/// <summary>
/// Updates the tracked ringStartEdge
/// if the given edge has a lower origin
/// (using the standard <see cref="Coordinate"/> ordering).
/// </summary>
/// <remarks>
/// Identifying the lowest starting node meets two goals:
/// * It ensures that isolated input rings are created using the original node and orientation.
/// * For isolated rings formed from multiple input linestrings,
/// it provides a canonical node and orientation for the output
/// (rather than essentially random, and thus hard to test).
/// </remarks>
/// <param name="e"></param>
private void UpdateRingStartEdge(DissolveHalfEdge e)
{
if (!e.IsStart)
{
e = (DissolveHalfEdge)e.Sym;
if (!e.IsStart)
{
return;
}
}
// here e is known to be a start edge
if (_ringStartEdge == null)
{
_ringStartEdge = e;
return;
}
Coordinate eOrig = e.Orig;
Coordinate rseOrig = _ringStartEdge.Orig;
int compareTo = eOrig.CompareTo(rseOrig);
if (compareTo < 0)
{
_ringStartEdge = e;
}
}
/// <summary>
/// Builds a line starting from the given edge.
/// The start edge origin is a node (valence = 1 or >= 3),
/// unless it is part of a pure ring.
/// </summary>
/// <remarks>
/// A pure ring has no other incident lines.
/// In this case the start edge may occur anywhere on the ring.
/// </remarks>
/// <remarks>
/// The line is built up to the next node encountered,
/// or until the start edge is re-encountered
/// (which happens if the edges form a ring).
/// </remarks>
/// <param name="eStart"></param>
private void BuildLine(HalfEdge eStart)
{
CoordinateList line = new CoordinateList();
DissolveHalfEdge e = (DissolveHalfEdge)eStart;
_ringStartEdge = null;
MarkHalfEdge.MarkBoth(e);
Coordinate orig = e.Orig;
line.Add(orig.Copy(), false);
// scan along the path until a node is found (if one exists)
while (e.Sym.Degree() == 2)
{
UpdateRingStartEdge(e);
DissolveHalfEdge eNext = (DissolveHalfEdge)e.Next;
// check if edges form a ring - if so, we're done
if (eNext == eStart)
{
BuildRing(_ringStartEdge);
return;
}
// add point to line, and move to next edge
orig = eNext.Orig;
line.Add(orig.Copy(), false);
e = eNext;
MarkHalfEdge.MarkBoth(e);
}
// add final node
Coordinate dest = e.Dest;
line.Add(dest.Copy(), false);
// queue up the final node edges
StackEdges(e.Sym);
// store the scanned line
AddLine(line);
}
