Graham McIntire
737de10bf4
Extend grid validation and decoding from 4/6 chars to unlimited even-length grids (8, 10, etc). Recursive decoder handles alternating letter(/24) and digit(/10) subdivision pairs after the field+square. Validated against gridmap-web's Maidenhead implementation.
69 lines
2.2 KiB
Elixir
69 lines
2.2 KiB
Elixir
defmodule Microwaveprop.Radio.Maidenhead do
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@moduledoc false
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# Maidenhead grid: Field (2 letters A-R) + Square (2 digits) + optional
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# repeating pairs of Subsquare (2 letters) and Extended Square (2 digits).
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# Minimum 4 chars, always even length. E.g. EM12, EM12kp, EM12kp37, EM12kp37ab
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@spec valid?(any()) :: boolean()
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def valid?(nil), do: false
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def valid?(grid) when is_binary(grid) do
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len = byte_size(grid)
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len >= 4 and rem(len, 2) == 0 and String.match?(grid, grid_regex())
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end
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def valid?(_), do: false
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# Field: [A-Ra-r]{2}, Square: [0-9]{2}, then alternating letter/digit pairs
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defp grid_regex do
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~r/^[A-Ra-r]{2}[0-9]{2}(?:[A-Xa-x]{2}(?:[0-9]{2}(?:[A-Xa-x]{2}(?:[0-9]{2})*)*)*)*$/
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end
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@spec to_latlon(any()) :: {:ok, {float(), float()}} | :error
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def to_latlon(nil), do: :error
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def to_latlon(grid) when is_binary(grid) do
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grid = String.upcase(grid)
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if valid?(grid) do
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<<f1, f2, s1, s2>> <> rest = grid
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# Field: 18 divisions (A-R), 20° lon / 10° lat each
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lon = (f1 - ?A) * 20.0 - 180.0
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lat = (f2 - ?A) * 10.0 - 90.0
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# Square: 10 divisions (0-9), 2° lon / 1° lat each
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lon = lon + (s1 - ?0) * 2.0
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lat = lat + (s2 - ?0) * 1.0
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chars = String.to_charlist(rest)
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{lat, lon, lat_size, lon_size} = decode_pairs(chars, lat, lon, 1.0, 2.0, :letter)
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{:ok, {lat + lat_size / 2, lon + lon_size / 2}}
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else
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:error
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end
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end
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def to_latlon(_), do: :error
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# Decode remaining alternating letter(24)/digit(10) pairs after field+square
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defp decode_pairs([c1, c2 | rest], lat, lon, lat_size, lon_size, :letter) do
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cell_lon = lon_size / 24
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cell_lat = lat_size / 24
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lon = lon + (c1 - ?A) * cell_lon
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lat = lat + (c2 - ?A) * cell_lat
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decode_pairs(rest, lat, lon, cell_lat, cell_lon, :digit)
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end
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defp decode_pairs([c1, c2 | rest], lat, lon, lat_size, lon_size, :digit) do
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cell_lon = lon_size / 10
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cell_lat = lat_size / 10
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lon = lon + (c1 - ?0) * cell_lon
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lat = lat + (c2 - ?0) * cell_lat
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decode_pairs(rest, lat, lon, cell_lat, cell_lon, :letter)
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end
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defp decode_pairs([], lat, lon, lat_size, lon_size, _type) do
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{lat, lon, lat_size, lon_size}
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end
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end
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