From 4bd9ec982b58aba0202794c0baea727da55a9df3 Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Mon, 17 Aug 2020 13:55:54 +1000
Subject: [PATCH] rand: add the splitmix64 generator

This is the suggested initialiser for xoshiro256plusplus state.
---
 CMakeLists.txt        |  1 +
 rand/splitmix64.hpp   | 44 +++++++++++++++++++++++++++++++++++++++++++
 test/rand/buckets.cpp |  2 ++
 3 files changed, 47 insertions(+)
 create mode 100644 rand/splitmix64.hpp

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1cb48acf..cccdd947 100644
--- a/CMakeLists.txt
+++ b/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
diff --git a/rand/splitmix64.hpp b/rand/splitmix64.hpp
new file mode 100644
index 00000000..6b3676ff
--- /dev/null
+++ b/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;
+    };
+}
diff --git a/test/rand/buckets.cpp b/test/rand/buckets.cpp
index 9dac1996..7e50d751 100644
--- a/test/rand/buckets.cpp
+++ b/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);