image.h

/*
  This file is part of the Fairytale project

  Copyright (C) 2018 Márcio Pais

  This library is free software: you can redistribute it and/or modify
  it under the terms of the GNU Lesser General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.

  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public License
  along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#ifndef IMAGE_H
#define IMAGE_H

#include "stream.h"

class Image {
public:
  static bool isGrayscalePalette(Stream* input, const int n = 256, const bool isRGBA = false) {
    off_t offset = input->curPos();
    int stride=3+(int)isRGBA, res=(n>0)<<8, order=1;
    for (int i=0; i<n*stride && (res>>8)>0; i++) {
      int b = input->getChar();
      if (b==EOF){
        res = 0;
        break;
      }
      if (i==0) {
        res = 0x100|b;
        order = 1-2*(b>0);
        continue;
      }

      //"j" is the index of the current byte in this color entry
      int j = i%stride;
      if (j==0){
        // load first component of this entry
        res = (res&((b-(res&0xFF)==order)<<8));
        res|=(res)?b:0;
      }
      else if (j==3)
        res&=((b==0 || b==0xFF)<<9)-1; // alpha/attribute component must be zero or 0xFF
      else
        res&=((b==(res&0xFF))<<9)-1;
    }
    input->setPos(offset);
    return (res>>8)>0;
  }
};

#endif