delphi - 如何吃掉从流中流出的字节？

Question

我正在修复一个ZIP库类。在内部，几乎所有 ZIP 实现都使用 DEFLATE compression (RFC1951) 。

问题是，在 Delphi 中，我无法访问任何 DEFLATE 压缩库。但我们确实拥有大量的一件事是 ZLIB compression code (RFC1950) 。它甚至随 Delphi 一起提供，并且还有六种其他实现。

在内部，ZLIB 还使用 DEFLATE 进行压缩。所以我想做每个人都做过的事情 - 使用 Delphi zlib 库来实现它的 DEFLATE 压缩功能。

问题是 ZLIB 在 DEFLATED 数据中添加了 2 字节前缀和 4 字节尾部:

[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 ，具体取决于您的源代码)的方法使用)流来压缩数据:

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;

这确实有效，只不过它写出了前导 2 字节和尾随 4 字节；我需要把它们去掉。

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;

该流重写 write 方法。吃掉前 2 个字节是很简单的。诀窍是吃掉尾随的 4 字节。

吃者流有一个 4 字节数组，我总是在缓冲区中保存每次写入的最后四个字节。当 EaterStream 被销毁时，尾随的四个字节也随之消失。

问题在于，通过此缓冲区进行数百万次写入会降低性能。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“末日已近”。我只是想让它更快。

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？鉴于您不知道最后一次写入将在哪一刻发生。

我正在修复的代码将整个压缩版本写入 TStringStream，然后仅抓取 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);

除非这会导致用户内存不足。最初我将其更改为写入 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，而不需要任何中间存储。

我的实现

并不是说它有什么帮助；因为我不需要一个甚至使用适应流来进行修剪的系统

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, FBuffer[0], 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^, FBuffer[0], 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, by definition, 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, 2, 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, 3333', 'Hello, ');
    Test('11Hello, W3333', 'Hello, W');
    Test('11Hello, Wo3333', 'Hello, Wo');
    Test('11Hello, Wor3333', 'Hello, Wor');
    Test('11Hello, Worl3333', 'Hello, Worl');
    Test('11Hello, World3333', 'Hello, World');
    Test('11Hello, World!3333', 'Hello, World!');
end;

Answer 1

只需要求 zlib 不包装 deflate 流即可避免整个问题。我在问题的代码中没有看到 zlib 的接口(interface)，但在某个地方使用 deflateInit() 或 deflateInit2() 进行了初始化。如果您使用 deflateInit2()，则可以为要请求的 windowBits 参数提供 -15 而不是 15展开的放气输出。