code: function entropy(str) {
      if (!str) return 0;
      return Math.round(str.length * (Math.log(alphabetSize(str)) / Math.log(2)));
  }
docs: Calculate [information entropy](https://en.wikipedia.org/wiki/Password_strength#Entropy_as_a_measure_of_password_strength) 
  @param {String} str String to calculate entropy for
  @returns {Number} entropy