Callback enumeration (no result vectors)

Several APIs can return many cells (or related indexes) in a Vector. For each of these, FastH3 also provides an overload whose first argument is a callback op. That overload returns only H3Error and invokes op once per output element in the same order as the vector-returning API would produce. The callback path does not allocate the result collection itself (your op may still allocate).

Use Julia’s do syntax so the callback reads as a trailing block:

using FastH3

err, h = latLngToCell(LatLng(0.0, 0.0), 8)
err == E_SUCCESS || error("latLngToCell failed")

# Collect children without a dedicated cellToChildren vector allocation
# (still allocates the collector `out` if you push!)
out = H3Index[]
err = cellToChildren(h, 9) do child
    push!(out, child)
end
err == E_SUCCESS || error("cellToChildren failed")

APIs with callback overloads

Callback form	Typical use
`cellToChildren(op, h, childRes)`	All children at finer resolution
`uncompactCells(op, compactedSet, res)`	Expand compacted set to `res`
`getPentagons(op, res)`	Pentagon base cells at `res`
`getRes0Cells(op)`	All resolution-0 indexes
`getIcosahedronFaces(op, h)`	Face indices (see note below)
`gridDiskDistancesUnsafe(op, origin, k)`	`op(cell, ring)` for disk + distance
`gridDiskUnsafe(op, origin, k)`	`op(cell)` only
`gridRingUnsafe(op, origin, k)`	Ring at exact distance `k`
`gridPathCells(op, start, end)`	Cells along interpolated path
`cellToVertexes(op, origin)`	Vertex indexes for a cell
`originToDirectedEdges(op, origin)`	Directed edges from a cell

Docstrings for these functions describe the callback signature and link to the H3 C API.

`getIcosahedronFaces` and the vector form

The Vector{Int} overload returns a fixed-length vector aligned with maxFaceCount(h), using -1 for unused slots (internal sentinel). The callback overload calls op(face) only for valid face indices (sentinel slots are skipped). If you need the raw buffer including sentinels, use the vector-returning method.

What does not have a callback overload yet

gridDisk and gridDiskDistances may fall back to an internal BFS when the fast “unsafe” path hits a pentagon. That path uses extra heap structures, so a zero-allocation callback story is not offered for those entry points yet. For allocation-free disk enumeration when the unsafe algorithm applies, use gridDiskUnsafe or gridDiskDistancesUnsafe with a callback.

compactCells, polygon-to-cells routines, and similar algorithms are also callback-free for now.

Callback enumeration (no result vectors)

APIs with callback overloads

getIcosahedronFaces and the vector form

What does not have a callback overload yet

See also

`getIcosahedronFaces` and the vector form