persistable sclass HexagonalIntegralImage {

  final int w, h;

  // 3 ints per pixel. 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(RGBImage img); }

  *(RGBImage img) {

    w = img.w();

    h = img.h();

    data1 = new int[w*h*3];

    int i = 0;

    for y to h: {

      int sumR = 0, sumG = 0, sumB = 0;

      int shift = iround(y*sqrtAThird());

      for x to w: {

        // grab pixels

        int rgb = img.getInt(x, y);

        sumR += (rgb >> 16) & 0xFF;

        sumG += (rgb >> 8) & 0xFF;

        sumB += rgb & 0xFF;

        // calc wedges

        data[0][i] = sumR;

        data[0][i+1] = sumG;

        data[0][i+2] = sumB;

        if (y != 0) {

          if (x >= shift) {

            data[0][i] += data[0][i-(w+shift)*3];

            data[0][i+1] += data[0][i-(w+shift)*3+1];

            data[0][i+2] += data[0][i-(w+shift)*3+2];

          }

          if (x+shift < w) {

            data[1][i] += data[1][i-(w-shift)*3];

            data[1][i+1] += data[1][i-(w-shift)*3+1];

            data[1][i+2] += data[1][i-(w-shift)*3+2];

          }

        }

        i += 3;

      }

    }

    columnSums[0] = new int[h];

    for y to h:

      for x to w: {

        c

    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; }

}