Splitting multipolygons by 180th meridian

[mngr777]

Adds h3_cells_to_multi_polygon_wkb function (and PostGIS wrappers) that allows to split multipoligons returned by cellsToLinkedMultiPolygon by 180th meridian.

Standalone tool using the same algorithm: https://github.com/konturio/h3-split

I noticed that a boolean flag has been removed for h3_cell_to_boundary_wkb, but WKB version of h3_cells_to_multi_polygon also greatly simplifies the interface, so maybe it's better to keep the flag in case the user doesn't need to split?

Non-convex polygon with holes:

\set polygon '\'POLYGON((-170 12.5, 170 12.5, 170 7.5, -175 7.5, -175 2.5, 170 2.5, 170 -12.5, -170 -12.5, -170 -7.5, 175 -7.5, 175 -2.5, -170 -2.5, -170 12.5), (-176 11.5, -179 11.5, -179 8.5, -176 8.5, -176 11.5), (174 11.5, 171 11.5, 171 8.5, 174 8.5, 174 11.5), (174 -3.5, 171 -3.5, 171 -6.5, 174 -6.5, 174 -3.5),(-176 -8.5, -179 -8.5, -179 -11.5, -176 -11.5, -176 -8.5))\''::geometry
SELECT ST_AsText(h3_cells_to_multi_polygon_wkb(array(SELECT h3_polygon_to_cells(:polygon, 4)), true)::geometry);

Polygon at North Pole:

\set north '\'point(0 90)\''::geometry
SELECT ST_AsText(h3_cells_to_multi_polygon_wkb(array(SELECT h3_cell_to_children(h3_lat_lng_to_cell(:north, 2), 4)), true)::geometry);

[zachasme]

This looks great! And thanks for adding _geometry and _geography variants for h3_cells_to_multi_polygon.

I ended up removing flags in favor of GUC variables. Furthermore, based on input from @Komzpa in #79 (review), I figured it was best to always split the PostGIS types (including the raw WKB).

Could you update the PR to not include the boolean flag, and always split?

[mngr777]

Updated.
Thank you for the explanation, I didn't see that discussion.

[zachasme]

Thank you for the contribution!

[mngr777]

@zachasme I'm also going to add aggregate versions of geometry/geography functions. Now that they don't have a second argument no C code is required to pass it.

[zachasme]

Interesting, I have not yet worked with aggregate functions. Would that be similar to ST_Extent in PostGIS, for example? What kind of aggregate functions can be made for h3 specifically?

[mngr777]

Yes, similar to ST_Extent. This is basically all it takes:

CREATE AGGREGATE h3_cells_to_multi_polygon_geometry(h3index) (
    sfunc = array_append,
    stype = h3index[],
    finalfunc = h3_cells_to_multi_polygon_geometry,
    parallel = safe
);

This allows to apply h3_cells_to_multi_polygon_geometry without explicitly creating an array:

select h3_cells_to_multi_polygon_geometry(h3) from (select h3_polygon_to_cells(:polygon, 4) as h3) as t;

Aggregate functions could be added for h3_compact_cells/h3_uncompact_cells since they take an array, not sure how it would work with h3_cells_to_multi_polygon. I think that's all.