class CaesarCipher

  def initialize(string, rot=13)
    @string = string
    @rot = rot
  end

  def perform
    small_alphabet = [*"a".."z"]
    big_alphabet = [*"A".."Z"]
    string = @string.split("")
    string.each.with_index do |letter, index|
      if small_alphabet.include?(letter)
        position = (small_alphabet.index(letter) + @rot) % 26
        string[index] = small_alphabet[position]
      elsif big_alphabet.include?(letter)
        position = (big_alphabet.index(letter) + @rot) % 26
        string[index] = big_alphabet[position]
      end
    end
    string.join("")
  end
end