In Splitmix.mixGamma, wrong test for weak Gamma values (#2)

The "popcount >= 24" test was taken from the original Splitmix paper, but is wrong: it should be "popcount < 24", as explained in the paper and here http://www.pcg-random.org/posts/bugs-in-splitmix.html Closes: #2
xavierleroy · May 14, 2018 · 44fe85b · 44fe85b
1 parent fa170bc
commit 44fe85b
Show file tree

Hide file tree

Showing 2 changed files with 2 additions and 1 deletion.
diff --git a/Changes b/Changes
@@ -0,0 +1 @@
+- #2: in Splitmix.mixGamma, wrong test for weak Gamma values
diff --git a/stubs.c b/stubs.c
@@ -74,7 +74,7 @@ static inline int popcount64(uint64_t x)
 CAMLprim uint64_t pringo_mixGamma_unboxed(uint64_t z)
 {
   z = mix64variant13(z) | 1ULL;
-  if (popcount64(z ^ (z >> 1)) >= 24) z ^= 0xaaaaaaaaaaaaaaaaULL;
+  if (popcount64(z ^ (z >> 1)) < 24) z ^= 0xaaaaaaaaaaaaaaaaULL;
   return z; 
 }