code: def __calculate_links(self, cluster1, cluster2):
          
          
          number_links = 0;
          
          for index1 in cluster1:
              for index2 in cluster2:
                  number_links += self.__adjacency_matrix[index1][index2];
                  
          return number_links;
docs: !
          @brief Returns number of link between two clusters. 
          @details Link between objects (points) exists only if distance between them less than connectivity radius.
          
          @param[in] cluster1 (list): The first cluster.
          @param[in] cluster2 (list): The second cluster.
          
          @return (uint) Number of links between two clusters.