Ruby Day 3 Livecode - Iterators and Blocks | Caesar's salad (encryption)

encrypt.rb

def encrypt(message, encryption_key = -3)
  # Creates an alphabet array using a range
  alphabet = ("A".."Z").to_a

  # Converts the message (a String) into an upcased
  # array of letters
  letters = message.upcase.chars

  # Iterate (with #map) over each letter
  letters.map do |letter|
    # Finds the index of the current letter in the alphabet
    # this returns an integer if the letter is found
    # or nil if not found
    index = alphabet.index(letter)

    # Checks if the letter was found or not
    if index.nil?
      # Returns (implicitly) just the same letter if not found
      # (this will be the case for spaces and punctuation marks)
      letter
    else
      # If the letter was indeed found we:
      # - Use the encryption key on the index
      # - and do a modulo (remainder) operation to limit
      # - the number to the amount of letter in the alphabet
      new_index = (index + encryption_key) % alphabet.size

      # Get and return (implicitly) the new equivalent letter
      # from the alphabet
      alphabet[new_index]
    end
  end.join # joins the array of letters into a string again
end

# The decrypt method uses the same encyption key
def decrypt(message, encryption_key = -3)
  # But calls encrypt again with its key sign (+/-) reversed
  encrypt(message, encryption_key * -1)
end

p decrypt("QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD", -3)
p decrypt(
  "FK ZOVMQLDOXMEV, X ZXBPXO ZFMEBO, XIPL HKLTK XP ZXBPXO'P ZFMEBO, QEB PEFCQ ZFMEBO, ZXBPXO'P ZLAB LO ZXBPXO PEFCQ, FP LKB LC QEB PFJMIBPQ XKA JLPQ TFABIV HKLTK BKZOVMQFLK QBZEKFNRBP. FQ FP X QVMB LC PRYPQFQRQFLK ZFMEBO FK TEFZE BXZE IBQQBO FK QEB MIXFKQBUQ FP OBMIXZBA YV X IBQQBO PLJB CFUBA KRJYBO LC MLPFQFLKP ALTK QEB XIMEXYBQ. CLO BUXJMIB, TFQE X IBCQ PEFCQ LC 3, A TLRIA YB OBMIXZBA YV X, B TLRIA YBZLJB Y, XKA PL LK. QEB JBQELA FP KXJBA XCQBO GRIFRP ZXBPXO, TEL RPBA FQ FK EFP MOFSXQB ZLOOBPMLKABKZB.",
  -3
)

encrypt_spec.rb

require_relative "encrypt"

describe "#encrypt" do
  it "returns a string" do
    actual = encrypt("hi").class
    expected = String
    expect(actual).to eq(expected)
  end

  it "returns an encrypted message" do
    actual = encrypt("QUICK")
    expected = "NRFZH"
    expect(actual).to eq(expected)
  end

  it "returns an encrypted message" do
    actual = encrypt("THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG")
    expected = "QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD"
    expect(actual).to eq(expected)
  end
end