defmodule Microwaveprop.Radio.Maidenhead do
  @moduledoc false

  @spec valid?(any()) :: boolean()
  def valid?(nil), do: false

  def valid?(grid) when is_binary(grid) do
    String.match?(grid, ~r/^[A-Ra-r]{2}[0-9]{2}$/) or
      String.match?(grid, ~r/^[A-Ra-r]{2}[0-9]{2}[A-Xa-x]{2}$/)
  end

  def valid?(_), do: false

  @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

      lon = (f1 - ?A) * 20 + (s1 - ?0) * 2 - 180
      lat = (f2 - ?A) * 10 + (s2 - ?0) * 1 - 90

      case rest do
        <<ss1, ss2>> ->
          lon = lon + (ss1 - ?A) * (5 / 60) + 5 / 120
          lat = lat + (ss2 - ?A) * (2.5 / 60) + 2.5 / 120
          {:ok, {lat, lon}}

        "" ->
          {:ok, {lat + 0.5, lon + 1.0}}
      end
    else
      :error
    end
  end

  def to_latlon(_), do: :error
end