# File lib/mirimiri/document.rb, line 81
    def entropy(s)
      en = 0.0
      counts = self.count_words

      s.split.each do |w|
        p_wi = counts[w].to_f/@words.count.to_f
        en += p_wi*Math.log2(p_wi)
      end

      en *= -1
      en
    end