hash.h

 
#ifndef HASH_H
#define HASH_H
 
#include <cstddef>
#include <type_traits>
 
namespace Data {
 
#ifndef DOXYGEN_SHOULD_SKIP_THIS
 
#ifndef _NODISCARD
#define _NODISCARD [[nodiscard]]
#endif
 
#if defined(_WIN64) || defined(__x86_64__) || defined(__aarch64__)
    inline constexpr size_t _FNV_offset_basis = 14695981039346656037ULL;
    inline constexpr size_t _FNV_prime = 1099511628211ULL;
#else  // defined(_WIN64)
    inline constexpr size_t _FNV_offset_basis = 2166136261U;
    inline constexpr size_t _FNV_prime = 16777619U;
#endif // defined(_WIN64)
 
    _NODISCARD inline size_t _Fnv1a_append_bytes(
        size_t _Val, const unsigned char* const _First,
        const size_t _Count) noexcept
    { // accumulate range [_First, _First + _Count) into partial FNV-1a hash
      // _Val
        for (size_t _Idx = 0; _Idx < _Count; ++_Idx) {
            _Val ^= static_cast<size_t>(_First[_Idx]);
            _Val *= _FNV_prime;
        }
 
        return _Val;
    }
 
    template <class _Kty>
    _NODISCARD size_t _Fnv1a_append_value(const size_t _Val,
                                          const _Kty& _Keyval) noexcept
    { // accumulate _Keyval into partial FNV-1a hash _Val
        static_assert(std::is_trivial_v<_Kty>,
                      "Only trivial types can be directly hashed.");
        return _Fnv1a_append_bytes(
            _Val, &reinterpret_cast<const unsigned char&>(_Keyval),
            sizeof(_Kty));
    }
 
    // FUNCTION TEMPLATE _Hash_representation
    template <class _Kty>
    _NODISCARD size_t _Hash_representation(const _Kty& _Keyval) noexcept
    { // bitwise hashes the representation of a key
        return _Fnv1a_append_value(_FNV_offset_basis, _Keyval);
    }
 
    // STRUCT TEMPLATE _Conditionally_enabled_hash
    template <class _Kty> struct Hash;
 
    template <class _Kty, bool _Enabled> struct _Conditionally_enabled_hash
    { // conditionally enabled hash base
        using argument_type = _Kty;
        using result_type = size_t;
 
        _NODISCARD size_t
 
        operator()(const _Kty& _Keyval) const
            noexcept(noexcept(Hash<_Kty>::_Do_hash(_Keyval))) /* strengthened */
        {
            return Hash<_Kty>::_Do_hash(_Keyval);
        }
    };
 
    // STRUCT TEMPLATE hash
    template <class _Kty>
    struct Hash
        : _Conditionally_enabled_hash<
              _Kty, !std::is_const_v<_Kty> && !std::is_volatile_v<_Kty> &&
                        (std::is_enum_v<_Kty> || std::is_integral_v<_Kty> ||
                         std::is_pointer_v<_Kty>)>
    {
        // hash functor primary template (handles enums, integrals, and
        // pointers)
        static size_t _Do_hash(const _Kty& _Keyval) noexcept
        {
            return _Hash_representation(_Keyval);
        }
    };
 
    template <> struct Hash<float>
    {
        using argument_type = float;
        using result_type = size_t;
        _NODISCARD size_t
 
        operator()(const float _Keyval) const noexcept
        {
            return _Hash_representation(
                _Keyval == 0.0F ? 0.0F : _Keyval); // map -0 to 0
        }
    };
 
    template <> struct Hash<double>
    {
        using argument_type = double;
        using result_type = size_t;
        _NODISCARD size_t
 
        operator()(const double _Keyval) const noexcept
        {
            return _Hash_representation(
                _Keyval == 0.0 ? 0.0 : _Keyval); // map -0 to 0
        }
    };
 
    template <> struct Hash<std::nullptr_t>
    {
        using argument_type = std::nullptr_t;
        using result_type = size_t;
        _NODISCARD size_t
 
        operator()(std::nullptr_t) const noexcept
        {
            void* _Null{};
            return _Hash_representation(_Null);
        }
    };
#endif // DOXYGEN_SHOULD_SKIP_THIS
} // namespace Data
 
#endif