geohash

geohash_encode.py

# https://en.wikipedia.org/wiki/Geohash

__base32 = '0123456789bcdefghjkmnpqrstuvwxyz'


def encode(lat, lon, precision):
    """(Double?, Double? Uint32) -> String?
    
    precision(km):
    1   ±2500
    2   ±630
    3   ±78
    4   ±20
    5   ±2.4
    6   ±0.61
    7   ±0.076
    8   ±0.019
    9   ±0.0024
    10  ±0.00060
    11  ±0.000074
    """

    if lat is None or lon is None:
        return None

    lat_interval, lon_interval = (-90.0, 90.0), (-180.0, 180.0)
    geohash = []
    bits = 16, 8, 4, 2, 1
    bit = 0
    ch = 0
    even = True
    while len(geohash) < precision:
        if even:
            mid = (lon_interval[0] + lon_interval[1]) / 2.0
            if lon > mid:
                ch |= bits[bit]
                lon_interval = (mid, lon_interval[1])
            else:
                lon_interval = (lon_interval[0], mid)
        else:
            mid = (lat_interval[0] + lat_interval[1]) / 2.0
            if lat > mid:
                ch |= bits[bit]
                lat_interval = (mid, lat_interval[1])
            else:
                lat_interval = (lat_interval[0], mid)
        even = not even
        if bit < 4:
            bit += 1
        else:
            geohash += __base32[ch]
            bit = 0
            ch = 0
    return ''.join(geohash)