Suggestion for improving performance

[tealloovvee]

Hi, I read your code and would like to suggest some minor edits to improve performance:

1. Self weight can be written directly into a vector
   

   libleidenalg/src/GraphHelper.cpp

   Lines 355 to 372 in a39ae4e

   	for (size_t v = 0; v < n; v++)
   	{
   	#ifdef DEBUG
   	cerr << "\t" << "Size node " << v << ": " << this->node_size(v) << endl;
   	#endif
   	double self_weight = 0.0;
   	// There should be only one self loop
   	igraph_integer_t eid;
   	// Get edge id for self loop
   	igraph_get_eid(this->_graph, &eid, v, v, this->is_directed(), false);
   	if (eid >= 0)
   	self_weight = this->edge_weight(eid);
   	this->_node_self_weights[v] = self_weight;
   	#ifdef DEBUG
   	cerr << "\t" << "Self weight node " << v << ": " << self_weight << endl;
   	#endif
   	}

2. The graph type check can be move outside the loop
   

   libleidenalg/src/GraphHelper.cpp

   Lines 401 to 425 in a39ae4e

   	for (size_t e = 0; e < m; e++) {
   	double w = this->edge_weight(e);
   	this->_total_weight += w;
   	size_t from, to;
   	this->edge(e, from, to);
   	if (this->is_directed()) {
   	this->_strength_in[to] += w;
   	this->_strength_out[from] += w;
   	this->_degree_in[to]++;
   	this->_degree_out[from]++;
   	this->_degree_all[to]++;
   	this->_degree_all[from]++;
   	} else {
   	// we only compute strength_in and degree_in for undirected graphs
   	this->_strength_in[to] += w;
   	this->_strength_in[from] += w;
   	// recall that igraph ignores the mode for undirected graphs
   	this->_degree_in[to]++;
   	this->_degree_in[from]++;
   	}
   	}

   
   

   libleidenalg/src/GraphHelper.cpp

   Lines 743 to 744 in a39ae4e

   	if (from == to && !this->is_directed())
   	w /= 2.0;

3. for each v_community, we reinit an empty vector. Two ideas: move this vector outside loop + clear after the iteration (capacity not changed); add vector reserve if we can simple find realistic upper bound for number of records
   

   libleidenalg/src/GraphHelper.cpp

   Lines 727 to 732 in a39ae4e

   	for (size_t v_comm = 0; v_comm < n_collapsed; v_comm++) {
   	vector<size_t> neighbour_communities;
   	for (size_t v : community_memberships[v_comm]) {
   	for (size_t e : this->get_neighbour_edges(v, IGRAPH_OUT)) {
   	size_t from, to;
   	this->edge(e, from, to);

[vtraag]

Thank you for your suggestions @tealloovvee ! If you open a PR it is easier for me to see what you exactly have in mind.