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Use formula with better random distribution for rand()

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Even better formula from random() could not be intetgrated because rand_r()
supposed to store its state in the single variable (but table needed for
random() algorithm integration).
This commit is contained in:
Andrey A. Chernov 2001-02-27 14:42:19 +00:00
parent 3bdced7c3c
commit cbcf73f998
1 changed files with 26 additions and 0 deletions
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26
lib/libc/stdlib/rand.c
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@ -31,6 +31,8 @@
* SUCH DAMAGE.
*
* Posix rand_r function added May 1999 by Wes Peters <wes@softweyr.com>.
*
* $FreeBSD$
*/
#if defined(LIBC_SCCS) && !defined(lint)
@ -47,7 +49,31 @@ static char sccsid[] = "@(#)rand.c 8.1 (Berkeley) 6/14/93";
static int
do_rand(unsigned long *ctx)
{
#ifdef USE_WEAK_SEEDING
/*
* Historic implementation compatibility.
* The random sequences do not vary much with the seed,
* even with overflowing.
*/
return ((*ctx = *ctx * 1103515245 + 12345) % ((u_long)RAND_MAX + 1));
#else /* !USE_WEAK_SEEDING */
/*
* Compute x = (7^5 * x) mod (2^31 - 1)
* wihout overflowing 31 bits:
* (2^31 - 1) = 127773 * (7^5) + 2836
* From "Random number generators: good ones are hard to find",
* Park and Miller, Communications of the ACM, vol. 31, no. 10,
* October 1988, p. 1195.
*/
long hi, lo, x;
hi = *ctx / 127773;
lo = *ctx % 127773;
x = 16807 * lo - 2836 * hi;
if (x <= 0)
x += 0x7fffffff;
return ((*ctx = x) % ((u_long)RAND_MAX + 1));
#endif /* !USE_WEAK_SEEDING */
}