ceasar_cipher_stream.exs

defmodule CeasarCipher do
  @lower Enum.to_list(?a..?z)
  @upper Enum.to_list(?A..?Z)

  defp map_within_range(c, range, shift) do
    if c in range do
      range
      |> Stream.cycle()
      |> Stream.drop(shift + (c - hd(range)))
      |> Stream.take(1)
    else
      [c]
    end
  end

  defp mapping(c, shift) when c in @lower,
    do: map_within_range(c, @lower, shift)
  defp mapping(c, shift) when c in @upper,
    do: map_within_range(c, @upper, shift)
  defp mapping(c, _shift),
    do: [c]

  def encode(string, shift) do
    string
    |> String.to_charlist()
    |> Stream.flat_map(&mapping(&1, shift))
    |> Enum.to_list()
    |> to_string()
  end
  def decode(string, shift) do
    encode(string, length(@lower) - shift)
  end
end

string = "Hello World"
target_shift = 10

string
|> CeasarCipher.encode(target_shift)
|> IO.inspect()
|> CeasarCipher.decode(target_shift)
|> IO.inspect()