Method SetWindCountForClosedPathEdge

CPP/Clipper2Lib/src/clipper.engine.cpp:1011–1086

  void ClipperBase::SetWindCountForClosedPathEdge(Active& e)
  {
    //Wind counts refer to polygon regions not edges, so here an edge's WindCnt
    //indicates the higher of the wind counts for the two regions touching the
    //edge. (NB Adjacent regions can only ever have their wind counts differ by
    //one. Also, open paths have no meaningful wind directions or counts.)

    Active* e2 = e.prev_in_ael;
    //find the nearest closed path edge of the same PolyType in AEL (heading left)
    PathType pt = GetPolyType(e);
    while (e2 && (GetPolyType(*e2) != pt || IsOpen(*e2))) e2 = e2->prev_in_ael;

    if (!e2)
    {
      e.wind_cnt = e.wind_dx;
      e2 = actives_;
    }
    else if (fillrule_ == FillRule::EvenOdd)
    {
      e.wind_cnt = e.wind_dx;
      e.wind_cnt2 = e2->wind_cnt2;
      e2 = e2->next_in_ael;
    }
    else
    {
      //NonZero, positive, or negative filling here ...
      //if e's WindCnt is in the SAME direction as its WindDx, then polygon
      //filling will be on the right of 'e'.
      //NB neither e2.WindCnt nor e2.WindDx should ever be 0.
      if (e2->wind_cnt * e2->wind_dx < 0)
      {
        //opposite directions so 'e' is outside 'e2' ...
        if (abs(e2->wind_cnt) > 1)
        {
          //outside prev poly but still inside another.
          if (e2->wind_dx * e.wind_dx < 0)
            //reversing direction so use the same WC
            e.wind_cnt = e2->wind_cnt;
          else
            //otherwise keep 'reducing' the WC by 1 (ie towards 0) ...
            e.wind_cnt = e2->wind_cnt + e.wind_dx;
        }
        else
          //now outside all polys of same polytype so set own WC ...
          e.wind_cnt = (IsOpen(e) ? 1 : e.wind_dx);
      }
      else
      {
        //'e' must be inside 'e2'
        if (e2->wind_dx * e.wind_dx < 0)
          //reversing direction so use the same WC
          e.wind_cnt = e2->wind_cnt;
        else
          //otherwise keep 'increasing' the WC by 1 (ie away from 0) ...
          e.wind_cnt = e2->wind_cnt + e.wind_dx;
      }
      e.wind_cnt2 = e2->wind_cnt2;
      e2 = e2->next_in_ael;  // ie get ready to calc WindCnt2