89 lines
2.6 KiB
Ruby
89 lines
2.6 KiB
Ruby
##
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# Encodes and decodes locations from Morton-coded "quad tile" strings. Each
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# variable-length string encodes to a precision of one pixel per tile (roughly,
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# since this computation is done in lat/lon coordinates, not mercator).
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# Each character encodes 3 bits of x and 3 of y, so there are extra characters
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# tacked on the end to make the zoom levels "work".
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module ShortLink
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# array of 64 chars to encode 6 bits. this is almost like base64 encoding, but
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# the symbolic chars are different, as base64's + and / aren't very
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# URL-friendly.
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ARRAY = ("A".."Z").to_a + ("a".."z").to_a + ("0".."9").to_a + ["_", "~"]
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class << self
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##
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# Given a string encoding a location, returns the [lon, lat, z] tuple of that
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# location.
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def decode(str)
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x = 0
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y = 0
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z = 0
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z_offset = 0
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# keep support for old shortlinks which use the @ character, now
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# replaced by the ~ character because twitter is horribly broken
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# and we can't have that.
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str.tr!("@", "~")
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str.each_char do |c|
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t = ARRAY.index c
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if t.nil?
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z_offset -= 1
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else
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3.times do
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x <<= 1
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x |= 1 unless (t & 32).zero?
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t <<= 1
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y <<= 1
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y |= 1 unless (t & 32).zero?
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t <<= 1
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end
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z += 3
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end
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end
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# pack the coordinates out to their original 32 bits.
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x <<= (32 - z)
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y <<= (32 - z)
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# project the parameters back to their coordinate ranges.
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[(x * 360.0 / (2**32)) - 180.0,
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(y * 180.0 / (2**32)) - 90.0,
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z - 8 - (z_offset % 3)]
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end
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##
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# given a location and zoom, return a short string representing it.
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def encode(lon, lat, z)
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code = interleave_bits(((lon + 180.0) * (2**32) / 360.0).to_i,
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((lat + 90.0) * (2**32) / 180.0).to_i)
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str = ""
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# add eight to the zoom level, which approximates an accuracy of
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# one pixel in a tile.
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((z + 8) / 3.0).ceil.times do |i|
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digit = (code >> (58 - (6 * i))) & 0x3f
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str << ARRAY[digit]
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end
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# append characters onto the end of the string to represent
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# partial zoom levels (characters themselves have a granularity
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# of 3 zoom levels).
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((z + 8) % 3).times { str << "-" }
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str
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end
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private
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##
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# interleaves the bits of two 32-bit numbers. the result is known
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# as a Morton code.
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def interleave_bits(x, y)
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c = 0
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31.downto(0) do |i|
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c = (c << 1) | ((x >> i) & 1)
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c = (c << 1) | ((y >> i) & 1)
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end
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c
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end
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end
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end
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