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Back out "drop first N values" method of removing monotonically increased

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seed->first value correlation. It breaks rand_r()... Other possible methods
like shuffling inside aray will breaks rand_r() too, because it assumes
only one word state, i.e. nothing extra can be added after seed assignment
in srand().

BTW, for old formulae seed->first value correlation is not so monotonically
increased as with other Linear Congruential Generators of this type only
becase arithmetic overflow happens. But overflow affects distribution
and lower bits very badly, as many articles says, such type of overflow
not improves PRNG.

So, monotonically increased seed->first value correlation problem remains...
This commit is contained in:
ache 2003-02-17 03:52:35 +00:00
parent eadbecd88b
commit 54d6accf9d
1 changed files with 1 additions and 7 deletions
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8
lib/libc/stdlib/rand.c
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@ -51,8 +51,6 @@ __FBSDID("$FreeBSD$");
#include <stdio.h>
#endif /* TEST */
#define NSHUFF 100 /* to drop part of seed -> 1st value correlation */
static int
do_rand(unsigned long *ctx)
{
@ -98,7 +96,7 @@ rand_r(unsigned int *ctx)
}
static u_long next = 892053144; /* after srand(1), NSHUFF counted */
static u_long next = 1;
int
rand()
@ -110,11 +108,7 @@ void
srand(seed)
u_int seed;
{
int i;
next = seed;
for (i = 0; i < NSHUFF; i++)
(void)rand();
}