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;
最佳答案
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。
关于delphi - 如何吃掉从流中流出的字节？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/19818363/