class SplittableRandom

[Martin Buchholz]

If we make the wraparound point for seeds Long.MIN_VALUE instead of 0L,
then we can optimize addGammaModGeorge.  Any reason the following won't
work?

--- src/main/java/util/SplittableRandom.java 14 Jul 2013 08:06:49 -0000 1.10
+++ src/main/java/util/SplittableRandom.java 15 Jul 2013 16:25:42 -0000
@@ -222,10 +222,10 @@
      */
     private static long addGammaModGeorge(long s, long g) {
         long p = s + g;
-        if (Long.compareUnsigned(p, g) >= 0)
+        if (p > s)
             return p;
-        long q = p - 13L;
-        return (Long.compareUnsigned(p, 13L) >= 0) ? q : (q + g);
+        p -= 13L;
+        return (p < s) ? p : p + g;
     }

     /**

Also, if gamma is much less than 2^64 (say 2^56; that number of gammas
"should be enough for anybody"), then the above will be a little more
efficient since the wraparound code will be rare and well-predicted.  The
bits that become available as a result can then be optionally donated to
the seed space!

Have we run DieHarder tests against adversarial gamma values, that provide
as few bits as possible?  Most obviously, try gamma == 16.