Bug #75170 [Opn->Csd]: mt_rand() bias on 64-bit machines

From: Date: Thu, 07 Sep 2017 18:06:05 +0000
Subject: Bug #75170 [Opn->Csd]: mt_rand() bias on 64-bit machines
References: 1  Groups: php.bugs 
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Edit report at https://bugs.php.net/bug.php?id=75170&edit=1

 ID:                 75170
 Updated by:         nikic@php.net
 Reported by:        nikic@php.net
 Summary:            mt_rand() bias on 64-bit machines
-Status:             Open
+Status:             Closed
 Type:               Bug
 Package:            Math related
 PHP Version:        7.1.9
 Block user comment: N
 Private report:     N

 New Comment:

Automatic comment on behalf of nikita.ppv@gmail.com
Revision: http://git.php.net/?p=php-src.git;a=commit;h=fd07302024bc47082b13b32217147fd39d1e9e61
Log: Fixed bug #75170


Previous Comments:
------------------------------------------------------------------------
[2017-09-07 17:55:04] nikic@php.net

Description:
------------
A bias in the PHP 7.1 implementation of mt_rand() on 64-bit systems has been reported by Solar
Designer on the mailing list. See https://externals.io/message/100229 for more
information.

Test script:
---------------
<?php
mt_srand(1234567890);
$total = 100000;
$max = 0x66666666;
$halves[0] = $halves[1] = 0;
for ($i = 0; $i < $total; $i++) {
    $halves[mt_rand(0, $max - 1) / ($max / 2)]++;
}
printf("%.1f%% vs. %.1f%%\n", 100. * $halves[0] / $total, 100. * $halves[1] / $total);
?>


Expected result:
----------------
50.0% vs. 50.0%

Actual result:
--------------
60.0% vs. 40.0%


------------------------------------------------------------------------



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Edit this bug report at https://bugs.php.net/bug.php?id=75170&edit=1

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