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/* 32b/64b xmx mix function.
*
* Copyright 2022 Peter Dimov.
* Copyright 2022 Joaquin M Lopez Munoz.
* Distributed under the Boost Software License, Version 1.0.
* (See accompanying file LICENSE_1_0.txt or copy at
* http://www.boost.org/LICENSE_1_0.txt)
*
* See https://www.boost.org/libs/unordered for library home page.
*/
#ifndef BOOST_UNORDERED_DETAIL_XMX_HPP
#define BOOST_UNORDERED_DETAIL_XMX_HPP
#include <boost/cstdint.hpp>
#include <climits>
#include <cstddef>
namespace boost{
namespace unordered{
namespace detail{
/* Bit mixer for improvement of statistical properties of hash functions.
* The implementation is different on 64bit and 32bit architectures:
*
* - 64bit: same as xmx function in
* http://jonkagstrom.com/bit-mixer-construction/index.html
* - 32bit: generated by Hash Function Prospector
* (https://github.com/skeeto/hash-prospector) and selected as the
* best overall performer in benchmarks of Boost.Unordered flat containers.
* Score assigned by Hash Prospector: 333.7934929677524
*/
#if defined(SIZE_MAX)
#if ((((SIZE_MAX >> 16) >> 16) >> 16) >> 15) != 0
#define BOOST_UNORDERED_64B_ARCHITECTURE /* >64 bits assumed as 64 bits */
#endif
#elif defined(UINTPTR_MAX) /* used as proxy for std::size_t */
#if ((((UINTPTR_MAX >> 16) >> 16) >> 16) >> 15) != 0
#define BOOST_UNORDERED_64B_ARCHITECTURE
#endif
#endif
static inline std::size_t xmx(std::size_t x)noexcept
{
#if defined(BOOST_UNORDERED_64B_ARCHITECTURE)
boost::uint64_t z=(boost::uint64_t)x;
z^=z>>23;
z*=0xff51afd7ed558ccdull;
z^=z>>23;
return (std::size_t)z;
#else /* 32 bits assumed */
x^=x>>18;
x*=0x56b5aaadu;
x^=x>>16;
return x;
#endif
}
#ifdef BOOST_UNORDERED_64B_ARCHITECTURE
#undef BOOST_UNORDERED_64B_ARCHITECTURE
#endif
} /* namespace detail */
} /* namespace unordered */
} /* namespace boost */
#endif
Version data entries
8 entries across 8 versions & 1 rubygems