unit DCT;

// FIXME.

{$MODE OBJFPC}

interface
uses sysutils, classes, custom_decoders, type_fixes;

type
  // The input format, signed 12-bit integers.
  TMatrix = {bitpacked} array[0..7] of array[0..7] of ShortInt; // [row, column].

  TDecoder = class(custom_decoders.TCustomDirectDecoder)
  private
    //procedure BufferByte(aValue : Byte); inline;
    //procedure BufferTuple(aValue : Cardinal; aDecodedCount : Cardinal); inline; // decoding shrinks from 5 byte to 4 byte.
    //procedure Decode(aInput : Byte); inline;
  published
    procedure Finish(); override;
    function FinishedP() : Boolean; override;
  protected
    procedure DecodeBlock(aEncodedBuffer : PByte; aEncodedBufferCount : Cardinal); override;
    function GetPermittedReadCount(aBufferAvailableSpace : Cardinal) : Cardinal; override;
  end;
  
implementation

function IDCTTransform(const aInput : TMatrix) : TMatrix;
var
  vN : Integer;
  vM : Integer; 
  vXF : Integer;
  vYF : Integer;
  
  function CellValue() : Single; inline;
  var
    vIX : Integer; // input.
    vIY : Integer; // input.
  begin
    Result := 0;
    for vIY := 0 to vN - 1 do begin
      for vIX := 0 to vM - 1 do begin
        // 2x+1 => 1, 3, 5, ...
        Result := Result + aInput[vIY][vIX] * cos(vXF * vIX * pi / (2 * vM)) * cos(vYF * vIY * pi / (2 * vN));
        if (vIX = 0) and (vIY = 0) then
          Result := Result / 2.0; // *1² / SQRT(2.0)²;
      end;
    end;
  end;

// For a different way, see <http://users.powernet.co.uk/kienzle/octave/matcompat/scripts/signal/idct.m>.
var
  vX : Integer;
  vY : Integer;
  vScale : Single;
  vValue : Single;
begin
  vN := Length(aInput); // number of rows.
  vM := Length(aInput[0]); // number of columns.
  // FIXME assert vN power of 2, vM power of 2, >0.
  
  vScale := 2.0 / SQRT(vM * vN);

  vYF := 1; // 2y+1.
  
  for vY := 0 to vN - 1 do begin
    vXF := 1; // 2x+1 first value.
    for vX := 0 to vM - 1 do begin
      Result[vY][vX] := Trunc(vScale * CellValue());
      Inc(vXF, 2);
    end;
    Inc(vYF, 2);
  end;
  
  //vXF := (vIX shl 1) + 1;
  //vYF := (vIY shl 1) + 1;


// DCT-III (IDCT)
//    X_k = \frac{1}{2} x_0 + \sum_{n=1}^{N-1} x_n \cos \left[\frac{\pi}{N} n \left(k+\frac{1}{2}\right) \right] \quad \quad k = 0, \dots, N-1. 
{
  f(x,y) = (2 / sqrt(MN)) * sum[m := 0,M)sum[n := 0,N) C(m)C(n)F(m,n)cos((2x+1)m*pi/2M)*cos((2y+1)n*pi/2N)
  where F(m,n) is the DCT of the signal f(x,y) and C(m),C(n) = 1/sqrt(2) for m,n = 0 and C(m),C(n) = 1 otherwise.
  M power of 2.
  N power of 2.
}
end;

procedure TDecoder.Finish();
begin
end;

function TDecoder.FinishedP() : Boolean;
begin
  Result := True;
end;

procedure TDecoder.DecodeBlock(aEncodedBuffer : PByte; aEncodedBufferCount : Cardinal);
begin
  raise EReadError.Create('not implemented yet.');
end;

function TDecoder.GetPermittedReadCount(aBufferAvailableSpace : Cardinal) : Cardinal;
begin
  Result := 0;
end;

end.