我正在修复 ZIP 库类。在内部，几乎所有ZIP实现都使用DEFLATE compression (RFC1951)。
问题在于，在Delphi中，我无法访问任何DEFLATE压缩库。但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可供使用。
在内部，ZLIB也使用DEFLATE进行压缩。所以我想做大家所做的事情 - 使用Delphi zlib 库来实现 DEFLATE 压缩功能。
问题是ZLIB在 DEFLATED 数据中添加了2字节前缀和4字节预告片：
[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;我需要去掉那些。
所以我写了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;
此流会覆盖write方法。吃第一个2字节是微不足道的。诀窍是吃掉尾随的4字节。
eater流有一个4字节的数组，我总是保存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去。
问题是通过这个缓冲区改写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指出“结束就在附近”。我只是希望它更快。
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);
除了使用户内存不足。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。
不是说它对任何事都有帮助;因为我不是要求一个甚至使用适应流来修剪的系统
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;
答案 0 :(得分：10)
只需要求zlib不包装deflate流就可以避免整个问题。我没有在问题的代码中看到zlib的接口，但在某处使用deflateInit()或deflateInit2()进行初始化。如果您使用deflateInit2()，则可以为-15参数提供15而不是windowBits来请求展开的deflate输出。
答案 1 :(得分：2)
您需要推迟写入，直到您确定要知道要写入的字节不是必须吃掉的尾随字节。这一观察结果使您认为缓冲将提供解决方案。
所以，我建议这样做：
上述方法要求的一个要求是缓冲区的大小必须大于要剥离的尾随字节数。