109 lines
3.9 KiB
Python
109 lines
3.9 KiB
Python
"""
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Copyright (C) 2008 Leonard Norrgard <leonard.norrgard@gmail.com>
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Copyright (C) 2015 Leonard Norrgard <leonard.norrgard@gmail.com>
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This file is part of Geohash.
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Geohash is free software: you can redistribute it and/or modify it
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under the terms of the GNU Affero General Public License as published
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by the Free Software Foundation, either version 3 of the License, or
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(at your option) any later version.
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Geohash is distributed in the hope that it will be useful, but WITHOUT
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ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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FITNESS FOR A PARTICULAR PURPOSE. See the GNU Affero General Public
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License for more details.
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You should have received a copy of the GNU Affero General Public
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License along with Geohash. If not, see
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<http://www.gnu.org/licenses/>.
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"""
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from math import log10
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# Note: the alphabet in geohash differs from the common base32
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# alphabet described in IETF's RFC 4648
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# (http://tools.ietf.org/html/rfc4648)
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__base32 = '0123456789bcdefghjkmnpqrstuvwxyz'
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__decodemap = { }
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for i in range(len(__base32)):
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__decodemap[__base32[i]] = i
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del i
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def decode_exactly(geohash):
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"""
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Decode the geohash to its exact values, including the error
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margins of the result. Returns four float values: latitude,
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longitude, the plus/minus error for latitude (as a positive
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number) and the plus/minus error for longitude (as a positive
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number).
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"""
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lat_interval, lon_interval = (-90.0, 90.0), (-180.0, 180.0)
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lat_err, lon_err = 90.0, 180.0
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is_even = True
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for c in geohash:
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cd = __decodemap[c]
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for mask in [16, 8, 4, 2, 1]:
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if is_even: # adds longitude info
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lon_err /= 2
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if cd & mask:
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lon_interval = ((lon_interval[0]+lon_interval[1])/2, lon_interval[1])
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else:
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lon_interval = (lon_interval[0], (lon_interval[0]+lon_interval[1])/2)
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else: # adds latitude info
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lat_err /= 2
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if cd & mask:
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lat_interval = ((lat_interval[0]+lat_interval[1])/2, lat_interval[1])
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else:
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lat_interval = (lat_interval[0], (lat_interval[0]+lat_interval[1])/2)
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is_even = not is_even
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lat = (lat_interval[0] + lat_interval[1]) / 2
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lon = (lon_interval[0] + lon_interval[1]) / 2
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return lat, lon, lat_err, lon_err
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def decode(geohash):
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"""
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Decode geohash, returning two strings with latitude and longitude
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containing only relevant digits and with trailing zeroes removed.
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"""
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lat, lon, lat_err, lon_err = decode_exactly(geohash)
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# Format to the number of decimals that are known
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lats = "%.*f" % (max(1, int(round(-log10(lat_err)))) - 1, lat)
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lons = "%.*f" % (max(1, int(round(-log10(lon_err)))) - 1, lon)
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if '.' in lats: lats = lats.rstrip('0')
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if '.' in lons: lons = lons.rstrip('0')
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return lats, lons
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def encode(latitude, longitude, precision=12):
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"""
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Encode a position given in float arguments latitude, longitude to
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a geohash which will have the character count precision.
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"""
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lat_interval, lon_interval = (-90.0, 90.0), (-180.0, 180.0)
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geohash = []
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bits = [ 16, 8, 4, 2, 1 ]
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bit = 0
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ch = 0
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even = True
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while len(geohash) < precision:
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if even:
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mid = (lon_interval[0] + lon_interval[1]) / 2
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if longitude > mid:
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ch |= bits[bit]
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lon_interval = (mid, lon_interval[1])
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else:
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lon_interval = (lon_interval[0], mid)
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else:
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mid = (lat_interval[0] + lat_interval[1]) / 2
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if latitude > mid:
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ch |= bits[bit]
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lat_interval = (mid, lat_interval[1])
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else:
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lat_interval = (lat_interval[0], mid)
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even = not even
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if bit < 4:
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bit += 1
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else:
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geohash += __base32[ch]
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bit = 0
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ch = 0
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return ''.join(geohash)
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