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

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 <stdlib.h>
#include <stdint.h>
static unsigned seed; static uint64_t seed;
void srand(unsigned s) void srand(unsigned s)
{ {
@ -9,5 +10,6 @@ void srand(unsigned s)
int rand(void) int rand(void)
{ {
return (seed = (seed+1) * 1103515245 + 12345 - 1)+1 & 0x7fffffff; seed = 6364136223846793005ULL*seed + 1;
return seed>>33;
} }