Home | History | Annotate | Download | only in lib 
      1 // rmfinalepsilon.h
      2 //
      3 // Licensed under the Apache License, Version 2.0 (the "License");
      4 // you may not use this file except in compliance with the License.
      5 // You may obtain a copy of the License at
      6 //
      7 //      http://www.apache.org/licenses/LICENSE-2.0
      8 //
      9 // Unless required by applicable law or agreed to in writing, software
     10 // distributed under the License is distributed on an "AS IS" BASIS,
     11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     12 // See the License for the specific language governing permissions and
     13 // limitations under the License.
     14 //
     15 //
     16 // \file
     17 // Function to remove of final states that have epsilon only input arcs.
     18 
     19 #ifndef FST_LIB_RMFINALEPSILON_H__
     20 #define FST_LIB_RMFINALEPSILON_H__
     21 
     22 #include <unordered_set>
     23 
     24 #include "fst/lib/connect.h"
     25 #include "fst/lib/mutable-fst.h"
     26 
     27 namespace fst {
     28 
     29 template<class A>
     30 void RmFinalEpsilon(MutableFst<A>* fst) {
     31   typedef typename A::StateId StateId;
     32   typedef typename A::Weight Weight;
     33 
     34   // Determine the coaccesibility of states.
     35   vector<bool> access;
     36   vector<bool> coaccess;
     37   uint64 props = 0;
     38   SccVisitor<A> scc_visitor(0, &access, &coaccess, &props);
     39   DfsVisit(*fst, &scc_visitor);
     40 
     41   // Find potential list of removable final states. These are final states
     42   // that have no outgoing transitions or final states that have a
     43   // non-coaccessible future. Complexity O(S)
     44   std::unordered_set<StateId> finals;
     45   for (StateIterator<Fst<A> > siter(*fst); !siter.Done(); siter.Next()) {
     46     StateId s = siter.Value();
     47     if (fst->Final(s) != Weight::Zero()) {
     48       bool future_coaccess = false;
     49       for (ArcIterator<Fst<A> > aiter(*fst, s); !aiter.Done(); aiter.Next()) {
     50         const A& arc = aiter.Value();
     51         if (coaccess[arc.nextstate]) {
     52           future_coaccess = true;
     53           break;
     54         }
     55       }
     56       if (!future_coaccess) {
     57         finals.insert(s);
     58       }
     59     }
     60   }
     61 
     62   // Move the final weight. Complexity O(E)
     63   vector<A> arcs;
     64   for (StateIterator<Fst<A> > siter(*fst); !siter.Done(); siter.Next()) {
     65     StateId s = siter.Value();
     66     Weight w(fst->Final(s));
     67 
     68     arcs.clear();
     69     for (ArcIterator<Fst<A> > aiter(*fst, s); !aiter.Done(); aiter.Next()) {
     70       const A& arc = aiter.Value();
     71       // is next state in the list of finals
     72       if (finals.find(arc.nextstate) != finals.end()) {
     73         // sum up all epsilon arcs
     74         if (arc.ilabel == 0 && arc.olabel == 0) {
     75           w = Plus(Times(fst->Final(arc.nextstate), arc.weight), w);
     76         } else {
     77           arcs.push_back(arc);
     78         }
     79       } else {
     80         arcs.push_back(arc);
     81       }
     82     }
     83 
     84     // If some arcs (epsilon arcs) were deleted, delete all
     85     // arcs and add back only the non epsilon arcs
     86     if (arcs.size() < fst->NumArcs(s)) {
     87       fst->DeleteArcs(s);
     88       fst->SetFinal(s, w);
     89       for (size_t i = 0; i < arcs.size(); ++i) {
     90         fst->AddArc(s, arcs[i]);
     91       }
     92     }
     93   }
     94 
     95   Connect(fst);
     96 }
     97 
     98 }
     99 
    100 #endif  // FST_LIB_RMFINALEPSILON_H__
    101