public double getSimilarity(FastIDSet cluster1, FastIDSet cluster2)
        {
            if (cluster1.isEmpty() || cluster2.isEmpty())
            {
                return(Double.NaN);
            }
            double leastSimilarity = Double.PositiveInfinity;
            var    someUsers       = SamplingLongPrimitiveIterator.maybeWrapIterator(cluster1.GetEnumerator(), samplingRate);

            while (someUsers.MoveNext())
            {
                long userID1 = someUsers.Current;
                var  it2     = cluster2.GetEnumerator();
                while (it2.MoveNext())
                {
                    double theSimilarity = similarity.userSimilarity(userID1, it2.Current);
                    if (theSimilarity < leastSimilarity)
                    {
                        leastSimilarity = theSimilarity;
                    }
                }
            }
            // We skipped everything? well, at least try comparing the first Users to get some value
            if (leastSimilarity == Double.PositiveInfinity)
            {
                return(similarity.userSimilarity(cluster1.GetEnumerator().Current, cluster2.GetEnumerator().Current));
            }
            return(leastSimilarity);
        }