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include/boost/container_hash/detail/hash_mix.hpp

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+// Copyright 2022 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#ifndef BOOST_HASH_DETAIL_HASH_MIX_HPP
+#define BOOST_HASH_DETAIL_HASH_MIX_HPP
+
+#include <cstdint>
+#include <cstddef>
+#include <climits>
+
+namespace boost
+{
+namespace hash_detail
+{
+
+template<std::size_t Bits> struct hash_mix_impl;
+
+// hash_mix for 64 bit size_t
+//
+// The general "xmxmx" form of state of the art 64 bit mixers originates
+// from Murmur3 by Austin Appleby, which uses the following function as
+// its "final mix":
+//
+//	k ^= k >> 33;
+//	k *= 0xff51afd7ed558ccd;
+//	k ^= k >> 33;
+//	k *= 0xc4ceb9fe1a85ec53;
+//	k ^= k >> 33;
+//
+// (https://github.com/aappleby/smhasher/blob/master/src/MurmurHash3.cpp)
+//
+// It has subsequently been improved multiple times by different authors
+// by changing the constants. The most well known improvement is the
+// so-called "variant 13" function by David Stafford:
+//
+//	k ^= k >> 30;
+//	k *= 0xbf58476d1ce4e5b9;
+//	k ^= k >> 27;
+//	k *= 0x94d049bb133111eb;
+//	k ^= k >> 31;
+//
+// (https://zimbry.blogspot.com/2011/09/better-bit-mixing-improving-on.html)
+//
+// This mixing function is used in the splitmix64 RNG:
+// http://xorshift.di.unimi.it/splitmix64.c
+//
+// We use Jon Maiga's implementation from
+// http://jonkagstrom.com/mx3/mx3_rev2.html
+//
+// 	x ^= x >> 32;
+//	x *= 0xe9846af9b1a615d;
+//	x ^= x >> 32;
+//	x *= 0xe9846af9b1a615d;
+//	x ^= x >> 28;
+//
+// An equally good alternative is Pelle Evensen's Moremur:
+//
+//	x ^= x >> 27;
+//	x *= 0x3C79AC492BA7B653;
+//	x ^= x >> 33;
+//	x *= 0x1C69B3F74AC4AE35;
+//	x ^= x >> 27;
+//
+// (https://mostlymangling.blogspot.com/2019/12/stronger-better-morer-moremur-better.html)
+
+template<> struct hash_mix_impl<64>
+{
+    inline static std::uint64_t fn( std::uint64_t x )
+    {
+        std::uint64_t const m = 0xe9846af9b1a615d;
+
+        x ^= x >> 32;
+        x *= m;
+        x ^= x >> 32;
+        x *= m;
+        x ^= x >> 28;
+
+        return x;
+    }
+};
+
+// hash_mix for 32 bit size_t
+//
+// We use the "best xmxmx" implementation from
+// https://github.com/skeeto/hash-prospector/issues/19
+
+template<> struct hash_mix_impl<32>
+{
+    inline static std::uint32_t fn( std::uint32_t x )
+    {
+        std::uint32_t const m1 = 0x21f0aaad;
+        std::uint32_t const m2 = 0x735a2d97;
+
+        x ^= x >> 16;
+        x *= m1;
+        x ^= x >> 15;
+        x *= m2;
+        x ^= x >> 15;
+
+        return x;
+    }
+};
+
+inline std::size_t hash_mix( std::size_t v )
+{
+    return hash_mix_impl<sizeof(std::size_t) * CHAR_BIT>::fn( v );
+}
+
+} // namespace hash_detail
+} // namespace boost
+
+#endif // #ifndef BOOST_HASH_DETAIL_HASH_MIX_HPP