MODULE GrayCode;

FROM STextIO IMPORT
  WriteString, WriteLn;
FROM SWholeIO IMPORT
  WriteInt;
FROM Conversions IMPORT
  LongBaseToStr;
FROM FormatString IMPORT
  FormatString; (* for justifying *)

VAR
  I, G, D: CARDINAL;
  Ok: BOOLEAN;
  BinS, OutBinS: ARRAY[0 .. 5] OF CHAR;

PROCEDURE Encode(V: CARDINAL): CARDINAL;
BEGIN
  RETURN V BXOR (V SHR 1)
END Encode;

PROCEDURE Decode(V: CARDINAL): CARDINAL;
VAR
  Result: CARDINAL;
BEGIN
  Result := 0;
  WHILE V > 0 DO
    Result := Result BXOR V;
    V := V SHR 1
  END;
  RETURN Result
END Decode;

BEGIN
  WriteString("decimal  binary   gray    decoded");
  WriteLn;
  FOR I := 0 TO 31 DO
    G := Encode(I);
    D := Decode(G);
    WriteInt(I, 4);
    WriteString("     ");
    Ok := LongBaseToStr(I, 2, BinS);
    Ok := FormatString("%'05s", OutBinS, BinS);
    (* Padded with 0; width: 5; type: string *)
    WriteString(OutBinS);
    WriteString("   ");
    Ok := LongBaseToStr(G, 2, BinS);
    Ok := FormatString("%'05s", OutBinS, BinS);
    WriteString(OutBinS);
    WriteString("   ");
    Ok := LongBaseToStr(D, 2, BinS);
    Ok := FormatString("%'05s", OutBinS, BinS);
    WriteString(OutBinS);
    WriteInt(D, 4);
    WriteLn;
  END
END GrayCode.