/// <summary>
/// Finds all intersections between the specified line segments, using a sweep line
/// algorithm.</summary>
/// <param name="lines">
/// An <see cref="Array"/> containing the <see cref="LineD"/> segments to intersect.
/// </param>
/// <returns>
/// A lexicographically sorted <see cref="List{T}"/> containing the final <see
/// cref="EventPoint"/> for every point of intersection between two or more <paramref
/// name="lines"/>.</returns>
/// <exception cref="ArgumentException">
/// <paramref name="lines"/> contains a <see cref="LineD"/> whose <see
/// cref="LineD.Start"/> and <see cref="LineD.End"/> coordinates are equal.</exception>
/// <exception cref="ArgumentNullException">
/// <paramref name="lines"/> is a null reference.</exception>
/// <exception cref="InvalidOperationException">
/// <paramref name="lines"/> contains coordinates that caused corruption to an internal
/// search structure.</exception>
/// <remarks>
/// <b>FindCore</b> creates the intermediate output which is further processed by one of
/// the <see cref="Find"/> overloads.</remarks>
internal List <EventPoint> FindCore(LineD[] lines)
{
BuildSchedule(lines);
while (Schedule.Count > 0)
{
var node = Schedule.RootNode._next;
Schedule.RemoveNode(node);
HandleEvent(node._value);
}
if (SweepLine.Count > 0)
{
ThrowHelper.ThrowInvalidOperationException(Strings.SearchStructureCorrupted);
}
return(Crossings);
}
/// <summary>
/// Handles the specified <see cref="EventPoint"/> that was just removed from the <see
/// cref="Schedule"/>.</summary>
/// <param name="e">
/// The <see cref="EventPoint"/> to handle.</param>
/// <remarks>
/// <b>HandleEvent</b> always updates the <see cref="SweepLine"/>, and possibly the <see
/// cref="Schedule"/> via <see cref="AddCrossing"/>.</remarks>
private void HandleEvent(EventPoint e)
{
BraidedTreeNode <Int32, Int32> node, previous = null, next = null;
Cursor = e.Shared;
bool adding = false;
// remove end point & crossing nodes
for (int i = 0; i < e.Locations.Count; i++)
{
switch (e.Locations[i])
{
case LineLocation.Start:
adding = true;
break;
case LineLocation.End:
node = SweepLine.FindNode(e.Lines[i]);
if (node == null)
{
ThrowHelper.ThrowInvalidOperationException(
Strings.SearchStructureCorrupted);
}
// remember surrounding lines
previous = node._previous;
next = node._next;
SweepLine.RemoveNode(node);
break;
case LineLocation.Between:
if (!SweepLine.Remove(e.Lines[i]))
{
ThrowHelper.ThrowInvalidOperationException(
Strings.SearchStructureCorrupted);
}
adding = true;
break;
}
}
if (!adding)
{
// intersect remaining neighbors of removed lines
if (previous._parent != null && next._parent != null)
{
AddCrossing(previous, next, e);
}
// record intersection event
var lines = e.Lines;
if (lines.Count < 2)
{
return;
}
/*
* The sweep line algorithm would normally record TWO intersections for
* overlapping lines that share the same lexicographic end point: one for the
* start point, and one for the end point. So when we encounter an event that
* contains only end points, we must check that its line segments arrive from at
* least two different directions, and only then record an intersection.
*/
double slope = Slopes[lines[0]];
for (int i = 1; i < lines.Count; i++)
{
if (slope != Slopes[lines[i]])
{
e.Normalize(Lines);
Crossings.Add(e);
break;
}
}
return;
}
// update remaining sweep line to prepare for insertion
var root = SweepLine.RootNode;
for (node = root._next; node != root; node = node._next)
{
int index = node.Key;
double slope = Slopes[index];
if (slope != Double.MaxValue)
{
PointD start = Lines[index].Start;
Positions[index] = slope * (Cursor.Y - start.Y) + start.X;
}
}
// (re-)insert start point & crossing nodes
previous = next = null;
for (int i = 0; i < e.Locations.Count; i++)
{
if (e.Locations[i] != LineLocation.End)
{
int index = e.Lines[i];
Positions[index] = Cursor.X;
SweepLine.TryAddNode(index, 0, out node);
// remember surrounding lines
if (previous == null)
{
previous = node._previous;
next = node._next;
}
}
}
// intersect outermost added lines with existing neighbors
if (previous._parent != null)
{
AddCrossing(previous, previous._next, e);
}
if (next._parent != null)
{
AddCrossing(next._previous, next, e);
}
// record intersection event
if (e.Lines.Count > 1)
{
e.Normalize(Lines);
Crossings.Add(e);
}
}