[Sync Iteration] elixir/rotational-cipher/1

solutions/elixir/rotational-cipher/1/lib/rotational_cipher.ex

@@ -0,0 +1,26 @@
+defmodule RotationalCipher do
+  @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 |> String.to_charlist() |> shift_letters(shift)
+
+  defp shift_letters(charlist, shift, encoded_text \\ "")
+  defp shift_letters([], _, encoded_text), do: encoded_text
+  defp shift_letters([char | tail], shift, encoded_text) when char in ?A..?Z do
+    encoded_char = if (char + shift) > ?Z, do: char - 26 + shift, else: char + shift
+    shift_letters(tail, shift, encoded_text <> <<encoded_char>>)
+  end
+
+  defp shift_letters([char | tail], shift, encoded_text) when char in ?a..?z do
+    encoded_char = if (char + shift) > ?z, do: char - 26 + shift, else: char + shift
+    shift_letters(tail, shift, encoded_text <> <<encoded_char>>)
+  end
+
+  defp shift_letters([char | tail], shift, encoded_text),
+    do: shift_letters(tail, shift, encoded_text <> <<char>>)
+end