Fnv.h

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// Copyright Epic Games, Inc. All Rights Reserved.
#pragma once
 
#include "Containers/StringView.h"
#include "CoreTypes.h"
#include "HAL/Platform.h"
 
namespace UE
{
    template<typename HashType, typename CharType>
    constexpr HashType HashStringFNV1a(TStringView<CharType> String)
    {
        static_assert(std::is_same_v<HashType, uint32> || std::is_same_v<HashType, uint64>, "HashType must be an unsigned 32 or 64 bit integer.");
        static_assert(sizeof(CharType) <= 4, "FNV1a only works with characters up to 32 bits.");
 
        // See https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function#FNV_hash_parameters
        constexpr HashType Offset = []() -> HashType
        {
            if constexpr (sizeof(HashType) == 4)
            {
                return 0x811c9dc5;
            }
            else
            {
                return 0xcbf29ce484222325;
            }
        }();
        constexpr HashType Prime = []() -> HashType
        {
            if constexpr (sizeof(HashType) == 4)
            {
                return 0x01000193;
            }
            else
            {
                return 0x100000001b3;
            }
        }();
 
        HashType Fnv = Offset;
        for (CharType Char : String)
        {
            // Operate on every character as if it's 4 bytes.
            // Characters < 4 bytes will be padded out with zeros.
            const uint32 Ch = static_cast<uint32>(Char);
 
            Fnv ^= (Ch >> 24) & 0xff;
            Fnv *= Prime;
            Fnv ^= (Ch >> 16) & 0xff;
            Fnv *= Prime;
            Fnv ^= (Ch >> 8) & 0xff;
            Fnv *= Prime;
            Fnv ^= (Ch) & 0xff;
            Fnv *= Prime;
        }
 
        return Fnv;
    }
 
    /* Version taking a string literal. Use this version to force hashing at compile time. */
    template<typename HashType, typename CharType, int32 Len>
    UE_CONSTEVAL HashType HashStringFNV1a(const CharType(&StringLiteral)[Len])
    {
        return HashStringFNV1a<HashType>(MakeStringView(StringLiteral, Len - 1));
    }
 
    /* Creates a 32-bit FNV1a hash for the given string. */
    template<typename CharType>
    constexpr uint32 HashStringFNV1a32(TStringView<CharType> String)
    {
        return HashStringFNV1a<uint32>(String);
    }
 
    template<typename CharType, int32 Len>
    UE_CONSTEVAL uint32 HashStringFNV1a32(const CharType(&StringLiteral)[Len])
    {
        return HashStringFNV1a<uint32>(StringLiteral);
    }
 
    /* Creates a 64-bit FNV1a hash for the given string. */
    template<typename CharType>
    constexpr uint64 HashStringFNV1a64(TStringView<CharType> String)
    {
        return HashStringFNV1a<uint64>(String);
    }
 
    template<typename CharType, int32 Len>
    UE_CONSTEVAL uint64 HashStringFNV1a64(const CharType(&StringLiteral)[Len])
    {
        return HashStringFNV1a<uint64>(StringLiteral);
    }
 
} // namespace UE