Use SplitMix64 for seeding test cases

Close #581.

tests/impl.cpp

@@ -350,10 +350,34 @@ static inline double bankersRounding(double val)
     return ret;
 }
 
-static float ranf(void)
-{
-    uint32_t ir = rand() & 0x7FFF;
-    return (float) ir * (1.0f / 32768.0f);
+/*  Written in 2015 by Sebastiano Vigna ([email protected])
+ *
+ *  To the extent possible under law, the author has dedicated all copyright
+ *  and related and neighboring rights to this software to the public domain
+ *  worldwide. This software is distributed without any warranty.
+ *
+ *  See <http://creativecommons.org/publicdomain/zero/1.0/>. */
+
+/* This is a fixed-increment version of Java 8's SplittableRandom generator
+ * See http://dx.doi.org/10.1145/2714064.2660195 and
+ * http://docs.oracle.com/javase/8/docs/api/java/util/SplittableRandom.html
+ *
+ * It is a very fast generator passing BigCrush, and it can be useful
+ * if for some reason you absolutely want 64 bits of state. */
+static uint64_t x;
+const double TWOPOWER64 = pow(2, 64);
+
+static double next()
+{
+    uint64_t z = (x += 0x9e3779b97f4a7c15);
+    z = (z ^ (z >> 30)) * 0xbf58476d1ce4e5b9;
+    z = (z ^ (z >> 27)) * 0x94d049bb133111eb;
+    return z ^ (z >> 31);
+}
+
+static float ranf()
+{
+    return next() / TWOPOWER64;
 }
 
 static float ranf(float low, float high)