caesar_cipher.rb

class String
  def encrypt(k)
    lower_alphabet = [*'a'..'z']
    lower_translation_map = lower_alphabet.zip(lower_alphabet.rotate(k%26)).to_h
    upper_alphabet = [*'A'..'Z']
    upper_translation_map = upper_alphabet.zip(upper_alphabet.rotate(k%26)).to_h
    translation_map = lower_translation_map.merge(upper_translation_map)

    chars.map { |char| translation_map[char] || char }.join
  end

  def decrypt(k)
    lower_alphabet = [*'a'..'z']
    lower_translation_map = lower_alphabet.rotate(k%26).zip(lower_alphabet).to_h
    upper_alphabet = [*'A'..'Z']
    upper_translation_map = upper_alphabet.rotate(k%26).zip(upper_alphabet).to_h
    translation_map = lower_translation_map.merge(upper_translation_map)

    chars.map { |char| translation_map[char] || char }.join
  end
end