Make Delaunay.find return -1 instead of NaN for edge cases

[akassaei]

Fixes #144

I've added only a test for the empty points array example because I couldn't find a way to actually have a single point array with inedges[0] that is -1. Feel free to modify the PR or give me feedback on how to achieve this 😄

The fix is straightforward, I've just added a new condition to return -1 instead of the operation done for the other cases.

[mourner]

Thanks for the fix, this looks good to me!

[mbostock]

I think this is correct when there are no points, but I don’t think this is correct when there’s a single point; in that case, we should return 0 instead of -1 from find. Let me take a closer look…

[mbostock]

Ah, right, so I forgot that points is a flat array of the form [x0, y0, x1, y2, …], so you should never have points.length === 1; it’s always a multiple of two.

So perhaps the bug here is only caused when you pass an odd-length points to the constructor? We should ignore the last point in that case… so maybe the bug is that all points.length === 0 tests should in fact be points.length < 2 because both 0 and 1 should be considered empty.

[mbostock]

I pushed my simplification, found another instance where we should round down if the points array has odd length, and added some tests for the half-a-point case (points.length === 1) which we should treat the same as no points.

[akassaei]

Thanks a lot for the improvements and the explanations @mbostock 🙏