/** flips an edge in the triangulation and returns the new edge */
public HalfedgeHandle<X> flipEdge(HalfedgeHandle<X> e) {
TriangulationDSFace_2<X> f1 = e.getFace();
TriangulationDSFace_2<X> f2 = e.getOpposite().getFace();
// retrieve vertices of quadrangle
ArrayList<TriangulationDSVertex_2<X>> vert = new ArrayList<TriangulationDSVertex_2<X>>();
vert.add(f1.vertex(e.index()));
vert.add(f1.vertex((e.index() + 1) % 3));
vert.add(f2.vertex(e.getOpposite().index()));
vert.add(f1.vertex((e.index() + 2) % 3));
// retrieve neighboring faces of quadrangle
ArrayList<TriangulationDSFace_2<X>> neighb = new ArrayList<TriangulationDSFace_2<X>>();
neighb.add(f1.neighbor((e.index() + 2) % 3));
neighb.add(f2.neighbor((e.getOpposite().index() + 1) % 3));
neighb.add(f2.neighbor((e.getOpposite().index() + 2) % 3));
neighb.add(f1.neighbor((e.index() + 1) % 3));
// reset the vertices of the 2 quadrangle faces
f1.setVertex(0, vert.get(0));
f1.setVertex(1, vert.get(1));
f1.setVertex(2, vert.get(2));
f2.setVertex(0, vert.get(0));
f2.setVertex(1, vert.get(2));
f2.setVertex(2, vert.get(3));
// reset the incident faces of the quandrangle vertices
vert.get(0).setFace(f1);
vert.get(1).setFace(f1);
vert.get(2).setFace(f2);
vert.get(3).setFace(f2);
// reset the neighbors of the 2 quadrangle faces
f1.setNeighbor(0, neighb.get(1));
f1.setNeighbor(1, f2);
f1.setNeighbor(2, neighb.get(0));
f2.setNeighbor(0, neighb.get(2));
f2.setNeighbor(1, neighb.get(3));
f2.setNeighbor(2, f1);
// reset the neighbors of the quadrangle neighbors (only modify what needs to be modified)
if (neighb.get(1) != null) neighb.get(1).setNeighbor(neighb.get(1).index(f2), f1);
if (neighb.get(3) != null) neighb.get(3).setNeighbor(neighb.get(3).index(f1), f2);
// reset the marks of the edges of the two faces
for (int i = 0; i < 3; i++) {
f1.unmark(i);
f2.unmark(i);
}
// restore the marks of the quadrangle edges (or rather their inner halfedges)
for (int i = 0; i < 3; i++) {
e = new HalfedgeHandle<X>(f1, i);
if (f1.neighbor(i) != null) e.setMark(e.getOpposite().isMarked());
e = new HalfedgeHandle<X>(f2, i);
if (f2.neighbor(i) != null) e.setMark(e.getOpposite().isMarked());
}
// return the new diagonal edge
return new HalfedgeHandle<X>(f1, 1);
}
