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prng: fix rand() to give good sequence with small state

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some applications rely on the low bits of rand() to be reasonably good
quality prng, so now it fixed by using the top bits of a 64 bit LCG,
this is simple, has small state and passes statistical tests.
D.E. Knuth attributes the multiplier to C.E. Haynes in TAOCP Vol2 3.3.4
This commit is contained in:
Szabolcs Nagy 2013-06-08 13:31:10 +00:00
parent fd1d7be35f
commit c79cd27e9e
1 changed files with 4 additions and 2 deletions
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6
src/prng/rand.c
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@ -1,6 +1,7 @@
#include <stdlib.h>
#include <stdint.h>
static unsigned seed;
static uint64_t seed;
void srand(unsigned s)
{
@ -9,5 +10,6 @@ void srand(unsigned s)
int rand(void)
{
return (seed = (seed+1) * 1103515245 + 12345 - 1)+1 & 0x7fffffff;
seed = 6364136223846793005ULL*seed + 1;
return seed>>33;
}