Implementation of OpenStreetmap Shortlinks in Python

osm_shortlink.py

# osm_shortlink.py - MAximillian Dornseif 2013 - Public Domain
# see http://wiki.openstreetmap.org/wiki/Shortlink
# https://github.com/openstreetmap/openstreetmap-website/blob/master/lib/short_link.rb
# and makeShortCode in
# https://github.com/openstreetmap/openstreetmap-website/blob/master/app/assets/javascripts/application.js

# array of 64 chars to encode 6 bits. this is almost like base64 encoding, but
# the symbolic chars are different, as base64's + and / aren't very
# URL-friendly.
ARRAY = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_~'

import math

def short_osm(lat, lon, zoom=16):
    """Return a short link representing a location in OpenStreetmap.

    Provide coordinates and optional zoom level. e.g.:

    >>> short_osm(50.671530961990356, 6.09715461730957)
    http://osm.org/go/0GAjIv8h
    >>> short_osm(0, 0, 3)
    http://osm.org/go/wAAA--
    >>> short_osm(0, 0, 4)
    http://osm.org/go/wAAA
    """
    return 'http://osm.org/go/' + _encode(lat, lon, zoom)


def _encode(lat, lon, z):
    """given a location and zoom, return a short string representing it."""
    x = long((lon + 180.0) * 2**32 / 360.0)
    y = long((lat +  90.0) * 2**32 / 180.0)
    code = _interleave(x, y)
    str = ''
    # add eight to the zoom level, which approximates an accuracy of
    # one pixel in a tile.
    for i in range(int(math.ceil((z + 8) / 3.0))):
        digit = (code >> (56 - 6 * i)) & 0x3f;
        str += ARRAY[digit]
    # append characters onto the end of the string to represent
    # partial zoom levels (characters themselves have a granularity
    # of 3 zoom levels).
    for i in range((z + 8) % 3):
        str += "-"
    return str


def _interleave(x, y):
    """combine 2 32 bit integers to a 64 bit integer"""
    c = 0
    for i in range(31, 0, -1):
      c = (c << 1) | ((x >> i) & 1)
      c = (c << 1) | ((y >> i) & 1)
    return c


if __name__ == '__main__':
    # testing
    print short_osm(50.671530961990356, 6.09715461730957)
    print short_osm(50.671530961990356, 6.09715461730957, 10)
    print short_osm(50.671530961990356, 6.09715461730957, 5)
    print short_osm(50.671530961990356, 6.09715461730957, 4)
    print short_osm(0, 0, 4)
    print short_osm(0, 0, 3)

Why 56 here? I all other sources there is 58.

If anyone's interested, thanks to the folks at the Python forum I now have a functioning shortlink decode routine in Python:

`def decode(sc):
index_of = {x: y for y, x in enumerate("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789~")}
x = y = 0
z = -8
i = 0
for ch in sc:
if (digit := index_of.get(ch)) is None:
break

    # distribute 6 bits into x and y
    x <<= 3
    y <<= 3
    for j in range(2, -1, -1):
        if digit & 1 << (2 * j + 1):
            x |= 1 << j
        if digit & 1 << (2 * j):
            y |= 1 << j
    z += 3
    i += 1

x = x * 2**(2 - 3 * i) * 90 - 180
y = y * 2**(2 - 3 * i) * 45 - 90

if i < len(sc) and sc[i] == "-":
    z -= 2
    if i + 1 < len(sc) and sc[i + 1] == "-":
        z += 1

return z, y, x

`