dcs.st-andrews.ac.uk/~history/HistTopics/Prime_numbers.html
+#
+
 sub findgteprime { # find the smallest prime integer greater than or equal to
     use integer;
 
-# It may be sufficient (and more efficient, IF IT IS CORRECT) to use
-# $max = 1 + int sqrt $num and calculate it once only, but is it correct?
-
     my $num = ceil(shift);
     return 2 if $num <= 2;
 
     $num++ unless $num % 2;
+    my $i;
+    my $sqrtnum = int sqrt $num;
+    my $sqrtnumsquared = $sqrtnum * $sqrtnum;
 
   NUM:
     for (;; $num += 2) {
-        my $max = int sqrt $num;
-        for ($i = 3; $i <= $max; $i += 2) {
+       if ($sqrtnumsquared < $num) {
+           $sqrtnum++;
+           $sqrtnumsquared = $sqrtnum * $sqrtnum;
+       }
+        for ($i = 3; $i <= $sqrtnum; $i += 2) {
             next NUM unless $num % $i;
         }
         return $num;