rot.ex

  @alphabet_lower ?a..?z
  @alphabet_upper ?A..?Z

  @spec rotate(text :: String.t(), shift :: integer) :: String.t()
  def rotate(text, shift) do
    text
    |> String.to_charlist()
    |> Enum.map(&(rot(&1, shift)))
    |> List.to_string()
  end

  defp rot(char, shift) when char in @alphabet_lower do
    rot(char, shift, @alphabet_lower)
  end
  defp rot(char, shift) when char in @alphabet_upper do
    rot(char, shift, @alphabet_upper)
  end
  defp rot(char, _), do: char

  defp rot(char, shift, alphabet) do
    case Enum.find_index(alphabet, & &1 == char) do
      nil -> raise ArgumentError
      index -> Enum.at(alphabet, Integer.mod(index + shift, 26))
    end
  end