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 (TODO)
  bool finalStep() {
    false;
  }
}

Began life as a copy of #1034983