/// <summary>
/// Computes the intersection of one or more line strings.
/// </summary>
public void Compute()
{
// source: http://geomalgorithms.com/a09-_intersect-3.html
_intersections = new List <Coordinate>();
_edgeIndices = new List <Tuple <Int32, Int32> >();
Event currentEvent = _eventQueue.Next();
SweepLineSegment segment;
IList <Coordinate> intersections;
while (currentEvent != null)
{
if (currentEvent is EndPointEvent)
{
EndPointEvent endPointEvent = (EndPointEvent)currentEvent;
switch (endPointEvent.Type)
{
// Left endpoint event: check for possible intersection with below and / or above segments.
case EventType.Left:
segment = _sweepLine.Add(endPointEvent);
if (segment.Above != null)
{
intersections = LineAlgorithms.Intersection(segment.LeftCoordinate, segment.RightCoordinate,
segment.Above.LeftCoordinate, segment.Above.RightCoordinate,
PrecisionModel);
if (intersections.Count > 0)
{
IntersectionEvent intersectionEvent = new IntersectionEvent
{
Vertex = intersections[0],
Below = segment,
Above = segment.Above
};
_eventQueue.Add(intersectionEvent);
}
}
if (segment.Below != null)
{
intersections = LineAlgorithms.Intersection(segment.LeftCoordinate, segment.RightCoordinate,
segment.Below.LeftCoordinate, segment.Below.RightCoordinate,
PrecisionModel);
if (intersections.Count > 0)
{
IntersectionEvent intersectionEvent = new IntersectionEvent
{
Vertex = intersections[0],
Below = segment.Below,
Above = segment
};
_eventQueue.Add(intersectionEvent);
}
}
break;
// Right endpoint event: check for possible intersection of the below and above segments.
case EventType.Right:
segment = _sweepLine.Search(endPointEvent);
if (segment != null)
{
if (segment.Above != null && segment.Below != null)
{
intersections = LineAlgorithms.Intersection(segment.Above.LeftCoordinate,
segment.Above.RightCoordinate,
segment.Below.LeftCoordinate,
segment.Below.RightCoordinate,
PrecisionModel);
if (intersections.Count > 0)
{
IntersectionEvent intersectionEvent = new IntersectionEvent
{
Vertex = intersections[0],
Below = segment.Below,
Above = segment.Above
};
if (!_eventQueue.Contains(intersectionEvent))
{
_eventQueue.Add(intersectionEvent);
}
}
}
_sweepLine.Remove(segment);
}
break;
}
}
// Intersection point event: switch the two concerned segments and check for possible intersection with their below and above segments.
else if (currentEvent is IntersectionEvent)
{
IntersectionEvent intersectionEvent = (IntersectionEvent)currentEvent;
/*
* Segment order before intersection: segmentBelow <-> segmentAbove <-> segment <-> segmentAboveAbove
* Segment order after intersection: segmentBelow <-> segment <-> segmentAbove <-> segmentAboveAbove
*/
segment = intersectionEvent.Above;
SweepLineSegment segmentAbove = intersectionEvent.Below;
// Handle closing intersection points when segments (partially) overlap each other.
if (intersectionEvent.IsClose)
{
if (!_sweepLine.IsAdjacent(segment.Edge, segmentAbove.Edge))
{
_intersections.Add(currentEvent.Vertex);
_edgeIndices.Add(Tuple.Create(Math.Min(segment.Edge, segmentAbove.Edge),
Math.Max(segment.Edge, segmentAbove.Edge)));
}
}
// It is possible that the previously detected intersection point is not a real intersection, because a new segment started between them,
// therefore a repeated check is necessary to carry out.
else if (_sweepLine.Add(intersectionEvent))
{
if (!_sweepLine.IsAdjacent(segment.Edge, segmentAbove.Edge))
{
_intersections.Add(currentEvent.Vertex);
_edgeIndices.Add(Tuple.Create(Math.Min(segment.Edge, segmentAbove.Edge),
Math.Max(segment.Edge, segmentAbove.Edge)));
intersections = LineAlgorithms.Intersection(segment.LeftCoordinate, segment.RightCoordinate,
segmentAbove.LeftCoordinate, segmentAbove.RightCoordinate,
PrecisionModel);
if (intersections.Count > 1 && !intersections[1].Equals(intersections[0]))
{
IntersectionEvent newIntersectionEvent = new IntersectionEvent
{
Vertex = intersections[1],
Below = segment,
Above = segmentAbove,
IsClose = true,
};
_eventQueue.Add(newIntersectionEvent);
}
}
if (segmentAbove.Above != null)
{
intersections = LineAlgorithms.Intersection(segmentAbove.LeftCoordinate, segmentAbove.RightCoordinate,
segmentAbove.Above.LeftCoordinate, segmentAbove.Above.RightCoordinate,
PrecisionModel);
if (intersections.Count > 0 && intersections[0].X >= intersectionEvent.Vertex.X)
{
IntersectionEvent newIntersectionEvent = new IntersectionEvent
{
Vertex = intersections[0],
Below = segmentAbove,
Above = segmentAbove.Above
};
if (!_eventQueue.Contains(newIntersectionEvent))
{
_eventQueue.Add(newIntersectionEvent);
}
}
}
if (segment.Below != null)
{
intersections = LineAlgorithms.Intersection(segment.LeftCoordinate, segment.RightCoordinate,
segment.Below.LeftCoordinate, segment.Below.RightCoordinate,
PrecisionModel);
if (intersections.Count > 0 && intersections[0].X >= intersectionEvent.Vertex.X)
{
IntersectionEvent newIntersectionEvent = new IntersectionEvent
{
Vertex = intersections[0],
Below = segment.Below,
Above = segment
};
if (!_eventQueue.Contains(newIntersectionEvent))
{
_eventQueue.Add(newIntersectionEvent);
}
}
}
}
}
currentEvent = _eventQueue.Next();
}
_hasResult = true;
}
