no more repeating code in xoshiro/xoroshiro

src/random/splitmix64/splitmix64.c

@@ -0,0 +1,34 @@
+#include "splitmix64.h"
+
+/* 
+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; otherwise, we
+rather suggest to use a xoroshiro128+ (for moderately parallel
+computations) or xorshift1024* (for massively parallel computations)
+generator. 
+*/
+
+// The state can be seeded with any (upto) 64 bit integer value.
+
+void* splitmix64_init(uint64 seed){
+    splitmix64_state* state=malloc(sizeof(splitmix64_state));
+    *state=seed;
+    return state;
+}
+
+uint64 splitmix64_next(void* _state) {
+    splitmix64_state* state=_state;
+    // increment the state variable
+    *state += 0x9e3779b97f4a7c15;
+    // copy the state to a working variable
+    uint64 z = *state;
+    // xor the variable with the variable right bit shifted 30 then multiply by a constant
+    z = (z ^ (z>>30)) * 0xbf58476d1ce4e5b9;
+    // xor the variable with the variable right bit shifted 27 then multiply by a constant
+    z = (z ^ (z>>27)) * 0x94d049bb133111eb;
+    // return the variable xored with itself right bit shifted 31
+    return z ^ (z>>31);
+}