unordered_ The hash function of map and the storage mode of hash bucket

//  The template parameters are   Key type , Value type ,  hash function , Equal to comparator ,  Memory allocator 
template <class _Kty, class _Ty,
	class _Hasher = hash<_Kty>, 
	class _Keyeq = equal_to<_Kty>,
    class _Alloc = allocator<pair<const _Kty, _Ty>>>
class unordered_map : public _Hash<_Umap_traits<_Kty, _Ty, _Uhash_compare<_Kty, _Hasher, _Keyeq>, _Alloc, false>> {
    }

// \2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\type_traits
// hash function
#if defined(_WIN64)
constexpr size_t _FNV_offset_basis = 14695981039346656037ULL;
constexpr size_t _FNV_prime        = 1099511628211ULL;
#else // defined(_WIN64)
constexpr size_t _FNV_offset_basis = 2166136261U;
constexpr size_t _FNV_prime        = 16777619U;
#endif // defined(_WIN64)

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;
}