Hypergraph FKAlgorithmA::transversal (const Hypergraph& H) const {
BOOST_LOG_TRIVIAL(debug) << "Starting FKA. Hypergraph has "
<< H.num_verts() << " vertices and "
<< H.num_edges() << " edges.";
Hypergraph G (H.num_verts());
Hypergraph Hmin = H.minimization();
Hypergraph::Edge V = Hmin.verts_covered();
bool still_searching_for_transversals = true;
while (still_searching_for_transversals) {
Hypergraph::Edge omit_set = find_omit_set(Hmin, G);
if (omit_set.none() and G.num_edges() > 0) {
BOOST_LOG_TRIVIAL(debug) << "Received empty omit_set, so we're done.";
still_searching_for_transversals = false;
} else {
Hypergraph::Edge new_hs = V - omit_set;
Hypergraph::Edge new_mhs = minimize_new_hs(H, G, new_hs);
BOOST_LOG_TRIVIAL(trace) << "Received witness."
<< "\nomit_set:\t" << omit_set
<< "\nMHS:\t\t" << new_mhs;
G.add_edge(new_mhs, true);
BOOST_LOG_TRIVIAL(debug) << "New G size: " << G.num_edges();
}
}
return G;
}
Hypergraph ParBMAlgorithm::transversal (const Hypergraph& H) const {
BOOST_LOG_TRIVIAL(debug) << "Starting BM with " << num_threads
<< " threads. Hypergraph has "
<< H.num_verts() << " vertices and "
<< H.num_edges() << " edges.";
// Set up inputs
Hypergraph Hmin = H.minimization();
Hypergraph G (H.num_verts());
Hypergraph::Edge V = Hmin.verts_covered();
// Initialize using any HS we can find
Hypergraph::Edge first_hs = FKAlgorithm::minimize_new_hs(Hmin, G, V);
G.add_edge(first_hs);
// Grow G until it covers all vertices
bool G_has_full_coverage = false;
while (not G_has_full_coverage) {
Hypergraph::Edge new_hs = V - coverage_condition_check(H, G);
if (new_hs.is_proper_subset_of(V)) {
Hypergraph::Edge new_mhs = FKAlgorithm::minimize_new_hs(Hmin, G, new_hs);
G.add_edge(new_mhs);
} else {
G_has_full_coverage = true;
}
}
// Apply the BM algorithm repeatedly, generating new transversals
// until duality is confirmed
bool still_searching_for_transversals = true;
Hypergraph::EdgeQueue new_hses, new_mhses;
#pragma omp parallel shared(Hmin, G, new_hses, new_mhses) num_threads(num_threads)
#pragma omp master
while (still_searching_for_transversals) {
find_new_hses(Hmin, G, Hmin.verts_covered(), new_hses);
if (new_hses.size_approx() == 0) {
still_searching_for_transversals = false;
} else {
minimize_new_hses(Hmin, G, new_hses, new_mhses);
Hypergraph::Edge new_mhs;
while (new_mhses.try_dequeue(new_mhs)) {
// The results will all be inclusion-minimal, but
// there may be some overlap. Thus, we try to add
// them...
try {
G.add_edge(new_mhs, true);
}
// But ignore any minimality_violated_exception
// that is thrown.
catch (minimality_violated_exception& e) {}
}
BOOST_LOG_TRIVIAL(debug) << "New |G|: " << G.num_edges();
}
}
return G;
}
Hypergraph::Edge FKAlgorithmA::find_omit_set (const Hypergraph& F,
const Hypergraph& G) {
/**
Test whether F and G are dual.
If so, return an empty edge as a notification.
If not, return a omit_set as per FK.
**/
BOOST_LOG_TRIVIAL(trace) << "Beginning run with "
<< "|F| = " << F.num_edges()
<< " and |G| = " << G.num_edges();
// Input specification
assert(F.num_verts() == G.num_verts());
// Create an empty omit_set to use as temporary storage
Hypergraph::Edge omit_set (F.num_verts());
// FK step 1: initialize if G is empty
if (G.num_edges() == 0) {
BOOST_LOG_TRIVIAL(trace) << "Returning empty omit_set since G is empty.";
return omit_set;
}
// FK step 2: consistency checks
// Check 1.1: hitting condition
omit_set = hitting_condition_check(F, G);
if (omit_set.any()) {
return omit_set;
}
// Check 1.2: same vertices covered
omit_set = coverage_condition_check(F, G);
if (omit_set.any()) {
return omit_set;
}
// Check 1.3: neither F nor G has edges too large
omit_set = edge_size_check(F, G);
if (omit_set.any()) {
return omit_set;
}
// Check 2.1: satisfiability count condition
omit_set = satisfiability_count_check(F, G);
if (omit_set.any()) {
return omit_set;
}
// FK step 3: Check whether F and G are small
// If either hypergraph is empty, they cannot be dual
omit_set = small_hypergraphs_check(F, G);
if (omit_set.any()) {
return omit_set;
}
// FK step 4: Recurse
// Find the most frequently occurring vertex
Hypergraph::EdgeIndex max_freq_vert = most_frequent_vertex(F, G);
// Then we compute the split hypergraphs F0, F1, G0, and G1
std::pair<Hypergraph, Hypergraph> Fsplit, Gsplit;
Hypergraph F0, F1, G0, G1;
Fsplit = split_hypergraph_over_vertex(F, max_freq_vert);
F0 = Fsplit.first;
F1 = Fsplit.second;
Gsplit = split_hypergraph_over_vertex(G, max_freq_vert);
G0 = Gsplit.first;
G1 = Gsplit.second;
// We will also need the unions F0∪F1 and G0∪G1
Hypergraph Fnew = minimized_union(F0, F1);
Hypergraph Gnew = minimized_union(G0, G1);
// And, finally, fire up the two recursions
if (F1.num_edges() > 0 and Gnew.num_edges() > 0) {
BOOST_LOG_TRIVIAL(trace) << "Side 1 recursion.";
Hypergraph::Edge omit_set = find_omit_set(F1, Gnew);
if (omit_set.any()) {
return omit_set;
}
}
if (Fnew.num_edges() > 0 and G1.num_edges() > 0) {
BOOST_LOG_TRIVIAL(trace) << "Side 2 recursion.";
Hypergraph::Edge omit_set = find_omit_set(Fnew, G1);
if (omit_set.any()) {
omit_set.set(max_freq_vert);
return omit_set;
}
}
// If we make it this far, we did not find a nonempty
// omit_set, so the pair is dual
return omit_set;
}
void ParBMAlgorithm::find_new_hses (const Hypergraph& H,
const Hypergraph& G,
const Hypergraph::Edge& c,
Hypergraph::EdgeQueue& results) const {
/**
Find any new hitting sets of H^c with respect to G_c, queueing the
results.
**/
// Construct the reduced hypergraphs
Hypergraph Hc = H.contraction(c, false);
Hypergraph Gc = G.restriction(c);
BOOST_LOG_TRIVIAL(trace) << "Starting transversal build with |Hc| = " << Hc.num_edges()
<< " and |Gc| = " << Gc.num_edges();
// Initialize a candidate hitting set to store intermediate results
Hypergraph::Edge new_hs = Hc.verts_covered();
// Step 1: Initialize Gc if it is empty by returning the set of all vertices
if (Gc.num_edges() == 0) {
// If any edge of Hc is empty, this is a dead end
for (const auto& e: Hc) {
if (e.none()) {
return;
}
}
// Otherwise, the support of Hc is a new hitting set
results.enqueue(new_hs);
return;
}
// Step 2: Handle |Gc| = 1
if (Gc.num_edges() == 1) {
assert(Hc.num_edges() != 0);
// Check whether H has a singleton for every vertex it covers
Hypergraph::Edge Hc_verts_to_cover = Gc[0];
for (const auto& e: Hc) {
if (e.count() == 1) {
Hc_verts_to_cover -= e;
}
}
if (Hc_verts_to_cover.none()) {
// If so, this is a dead end
return;
} else {
// If not, we choose some vertex that was hit by Gc but was
// not a singleton in Hc
Hypergraph::EdgeIndex uncovered_vertex = Hc_verts_to_cover.find_first();
new_hs.reset(uncovered_vertex);
results.enqueue(new_hs);
return;
}
// We definitely shouldn't ever be here!
throw std::runtime_error("invalid state in |Gc|=1 case.");
}
// Step 3: Split up subcases using a full cover
// Construct I according to lemmas 7 and 8
Hypergraph::Edge I = Gc.vertices_with_degree_above_threshold(0.5);
// Per lemma 7, look for an edge that does not intersect I
Hypergraph::Edge missed_edge = find_missed_edge(Hc, I);
// If found, form a full cover
if (missed_edge.any()) {
Hypergraph C = l4_full_cover(Hc, missed_edge);
find_new_hses_fork(H, G, C, results);
return;
}
// Per lemma 8, look for a transversal that covers I
Hypergraph::Edge subtrans_edge = find_subset_edge(Gc, I);
// If found, form a full cover
if (subtrans_edge.any()) {
Hypergraph C = l5_full_cover(Hc, subtrans_edge);
find_new_hses_fork(H, G, C, results);
return;
}
// If we reach this point, I itself is a new HS
results.enqueue(I);
return;
}
