code: def calculate_entropy(self, entropy_string):
          
          total = 0
          for char in entropy_string:
              if char.isalpha():
                  prob = self.frequency[char.lower()]
                  total += - math.log(prob) / math.log(2)
          logging.debug("Entropy score: {0}".format(total))
          return total
docs: Calculates the entropy of a string based on known frequency of
          English letters.
  
          Args:
              entropy_string: A str representing the string to calculate.
  
          Returns:
              A negative float with the total entropy of the string (higher
              is better).