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