Add _idx variants for ConvexHull and SimplifyVwPreserve to enable dimension passthrough downstream

[willcohen]

I'm looking at incorporating geoarrow-rs geospatial algorithms as a performance optimization compared to vanilla JTS / a webassembly build of JTS when they're available, which, as I understand it, wraps the core operations from georust/geo.

I'm noticing that JTS can generally operate on coordinate sequence/buffers with extra dimensions (XYZ, XYM, XYZM), and can always pass them through.

Simplify and SimplifyVw have their SimplifyIdx and SimplifyVwIdx variants, so geoarrow-rs can just nab the relevant indices from those and pull the extra dimensions through in their output.

SimplifyVwPreserve and ConvexHull, however, don't have idx variants, and so geoarrow-rs has no way to route the Z/M values through.

Would a PR to add this be welcomed?

[kylebarron]

Not sure but I thought maybe SimplifyVwPreserve would occasionally have to add new points to ensure the output is valid? Like perhaps it wouldn't always be able to define just indices into the existing coordinates?

[willcohen]

I'm happy to be wrong but I'm not seeing any addition of new vertices, only the dropping of them (via

geo/geo/src/algorithm/simplify_vw.rs

Line 366 in 8126f0a

adjacent[smallest.current] = (0, 0);

).

[urschrei]

Just to confirm the preserve variant never adds coords: it skips removal to ensure validity.