set_locale

defmodule SetLocale.Headers do

  def extract_accept_language(conn) do
    case Plug.Conn.get_req_header(conn, "accept-language") do
      [value | _] ->
        value
        |> String.split(",")
        |> Enum.map(&parse_language_option/1)
        |> Enum.sort(&(&1.quality > &2.quality))
        |> Enum.map(&(&1.tag))
        |> Enum.reject(&is_nil/1)
        |> ensure_language_fallbacks()

      _ ->
        []
    end
  end

  defp parse_language_option(string) do
    captures = Regex.named_captures(~r/^\s?(?<tag>[\w\-]+)(?:;q=(?<quality>[\d\.]+))?$/i, string)

    quality = case Float.parse(captures["quality"] || "1.0") do
      {val, _} -> val
      _ -> 1.0
    end

    %{tag: captures["tag"], quality: quality}
  end

  defp ensure_language_fallbacks(tags) do
    Enum.flat_map tags, fn tag ->
      case String.split(tag, "-") do
        [language, _country_variant] -> if Enum.member?(tags, language), do: [tag], else: [tag, language]
        [_language] -> [tag]
      end
    end
  end
end