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authorSzabolcs Nagy <nsz@port70.net>2013-06-08 13:31:10 +0000
committerSzabolcs Nagy <nsz@port70.net>2013-06-08 13:31:10 +0000
commitc79cd27e9e81eb5e223728094f1233ee2fc12dda (patch)
treec210ea1472b25519a62fd33e04e5809a16e76cbc
parentfd1d7be35f1b7f083071e246208498aa3b5ced3e (diff)
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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
-rw-r--r--src/prng/rand.c6
1 files changed, 4 insertions, 2 deletions
diff --git a/src/prng/rand.c b/src/prng/rand.c
index e3ce6347..c000cd24 100644
--- a/src/prng/rand.c
+++ b/src/prng/rand.c
@@ -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;
}