Caesar Cipher

caesar_cipher.rb

class CaesarCipher
  def initialize(shift)
    @shift = shift
  end

  def call(input)
    [capital_characters, small_characters].each do |chars|
      input = input.tr(chars.join, encrypt(chars).join)  
    end

    input
  end

  private
  def caesar_shift(char_index)
    (char_index - @shift) % 26
  end

  def capital_characters
    ('A'..'Z').to_a
  end

  def small_characters
    ('a'..'z').to_a
  end

  def encrypt(chars)
    chars.each_with_index.map do |c, char_index|
      chars[caesar_shift(char_index)]
    end
  end
end

puts CaesarCipher.new(3).("This is so interesting")