persistable sclass HexagonalBWIntegralImage {
  final int w, h;
  
  // data[0] is the sharp edge, data[1] the blunt edge
  final int[2][] data;
  
  // the column sums in three directions
  // north, north west, north east
  final int[][] columnSums = new int[3][];
  
  *(BufferedImage img) { this(BWImage img); }
  
  *(BWImage img) {
    w = img.w();
    h = img.h();
    for i to 2:
      data[0] = new int[w*h];
    columnSums[0] = new int[h];
    int i = 0;
    int lastShift = 0;
    for y to h: {
      int sum = 0;
      int shift = iround(y*sqrtAThird());
      int inc = shift-lastShift;
      for x to w: {
        // grab pixels
        int pixel = img.getInt(x, y);
        sum += pixel;

        // calc wedges
        data[0][i] = data[1][i] = sum;

        if (y != 0) {
          if (x >= inc)
            data[0][i] += data[0][i-w-inc];

          if (x+inc < w)
            data[1][i] += data[1][i-w+inc];
        }
          
        i++;
      }
      
      columnSums[0][y] = sum;
    }
    
    for i over columnSums:
      columnSums[i] = new int[max(w, h)*2]; // TODO: find actual upper bound
      
    int i = 0, j = 0, sumR = 0, sumG = 0, sumB = 0;
    for x to w: {
      int rgb = pixels[j++];
      data[i++] = (sumR += (rgb >> 16) & 0xFF);
      data[i++] = (sumG += (rgb >> 8) & 0xFF);
      data[i++] = (sumB += rgb & 0xFF);
    }
    for (int y = 1; y < h; y++) {
      sumR = sumG = sumB = 0;
      for x to w: {
        int rgb = pixels[j++];
        sumR += (rgb >> 16) & 0xFF;
        sumG += (rgb >> 8) & 0xFF;
        sumB += rgb & 0xFF;
        data[i] = sumR + data[i-w*3];
        data[i+1] = sumG + data[i-w*3+1];
        data[i+2] = sumB + data[i-w*3+2];
        i += 3;
      }
    }
  }
  
  // gets the integral value at x/y for given RGB channel
  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 "HexagonalIntegralImage " + w + "*" + h + ", brightness: " + averageBrightness();
  }
  
  public int getWidth() { ret w; }
  public int getHeight() { ret h; }
  int w() { ret w; }
  int h() { ret h; }
}