NonBufferingReadOnlyArchive.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include "Containers/StringFwd.h"
#include "HAL/FileManager.h"
#include "Serialization/ArchiveUObject.h"
 
/*
* This archive uses non-buffered IO to avoid overhead caused by reading into the OS cache manager 
* rather than not reading into an archive owned buffer (it does that). It is intended to be 
* used when reads will occur in parallel, contention would be caused at an OS level to allocate cache pages, 
* and typically the whole file is not read sequentially but instead only a smaller portion of the file 
* is needed (otherwise OS caching likely performs better since it can predict more accurately without waste).
* This archive is intended to be re-usable by opening new files on top so the internal buffer can be re-used.
*/
class FNonBufferingReadOnlyArchive : public FArchive
{
public:
    FNonBufferingReadOnlyArchive(int64 InBufferSize = 4096, uint32 InFlags = 0);
    ~FNonBufferingReadOnlyArchive();
 
    // FArchive API
    virtual void Seek(int64 InPos) override final;
    virtual int64 Tell() override final
    {
        return Pos;
    }
    virtual int64 TotalSize() override final
    {
        return Size;
    }
    virtual bool Close() override final;
    virtual void Serialize(void* V, int64 Length) override final;
    virtual FString GetArchiveName() const override
    {
        return Filename;
    }
    virtual bool Precache(int64 PrecacheOffset, int64 PrecacheSize) override;
    //End FArchive API
 
    bool OpenFile(const TCHAR* InFilename);
 
protected:
    bool InternalPrecache(int64 PrecacheOffset, int64 PrecacheSize);
 
    bool SeekLowLevel(int64 InPos);
 
    void OpenFileLowLevel(const TCHAR* InFilename);
 
    void CloseLowLevel();
 
    void ReadLowLevel(uint8* Dest, int64 CountToRead, int64& OutBytesRead);
 
    bool IsSilent() const
    {
        return !!(Flags & EFileRead::FILEREAD_Silent);
    }
 
    FString Filename;
    int64 Size;
    int64 Pos;
    int64 BufferBase;
    TUniquePtr<IFileHandle> Handle;
 
    uint8* Buffer;
    int64 BufferReadAlignment;
    int64 BufferSize;
    uint32 Flags;
};