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

diff --git a/src/prng/rand.c b/src/prng/rand.c
index e3ce6347331608ce663a2ac6006c6bc08e2cd800..c000cd248cef667bab257940768616c9349f7d70 100644 (file)
--- 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;
 }