/// <summary>
/// Calculate the Diagonals to add inside the Polygon.
/// </summary>
/// <param name="events">The Events in sorted Form</param>
/// <param name="sweepDown">True in the first Stage (sweeping down), false in the following Stages (sweeping up)</param>
/// <returns></returns>
private static List <Tuple <int, int> > CalculateDiagonals(List <PolygonPoint> events, Boolean sweepDown = true)
{
// Diagonals to add to the Polygon to make it monotone after the Down- and Up-Sweeps
var diagonals = new List <Tuple <int, int> >();
// Construct Status and Helper, a List of Edges left of every Point of the Polygon
// by shooting a Ray from the Vertex to the left.
// The Helper Point of that Edge will be used to create Monotone Polygons
// by adding Diagonals from/to Split- and Merge-Points
var statusAndHelper = new StatusHelper();
// Sweep through the Polygon using the sorted Polygon Points
for (int i = 0; i < events.Count; i++)
{
var ev = events[i];
// Get the Class of this event (depending on the sweeping direction)
var evClass = ev.PointClass(!sweepDown);
// Temporary StatusHelperElement
StatusHelperElement she = null;
// Handle the different Point-Classes
switch (evClass)
{
case PolygonPointClass.Start:
// Just add the left Edge (depending on the sweeping direction)
statusAndHelper.Add(new StatusHelperElement(sweepDown ? ev.EdgeTwo : ev.EdgeOne, ev));
break;
case PolygonPointClass.Stop:
// Just remove the left Edge (depending on the sweeping direction)
statusAndHelper.Remove(sweepDown ? ev.EdgeOne : ev.EdgeTwo);
break;
case PolygonPointClass.Regular:
// If the Polygon is positioned on the right Side of this Event
if (ev.Last > ev.Next)
{
// Replace the corresponding (old) StatusHelperElement with the new one
statusAndHelper.Remove(sweepDown ? ev.EdgeOne : ev.EdgeTwo);
statusAndHelper.Add(new StatusHelperElement(sweepDown ? ev.EdgeTwo : ev.EdgeOne, ev));
}
else
{
// Search Edge left of the Event and set Event as it's Helper
she = statusAndHelper.SearchLeft(ev);
she.Helper = ev;
}
break;
case PolygonPointClass.Merge:
// Just remove the left Edge (depending on the sweeping direction)
statusAndHelper.Remove(sweepDown ? ev.EdgeOne : ev.EdgeTwo);
// Search Edge left of the Event and set Event as it's Helper
she = statusAndHelper.SearchLeft(ev);
she.Helper = ev;
break;
case PolygonPointClass.Split:
// Search Edge left of the Event
she = statusAndHelper.SearchLeft(ev);
// Chose diagonal from Helper of Edge to Event.
var minP = Math.Min(she.Helper.Index, ev.Index);
var maxP = Math.Max(she.Helper.Index, ev.Index);
var diagonal = new Tuple <int, int>(minP, maxP);
diagonals.Add(diagonal);
// Replace the Helper of the StatusHelperElement by Event
she.Helper = ev;
// Insert the right Edge from Event
statusAndHelper.Add(new StatusHelperElement(sweepDown ? ev.EdgeTwo : ev.EdgeOne, ev));
break;
}
}
return(diagonals);
}