Boost C++ Libraries

boost/container_hash/detail/hash_mix.hpp

// 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 <boost/cstdint.hpp>
#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 boost::uint64_t fn( boost::uint64_t x )
    {
        boost::uint64_t const m = (boost::uint64_t(0xe9846af) << 32) + 0x9b1a615d;

        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 boost::uint32_t fn( boost::uint32_t x )
    {
        boost::uint32_t const m1 = 0x21f0aaad;
        boost::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