Author: Andrew Williams
Date: 15:17:08 08/17/02
Go up one level in this thread
On August 17, 2002 at 17:41:52, Dieter Buerssner wrote:
>On August 17, 2002 at 06:35:16, Andrew Williams wrote:
>
>>I use the Mersenne-Twister random number generator.
>
>I agree, that Mersenne-Twister is a very good generator:
>
>>long long random64() {
>> long long rand;
>> int r1, r2;
>>
>> r1 = randomMT();
>> r2 = randomMT();
>>
>> rand = r1;
>> rand = rand << 32;
>> rand = rand | r2;
>>
>> return rand;
>>}
>
>To me, it seems this is buggy. To show an example with hard coded values instead
>of calls to the Mersenne twister:
>
>long long random64() {
> long long rand;
> int r1, r2;
>
> r1 = 0x12345678;
> r2 = 0xffffffff; /* A valid return from Mersenne twister */
>
> rand = r1;
> rand = rand << 32;
> rand = rand | r2;
>
> /* We exect 0x12345678ffffffff */
> return rand;
>}
>
>#include <stdio.h>
>
>int main(void)
>{
> unsigned long long r = random64();
> printf("%0LLx\n", r);
> return 0;
>}
>
>When I start the program, I get
>
>ffffffffffffffff
>
>Perhaps, this late at night, I have overlooked something ...
>
>BTW. Using unsigned types yields the expected result:
>
>unsigned long long random64() {
> long long rand;
> unsigned long r1, r2; /* why use int, when long is more protable? */
>
> r1 = 0x12345678;
> r2 = 0xffffffff; /* A valid return from Mersenne twister */
>
> rand = r1;
> rand = rand << 32;
> rand = rand | r2;
>
> /* We exect 0x12345678ffffffff */
> return rand;
>}
>
>Regards,
>Dieter
>
>
>12345678ffffffff
Ugh. You're correct of course. What a mess.
Thanks
Andrew
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