- *
- * Vertices in strip are numbered as follows:
- *
- * ___ X-2 ___ X-1 ___ 0 ___ 1 ___ 2 ___ 3 ___ 4 __
- * Y-1 Y-1 0 0 0 0 0
- * / \ / \ / \ / \ / \ / \ / \
- * / \ / \ / \ / \ / \ / \ / \
- * X-3 ___ X-2 ___ X-1 ___ 0 ___ 1 ___ 2 ___ 3 ___ 4
- * Y-2 Y-2 Y-2 1 1 1 1 1
- * \ / \ / \ / \ / \ / \ / \ /
- * \ / \ / \ / \ / \ / \ / \ /
- * ___ X-3 ___ X-2 ___ X-1 ___ 0 ___ 1 ___ 2 ___ 3 __
- * Y-3 Y-3 Y-3 2 2 2 2
- *
- * . . . . . . . . . . . . . . .
- *
- * X-4 ___ X-3 ___ X-2 ___ X-1 ___ 0 ___ 1 ___ 2 ___ 3
- * 1 1 1 1 Y-2 Y-2 Y-2 Y-2
- * \ / \ / \ / \ / \ / \ / \ /
- * \ / \ / \ / \ / \ / \ / \ /
- * ___ X-4 ___ X-3 ___ X-2 ___ X-1 ___ 0 ___ 1 ___ 2 __
- * 0 0 0 0 Y-1 Y-1 Y-1
- *
- * Node x,y for
- * 0 <= x < X x = distance along
- * 0 <= y < Y y = distance across
- *
- * Vertices are in reading order from diagram above ie x varies fastest.
- *
- * Y must be even. The actual location opposite (0,0) is (X-(Y-1)/2,0),
- * and likewise opposite (0,Y-1) is ((Y-1)/2,0).
- *
- * To label edges, we counte anticlockwise[*] from to-the-right:
- *
- * \2 /1
- * \ /
- * ___ 0 __
- * 3 1 0
- * / \
- * 4/ 5\
- *
- * [*] That is, in the direction represented as anticlockwise for
- * the vertices (0,*)..(4,*) in the diagram above; and of course
- * that is clockwise for the vertices (X-5,*)..(X-1,*). The
- * numbering has an actual discontinuity between (X-1,*) and (0,*).
- *
- * When we iterate over edges, we iterate first over vertices and then
- * over edges 0 to 2, disregarding edges 3 to 5.