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

def short_decode(s):
    """ return coordinates of tile described by short link

    >>> short_decode("0GAjIv8h")
    (50.671536326408386, 6.09716534614563)

    Note: A short link describes a tile not a point.  I.e. the result will only be
          approximately the same as the values you entered into osm_short to get
          the short link in the first place.
    """
    slon = 0.0
    slat = 0.0
    lonw = 360.0
    latw = 180.0
    z = 0
    z_offset = 0

    for c in s:
        a = ARRAY.find(c)

        if a == -1:
            z_offset -= 1
        else:
            z += 3
            # scan the 3 * 2 bits of the decoded char
            mask = 0x20
            for x in range(3):
                lonw = lonw / 2
                latw = latw / 2
                # if set, go to the middle of that tile horizontally
                if (a & mask): slon += lonw
                mask = mask >> 1
                # if set, go to the middle of that tile vertically
                if (a & mask): slat += latw
                mask = mask >> 1

    # go to the middle of that tile
    lonw = lonw / 2
    latw = latw / 2
    slon += lonw
    slat += latw

    slon -= 180   # transform 0...360 -> -180 ... +180
    slat -= 90    # transform 0...180 -> -90 ... +90

    return slat, slon, z - 8 -(z_offset % 3)

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)

    # testing
    w = "0GAjIv8h"
    lat, lon, zoom = short_decode(w)
    print lat, lon, zoom
    res = short_osm(lat, lon, zoom)
    print "http://osm.org/go/" + w
    print res

Note that the calls to long() in encode() are Python2-specific.
For Python3, replace by calls to int()
Brian