exercism/elixir/rotational-cipher/lib/rotational_cipher.ex

defmodule RotationalCipher do
  @lower ?a..?z
  @upper ?A..?Z

  defguard is_lower(char) when char in @lower
  defguard is_upper(char) when char in @upper
  defguard is_letter(char) when char in @lower or char in @upper

  defp rotate_char(char, shift) when is_lower(char) and is_lower(char + shift), do: char + shift
  defp rotate_char(char, shift) when is_upper(char) and is_upper(char + shift), do: char + shift
  defp rotate_char(char, shift) when is_letter(char), do: char + shift - 26
  defp rotate_char(char, _shift) when not is_letter(char), do: char

  @doc """
  Given a plaintext and amount to shift by, return a rotated string.

  Example:
  iex> RotationalCipher.rotate("Attack at dawn", 13)
  "Nggnpx ng qnja"
  """

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