// 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