abingo.rb

  #read more about abingo here: http://www.bingocardcreator.com/abingo/

  def self.find_alternative_for_user(test_name, alternatives)
    alternatives_array = retrieve_alternatives(test_name, alternatives)
    alternatives_array[self.modulo_choice(test_name, alternatives_array.size)]
  end

  #Quickly determines what alternative to show a given user.  Given a test name
  #and their identity, we hash them together (which, for MD5, provably introduces
  #enough entropy that we don't care) otherwise
  def self.modulo_choice(test_name, choices_count)
    Digest::MD5.hexdigest(Abingo.salt.to_s + test_name + self.identity.to_s).to_i(16) % choices_count
  end