unit GIFLZW;

// <http://www.martinreddy.net/gfx/2d/GIF-comp.txt>.

{$MODE OBJFPC}
{$M+}

interface
uses classes, type_fixes, custom_decoders, bitstreams, LZW;

{
GIF LZW:
  1. A special Clear code is defined which resets all compression/decompression parameters and tables to a start-up state. The value of this code is 2**<code size>. For example if the code size indicated was 4 (image was 4 bits/pixel) the Clear code value would be 16 (10000 binary). The Clear code can appear at any point in the image data stream and therefore requires the LZW algorithm to process succeeding codes as if a new data stream was starting.
  2. An End of Information code is defined that explicitly indicates the end of the image data stream. LZW processing terminates when this code is encountered. It must be the last code output by the encoder for an image. The value of this code is +1.
  3. The first available compression code value is <clear code>+2.
  4. The output codes are of variable length, starting at <code size>+1 bits per code, up to 12 bits per code. This defines a maximum code value of 4095 (0xFFF). Whenever the LZW code value would exceed the current code length, the code length is increased by one. The packing/unpacking of these codes must then be altered to reflect the new code length. 
}

type
  TDecoder = class(LZW.TDecoder)
  private
    fClearCode : TUINT16;
    fEOFCode : TUINT16;
  published
    constructor Create(aSource : TStream; abOwnsSource : Boolean; aCodeSize : TByte);
  protected
    function HandleSpecialCode(aCode : TUINT16) : TSpecialHandlingVerdict; override;
  end;

implementation
uses sysutils;

constructor TDecoder.Create(aSource : TStream; aBOwnsSource : Boolean; aCodeSize : TByte);
begin
  if aCodeSize > 12 then
    raise EFormatError.Create('root size > 12 is not supported by GIF. The file is damaged.');
    
  inherited Create(aSource, aBOwnsSource, aCodeSize { that's root size, actually }, 1, 13 {FIXME or is that 12?});
  IncreaseCodeSize();
  fClearCode := Self.AddTranslation(-1, 0);
  fEOFCode := Self.AddTranslation(-1, 0);
end;

function TDecoder.HandleSpecialCode(aCode : TUINT16) : TSpecialHandlingVerdict;
begin
  if aCode = fClearCode then
    Result := shReinitialize
  else if aCode = fEOFCode then
    Result := shStopParsing
  else
    Result := shNothingSpecial;
end;

end.