Background

I have come up with a solution to the Caeser Cipher challenge in Exercism.io. This solution works and passes all tests, but I am still not happy.

Code

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()
    |> Enum.map( fn char ->
      cond do
        is_lower_case?( char ) -> rotate_lower_case( char, shift )
        is_upper_case?( char ) -> rotate_upper_case( char, shift )
        true -> char
      end
    end )
    |> to_string()
  end

  defp is_lower_case?( char ), do: char >= 97 && char <= 122
  defp rotate_lower_case( char, shift ), do: 97 + rem( char - 71 + shift, 26 )

  defp is_upper_case?( char ), do: char >= 65 && char <= 90
  defp rotate_upper_case( char, shift ), do: 65 + rem( char - 39 + shift, 26 )
end

So, given a string and a shift, I have to shift all the letters. Punctuation, numbers and spaces remain the same.

I achieve this by converting the string to a charlist, mapping over it, and then converting the result into a string again. But in my eyes, there is room for better.

Possible improvements

1. I had to define is_upper_case? and is_lower_case? for characters. Isn’t there a built in function that already does this?
2. In my Enum.map I have a function that I would like to extract into a defp, for the sake of clarity. But I still don’t know how to do it and I only found answer that tell me how to use anonymous functions with the terse syntax. How would I call an already defined function that takes more than 1 parameter inside an Enum.map ?
3. I am using a conditional, even though I have the feeling I shouldn’t be using one yet. The exercise is supposed to be about string only but I am already using control flow. I have the feeling I am over complicating. Perhaps there is a simpler solution?

Let me know what you guys think!