Rot13 in Elixir

rot13.ex

defmodule Rotator do
  def encode(string, shift \\ 13)
  def encode(<<char>> <> tail, shift) do
    <<rotate(char, shift)>> <> encode(tail, shift)
  end

  def encode(_, _), do: ""

  def rotate(char, shift) when ((char >= ?a) and (char <= ?z)) do
    ?a + rem(char - ?a + shift, 26)
  end

  def rotate(char, shift) when ((char >= ?A) and (char <= ?Z)) do
    ?A + rem(char - ?A + shift, 26)
  end

  def rotate(char, _), do: char
end

IO.puts Rotator.encode("Attack at dawn!") #=> "Nggnpx ng qnja!"
IO.puts Rotator.encode("Attack at dawn!", 1) #=> "Buubdl bu ebxo!"
IO.puts Rotator.encode("Attack at dawn!", 0) #=> "Attack at dawn!"