[Sync Iteration] elixir/atbash-cipher/1

solutions/elixir/atbash-cipher/1/lib/atbash.ex

@@ -0,0 +1,40 @@
+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.to_charlist |> a_to_z()
+
+  defp a_to_z(charlist, cipher \\ "", group_pos \\ 0)
+  defp a_to_z([], cipher, _), do: cipher
+  defp a_to_z([char | _] = charlist, cipher, 5)
+    when char in ?0..?9 or char in ?a..?z or char in ?A..?Z,
+    do: a_to_z(charlist, cipher <> " ", 0)
+
+  defp a_to_z([char | tail], cipher, group_pos) when char in ?0..?9,
+    do: a_to_z(tail, cipher <> <<char>>, group_pos + 1)
+
+  defp a_to_z([char | tail], cipher, group_pos) when char in ?a..?z,
+    do: a_to_z(tail, cipher <> <<?a + ?z - char>>, group_pos + 1)
+
+  defp a_to_z([char | tail], cipher, group_pos) when char in ?A..?Z,
+    do: a_to_z(tail, cipher <> <<?a + ?Z - char>>, group_pos + 1)
+
+  defp a_to_z([_ | tail], cipher, group_pos), do: a_to_z(tail, cipher, group_pos)
+
+  @spec decode(String.t()) :: String.t()
+  def decode(cipher), do: cipher |> String.to_charlist |> z_to_a()
+
+  defp z_to_a(charlist, plaintext \\ "")
+  defp z_to_a([], plaintext), do: plaintext
+  defp z_to_a([char | tail], plaintext) when char in ?0..?9, do: z_to_a(tail, plaintext <> <<char>>)
+  defp z_to_a([char | tail], plaintext) when char in ?a..?z,
+    do: z_to_a(tail, plaintext <> <<?a + ?z - char>>)
+
+  defp z_to_a([_ | tail], plaintext), do: z_to_a(tail, plaintext)
+end