Indexing

Cells are uniquely identified by a UInt64 index. The only global indexing system is through these values.
- You can create cells from its index or from coordinates that are contained in the cell: Cell(index) or Cell((lon, lat), res=10).
However, H3 supports several other indexing systems that are valid within a single face of the icosahedron.
HexEarth uses several different getindex methods to expose these indexing systems.

Index Ordering

Index ordering is not consistent. I.e. there are no directional guarantees as to where cell[1] is relative to cell.

julia> using HexEarth
julia> new_york = (-74, 40.7)(-74, 40.7)
julia> cell = Cell(new_york)⬡ Cell 10 (-74.00025337207035, 40.699690184228594)
julia> cell[1]  # spiral indexing (1-6) for immediate neighbors⬡ Cell 10 (-74.00179375894132, 40.70002575242847)
julia> cell[0, 1]  # https://h3geo.org/docs/core-library/coordsystems#local-ij-coordinates⬡ Cell 10 (-74.00179375894132, 40.70002575242847)
julia> cell[0, 1, 0]  # https://h3geo.org/docs/core-library/coordsystems#ijk-coordinates⬡ Cell 10 (-74.00179375894132, 40.70002575242847)

Spiral Indexing

import GeoInterface as GI
using CairoMakie

fig = Figure()
ax = Axis(fig[1,1])
poly!(ax, cell)
for i in 1:6
    lines!(ax, cell[i])
    text!(ax, GI.centroid(cell[i]), text=string(i))
end
fig

IJ Indexing

Note

For IJ/IJK indexing, HexEarth uses the conventions that (0,0) and (0,0,0) refer to the origin cell. For whatever reason, this is not the case in the original h3 library.

Reference: https://h3geo.org/docs/core-library/coordsystems/#local-ij-coordinates

fig = Figure()
ax = Axis(fig[1,1])
poly!(ax, cell)
for i in [(0,1), (0,-1), (-1,-1), (1,1), (1,0), (-1,0)]
    lines!(ax, cell[i...])
    text!(ax, GI.centroid(cell[i...]), text=string(i))
end
fig

IJK Indexing

Reference: https://h3geo.org/docs/core-library/coordsystems#ijk-coordinates
IJK addresses are not unique unless "normalized"
Note: normalized IJK coordinates have at most two non-zero coordinates.

fig = Figure()
ax = Axis(fig[1,1])
poly!(ax, cell)
for i in HexEarth.neighbor_indices
    @info i
    lines!(ax, cell[i...])
    text!(ax, GI.centroid(cell[i...]), text=string(i))
end
fig