// for documentation see technical reports of christian schulz
void contraction::contract(const PartitionConfig & partition_config,
graph_access & G,
graph_access & coarser,
const Matching & edge_matching,
const CoarseMapping & coarse_mapping,
const NodeID & no_of_coarse_vertices,
const NodePermutationMap & permutation) const {
if(partition_config.combine) {
coarser.resizeSecondPartitionIndex(no_of_coarse_vertices);
}
std::vector<NodeID> new_edge_targets(G.number_of_edges());
forall_edges(G, e) {
new_edge_targets[e] = coarse_mapping[G.getEdgeTarget(e)];
} endfor
std::vector<EdgeID> edge_positions(no_of_coarse_vertices, UNDEFINED_EDGE);
//we dont know the number of edges jet, so we use the old number for
//construction of the coarser graph and then resize the field according
//to the number of edges we really got
coarser.start_construction(no_of_coarse_vertices, G.number_of_edges());
NodeID cur_no_vertices = 0;
forall_nodes(G, n) {
NodeID node = permutation[n];
//we look only at the coarser nodes
if(coarse_mapping[node] != cur_no_vertices)
continue;
NodeID coarseNode = coarser.new_node();
coarser.setNodeWeight(coarseNode, G.getNodeWeight(node));
if(partition_config.combine) {
coarser.setSecondPartitionIndex(coarseNode, G.getSecondPartitionIndex(node));
}
// do something with all outgoing edges (in auxillary graph)
forall_out_edges(G, e, node) {
visit_edge(G, coarser, edge_positions, coarseNode, e, new_edge_targets);
} endfor
// implements our version of gal combine
// compute a matching between blocks (greedily)
// extend to partition
// apply our refinements and tabu search
void gal_combine::perform_gal_combine( PartitionConfig & config, graph_access & G) {
//first greedily compute a matching of the partitions
std::vector< std::unordered_map<PartitionID, unsigned> > counters(config.k);
forall_nodes(G, node) {
//boundary_pair bp;
if(counters[G.getPartitionIndex(node)].find(G.getSecondPartitionIndex(node)) != counters[G.getPartitionIndex(node)].end()) {
counters[G.getPartitionIndex(node)][G.getSecondPartitionIndex(node)] += 1;
} else {
counters[G.getPartitionIndex(node)][G.getSecondPartitionIndex(node)] = 1;
}
} endfor
std::vector< PartitionID > permutation(config.k);
for( unsigned i = 0; i < permutation.size(); i++) {
permutation[i] = i;
}
random_functions::permutate_vector_good_small(permutation);
std::vector<bool> rhs_matched(config.k, false);
std::vector<PartitionID> bipartite_matching(config.k);
for( unsigned i = 0; i < permutation.size(); i++) {
PartitionID cur_partition = permutation[i];
PartitionID best_unassigned = config.k;
NodeWeight best_value = 0;
for( std::unordered_map<PartitionID, unsigned>::iterator it = counters[cur_partition].begin();
it != counters[cur_partition].end(); ++it) {
if( rhs_matched[it->first] == false && it->second > best_value ) {
best_unassigned = it->first;
best_value = it->second;
}
}
bipartite_matching[cur_partition] = best_unassigned;
if( best_unassigned != config.k ) {
rhs_matched[best_unassigned] = true;
}
}
std::vector<bool> blocked_vertices(G.number_of_nodes(), false);
forall_nodes(G, node) {
if( bipartite_matching[G.getPartitionIndex(node)] == G.getSecondPartitionIndex(node) ){
blocked_vertices[node] = true;
} else {
// we will reassign this vertex since the partitions do not agree on it
G.setPartitionIndex(node, config.k);
}
} endfor
construct_partition cp;
cp.construct_starting_from_partition( config, G );
refinement* refine = new mixed_refinement();
double real_epsilon = config.imbalance/100.0;
double epsilon = random_functions::nextDouble(real_epsilon+0.005,real_epsilon+config.kabaE_internal_bal);
PartitionConfig copy = config;
copy.upper_bound_partition = (1+epsilon)*ceil(config.largest_graph_weight/(double)config.k);
complete_boundary boundary(&G);
boundary.build();
tabu_search ts;
ts.perform_refinement( copy, G, boundary);
//now obtain the quotient graph
complete_boundary boundary2(&G);
boundary2.build();
copy = config;
copy.upper_bound_partition = (1+epsilon)*ceil(config.largest_graph_weight/(double)config.k);
refine->perform_refinement( copy, G, boundary2);
copy = config;
cycle_refinement cr;
cr.perform_refinement(config, G, boundary2);
delete refine;
}