+##
+# Encodes and decodes locations from Morton-coded "quad tile" strings. Each
+# variable-length string encodes to a precision of one pixel per tile (roughly,
+# since this computation is done in lat/lon coordinates, not mercator).
+# Each character encodes 3 bits of x and 3 of y, so there are extra characters
+# tacked on the end to make the zoom levels "work".
+module ShortLink
+
+ # 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 = ('A'..'Z').to_a + ('a'..'z').to_a + ('0'..'9').to_a + ['_','@']
+
+ ##
+ # Given a string encoding a location, returns the [lon, lat, z] tuple of that
+ # location.
+ def self.decode(str)
+ x = 0
+ y = 0
+ z = 0
+ z_offset = 0
+
+ str.each_char do |c|
+ t = ARRAY.index c
+ if t.nil?
+ z_offset -= 1
+ else
+ 3.times do
+ x <<= 1; x = x | 1 unless (t & 32).zero?; t <<= 1
+ y <<= 1; y = y | 1 unless (t & 32).zero?; t <<= 1
+ end
+ z += 3
+ end
+ end
+ # pack the coordinates out to their original 32 bits.
+ x <<= (32 - z)
+ y <<= (32 - z)
+
+ # project the parameters back to their coordinate ranges.
+ [(x * 360.0 / 2**32) - 180.0,
+ (y * 180.0 / 2**32) - 90.0,
+ z - 8 - (z_offset % 3)]
+ end
+
+ ##
+ # given a location and zoom, return a short string representing it.
+ def self.encode(lon, lat, z)
+ code = interleave_bits(((lon + 180.0) * 2**32 / 360.0).to_i,
+ ((lat + 90.0) * 2**32 / 180.0).to_i)
+ str = ""
+ # add eight to the zoom level, which approximates an accuracy of
+ # one pixel in a tile.
+ ((z + 8)/3.0).ceil.times do |i|
+ digit = (code >> (58 - 6 * i)) & 0x3f
+ str << ARRAY[digit]
+ end
+ # append characters onto the end of the string to represent
+ # partial zoom levels (characters themselves have a granularity
+ # of 3 zoom levels).
+ ((z + 8) % 3).times { str << "=" }
+
+ return str
+ end
+
+ private
+
+ ##
+ # interleaves the bits of two 32-bit numbers. the result is known
+ # as a Morton code.
+ def self.interleave_bits(x, y)
+ c = 0
+ 31.downto(0) do |i|
+ c = (c << 1) | ((x >> i) & 1)
+ c = (c << 1) | ((y >> i) & 1)
+ end
+ c
+ end
+
+end