Geohash encoder and decoder with neighbour cells

Geohash encoder and decoder: encode a WGS-84 latitude and longitude to a geohash at a chosen precision, or decode a geohash back to its bounding box, centre point and error bounds in metres. Encoding also returns the eight neighbouring cells, which is what a caller needs to do proximity search over a geohash-indexed store without a spatial database.

Output (data): encode: geohash, precision, bbox {min_lat, min_lon, max_lat, max_lon}, center {lat, lon}, error_m {lat, lon}, neighbours {n, ne, e, se, s, sw, w, nw} (each string|null: null where the cell would lie beyond a pole). decode: the same without neighbours, plus cell_width_m, cell_height_m (metres on WGS-84). Neighbour cells that would lie beyond a pole are null; longitude wraps at the antimeridian.

Deterministic: identical input returns identical output, so the response is served with Cache-Control: private, max-age=31536000, immutable (the paying client may cache it indefinitely; shared caches must not, because the response is paid).

Price: $0.05 USDC per call (x402, Base). Route contract version 1.0.0. Docs: https://api.duoleads.com/docs#geo-geohash