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  TCompressStream = class(TObject)
  private
    FSignature: array[0..5] of Char;
  public
    constructor Create; virtual;
    procedure Compress(InStream, OutStream: TStream; InSize: LongInt);
    procedure DeCompress(InStream, OutStream: TStream);
  end;

{ TCompressStream }

constructor TCompressStream.Create;
begin
  FSignature := 'XY2008';
end;

procedure TCompressStream.Compress(InStream, OutStream: TStream; InSize: LongInt);
var InBuffer, OutBuffer: BufferArray;
  CompressedSize, BytesRead, FinalPos, SizePos, TotalSize: LongInt;
begin
  TotalSize := 0;
  OutStream.WriteBuffer(FSignature, SizeOf(FSignature));
  SizePos := OutStream.Position;
  OutStream.WriteBuffer(TotalSize, SizeOf(TotalSize));
  while InSize > 0 do
  begin
    BytesRead := InStream.Read(InBuffer, SizeOf(InBuffer));
    CompressedSize := Compression(@InBuffer, @OutBuffer, BytesRead);
    OutStream.WriteBuffer(CompressedSize, SizeOf(CompressedSize));
    OutStream.WriteBuffer(OutBuffer, CompressedSize);
    TotalSize := TotalSize + CompressedSize + SizeOf(CompressedSize);
    InSize := InSize - BytesRead;
  end;
  FinalPos := OutStream.Position;
  OutStream.Position := SizePos;
  OutStream.WriteBuffer(TotalSize, SizeOf(TotalSize));
  OutStream.Position := FinalPos;
end;

procedure TCompressStream.DeCompress(InStream, OutStream: TStream);
var InBuffer, OutBuffer: BufferArray;
    CompressedSize, UnCompressedSize, InSize: LongInt;
    Sig: array[0..SizeOf(FSignature)-1] of Char;
begin
  InStream.ReadBuffer(Sig, SizeOf(FSignature));
  if Sig <> FSignature then
    raise Exception.Create('Wrong file type');
  InStream.ReadBuffer(InSize, SizeOf(InSize));
  while InSize > 0 do
  begin
    InStream.ReadBuffer(CompressedSize, SizeOf(CompressedSize));
    InStream.ReadBuffer(InBuffer, CompressedSize);
    UnCompressedSize := DeCompression(@InBuffer, @OutBuffer, CompressedSize);
    OutStream.WriteBuffer(OutBuffer, UnCompressedSize);
    InSize := InSize - CompressedSize - SizeOf(CompressedSize);
  end;
end;					

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    DataFileStream.Position := 0;
    with TCompressStream.Create do
      try
        DeCompress(DataFileStream, DataStream);
      finally
        Free; // with TCompressStream.Create
        DataFileStream.Free;
      end;