[Sync Iteration] elixir/atbash-cipher/2

solutions/elixir/atbash-cipher/2/lib/atbash.ex

@@ -0,0 +1,29 @@
+defmodule Atbash do  
+  @doc """
+  Encode a given plaintext to the corresponding ciphertext
+
+  ## Examples
+
+  iex> Atbash.encode("completely insecure")
+  "xlnko vgvob rmhvx fiv"
+  """
+  @spec encode(String.t()) :: String.t()
+  def encode(plaintext) do
+    plaintext
+    |> String.downcase()
+    |> String.to_charlist()
+    |> Enum.map_join(&transpose/1)
+    |> String.replace(~r/(.{5})\B/, "\\1 ")
+  end
+
+  @spec decode(String.t()) :: String.t()
+  def decode(cipher) do
+    cipher
+    |> String.to_charlist()
+    |> Enum.map_join(&transpose/1)
+  end
+
+  defp transpose(char) when char in ?0..?9, do: <<char>>
+  defp transpose(char) when char in ?a..?z, do: <<?a + ?z - char>>
+  defp transpose(_), do: ""
+end