Geohex aggregation on geo_shape field

[iverase]

Implementation of geohex aggregation on geo_shape fields. It projects the H3 grid system into a equirectangular projection (see H3CartesianUtil) and uses it to rasterise the geometries indexed on a geo_shape field. One important side effect of this projection is that the relationship between the cell containing a pole and its children maybe lost so it requires special treatment.

Most the rasterization logic is inside AbstractGeoHexGridTiler. It is important to understand that children of an H3 bin are not fully contain in the parent and an H3 bin intersects other h3 bin in the next level which are not in the children set. Those bins can be found using H3 method H3.h3ToNoChildrenIntersecting. In order to make sure we visit all necessary cells we add the following call when navigating down the H3 hierarchy:

// H3 cells do intersect with other cells that are not part of the children cells. If the parent cell of those
// cells is disjoint, they will not be visited, therefore visit them here.
for (long child : H3.h3ToNoChildrenIntersecting(h3)) {
   if (relateTile(geoValue, H3.h3ToParent(child)) == GeoRelation.QUERY_DISJOINT) {
      valueIndex = setValuesByRecursion(values, geoValue, child, precision + 1, valueIndex);
   }
}

[elasticsearchmachine]

Hi @iverase, I've created a changelog YAML for you.

[elasticsearchmachine]

Pinging @elastic/es-analytics-geo (Team:Analytics)

[iverase]

@elasticmachine update branch

[craigtaverner]

Mostly I like this a lot. I have, however, many comments, questions and suggestions. I'm approving this as a very good first step, with the expectation that we do more work before making this feature available. In particular increasing test coverage and fixing any bugs found by that process.

[craigtaverner]

Could just have used the shared interface GeoShapeQueryable