All cells within k grid steps of the origin

gridDisk(h3index,integer) → h3indexset
gridDisk(quadbin,integer) → quadbinset
s2EdgeNeighbors(s2cell) → s2cellset

The disk includes the origin, which is the whole of it at k = 0. Its shape follows the grid: on the QUADBIN square grid the disk at step k is the (2k+1)×(2k+1) block of cells centred on the origin, while on the H3 hexagonal grid it is the ring of hexagons at that many steps and everything inside it. S2 states the four cells sharing an edge with the origin and takes no step count, so it answers the disk at k = 1 less the origin itself.

SELECT gridDisk(h3index '871fa44a8ffffff', 1);
/* {"871fa44a8ffffff", "871fa44a9ffffff", "871fa44aaffffff", "871fa44abffffff",
   "871fa44acffffff", "871fa44adffffff", "871fa44aeffffff"} */
SELECT gridDisk(quadbin '48a6227affffffff', 1);
/* {"48a6226dffffffff", "48a6226fffffffff", "48a62278ffffffff", "48a62279ffffffff",
   "48a6227affffffff", "48a6227bffffffff", "48a622c5ffffffff", "48a622d0ffffffff",
   "48a622d1ffffffff"} */
SELECT s2EdgeNeighbors(s2cell '47c3c');
-- {"47c14", "47c24", "47c34", "47c44"}

All children of a cell at the given target resolution

cellToChildren(h3index,integer) → h3indexset
cellToChildren(quadbin,integer) → quadbinset
cellToChildren(s2cell,integer) → s2cellset

The number of children is the grid's branching factor: four for a QUADBIN or an S2 quadtree cell, seven for an H3 cell. The helper is static, children being a per-instant set whose temporal lift is deferred pending a tset<T> primitive.

SELECT cellToChildren(h3index '831c02fffffffff', 4);
/* {"841c021ffffffff", "841c023ffffffff", "841c025ffffffff", "841c027ffffffff",
   "841c029ffffffff", "841c02bffffffff", "841c02dffffffff"} */
SELECT cellToChildren(quadbin '48a6227affffffff', 11);
-- {"48b6227a3fffffff", "48b6227a7fffffff", "48b6227abfffffff", "48b6227affffffff"}
SELECT cellToChildren(s2cell '47c3c', 8);
-- {"47c39", "47c3b", "47c3d", "47c3f"}

Compacted representation of a cell set, finer cells being merged into their parent whenever the parent's full set of children is present

h3CompactCells(h3indexset) → h3indexset

Compaction is stated for H3 alone: it is the one grid MobilityDB carries the kernel for, and the QUADBIN and S2 forms are not yet implemented.

SELECT h3CompactCells(cellToChildren(h3index '831c02fffffffff', 4));
-- {"831c02fffffffff"}

Uncompacted representation at the given resolution, the inverse of compactCells

h3UncompactCells(h3indexset,integer) → h3indexset

SELECT numValues(h3UncompactCells(h3CompactCells(
  cellToChildren(h3index '831c02fffffffff', 4)), 4));
-- 7