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sclass IntegralImage {
  int w, h;
  int[] data; // 3 ints per pixel
  
  *() {}
  *(BufferedImage img) { this(RGBImage(img)); }
  
  *(RGBImage img) {
    w = img.w();
    h = img.h();
    if (longMul(w, h) > 8000000)
      fail("Image too big: " + w + "*" + h);
    data = new int[w*h*3];
    int i = 0;
    for y to h: {
      int sumR = 0, sumG = 0, sumB = 0;
      for x to w: {
        int rgb = img.getInt(x, y);
        sumR += (rgb >> 16) & 0xFF;
        sumG += (rgb >> 8) & 0xFF;
        sumB += rgb & 0xFF;
        data[i] = y > 0 ? sumR + data[i-w*3] : sumR;
        data[i+1] = y > 0 ? sumG + data[i-w*3+1] : sumG;
        data[i+2] = y > 0 ? sumB + data[i-w*3+2] : sumB;
        i += 3;
      }
    }
  }
  
  int get(int x, int y, int channel) {
    ret x < 0 || y < 0 || x >= w || y >= h ? 0
      : data[(y*w+x)*3+channel];
  }
  
  // gets sum of the 3 channels
  int get(int x, int y) {
    if (x < 0 || y < 0 || x >= w || y >= h) ret 0;
    int i = (y*w+x)*3;
    ret data[i]+data[i+1]+data[i+2];
  }
  
  double averageBrightness() {
    ret doubleRatio(get(w-1, h-1), w*h*3*255.0);
  }
  
  toString {
    ret "IntegralImage " + w + "*" + h + ", brightness: " + averageBrightness();
  }
}