Hello,

I am trying to use TCompressionStream to compress a string. But the destination stream is empty. Here is by code, thanks:

program stringcompressor;
{$apptype console}
uses
  Classes, SysUtils, zstream;
var
  Compressor: TCompressionStream;
  Source: TStringStream;
  Dest: TMemoryStream;
  i: Integer;
begin
  Source := TStringStream.Create('The string to be compressed');
  Dest := TMemoryStream.Create();
  Compressor := TCompressionStream.Create(clDefault, Dest);
  try
    Compressor.CopyFrom(Source, Source.Size);
    Write('The result is: ');
    Dest.Position := 0;
    for i := 0 to Dest.Size - 1 do
      IntToHex(Dest.ReadByte, 2);
    WriteLn('');
  finally
    Compressor.Free;
    Dest.Free;
    Source.Free;
  end;
end.

What about use Compressor intead of Dest?

Joao Morais

for i := 0 to Dest.Size - 1 do IntToHex(Dest.ReadByte, 2);

Personally, I'd write() the strings that come from inttohex, instead of discarding them :_)

Marco van de Voort wrote

Personally, I'd write() the strings that come from inttohex, instead of discarding them :_)

Indeed ... but that doesn't change anything =) It doesn't enter the loop ... I checked with the debugger.

What about use Compressor intead of Dest?

What is the use for Dest then if not to hold the destination data?

Also, I cannot use the Compressor because I cannot seek it and I cannot read it. I can only write to it.

thanks,

Call Compressor.flush first, only then all data will be in dest. This is done automatically when it is destroyed:

So it should also work if you destroy compressor prior to examining the result.

Michael.

Michael Van Canneyt wrote

Call Compressor.flush first, only then all data will be in dest. This is done automatically when it is destroyed:

There is no Compressor.Flush. But freeing it seams to work.

Now I am having trouble using TDecompressor. Should I use CopyFrom from another stream to get the decompressed data? This crashes the app. Also, CopyFrom expects the size, but I have no size ... I just want everything!

program stringcompressor;
{$apptype console}
uses
  Classes, SysUtils, zstream;
const
  Str_Original_String = 'The string to be compressed';
var
  Compressor: TCompressionStream;
  Decompressor: TDecompressionStream;
  StringStream: TStringStream;
  CompressedData: TMemoryStream;
  i: Integer;
begin
  WriteLn('The string to be compressed is: ', Str_Original_String);
  StringStream := TStringStream.Create(Str_Original_String);
  CompressedData := TMemoryStream.Create();
  Compressor := TCompressionStream.Create(clDefault, CompressedData);
  Decompressor := TDecompressionStream.Create(CompressedData);
  { Compress the string }
  Compressor.CopyFrom(StringStream, StringStream.Size);
  Compressor.Free;
  { Write the compressed string to the console }
  Write('The compressed data is: ');
  CompressedData.Position := 0;
  for i := 0 to CompressedData.Size - 1 do
    Write(IntToHex(CompressedData.ReadByte, 2));
  WriteLn('');
  { Decompress the string }
  StringStream.Size := 0;
  StringStream.CopyFrom(Decompressor, Decompressor.Size); // <----  crash
  WriteLn('The string after being compressed and decompressed is: ',
StringStream.DataString);
  Decompressor.Free;
  CompressedData.Free;
  StringStream.Free;
end.

Michael Van Canneyt wrote

Copy & paste from zstream:

You are certainly not using FPC 2.2.0!!

You cannot use CopyFrom. You must do a read till there is no data anymore.

ok ... thanks

Copy & paste from zstream:

        Tcompressionstream=class(Tcustomzlibstream)
        protected
          raw_written,compressed_written:longint;
        public
          constructor create(level:Tcompressionlevel;
                             dest:Tstream;
                             Askipheader:boolean=false);
          destructor destroy;override;
          function write(const buffer;count:longint):longint;override;
          procedure flush; //// <-------------------- Flush ?
          function get_compressionrate:single;
        end;

You cannot use CopyFrom.
You must do a read till there is no data anymore.
See TStrings.LoadFromStream for instance.

Michael.

Michael Van Canneyt wrote

You cannot use CopyFrom. You must do a read till there is no data anymore. See TStrings.LoadFromStream for instance.

Indeed. TStrings.LoadFromStream works in the decompressor =)

thanks,

Correct, I'm using 2.2.2 or higher.

Michael.