public static RecommenderResult RankUsers(
            this IMatrixFactorizationRecommender recommender,
            string userId,
            IEnumerable <KeyValuePair <string, Dictionary <string, double> > > userItems)
        {
            if (recommender == null)
            {
                throw new ArgumentNullException(nameof(recommender));
            }

            if (userId == null)
            {
                throw new ArgumentNullException(nameof(userId));
            }

            if (userItems == null)
            {
                throw new ArgumentNullException(nameof(userItems));
            }

            return(recommender.RankUsers(userId, userItems.Select(o => new KeyValuePair <string, UserFeatures>(o.Key, recommender.ComputeUserFeatures(o.Value))).ToList()));
        }
        public static RecommenderResult RankUsers(
            this IMatrixFactorizationRecommender recommender,
            UserFeatures user,
            List <KeyValuePair <string, UserFeatures> > users)
        {
            if (recommender == null)
            {
                throw new ArgumentNullException(nameof(recommender));
            }

            if (user == null)
            {
                throw new ArgumentNullException(nameof(user));
            }

            if (users == null)
            {
                throw new ArgumentNullException(nameof(users));
            }

            return(recommender.RankUsers(user, users, RecommenderResultBuilderFactory.Instance));
        }