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][];
  
  int offset1;
  
  final static double slant = sqrtAThird();
  
  *(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*slant);
      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;
    }
    
    offset1 = iceil(h*slant);
    columnSums[1] = new int[offset1+w];
    //columnSums[2] = ...
    
    int end = -h*slant;
    for (int x = w-1; x >= -offset1; x--) {
      int y = 0;
      double xfrac = x;
      do {
        int x2 = iround(xfrac);
        if (x2 >= w)
          break;
        sum += img.getInt(x2, y);
        y++;
        xfrac += slant;
      }
      columnSums[1][x-offset1] = sum;
    }
    
    // TODO: columnSums[2]
  }
  
  public int getWidth() { ret w; }
  public int getHeight() { ret h; }
  int w() { ret w; }
  int h() { ret h; }
}