srecord noeq G22SkeletonWalker<Img extends WidthAndHeight>(IImageRegion<Img> region) is Steppable {

  event foundLink(Pt a, Pt b);

  settable int minDist = 3;

  // point to "registered" point close by

  new Map<Pt> ptMap;

  // TEMPORARY DATA

  bool initialized;

  // each pair is (registered point, point-to-explore)

  new LinkedList<Pair<Pt>> queue;

  public bool step() {

    // find first pixel

    if (!initialized) {

      set initialized;

      Pt p = region.firstPixel();

      if (p == null) false;

      queue.add(pair(null, p));

    }

    if (empty(queue)) false;

    Pair<Pt> pair = popFirst(queue);

    Pt prev = pair.a, p = pair.b;

    Pt mapped = lookupOrKeep(ptMap, p);

    bool seen = ptMap.containsKey(p);

    Pt mapTo = p;

    if (prev != null) {

      if (distantEnough(prev, mapped))

        foundLink(prev, mapped);

      else

        mapTo = prev;

    }

    if (!seen) {

      ptMap.put(p, mapTo);

      Pt linkFrom = mapTo;

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

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

        if (region.contains(p2))

          queue.add(pair(linkFrom, p2));

      }

    }

    true;

  }

  bool distantEnough(Pt a, Pt b) {

    ret a != null && b != null

      && (absDiff(a.x, b.x) >= minDist || absDiff(a.y, b.y) >= minDist);

  }

}