rand: add the splitmix64 generator

This is the suggested initialiser for xoshiro256plusplus state.

CMakeLists.txt

@@ -487,6 +487,7 @@ list (
     rand/pcg.hpp
     rand/rdrand.cpp
     rand/rdrand.hpp
+    rand/splitmix64.hpp
     rand/system.hpp
     random.cpp
     random.hpp

rand/splitmix64.hpp

@@ -0,0 +1,44 @@
+/*  Written in 2015 by Sebastiano Vigna (vigna@acm.org)
+ *
+ * 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.
+ */
+
+#include "../std.hpp"
+
+#include <limits>
+
+
+namespace cruft::rand {
+    class splitmix64 {
+    public:
+        using result_type = u64;
+
+        splitmix64 (u64 seed):
+            m_state (seed)
+        { ; }
+
+        result_type operator() (void)
+        {
+            u64 z = (m_state += 0x9e3779b97f4a7c15);
+            z = (z ^ (z >> 30)) * 0xbf58476d1ce4e5b9;
+            z = (z ^ (z >> 27)) * 0x94d049bb133111eb;
+            return z ^ (z >> 31);
+        }
+
+        static constexpr result_type min (void)
+        {
+            return std::numeric_limits<result_type>::min ();
+        }
+
+        static constexpr result_type max (void)
+        {
+            return std::numeric_limits<result_type>::max ();
+        }
+
+
+    private:
+        u64 m_state;
+    };
+}

test/rand/buckets.cpp

@@ -4,6 +4,7 @@
 #include "rand/pcg.hpp"
 #include "rand/rdrand.hpp"
 #include "rand/system.hpp"
+#include "rand/splitmix64.hpp"
 
 #include "tap.hpp"
 #include "maths.hpp"
@@ -49,6 +50,7 @@ main (int,char**)
 {
     cruft::TAP::logger tap;
 
+    test_buckets<cruft::rand::splitmix64> (tap, 0x1234u);
     test_buckets<cruft::rand::xorshift<uint32_t>> (tap, 0x1234u);
     test_buckets<cruft::rand::xorshift<uint64_t>> (tap, 0x1234u);
     test_buckets<cruft::rand::lcg_t> (tap, 0x1234u);