Returns six entries indexed by edge: -1 for the two edges the river runs
through, 0 for the edges on one bank and 1 for the other. Returns null
when the cell has no through-river — no river, a source or mouth (the
channel ends at the centre, so there is only one bank), a confluence (three
or more channels, no road is drawn there at all), or a hairpin where the
river enters and leaves through adjacent edges, which leaves one bank with
no edges on it.
Which bank each edge of a river cell lies on.
Returns six entries indexed by edge:
-1for the two edges the river runs through,0for the edges on one bank and1for the other. Returnsnullwhen the cell has no through-river — no river, a source or mouth (the channel ends at the centre, so there is only one bank), a confluence (three or more channels, no road is drawn there at all), or a hairpin where the river enters and leaves through adjacent edges, which leaves one bank with no edges on it.