Caesar cipher in Ruby

Question

I must implement a simple Caesar cipher class. Initializer takes two parameters - number of positions to shift the alphabet (integer) and the alphabet used (as string, not necessarily the standard English alphabet). The class must have two instance methods encrypt and decrypt, which take a string and do the respective action based on the parameters of the initializer. These are my rspec tests:

describe "Caesar" do

  latin_encrypter = Caesar.new 4
  dna_encrypter = Caesar.new 3, 'ACTG'

  it 'encrypts correctly' do
    expect(latin_encrypter.encrypt 'hello').to eq "lipps"
  end

  it 'encrypts correctly using DNA alphabet' do
    expect(dna_encrypter.encrypt 'ACCTGA').to eq "GAACTG"
  end

  it 'decrypts correctly' do
    expect(latin_encrypter.decrypt 'lipps').to eq 'hello'
  end

  it 'decrypts correctly using DNA alphabet' do
    expect(dna_encrypter.decrypt 'GAACTG').to eq 'ACCTGA'
  end
end

and this is my implementation:

class Caesar
  def initialize(shift, alphabet = 'abcdefghijklmnopqrstuvwxyz')
    @shift = shift % alphabet.size
    @alphabet = alphabet
  end

  def encrypt(string)
    string.chars.map { |char| @alphabet[@alphabet.index(char) + @shift - @alphabet.size] }.join
  end

  def decrypt(string)
    string.chars.map { |char| @alphabet[@alphabet.index(char) - @shift] }.join
  end
end

Is this an optimal algorithm for encryption/decryption or is there a better one (perhaps using gsub)?

Answer 1

Ruby has tr, a very efficient method for substituting one character to another. It does not error on "unknown" characters, like spaces etc. Using that, the Caesar class becomes:

class Caesar
  def initialize(shift, alphabet = ('a'..'z').to_a.join)
    i = shift % alphabet.size #I like this
    @decrypt = alphabet
    @encrypt = alphabet[i..-1] + alphabet[0...i]
  end

  def encrypt(string)
    string.tr(@decrypt, @encrypt)
  end

  def decrypt(string)
    string.tr(@encrypt, @decrypt)
  end
end

Answer 2

The only (conceptual) problem I see in your code is that both encrypt/decrypt perform a O(n) operation (detect) for every character (so at then end it's a O(n*m) algorithm), that's unnecessarily inefficient. Build a hash object to use as an indexer:

class Caesar
  def initialize(shift, alphabet = ('a'..'z').to_a.join)
    @shift = shift
    @alphabet = alphabet
    @indexes = alphabet.chars.map.with_index.to_h
  end

  def encrypt(string)
    string.chars.map { |c| @alphabet[(@indexes[c] + @shift) % @alphabet.size] }.join
  end

  def decrypt(string)
    string.chars.map { |c| @alphabet[(@indexes[c] - @shift) % @alphabet.size] }.join
  end
end

You can also build encrypter/decrypter hash tables at initialization:

class Caesar
  def initialize(shift, alphabet = ('a'..'z').to_a.join)
    chars = alphabet.chars.to_a
    @encrypter = chars.zip(chars.rotate(shift)).to_h
    @decrypter = @encrypter.invert
  end

  def encrypt(string)
    @encrypter.values_at(*string.chars).join
  end

  def decrypt(string)
    @decrypter.values_at(*string.chars).join
  end
end

Answer 3

Your code does not work with letters like "A..Z". The following does:

puts "Text for encrypt please: "
text_for_encrypt = gets.chomp

puts "Input encrypt key: "
key = gets.chomp.to_i

text_arrow = text_for_encrypt.split('').to_a
alphabet_az = ("a".."z").to_a.join
alphabet_AZ = ("A".."Z").to_a.join
i = key % alphabet_az.size


encrypt_az = alphabet_az.chars.rotate(i).join
encrypt_AZ = alphabet_AZ.chars.rotate(i).join

res = []
text_arrow.each do |letter|
  if ("a".."z") === letter
    letter = letter.tr( alphabet_az, encrypt_az )
    res << letter
  else
    letter = letter.tr( alphabet_AZ, encrypt_AZ )
    res << letter
  end
end

puts "Your code: #{res.join}"