// from https://rosettacode.org/wiki/Zhang-Suen_thinning_algorithm#Java

// each array element is one pixel (0 = background, anything else = foreground)

srecord noeq ZhangSuenThinner_byteArray(int w, int h, byte[] pixels) {

  static final int[][] nbrs = {{0, -1}, {1, -1}, {1, 0}, {1, 1}, {0, 1},

    {-1, 1}, {-1, 0}, {-1, -1}, {0, -1}};

  static final int[][][] nbrGroups = {

    {{0, 2, 4}, {2, 4, 6}},

    {{0, 2, 6}, {0, 4, 6}}};

  new PtBuffer toWhite;

  run {

    bool firstStep;

    while ping (true) {

      firstStep = !firstStep;

      for (int r = 1; r < h - 1; r++)

        for (int c = 1; c < w - 1; c++) {

          if (!getPixel(c, r))

            continue;

          int nn = numNeighbors(r, c);

          if (nn < 2 || nn > 6)

            continue;

          if (numTransitions(r, c) != 1)

            continue;

          if (!atLeastOneIsWhite(r, c, firstStep ? 0 : 1))

            continue;

          toWhite.add(c, r);

        }

      if (!firstStep && empty(toWhite)) break;

      for (Pt p : toWhite)

        clearPixel(p.x, p.y);

      toWhite.clear();

    }

  }

  int numNeighbors(int r, int c) {

    int count = 0;

    for (int i = 0; i < nbrs.length - 1; i++)

      if (getPixel(c + nbrs[i][0], r + nbrs[i][1]))

        count++;

    ret count;

  }

  int numTransitions(int r, int c) {

    int count = 0;

    for (int i = 0; i < nbrs.length - 1; i++)

      if (!getPixel(c + nbrs[i][0], r + nbrs[i][1])

        && getPixel(c + nbrs[i+1][0], r + nbrs[i+1][1]))

          count++;

    ret count;

  }

  bool atLeastOneIsWhite(int r, int c, int step) {

    int count = 0;

    int[][] group = nbrGroups[step];

    for (int i = 0; i < 2; i++)

      for (int j = 0; j < group[i].length; j++) {

        int[] nbr = nbrs[group[i][j]];

        if (!getPixel(c + nbr[0], r + nbr[1])) {

          count++;

          break;

        }

      }

    ret count > 1;

  }

  bool getPixel(int x, int y) {

    ret pixels[y*w+x] != 0;

  }

  void clearPixel(int x, int y) {

    pixels[y*w+x] = 0;

  }

}