UnrealEngineUWP/Engine/Source/Developer/DerivedDataCache/Private/DerivedDataLimitKeyLengthWrapper.h

// Copyright 1998-2015 Epic Games, Inc. All Rights Reserved.

#pragma once

/** 
 * A backend wrapper that limits the key size and uses hashing...in this case it wraps the payload and the payload contains the full key to verify the integrity of the hash
**/
class FDerivedDataLimitKeyLengthWrapper : public FDerivedDataBackendInterface
{
public:

	/**
	 * Constructor
	 *
	 * @param	InInnerBackend	Backend to use for storage, my responsibilities are about key length
	 */
	FDerivedDataLimitKeyLengthWrapper(FDerivedDataBackendInterface* InInnerBackend, int32 InMaxKeyLength)
		: InnerBackend(InInnerBackend)
		, MaxKeyLength(InMaxKeyLength)
	{
		check(InnerBackend);
	}

	/** return true if this cache is writable **/
	virtual bool IsWritable() override
	{
		return InnerBackend->IsWritable();
	}

	/**
	 * Synchronous test for the existence of a cache item
	 *
	 * @param	CacheKey	Alphanumeric+underscore key of this cache item
	 * @return				true if the data probably will be found, this can't be guaranteed because of concurrency in the backends, corruption, etc
	 */
	virtual bool CachedDataProbablyExists(const TCHAR* CacheKey) override
	{
		FString NewKey;
		ShortenKey(CacheKey, NewKey);
		return InnerBackend->CachedDataProbablyExists(*NewKey);
	}
	/**
	 * Synchronous retrieve of a cache item
	 *
	 * @param	CacheKey	Alphanumeric+underscore key of this cache item
	 * @param	OutData		Buffer to receive the results, if any were found
	 * @return				true if any data was found, and in this case OutData is non-empty
	 */
	virtual bool GetCachedData(const TCHAR* CacheKey, TArray<uint8>& OutData) override
	{
		FString NewKey;
		bool bOk;
		if (!ShortenKey(CacheKey, NewKey))
		{
			// no shortening needed
			bOk = InnerBackend->GetCachedData(CacheKey, OutData);
			// look for old bug
			if (FString(CacheKey).StartsWith(TEXT("TEXTURE2D_0002")))
			{
				int32 KeyLen = FCString::Strlen(CacheKey) + 1;
				if (OutData.Num() > KeyLen && OutData.Last() == 0)
				{
					int32 Compare = FCStringAnsi::Strcmp(TCHAR_TO_ANSI(CacheKey), (char*)&OutData[OutData.Num() - KeyLen]);
					if (Compare == 0)
					{
						UE_LOG(LogDerivedDataCache, Warning, TEXT("FDerivedDataLimitKeyLengthWrapper: Fixed old bug %s."), CacheKey);
						OutData.RemoveAt(OutData.Num() - KeyLen, KeyLen);
					}
				}
			}
		}
		else
		{
			bOk = InnerBackend->GetCachedData(*NewKey, OutData);
			if (bOk)
			{
				int32 KeyLen = FCString::Strlen(CacheKey) + 1;
				if (OutData.Num() < KeyLen)
				{
					UE_LOG(LogDerivedDataCache, Warning, TEXT("FDerivedDataLimitKeyLengthWrapper: Short file or Hash Collision, ignoring and deleting %s."), CacheKey);
					bOk	= false;
				}
				else
				{
					int32 Compare = FCStringAnsi::Strcmp(TCHAR_TO_ANSI(CacheKey), (char*)&OutData[OutData.Num() - KeyLen]);
					OutData.RemoveAt(OutData.Num() - KeyLen, KeyLen);
					if (Compare == 0)
					{
						UE_LOG(LogDerivedDataCache, Verbose, TEXT("FDerivedDataLimitKeyLengthWrapper: cache hit, key match is ok %s"), CacheKey);
					}
					else
					{
						UE_LOG(LogDerivedDataCache, Warning, TEXT("FDerivedDataLimitKeyLengthWrapper: HASH COLLISION, ignoring and deleting %s."), CacheKey);
						bOk	= false;
					}
				}
				if (!bOk)
				{
					// _we_ detected corruption, so _we_ will force a flush of the corrupted data
					InnerBackend->RemoveCachedData(*NewKey, /*bTransient=*/ false);
				}
			}
		}
		if (!bOk)
		{
			OutData.Empty();
		}
		return bOk;
	}
	/**
	 * Asynchronous, fire-and-forget placement of a cache item
	 *
	 * @param	CacheKey	Alphanumeric+underscore key of this cache item
	 * @param	InData		Buffer containing the data to cache, can be destroyed after the call returns, immediately
	 * @param	bPutEvenIfExists	If true, then do not attempt skip the put even if CachedDataProbablyExists returns true
	 */
	virtual void PutCachedData(const TCHAR* CacheKey, TArray<uint8>& InData, bool bPutEvenIfExists) override
	{
		if (!InnerBackend->IsWritable())
		{
			return; // no point in continuing down the chain
		}
		FString NewKey;
		if (!ShortenKey(CacheKey, NewKey))
		{
			// no shortening needed
			InnerBackend->PutCachedData(CacheKey, InData, bPutEvenIfExists);
			return;
		}
		TArray<uint8> Data(InData);
		check(Data.Num());
		int32 KeyLen = FCString::Strlen(CacheKey) + 1;
		Data.AddUninitialized(KeyLen);
		FCStringAnsi::Strcpy((char*)&Data[Data.Num() - KeyLen], KeyLen, TCHAR_TO_ANSI(CacheKey));
		check(Data.Last()==0);
		InnerBackend->PutCachedData(*NewKey, Data, bPutEvenIfExists);
	}

	virtual void RemoveCachedData(const TCHAR* CacheKey, bool bTransient) override
	{
		if (!InnerBackend->IsWritable())
		{
			return; // no point in continuing down the chain
		}
		FString NewKey;
		ShortenKey(CacheKey, NewKey);
		return InnerBackend->RemoveCachedData(*NewKey, bTransient);
	}
private:

	/** Shorten the cache key and return true if shortening was required **/
	bool ShortenKey(const TCHAR* CacheKey, FString& Result)
	{
		Result = FString(CacheKey);
		if (Result.Len() <= MaxKeyLength)
		{
			return false;
		}

		FSHA1 HashState;
		int32 Length = Result.Len();
		HashState.Update((const uint8*)&Length, sizeof(int32));

		auto ResultSrc = StringCast<UCS2CHAR>(*Result);
		uint32 CRCofPayload(FCrc::MemCrc32(ResultSrc.Get(), Length * sizeof(UCS2CHAR)));

		HashState.Update((const uint8*)&CRCofPayload, sizeof(uint32));
		HashState.Update((const uint8*)ResultSrc.Get(), Length * sizeof(UCS2CHAR));

		HashState.Final();
		uint8 Hash[FSHA1::DigestSize];
		HashState.GetHash(Hash);
		FString HashString = BytesToHex(Hash, FSHA1::DigestSize);

		int32 HashStringSize = HashString.Len();
		int32 OriginalPart = MaxKeyLength - HashStringSize - 2;
		Result = Result.Left(OriginalPart) + TEXT("__") + HashString;
		check(Result.Len() == MaxKeyLength && Result.Len() > 0);
		return true;
	}

	/** Backend to use for storage, my responsibilities are about key length **/
	FDerivedDataBackendInterface* InnerBackend;

	/** Maximum size of the backend key length **/
	int32 MaxKeyLength;
};