static List<Tuple<int, double>> NearestNeighbours(int user, int k, double threshold, IDataSet data)
        {
            var userRated = data.Data[user].Keys;
            var similarity = data.Data
                .Where(u => u.Key != user && u.Value.Keys.Except(userRated).Any())
                .Select(u => new Tuple<int, double>(u.Key, data.EuclideanSimilarity(user, u.Key)))
                .Where(u => u.Item2 >= threshold)
                .OrderByDescending(x => x.Item2)
                .Take(k)
                .ToList();
            return similarity;
        }