srecord noeq RegionToGSI(IImageRegion region) extends Meta {
  Color color;
  L<SSI> ssis;
  gettable GSI gsi;

  private int x1, y;
  
  class GrowingSSI {
    settable int y1;
    new ShortBuffer data;
    
    int y2() { ret y1+l(data)/2; }
    
    SSI finish() {
      ret addAndReturn(ssis, new SSI(y1, y2()).data(data.toArray())
        .color(color));
    }
    
    bool isEmpty() { ret data.isEmpty(); }
    int lastx1() { ret data.get(l(data)-2); }
    int lastx2() { ret data.get(l(data)-1); }
    IntRange lastRange() { ret isEmpty() ? null : intRange(lastx1(), lastx2()); }
    
    // can add this line without breaking coherence?
    bool canAdd(IntRange r) {
      ret isEmpty()
        || intRangesOverlapNempty(r.start, r.end, lastx1()-diag(), lastx2()+diag());
    }
    
    void add(IntRange r) {
      if (checkCoherence() && !canAdd(r))
        fail("Coherence fail", +lastx1(), +lastx2(), +r);
      
      data.add(toShort_enforce(r.start));
      data.add(toShort_enforce(r.end));
    }
  }
  
  void finishSSI(int iSSI) {
    growingSSIs.remove(iSSI).finish();
  }
  
  private GrowingSSI startSSI(int iSSI default l(growingSSIs), IntRange range) {
    if (scaffoldingEnabled()) printVars("startSSI", +iSSI, +range);
    var ssi = new GrowingSSI().y1(y);
    ssi.add(range);
    ret addAndReturn(growingSSIs, iSSI, ssi);
  }
  
  L<IntRange> lastStreaks() {
    ret reallyLazyMap(growingSSIs, -> .lastRange());
  }
  
  int diag() { ret withDiagonals ? 1 : 0; }
  
  L<SSI> get() {
    if (region == null) null;
    
    color = region.color();
    ssis = new L;
    Rect r = region.bounds();
    int x1 = this.x1 = r.x1(), y1 = r.y1(), y2 = r.y2(), h = y2-y1, w = r.w;
    bool scaff = scaffoldingEnabled();
    
    for (y = y1; y < y2; y++) {
      reMutable y;
      L<IntRange> streaks = shiftIntRanges(x1, genericStreaks(w, x -> region.contains(x1+x, y)));
      

        // add line to SSI
        growingSSIs.get(iSSI++).add(streaks.get(iStreak++));
      }
      
      // remaining SSIs are unpaired, close them
      while (iSSI < l(growingSSIs)) finishSSI(iSSI);
    }
    
    ret gsi = new GSI(y1, rowStarts, xData);
  }
}