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// prune.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.
//
// Author: allauzen@cs.nyu.edu (Cyril Allauzen)
//
// \file
// Functions implementing pruning.
#ifndef FST_LIB_PRUNE_H__
#define FST_LIB_PRUNE_H__
#include "fst/lib/arcfilter.h"
#include "fst/lib/shortest-distance.h"
namespace fst {
template <class A, class ArcFilter>
class PruneOptions {
public:
typedef typename A::Weight Weight;
// Pruning threshold.
Weight threshold;
// Arc filter.
ArcFilter filter;
// If non-zero, passes in pre-computed shortest distance from initial state
// (possibly resized).
vector<Weight> *idistance;
// If non-zero, passes in pre-computed shortest distance to final states
// (possibly resized).
vector<Weight> *fdistance;
PruneOptions(const Weight& t, ArcFilter f, vector<Weight> *id = 0,
vector<Weight> *fd = 0)
: threshold(t), filter(f), idistance(id), fdistance(fd) {}
};
// Pruning algorithm: this version modifies its input and it takes an
// options class as an argment. Delete states and arcs in 'fst' that
// do not belong to a successful path whose weight is no more than
// 'opts.threshold' Times() the weight of the shortest path. Weights
// need to be commutative and have the path property.
template <class Arc, class ArcFilter>
void Prune(MutableFst<Arc> *fst,
const PruneOptions<Arc, ArcFilter> &opts) {
typedef typename Arc::Weight Weight;
typedef typename Arc::StateId StateId;
if ((Weight::Properties() & (kPath | kCommutative))
!= (kPath | kCommutative))
LOG(FATAL) << "Prune: Weight needs to have the path property and"
<< " be commutative: "
<< Weight::Type();
StateId ns = fst->NumStates();
if (ns == 0) return;
vector<Weight> *idistance = opts.idistance;
vector<Weight> *fdistance = opts.fdistance;
if (!idistance) {
idistance = new vector<Weight>(ns, Weight::Zero());
ShortestDistance(*fst, idistance, false);
} else {
idistance->resize(ns, Weight::Zero());
}
if (!fdistance) {
fdistance = new vector<Weight>(ns, Weight::Zero());
ShortestDistance(*fst, fdistance, true);
} else {
fdistance->resize(ns, Weight::Zero());
}
vector<StateId> dead;
dead.push_back(fst->AddState());
NaturalLess<Weight> less;
Weight ceiling = Times((*fdistance)[fst->Start()], opts.threshold);
for (StateId state = 0; state < ns; ++state) {
if (less(ceiling, Times((*idistance)[state], (*fdistance)[state]))) {
dead.push_back(state);
continue;
}
for (MutableArcIterator< MutableFst<Arc> > it(fst, state);
!it.Done();
it.Next()) {
Arc arc = it.Value();
if (!opts.filter(arc)) continue;
Weight weight = Times(Times((*idistance)[state], arc.weight),
(*fdistance)[arc.nextstate]);
if(less(ceiling, weight)) {
arc.nextstate = dead[0];
it.SetValue(arc);
}
}
if (less(ceiling, Times((*idistance)[state], fst->Final(state))))
fst->SetFinal(state, Weight::Zero());
}
fst->DeleteStates(dead);
if (!opts.idistance)
delete idistance;
if (!opts.fdistance)
delete fdistance;
}
// Pruning algorithm: this version modifies its input and simply takes
// the pruning threshold as an argument. Delete states and arcs in
// 'fst' that do not belong to a successful path whose weight is no
// more than 'opts.threshold' Times() the weight of the shortest
// path. Weights need to be commutative and have the path property.
template <class Arc>
void Prune(MutableFst<Arc> *fst, typename Arc::Weight threshold) {
PruneOptions<Arc, AnyArcFilter<Arc> > opts(threshold, AnyArcFilter<Arc>());
Prune(fst, opts);
}
// Pruning algorithm: this version writes the pruned input Fst to an
// output MutableFst and it takes an options class as an argument.
// 'ofst' contains states and arcs that belong to a successful path in
// 'ifst' whose weight is no more than 'opts.threshold' Times() the
// weight of the shortest path. Weights need to be commutative and
// have the path property.
template <class Arc, class ArcFilter>
void Prune(const Fst<Arc> &ifst,
MutableFst<Arc> *ofst,
const PruneOptions<Arc, ArcFilter> &opts) {
typedef typename Arc::Weight Weight;
typedef typename Arc::StateId StateId;
if ((Weight::Properties() & (kPath | kCommutative))
!= (kPath | kCommutative))
LOG(FATAL) << "Prune: Weight needs to have the path property and"
<< " be commutative: "
<< Weight::Type();
ofst->DeleteStates();
if (ifst.Start() == kNoStateId)
return;
vector<Weight> *idistance = opts.idistance;
vector<Weight> *fdistance = opts.fdistance;
if (!idistance) {
idistance = new vector<Weight>;
ShortestDistance(ifst, idistance, false);
}
if (!fdistance) {
fdistance = new vector<Weight>;
ShortestDistance(ifst, fdistance, true);
}
vector<StateId> copy;
NaturalLess<Weight> less;
while (fdistance->size() <= ifst.Start())
fdistance->push_back(Weight::Zero());
Weight ceiling = Times((*fdistance)[ifst.Start()], opts.threshold);
for (StateIterator< Fst<Arc> > sit(ifst);
!sit.Done();
sit.Next()) {
StateId state = sit.Value();
while (idistance->size() <= state)
idistance->push_back(Weight::Zero());
while (fdistance->size() <= state)
fdistance->push_back(Weight::Zero());
while (copy.size() <= state)
copy.push_back(kNoStateId);
if (less(ceiling, Times((*idistance)[state], (*fdistance)[state])))
continue;
if (copy[state] == kNoStateId)
copy[state] = ofst->AddState();
if (!less(ceiling, Times((*idistance)[state], ifst.Final(state))))
ofst->SetFinal(copy[state], ifst.Final(state));
for (ArcIterator< Fst<Arc> > ait(ifst, state);
!ait.Done();
ait.Next()) {
Arc arc = ait.Value();
if (!opts.filter(arc)) continue;
while (idistance->size() <= arc.nextstate)
idistance->push_back(Weight::Zero());
while (fdistance->size() <= arc.nextstate)
fdistance->push_back(Weight::Zero());
while (copy.size() <= arc.nextstate)
copy.push_back(kNoStateId);
Weight weight = Times(Times((*idistance)[state], arc.weight),
(*fdistance)[arc.nextstate]);
if (!less(ceiling, weight)) {
if (copy[arc.nextstate] == kNoStateId)
copy[arc.nextstate] = ofst->AddState();
arc.nextstate = copy[arc.nextstate];
ofst->AddArc(copy[state], arc);
}
}
}
ofst->SetStart(copy[ifst.Start()]);
if (!opts.idistance)
delete idistance;
if (!opts.fdistance)
delete fdistance;
}
// Pruning algorithm: this version writes the pruned input Fst to an
// output MutableFst and simply takes the pruning threshold as an
// argument. 'ofst' contains states and arcs that belong to a
// successful path in 'ifst' whose weight is no more than
// 'opts.threshold' Times() the weight of the shortest path. Weights
// need to be commutative and have the path property.
template <class Arc>
void Prune(const Fst<Arc> &ifst,
MutableFst<Arc> *ofst,
typename Arc::Weight threshold) {
PruneOptions<Arc, AnyArcFilter<Arc> > opts(threshold, AnyArcFilter<Arc>());
Prune(ifst, ofst, opts);
}
} // namespace fst
#endif // FST_LIB_PRUNE_H_
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