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tools/openfst-1.6.7/src/include/fst/rmfinalepsilon.h
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// See www.openfst.org for extensive documentation on this weighted
// finite-state transducer library.
//
// Function to remove of final states that have epsilon-only input arcs.
#ifndef FST_RMFINALEPSILON_H_
#define FST_RMFINALEPSILON_H_
#include <unordered_set>
#include <vector>
#include <fst/connect.h>
#include <fst/mutable-fst.h>
namespace fst {
// Removes final states that have epsilon-only input arcs.
template <class Arc>
void RmFinalEpsilon(MutableFst<Arc> *fst) {
using StateId = typename Arc::StateId;
using Weight = typename Arc::Weight;
// Determines the coaccesibility of states.
std::vector<bool> access;
std::vector<bool> coaccess;
uint64 props = 0;
SccVisitor<Arc> scc_visitor(nullptr, &access, &coaccess, &props);
DfsVisit(*fst, &scc_visitor);
// Finds potential list of removable final states. These are final states that
// have no outgoing transitions or final states that have a non-coaccessible
// future.
std::unordered_set<StateId> finals;
for (StateIterator<Fst<Arc>> siter(*fst); !siter.Done(); siter.Next()) {
const auto s = siter.Value();
if (fst->Final(s) != Weight::Zero()) {
bool future_coaccess = false;
for (ArcIterator<Fst<Arc>> aiter(*fst, s); !aiter.Done(); aiter.Next()) {
const auto &arc = aiter.Value();
if (coaccess[arc.nextstate]) {
future_coaccess = true;
break;
}
}
if (!future_coaccess) finals.insert(s);
}
}
// Moves the final weight.
std::vector<Arc> arcs;
for (StateIterator<Fst<Arc>> siter(*fst); !siter.Done(); siter.Next()) {
const auto s = siter.Value();
auto weight = fst->Final(s);
arcs.clear();
for (ArcIterator<Fst<Arc>> aiter(*fst, s); !aiter.Done(); aiter.Next()) {
const auto &arc = aiter.Value();
// Next state is in the list of finals.
if (finals.find(arc.nextstate) != finals.end()) {
// Sums up all epsilon arcs.
if (arc.ilabel == 0 && arc.olabel == 0) {
weight = Plus(Times(fst->Final(arc.nextstate), arc.weight), weight);
} else {
arcs.push_back(arc);
}
} else {
arcs.push_back(arc);
}
}
// If some arcs (epsilon arcs) were deleted, delete all arcs and add back
// only the non-epsilon arcs.
if (arcs.size() < fst->NumArcs(s)) {
fst->DeleteArcs(s);
fst->SetFinal(s, weight);
for (const auto &arc : arcs) fst->AddArc(s, arc);
}
}
Connect(fst);
}
} // namespace fst
#endif // FST_RMFINALEPSILON_H_
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