#22988 [Opn]: I'm feeling lucky - rand() bug

ID: 22988 Updated by: pollita@php.net Reported By: vakmajom at freemail dot hu Status: Open Bug Type: Math related Operating System: Windows 2000 PHP Version: 4.3.2RC1 New Comment: I can see where you were going with that, unfortunately it has the unexpected result of simply chaning the bug so that only *odd* numbers get returned.... I'm still thinking over this one and hope you give it some more thought too.... :) Previous Comments: ------------------------------------------------------------------------ [2003-04-17 11:06:09] vakmajom at freemail dot hu About the original bug... Nothing has happened since April 1st, so i browsed the source code a little bit: I think it is not just a carryover bug, it is more than that. It is because RAND_MAX on Windows, and RAND_RANGE. The problem is the follownig: On Windows systems RAND_MAX is 32767, so only 32768 values can be returned if I call rand(1, 2147483647) from a PHP script. That's why rand(1, 2147483647) or rand(1, 65535) returns always an even number. Maybe a RAND_MAX specific hack to the code: e.g. calling rand() three times and creating a 45 bit random value and then reducing it to a 32bit one, or somthing like that would help, but this would dirty the code... Alright, let's use mt_rand(), it is a 31 bit random number on every system, and it is better than a 15 bit random number... So if I call mt_rand(-2147483648, 2147483647) then I should get 2147483648 results. This is less than it should (4294967296), but more than 32768. And now, the suprise: the result is always the same: -2147483648. This IS a carryover bug, and unless I am mistaken it can be solved by changing line 43 in file ext/standard/php_rand.h from (__n) = (__min) + (long) ((double) ((__max) - (__min) + 1.0) * ((__n) / ((__tmax) + 1.0))) to (__n) = (__min) + (long) ((double) ((double)(__max) - (__min) + 1.0) * ((__n) / ((__tmax) + 1.0))) ------------------------------------------------------------------------ [2003-04-01 08:01:28] vakmajom at freemail dot hu Sorry: srand((double)microtime()*100000); ------------------------------------------------------------------------ [2003-04-01 07:59:49] vakmajom at freemail dot hu The same case. Only even numbers. And a new rand() bug has also appeared: Without srand(microtime()*100000), always the same 10-20 numbers are returned. You should try this bug after the restart of Apache, to be sure that none of the php scripts called srand() before. ------------------------------------------------------------------------ [2003-03-31 17:49:50] sniper@php.net Please try using this CVS snapshot: http://snaps.php.net/php4-STABLE-latest.tar.gz For Windows: http://snaps.php.net/win32/php4-win32-STABLE-latest.zip ------------------------------------------------------------------------ [2003-03-31 15:22:49] pollita@php.net This is being caused by a carryover (2147483647 + 1 == -2147483648). The short answer is: "If it hurts, don't do it!" Of course that's not a proper answer. The API Macro could probably use some tweaking... ------------------------------------------------------------------------ The remainder of the comments for this report are too long. To view the rest of the comments, please view the bug report online at http://bugs.php.net/22988 -- Edit this bug report at http://bugs.php.net/?id=22988&edit=1