defmodule Microwaveprop.Radio.Maidenhead do
  @moduledoc false

  # Maidenhead grid: Field (2 letters A-R) + Square (2 digits) + optional
  # repeating pairs of Subsquare (2 letters) and Extended Square (2 digits).
  # Minimum 4 chars, always even length. E.g. EM12, EM12kp, EM12kp37, EM12kp37ab
  @spec valid?(any()) :: boolean()
  def valid?(nil), do: false

  def valid?(grid) when is_binary(grid) do
    len = byte_size(grid)
    len >= 4 and rem(len, 2) == 0 and String.match?(grid, grid_regex())
  end

  def valid?(_), do: false

  # Field: [A-Ra-r]{2}, Square: [0-9]{2}, then alternating letter/digit pairs
  defp grid_regex do
    ~r/^[A-Ra-r]{2}[0-9]{2}(?:[A-Xa-x]{2}(?:[0-9]{2}(?:[A-Xa-x]{2}(?:[0-9]{2})*)*)*)*$/
  end

  @spec to_latlon(any()) :: {:ok, {float(), float()}} | :error
  def to_latlon(nil), do: :error

  def to_latlon(grid) when is_binary(grid) do
    grid = String.upcase(grid)

    if valid?(grid) do
      <<f1, f2, s1, s2>> <> rest = grid

      # Field: 18 divisions (A-R), 20° lon / 10° lat each
      lon = (f1 - ?A) * 20.0 - 180.0
      lat = (f2 - ?A) * 10.0 - 90.0

      # Square: 10 divisions (0-9), 2° lon / 1° lat each
      lon = lon + (s1 - ?0) * 2.0
      lat = lat + (s2 - ?0) * 1.0

      chars = String.to_charlist(rest)
      {lat, lon, lat_size, lon_size} = decode_pairs(chars, lat, lon, 1.0, 2.0, :letter)
      {:ok, {lat + lat_size / 2, lon + lon_size / 2}}
    else
      :error
    end
  end

  def to_latlon(_), do: :error

  # Decode remaining alternating letter(24)/digit(10) pairs after field+square
  defp decode_pairs([c1, c2 | rest], lat, lon, lat_size, lon_size, :letter) do
    cell_lon = lon_size / 24
    cell_lat = lat_size / 24
    lon = lon + (c1 - ?A) * cell_lon
    lat = lat + (c2 - ?A) * cell_lat
    decode_pairs(rest, lat, lon, cell_lat, cell_lon, :digit)
  end

  defp decode_pairs([c1, c2 | rest], lat, lon, lat_size, lon_size, :digit) do
    cell_lon = lon_size / 10
    cell_lat = lat_size / 10
    lon = lon + (c1 - ?0) * cell_lon
    lat = lat + (c2 - ?0) * cell_lat
    decode_pairs(rest, lat, lon, cell_lat, cell_lon, :letter)
  end

  defp decode_pairs([], lat, lon, lat_size, lon_size, _type) do
    {lat, lon, lat_size, lon_size}
  end
end