code: public static <E> double jaccardCoefficient(Counter<E> c1, Counter<E> c2) {
      double minCount = 0.0, maxCount = 0.0;
      for (E key : Sets.union(c1.keySet(), c2.keySet())) {
        double count1 = c1.getCount(key);
        double count2 = c2.getCount(key);
        minCount += (count1 < count2 ? count1 : count2);
        maxCount += (count1 > count2 ? count1 : count2);
      }
      return minCount / maxCount;
    }
docs: Returns the Jaccard Coefficient of the two counters. Calculated as |c1
  intersect c2| / ( |c1| + |c2| - |c1 intersect c2|
  
  @return The Jaccard Coefficient of the two counters