invertible hash functions

invertible_hash_functions.hpp

// MIT License
//
// Copyright (c) 2018 degski
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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// SOFTWARE.

#pragma once

#include <cstddef>


inline constexpr std::uint32_t hash ( std::uint32_t x ) noexcept {
    x = ( ( x >> 16 ) ^ x ) * 0X45D9F3B;
    x = ( ( x >> 16 ) ^ x ) * 0X45D9F3B; // repeat iff required
    x = ( ( x >> 16 ) ^ x );
    return x;
}

inline constexpr std::uint32_t unhash ( std::uint32_t x ) noexcept {
    x = ( ( x >> 16 ) ^ x ) * 0X119DE1F3;
    x = ( ( x >> 16 ) ^ x ) * 0X119DE1F3; // repeat iff required (as per above)
    x = ( ( x >> 16 ) ^ x );
    return x;
}

inline constexpr std::uint64_t hash ( std::uint64_t x ) noexcept {
    x = ( ( x >> 32 ) ^ x ) * 0xD6E8FEB86659FD93;
    x = ( ( x >> 32 ) ^ x ) * 0xD6E8FEB86659FD93;
    x = ( ( x >> 32 ) ^ x );
    return x;
}

inline constexpr std::uint64_t unhash ( std::uint64_t x ) noexcept {
    x = ( ( x >> 32 ) ^ x ) * 0xCFEE444D8B59A89B;
    x = ( ( x >> 32 ) ^ x ) * 0xCFEE444D8B59A89B;
    x = ( ( x >> 32 ) ^ x );
    return x;
}