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// reweight.h
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// Copyright 2005-2010 Google, Inc.
// Author: allauzen@google.com (Cyril Allauzen)
//
// \file
// Function to reweight an FST.
#ifndef FST_LIB_REWEIGHT_H__
#define FST_LIB_REWEIGHT_H__
#include <vector>
using std::vector;
#include <fst/mutable-fst.h>
namespace fst {
enum ReweightType { REWEIGHT_TO_INITIAL, REWEIGHT_TO_FINAL };
// Reweight FST according to the potentials defined by the POTENTIAL
// vector in the direction defined by TYPE. Weight needs to 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\wq when
// reweighting towards the initial state and by pw/q when reweighting
// towards the final states.
template <class Arc>
void Reweight(MutableFst<Arc> *fst,
const vector<typename Arc::Weight> &potential,
ReweightType type) {
typedef typename Arc::Weight Weight;
if (fst->NumStates() == 0)
return;
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;
}
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> > sit(*fst);
for (; !sit.Done(); sit.Next()) {
typename Arc::StateId state = sit.Value();
if (state == potential.size())
break;
typename Arc::Weight weight = potential[state];
if (weight != Weight::Zero()) {
for (MutableArcIterator< MutableFst<Arc> > ait(fst, state);
!ait.Done();
ait.Next()) {
Arc arc = ait.Value();
if (arc.nextstate >= potential.size())
continue;
typename Arc::Weight 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);
ait.SetValue(arc);
}
if (type == REWEIGHT_TO_INITIAL)
fst->SetFinal(state, Divide(fst->Final(state), weight, DIVIDE_LEFT));
}
if (type == REWEIGHT_TO_FINAL)
fst->SetFinal(state, Times(weight, fst->Final(state)));
}
// This handles elements past the end of the potentials array.
for (; !sit.Done(); sit.Next()) {
typename Arc::StateId state = sit.Value();
if (type == REWEIGHT_TO_FINAL)
fst->SetFinal(state, Times(Weight::Zero(), fst->Final(state)));
}
typename Arc::Weight startweight = fst->Start() < potential.size() ?
potential[fst->Start()] : Weight::Zero();
if ((startweight != Weight::One()) && (startweight != Weight::Zero())) {
if (fst->Properties(kInitialAcyclic, true) & kInitialAcyclic) {
typename Arc::StateId state = fst->Start();
for (MutableArcIterator< MutableFst<Arc> > ait(fst, state);
!ait.Done();
ait.Next()) {
Arc arc = ait.Value();
if (type == REWEIGHT_TO_INITIAL)
arc.weight = Times(startweight, arc.weight);
else
arc.weight = Times(
Divide(Weight::One(), startweight, DIVIDE_RIGHT),
arc.weight);
ait.SetValue(arc);
}
if (type == REWEIGHT_TO_INITIAL)
fst->SetFinal(state, Times(startweight, fst->Final(state)));
else
fst->SetFinal(state, Times(Divide(Weight::One(), startweight,
DIVIDE_RIGHT),
fst->Final(state)));
} else {
typename Arc::StateId state = fst->AddState();
Weight w = type == REWEIGHT_TO_INITIAL ? startweight :
Divide(Weight::One(), startweight, DIVIDE_RIGHT);
Arc arc(0, 0, w, fst->Start());
fst->AddArc(state, arc);
fst->SetStart(state);
}
}
fst->SetProperties(ReweightProperties(
fst->Properties(kFstProperties, false)),
kFstProperties);
}
} // namespace fst
#endif // FST_LIB_REWEIGHT_H_
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