[FEA] Nearest point and segment index of linestring to a point (pairwise)

[harrism]

Is your feature request related to a problem? Please describe.

For linestring/point pairs, I would like to calculate not only the distance between points and linestrings, but also the nearest point on the linestring and possibly the index of the segment within the linestring that contains that nearest point.

Describe the solution you'd like

Ingestivate leveraging the functionality in #573 to provide the nearest point and segment index.

[isVoid]

To clarify, if a point is the same as a vertex, what's the result segment idx?

[harrism]

I think that is easily answered by considering the cases where the point is the first point or the last point in the linestring. Based on that, I think this is a good answer:

segment_idx = min(point_idx, num_segments - 1);

Where point_idx is the index of the point within the linestring, and num_segments is the number of segments in the linestring. (Note num_segments == num_points - 1, where num_points is the number of points in the linestring.)

So if we have 5 points and 4 segments in a linestring:

closest point index	closest segment index
0	0
1	1
2	2
3	3
4	3

[isVoid]

Another clarification: should the function handle multi-linestring in this method?

[harrism]

For the current request I haven't heard that this is needed. However for completeness I guess we should. The question is what do you return if so?

[isVoid]

My prototype returns a third column that provides the "part index" of the linestring in a multi-linestring.

[isVoid]

A third clarification: in case there are multiple segments having equal distance to the point, how do you break the tie?

[harrism]

If we are only returning one, I would suggest to not guarantee any ordering. This gives the most flexibility in a parallel implementation.