delphi – 如何吃流出流的字节？

问题是,在Delphi中,我无法访问任何DEFLATE压缩库.但是我们做的一件事就是ZLIB compression code (RFC1950).它甚至还附带Delphi,还有其他六种实现方式.

在内部,ZLIB也使用DEFLATE进行压缩.所以我想做每个人都做过的事情 – 使用Delphi zlib库来实现其DEFLATE压缩功能.

问题是ZLIB在DEFLATED数据中添加了一个2字节的前缀和4字节的尾部：

[CMF]                1 byte
[FLG]                1 byte
[...deflate compressed data...]
[Adler-32 checksum]  4 bytes

所以我需要的是一种使用标准TCompressionStream(或TZCompressionStream,或TZCompressionStreamEx,取决于您正在使用的源代码)流来压缩数据的方法：

procedure CompressDataToTargetStream(sourceStream: TStream; targetStream: TStream);
var
   compressor: TCompressionStream;
begin
   compressor := TCompressionStream.Create(clDefault,targetStream); //clDefault = CompressionLevel
   try
      compressor.CopyFrom(sourceStream,sourceStream.Length)
   finally
      compressor.Free; 
   end;
end;

这是有效的,除了它写出前导2字节和尾随4字节;我需要去除那些.

所以我写了一个TByteEaterStream：

TByteEaterStream = class(TStream)
public
   constructor Create(TargetStream: TStream; 
         LeadingBytesToEat,TrailingBytesToEat: Integer);
end;

例如

procedure CompressDataToTargetStream(sourceStream: TStream; targetStream: TStream);
var
   byteEaterStream: TByteEaterStream;
   compressor: TCompressionStream;
begin
   byteEaterStream := TByteEaterStream.Create(targetStream,2,4); //2 leading bytes,4 trailing bytes
   try
      compressor := TCompressionStream.Create(clDefault,byteEaterStream); //clDefault = CompressionLevel
      try
         compressor.CopyFrom(sourceStream,sourceStream.Length)
      finally
         compressor.Free; 
      end;
   finally
      byteEaterStream.Free;
   end;
end;

此流将覆盖write方法.吃前2个字节是微不足道的.诀窍是吃掉4个字节.

食者流有一个4字节的数组,我总是保持缓冲区中每次写入的最后四个字节.当EaterStream被销毁时,尾随的四个字节随之而来.

问题是通过这个缓冲区洗几百万次写入会破坏性能.上游的典型用途是：

for each of a million data rows
    stream.Write(s,Length(s)); //30-90 character string

我绝对不希望上游用户必须表明“结束就在附近”.我只是希望它更快.

问题

观察流过的字节流,保留最后四个字节的最佳方法是什么;鉴于你不知道什么时候写作将是最后一次.

我正在修复的代码将整个压缩版本写入TStringStream,然后只抓取900MB – 6个字节来获取内部DEFLATE数据：

cs := TStringStream.Create('');
....write compressed data to cs
S := Copy(CS.DataString,3,Length(CS.DataString) - 6);

除了运行用户内存不足.最初我改变它以写入TFileStream,然后我可以执行相同的技巧.

但我想要更好的解决方案;流解决方案.我希望数据进入压缩的最终流,没有任何中间存储.

我的实施

并不是说它有所帮助;因为我不是要求系统甚至使用适应流来进行修剪

TByteEaterStream = class(TStream)
private
    FTargetStream: TStream;
    FTargetStreamOwnership: TStreamOwnership;
    FLeadingBytesToEat: Integer;
    FTrailingBytesToEat: Integer;
    FLeadingBytesRemaining: Integer;

    FBuffer: array of Byte;
    FValidBufferLength: Integer;
    function GetBufferValidLength: Integer;
public
    constructor Create(TargetStream: TStream; LeadingBytesToEat,TrailingBytesToEat: Integer; StreamOwnership: TStreamOwnership=soReference);
    destructor Destroy; override;

    class procedure SelfTest;

    procedure Flush;

    function Read(var Buffer; Count: Longint): Longint; override;
    function Write(const Buffer; Count: Longint): Longint; override;
    function Seek(Offset: Longint; Origin: Word): Longint; override;
end;

{ TByteEaterStream }

constructor TByteEaterStream.Create(TargetStream: TStream; LeadingBytesToEat,TrailingBytesToEat: Integer; StreamOwnership: TStreamOwnership=soReference);
begin
    inherited Create;

    //User requested state
    FTargetStream := TargetStream;
    FTargetStreamOwnership := StreamOwnership;
    FLeadingBytesToEat := LeadingBytesToEat;
    FTrailingBytesToEat := TrailingBytesToEat;

    //internal housekeeping
    FLeadingBytesRemaining := FLeadingBytesToEat;

    SetLength(FBuffer,FTrailingBytesToEat);
    FValidBufferLength := 0;
end;

destructor TByteEaterStream.Destroy;
begin
    if FTargetStreamOwnership = soOwned then
        FTargetStream.Free;
    FTargetStream := nil;

    inherited;
end;

procedure TByteEaterStream.Flush;
begin
    if FValidBufferLength > 0 then
    begin
        FTargetStream.Write(FBuffer[0],FValidBufferLength);
        FValidBufferLength  := 0;
    end;
end;

function TByteEaterStream.Write(const Buffer; Count: Integer): Longint;
var
    newStart: Pointer;
    totalCount: Integer;
    addIndex: Integer;
    bufferValidLength: Integer;
    bytesToWrite: Integer;
begin
    Result := Count;

    if Count = 0 then
        Exit;

    if FLeadingBytesRemaining > 0 then
    begin
        newStart := Addr(Buffer);
        Inc(Cardinal(newStart));
        Dec(Count);
        Dec(FLeadingBytesRemaining);
        Result := Self.Write(newStart^,Count)+1; //tell the upstream guy that we wrote it

        Exit;
    end;

    if FTrailingBytesToEat > 0 then
    begin
        if (Count < FTrailingBytesToEat) then
        begin
            //There's less bytes incoming than an entire buffer
            //But the buffer might overfloweth
            totalCount := FValidBufferLength+Count;

            //If it could all fit in the buffer,then let it
            if (totalCount <= FTrailingBytesToEat) then
            begin
                Move(Buffer,FBuffer[FValidBufferLength],Count);
                FValidBufferLength := totalCount;
            end
            else
            begin
                //We're going to overflow the buffer.

                //Purge from the buffer the amount that would get pushed
                FTargetStream.Write(FBuffer[0],totalCount-FTrailingBytesToEat);

                //Shuffle the buffer down (overlapped move)
                bufferValidLength := bufferValidLength - (totalCount-FTrailingBytesToEat);
                Move(FBuffer[totalCount-FTrailingBytesToEat],FBuffer[0],bufferValidLength);

                addIndex := bufferValidLength ; //where we will add the data to
                Move(Buffer,FBuffer[addIndex],Count);
            end;
        end
        else if (Count = FTrailingBytesToEat) then
        begin
            //The incoming bytes exactly fill the buffer. Flush what we have and eat the incoming amounts
            Flush;
            Move(Buffer,FTrailingBytesToEat);
            FValidBufferLength := FTrailingBytesToEat;
            Result := FTrailingBytesToEat; //we "wrote" n bytes
        end
        else
        begin
            //Count is greater than trailing buffer eat size
            Flush;

            //Write the data that definitely not to be eaten
            bytesToWrite := Count-FTrailingBytesToEat;
            FTargetStream.Write(Buffer,bytesToWrite);

            //Buffer the remainder
            newStart := Addr(Buffer);
            Inc(Cardinal(newStart),bytesToWrite);

            Move(newStart^,FTrailingBytesToEat);
            FValidBufferLength := 4;
        end;
    end;
end;

function TByteEaterStream.Seek(Offset: Integer; Origin: Word): Longint;
begin
    //what does it mean if they want to seek around when i'm supposed to be eating data?
    //i don't know; so results are,by definition,undefined. Don't use at your own risk
    Result := FTargetStream.Seek(Offset,Origin);
end;

function TByteEaterStream.Read(var Buffer; Count: Integer): Longint;
begin
    //what does it mean if they want to read back bytes when i'm supposed to be eating data?
    //i don't know; so results are,undefined. Don't use at your own risk
    Result := FTargetStream.Read({var}Buffer,Count);
end;

class procedure TByteEaterStream.SelfTest;

    procedure CheckEquals(Expected,Actual: string; Message: string);
    begin
        if Actual <> Expected then
            raise Exception.CreateFmt('TByteEaterStream self-test failed. Expected "%s",but was "%s". Message: %s',[Expected,Actual,Message]);
    end;

    procedure Test(const InputString: string; ExpectedString: string);
    var
        s: TStringStream;
        eater: TByteEaterStream;
    begin
        s := TStringStream.Create('');
        try
            eater := TByteEaterStream.Create(s,4,soReference);
            try
                eater.Write(InputString[1],Length(InputString));
            finally
                eater.Free;
            end;
            CheckEquals(ExpectedString,s.DataString,InputString);
        finally
            s.Free;
        end;
    end;
begin
    Test('1','');
    Test('11','');
    Test('113','');
    Test('1133','');
    Test('11333','');
    Test('113333','');
    Test('11H3333','H');
    Test('11He3333','He');
    Test('11Hel3333','Hel');
    Test('11Hell3333','Hell');
    Test('11Hello3333','Hello');
    Test('11Hello,3333','Hello,');
    Test('11Hello,W3333',W');
    Test('11Hello,Wo3333',Wo');
    Test('11Hello,Wor3333',Wor');
    Test('11Hello,Worl3333',Worl');
    Test('11Hello,World3333',World');
    Test('11Hello,World!3333',World!');
end;