Atbash Cipher

atbash.rb

class Atbash
  LETTERS = %w(A B C D E F G H I J K L M N O P Q R S T U V W X Y Z)
  NUMBERS = %w(0 1 2 3 4 5 6 7 8 9)
  PUNCTUATION = [' ', ',', '.']

  def self.encode(str)
    cipher = []

    str.chars.each do |char|
      next if PUNCTUATION.include?(char)
      cipher << char if NUMBERS.include?(char)
      cipher << encode_character(char).downcase if LETTERS.include?(char.upcase)
    end

    cipher.join('').scan(/.{5}|.+/).join(' ')
  end

  def self.encode_character(char)
    char_index = LETTERS.index(char.upcase)
    half_alphabet_size = (LETTERS.size / 2) # 13
    first_half_of_alphabet  = LETTERS[0...half_alphabet_size]
    second_half_of_alphabet = LETTERS[half_alphabet_size...LETTERS.size]
    encoded_char_index = (half_alphabet_size - 1) - char_index

    if char_index >= half_alphabet_size
      return first_half_of_alphabet[encoded_char_index]
    else
      return second_half_of_alphabet[encoded_char_index]
    end

    nil
  end

  private_class_method :encode_character
end