srecord noeq G22RegionThinner_v2<Img extends WidthAndHeight>(IImageRegion<Img> originalRegion) is Steppable {

  Rect bounds;

  IImageRegion<Img> region;

  bool phase1done;

  // 0 = outside of region

  // 1 = inner pixel

  // 2 = border pixel

  // index is in bounds coordinates

  byte[] pixels;

  int idx(int x, int y) {

    ret (y-bounds.y)*bounds.w+x-bounds.x;

  }

  int idx(Pt p) { ret idx(p.x, p.y); }

  Pt idxToPt(int idx) {

    ret pt(bounds.x+(idx % bounds.w), bounds.y+idx/bounds.w);

  }

  byte getPixel(Pt p) {

    ret !containsPt(bounds, p) ? 0 : pixels[idx(p)];

  }

  byte getPixel(int x, int y) {

    ret !containsPt(bounds, x, y) ? 0 : pixels[idx(x, y)];

  }

  bool contains(int x, int y) {

    ret getPixel(x, y) != 0;

  }

  bool clearPixel(int x, int y) {

    if (!region.contains(x, y)) false;

    pixels[idx(x, y)] = 0;

    true;

  }

  void init {

    if (bounds != null) ret;

    bounds = originalRegion.bounds();

    pixels = new byte[area(bounds)];

    for (Pt p : originalRegion.pixelIterator())

      pixels[idx(p.x, p.y)] = 1;

    region = new ThinnedRegion;

  }

  class ThinnedRegion is IImageRegion<Img> {

    public Img image() { ret originalRegion.image(); }

    public Rect bounds() { ret bounds; }

    public bool contains(int x, int y) {

      ret containsPt(bounds, x, y) && pixels[idx(x, y)] > 0;

    }

    public ItIt<Pt> pixelIterator() {

      ret iff_null(new IF0<Pt> {

        int idx = 0;

        public Pt get() {

          for (; idx < pixels.length; idx++)

            if (pixels[idx] > 0)

              ret idxToPt(idx++);

          null;

        }

      });

    }

  }

  public bool step() {

    init();

    if (phase1done)

      ret finalStep();

    L<PtBuffer> traces = g22_allBorderTraces_withDiagonals(region);

    for (points : traces)

      for (p : points)

        pixels[idx(p)] = 2;

    new PtBuffer toDelete;

    for (points : traces) {

      int nPoints = l(points);

      BitSet deletable = emptyBitSet(nPoints);

      for i to nPoints: {

        ping();

        if (deletableBorderPoint(points, i))

          deletable.set(i);

      }

      // handle special cases      

      for (int i = 1; i < nPoints-1; i++)

        if (deletable.get(i-1)

          && !deletable.get(i)

          && deletable.get(i+1)) {

          Pt p = points.get(i);

          Pt prev = ptMinus(points.get(i-1), p);

          Pt next = ptMinus(points.get(i+1), p);

          int dir1 = onePathLookupDirection(prev);

          int dir2 = onePathLookupDirection(next);

          int diff = mod(dir2-dir1, 8);

          printVars ifdef G22RegionThinner_debug(+p, +prev, +next, +dir1, +dir2, +diff);

          if (diff == 1 || diff == 7)

            deletable.set(i);

        }

      for i to nPoints:

        if (deletable.get(i))

          toDelete.add(points.get(i));

    }

    for (p : toDelete)

      pixels[idx(p)] = 0;

    if (empty(toDelete))

      set phase1done;

    true;

  }

  bool deletableBorderPoint(PtBuffer points, int i) {

    L<Pt> range = cyclicSubList_incl(points, i-3, i+3);

    Pt p = points.get(i);

    // special case

    if (eq(range.get(1), p) || eq(range.get(5), p)) false;

    int surroundingBorderPixels = 0, surroundingInnerPixels = 0;

    for (int dir = 1; dir <= 8; dir++) {

      Pt p2 = ptPlus(p, onePathDirection(dir));

      byte value = getPixel(p2);

      if (value == 2 && !range.contains(p2))

        surroundingBorderPixels++;

      else if (value == 1)

        surroundingInnerPixels++;

    }

    bool deletable = surroundingInnerPixels > 0 && surroundingBorderPixels == 0;

    printVars ifdef G22RegionThinner_debug(+p, +surroundingInnerPixels, +surroundingBorderPixels, +deletable, +range);

    ret deletable;

  }

  IImageRegion region aka get() { ret or(region, originalRegion); }

  // go from 2 pixels wide to 1 pixel wide 

  bool finalStep() {

    bool change;

    int x1 = bounds.x, x2 = bounds.x2();

    int y1 = bounds.y, y2 = bounds.y2();

    printVars ifdef G22RegionThinner_debug("finalStep", +x1, +y1, +x2, +y2);

    for (int y = y1; y < y2; y++)

      for (int x = x1; x < x2; x++) {

        if (!contains(x, y)) continue;

        // found a set pixel. check neighboorhod for thinning candidates

        bool right1 = contains(x+1, y), right2 = contains(x+2, y);

        bool down1 = contains(x, y+1), down2 = contains(x, y+2);

        printVars ifdef G22RegionThinner_debug(+x, +y, +right1, +right2, +down1, +down2);

        if (right1 && down1) {

          if (!right2)

            change |= clearPixel(x+1, y);

          else if (!down2)

            change |= clearPixel(x, y+1);

        }

      }

    ret change;

  }

}