HashBuilder.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include "CoreTypes.h"
#include "Crc.h"
#include "Concepts/GetTypeHashable.h"
#include "Containers/UnrealString.h"
 
/**
 * Class for computing a hash of multiple types, going through GetTypeHash when the type implements it, and
 * fallbacks to CRC32 when the type doesn't.
 *
 * Note: this hash builder should be used for transient hashes, as some types implements run-dependent hash
 * computations, such as GetTypeHash(FName).
 */
class FHashBuilder
{
public:
    explicit FHashBuilder(uint32 InHash = 0)
        : Hash(~InHash)
    {}
 
    void AppendRaw(const void* Data, int64 Num)
    {
        Hash = FCrc::MemCrc32(Data, static_cast<int32>(Num), Hash);     // TODO: Update MemCrc32 to take an int64 Length?
    }
 
    template <typename T>
    typename TEnableIf<TIsPODType<T>::Value, FHashBuilder&>::Type AppendRaw(const T& InData)
    {
        AppendRaw(&InData, sizeof(T), Hash);
        return *this;
    }
 
    template <typename T>
    typename TEnableIf<!TModels<CGetTypeHashable, T>::Value, FHashBuilder&>::Type Append(const T& InData)
    {
        return AppendRaw(InData);
    }
 
    template <typename T>
    typename TEnableIf<TModels<CGetTypeHashable, T>::Value, FHashBuilder&>::Type Append(const T& InData)
    {
        Hash = HashCombineFast(Hash, GetTypeHash(InData));
        return *this;
    }
 
    template <typename T>
    FHashBuilder& Append(const TArray<T>& InArray)
    {
        for (auto& Value: InArray)
        {
            Append(Value);
        }
        return *this;
    }
 
    template <typename T>
    FHashBuilder& Append(const TSet<T>& InArray)
    {
        for (auto& Value: InArray)
        {
            Append(Value);
        }
        return *this;
    }
 
    template <typename T>
    FHashBuilder& operator<<(const T& InData)
    {
        return Append(InData);
    }
 
    uint32 GetHash() const
    {
        return ~Hash;
    }
 
private:
    uint32 Hash;
};