Caesar Cipher Implementation in Ruby

caesar_cipher.rb

# CaesarCipher
# Example:
#   cipher = CaesarCipher.new "Ruby is a dynamic, open source programming language!"
#   cipher.encrypt(13)
#   => "ehol vf n qlanzvp, bcra fbhepr cebtenzzvat ynathntr!"
class CaesarCipher
  attr_accessor :sentence

  def initialize(sentence)
    @sentence = sentence
  end

  def decrypt(i)
    compute { |index| index - i }
  end

  def encrypt(i)
    compute { |index| index + i }
  end

  private

  def compute
    @sentence.downcase.each_char.each_with_object(String.new) do |char, str|
      if alphabet.has_key?(char)
        str << alphabet.key(yield(alphabet.fetch(char)) % 26)
      else
        str << char
      end
    end
  end

  def alphabet
    @alphabet ||= ("a".."z").each_with_index.to_h
  end
end

caesar_cipher_test.rb

require 'minitest/spec'
require 'minitest/autorun'

$LOAD_PATH.unshift File.expand_path('.', File.dirname(__FILE__))
require 'caesar_cipher.rb'

describe CaesarCipher do
  subject { CaesarCipher.new "Hello World!" }

  it { subject.must_be_instance_of CaesarCipher }
  it { subject.sentence.must_equal "Hello World!" }

  describe "#encrypt" do
    subject { CaesarCipher.new "Hello World!" }
    it { subject.encrypt(1).must_equal "ifmmp xpsme!" }
    it { subject.encrypt(13).must_equal "uryyb jbeyq!" }
  end

  describe "#decrypt" do
    subject { CaesarCipher.new "uryyb jbeyq!" }
    it { subject.decrypt(13).must_equal "hello world!" }
  end
end

example.rb

$LOAD_PATH.unshift File.expand_path('.', File.dirname(__FILE__))
require 'caesar_cipher.rb'

WORDS = ["what", "how", "why", "when", "where", "which", "is", "the", "of", "by", "on"]

question = "jung vf gur orfg jro senzrjbex?"
response = "Qtax Nm Qzhkr hr ne bntqrd sgd adrs vda eqzldvnqj!"

def detect(cipher)
  26.times.each do |i|
    decrypt = cipher.decrypt(i)
    puts decrypt if WORDS.detect { |word| decrypt.include?(word) }
  end
end

cipher = CaesarCipher.new(question)
detect(cipher)

cipher.sentence = response
detect(cipher)