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tools/openfst-1.6.7/include/fst/reweight.h
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// See www.openfst.org for extensive documentation on this weighted
// finite-state transducer library.
//
// Function to reweight an FST.
#ifndef FST_REWEIGHT_H_
#define FST_REWEIGHT_H_
#include <vector>
#include <fst/log.h>
#include <fst/mutable-fst.h>
namespace fst {
enum ReweightType { REWEIGHT_TO_INITIAL, REWEIGHT_TO_FINAL };
// Reweights an FST according to a vector of potentials in a given direction.
// The weight must be left distributive when reweighting towards the initial
// state and right distributive when reweighting towards the final states.
//
// An arc of weight w, with an origin state of potential p and destination state
// of potential q, is reweighted by p^-1 \otimes (w \otimes q) when reweighting
// torwards the initial state, and by (p \otimes w) \otimes q^-1 when
// reweighting towards the final states.
template <class Arc>
void Reweight(MutableFst<Arc> *fst,
const std::vector<typename Arc::Weight> &potential,
ReweightType type) {
using Weight = typename Arc::Weight;
if (fst->NumStates() == 0) return;
// TODO(kbg): Make this a compile-time static_assert once we have a pleasant
// way to "deregister" this operation for non-distributive semirings so an
// informative error message is produced.
if (type == REWEIGHT_TO_FINAL && !(Weight::Properties() & kRightSemiring)) {
FSTERROR() << "Reweight: Reweighting to the final states requires "
<< "Weight to be right distributive: " << Weight::Type();
fst->SetProperties(kError, kError);
return;
}
// TODO(kbg): Make this a compile-time static_assert once we have a pleasant
// way to "deregister" this operation for non-distributive semirings so an
// informative error message is produced.
if (type == REWEIGHT_TO_INITIAL && !(Weight::Properties() & kLeftSemiring)) {
FSTERROR() << "Reweight: Reweighting to the initial state requires "
<< "Weight to be left distributive: " << Weight::Type();
fst->SetProperties(kError, kError);
return;
}
StateIterator<MutableFst<Arc>> siter(*fst);
for (; !siter.Done(); siter.Next()) {
const auto s = siter.Value();
if (s == potential.size()) break;
const auto &weight = potential[s];
if (weight != Weight::Zero()) {
for (MutableArcIterator<MutableFst<Arc>> aiter(fst, s); !aiter.Done();
aiter.Next()) {
auto arc = aiter.Value();
if (arc.nextstate >= potential.size()) continue;
const auto &nextweight = potential[arc.nextstate];
if (nextweight == Weight::Zero()) continue;
if (type == REWEIGHT_TO_INITIAL) {
arc.weight =
Divide(Times(arc.weight, nextweight), weight, DIVIDE_LEFT);
}
if (type == REWEIGHT_TO_FINAL) {
arc.weight =
Divide(Times(weight, arc.weight), nextweight, DIVIDE_RIGHT);
}
aiter.SetValue(arc);
}
if (type == REWEIGHT_TO_INITIAL) {
fst->SetFinal(s, Divide(fst->Final(s), weight, DIVIDE_LEFT));
}
}
if (type == REWEIGHT_TO_FINAL) {
fst->SetFinal(s, Times(weight, fst->Final(s)));
}
}
// This handles elements past the end of the potentials array.
for (; !siter.Done(); siter.Next()) {
const auto s = siter.Value();
if (type == REWEIGHT_TO_FINAL) {
fst->SetFinal(s, Times(Weight::Zero(), fst->Final(s)));
}
}
const auto startweight = fst->Start() < potential.size()
? potential[fst->Start()]
: Weight::Zero();
if ((startweight != Weight::One()) && (startweight != Weight::Zero())) {
if (fst->Properties(kInitialAcyclic, true) & kInitialAcyclic) {
const auto s = fst->Start();
for (MutableArcIterator<MutableFst<Arc>> aiter(fst, s); !aiter.Done();
aiter.Next()) {
auto arc = aiter.Value();
if (type == REWEIGHT_TO_INITIAL) {
arc.weight = Times(startweight, arc.weight);
} else {
arc.weight = Times(Divide(Weight::One(), startweight, DIVIDE_RIGHT),
arc.weight);
}
aiter.SetValue(arc);
}
if (type == REWEIGHT_TO_INITIAL) {
fst->SetFinal(s, Times(startweight, fst->Final(s)));
} else {
fst->SetFinal(s, Times(Divide(Weight::One(), startweight, DIVIDE_RIGHT),
fst->Final(s)));
}
} else {
const auto s = fst->AddState();
const auto weight =
(type == REWEIGHT_TO_INITIAL)
? startweight
: Divide(Weight::One(), startweight, DIVIDE_RIGHT);
fst->AddArc(s, Arc(0, 0, weight, fst->Start()));
fst->SetStart(s);
}
}
fst->SetProperties(ReweightProperties(fst->Properties(kFstProperties, false)),
kFstProperties);
}
} // namespace fst
#endif // FST_REWEIGHT_H_
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