Delphi ZLib Compress / Decompress - delphi

Question

I've got a minor issue with decompressing using the ZLib unit in Delphi
unit uZCompression;
interface
uses
uCompression;
type
TZZipCompression = class(TInterfacedObject, ICompression)
public
function DoCompression(aContent: TArray<Byte>): TArray<Byte>;
function DoDecompression(aContent: TArray<Byte>): TArray<Byte>;
function GetWindowsBits: Integer; virtual;
end;
TZGZipCompression = class(TZZipCompression)
function GetWindowsBits: Integer; override;
end;
implementation
uses
System.ZLib, System.Classes, uMxKxUtils;
{ TZCompression }
function TZZipCompression.DoCompression(aContent: TArray<Byte>): TArray<Byte>;
var
LContentStream, LOutputStream: TMemoryStream;
LCompressedStream: TZCompressionStream;
begin
LContentStream := ByteArrayToStream(aContent);
LOutputStream := TMemoryStream.Create;
LCompressedStream := TZCompressionStream.Create(LOutputStream, zcDefault, GetWindowsBits);
LCompressedStream.CopyFrom(LContentStream, LContentStream.Size);
LCompressedStream.Free;
Result := StreamToByteArray(LOutputStream);
LOutputStream.Free;
LContentStream.Free;
end;
function TZZipCompression.DoDecompression(aContent: TArray<Byte>): TArray<Byte>;
var
LContentStream, LOutputStream: TMemoryStream;
LDecompressedStream: TZDecompressionStream;
begin
LContentStream := ByteArrayToStream(aContent);
LOutputStream := TMemoryStream.Create;
LDecompressedStream := TZDecompressionStream.Create(LContentStream);
LOutputStream.CopyFrom(LDecompressedStream, LDecompressedStream.Size);
LDecompressedStream.Free;
Result := StreamToByteArray(LOutputStream);
LOutputStream.Free;
LContentStream.Free;
end;
function TZZipCompression.GetWindowsBits: Integer;
begin
Result := 15;
end;
{ TZGZipCompression }
function TZGZipCompression.GetWindowsBits: Integer;
begin
Result := inherited;
Result := Result + 16;
end;
end.
This is my unit which is driven by an interface (which you don't need to know about), and data is passed in and out through a TArray variables.
I've coded it to be able to do 2 types of compression, standard zip and gzip which is determined by the windowsbits passed in to the functions.
Here are a couple of other functions being used to convert the TArray to TMemoryStream
function ByteArrayToStream(aContent: TArray<Byte>): TMemoryStream;
begin
Result := TMemoryStream.Create;
Result.Write(aContent, length(aContent)*SizeOf(aContent[0]));
Result.Position := 0;
end;
function StreamToByteArray(aStream: TMemoryStream): TArray<Byte>;
var
LStreamPos: Int64;
begin
if Assigned(aStream) then
begin
LStreamPos := aStream.Position;
aStream.Position := 0;
SetLength(Result, aStream.Size);
aStream.Read(Result, aStream.Size);
aStream.Position := LStreamPos;
end
else
SetLength(Result, 0);
end;
Now I can compress and decompress to .zip using the TZZipCompression class perfectly fine (it doesn't open up as a zip file, but it does decompress back to the original file which I can open and edit).
I can also compress to .gz using the TZGZipCompression class fine as well (interestingly I can open this gzip file perfectly well).
My issue however is that it won't decompress back from the .gz file and throws and error as soon as it hits
LOutputStream.CopyFrom(LDecompressedStream, LDecompressedStream.Size)
Funnily enough the Help file example has it as below
LOutputStream.CopyFrom(LDecompressedStream, 0)
But this doesn't work either.
Can anyone spot the issue?

Answer 1

Your conversion functions between TArray<Byte> and TMemoryStream are wrong, as you are not accessing the array content correctly. TArray is a dynamic array. When calling TMemoryStream.Write() and TMemoryStream.Read(), you are passing the memory address of the TArray itself, not the memory address of the data that the TArray points at. You need to reference the TArray to get the correct memory address, eg:
function ByteArrayToStream(const aContent: TArray<Byte>): TMemoryStream;
begin
Result := TMemoryStream.Create;
try
if Length(aContent) > 0 then
Result.WriteBuffer(aContent[0], Length(aContent));
Result.Position := 0;
except
Result.Free;
raise;
end;
end;
function StreamToByteArray(aStream: TMemoryStream): TArray<Byte>;
begin
if Assigned(aStream) then
begin
SetLength(Result, aStream.Size);
if Length(Result) > 0 then
Move(aStream.Memory^, Result[0], aStream.Size);
end
else
SetLength(Result, 0);
end;
Alternatively:
function ByteArrayToStream(const aContent: TArray<Byte>): TMemoryStream;
begin
Result := TMemoryStream.Create;
try
Result.WriteBuffer(PByte(aContent)^, Length(aContent));
Result.Position := 0;
except
Result.Free;
raise;
end;
end;
function StreamToByteArray(aStream: TMemoryStream): TArray<Byte>;
begin
if Assigned(aStream) then
begin
SetLength(Result, aStream.Size);
Move(aStream.Memory^, PByte(Result)^, aStream.Size);
end
else
SetLength(Result, 0);
end;
That being said, you don't need to waste memory making copies of the array data using TMemoryStream. You can use TBytesStream instead (since dynamic arrays are reference counted), eg:
function TZZipCompression.DoCompression(aContent: TArray<Byte>): TArray<Byte>;
var
LContentStream, LOutputStream: TBytesStream;
LCompressedStream: TZCompressionStream;
begin
LContentStream := TBytesStream.Create(aContent);
try
LOutputStream := TBytesStream.Create(nil);
try
LCompressedStream := TZCompressionStream.Create(LOutputStream, zcDefault, GetWindowsBits);
try
LCompressedStream.CopyFrom(LContentStream, 0);
finally
LCompressedStream.Free;
end;
Result := Copy(LOutputStream.Bytes, 0, LOutputStream.Size);
finally
LOutputStream.Free;
end;
finally
LContentStream.Free;
end;
end;
function TZZipCompression.DoDecompression(aContent: TArray<Byte>): TArray<Byte>;
var
LContentStream, LOutputStream: TBytesStream;
LDecompressedStream: TZDecompressionStream;
begin
LContentStream := TBytesStream.Create(aContent);
try
LOutputStream := TBytesStream.Create(nil);
try
LDecompressedStream := TZDecompressionStream.Create(LContentStream, GetWindowsBits);
try
LOutputStream.CopyFrom(LDecompressedStream, 0);
finally
LDecompressedStream.Free;
end;
Result := Copy(LOutputStream.Bytes, 0, LOutputStream.Size);
finally
LOutputStream.Free;
end;
finally
LContentStream.Free;
end;
end;