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      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 // Copyright 2005-2010 Google, Inc.
     16 // Author: johans (at) google.com (Johan Schalkwyk)
     17 //
     18 // \file
     19 // Function to remove of final states that have epsilon only input arcs.
     20 
     21 #ifndef FST_LIB_RMFINALEPSILON_H__
     22 #define FST_LIB_RMFINALEPSILON_H__
     23 
     24 #include <tr1/unordered_set>
     25 using std::tr1::unordered_set;
     26 using std::tr1::unordered_multiset;
     27 #include <vector>
     28 using std::vector;
     29 
     30 #include <fst/connect.h>
     31 #include <fst/mutable-fst.h>
     32 
     33 
     34 namespace fst {
     35 
     36 template<class A>
     37 void RmFinalEpsilon(MutableFst<A>* fst) {
     38   typedef typename A::StateId StateId;
     39   typedef typename A::Weight Weight;
     40 
     41   // Determine the coaccesibility of states.
     42   vector<bool> access;
     43   vector<bool> coaccess;
     44   uint64 props = 0;
     45   SccVisitor<A> scc_visitor(0, &access, &coaccess, &props);
     46   DfsVisit(*fst, &scc_visitor);
     47 
     48   // Find potential list of removable final states. These are final states
     49   // that have no outgoing transitions or final states that have a
     50   // non-coaccessible future. Complexity O(S)
     51   unordered_set<StateId> finals;
     52   for (StateIterator<Fst<A> > siter(*fst); !siter.Done(); siter.Next()) {
     53     StateId s = siter.Value();
     54     if (fst->Final(s) != Weight::Zero()) {
     55       bool future_coaccess = false;
     56       for (ArcIterator<Fst<A> > aiter(*fst, s); !aiter.Done(); aiter.Next()) {
     57         const A& arc = aiter.Value();
     58         if (coaccess[arc.nextstate]) {
     59           future_coaccess = true;
     60           break;
     61         }
     62       }
     63       if (!future_coaccess) {
     64         finals.insert(s);
     65       }
     66     }
     67   }
     68 
     69   // Move the final weight. Complexity O(E)
     70   vector<A> arcs;
     71   for (StateIterator<Fst<A> > siter(*fst); !siter.Done(); siter.Next()) {
     72     StateId s = siter.Value();
     73     Weight w(fst->Final(s));
     74 
     75     arcs.clear();
     76     for (ArcIterator<Fst<A> > aiter(*fst, s); !aiter.Done(); aiter.Next()) {
     77       const A& arc = aiter.Value();
     78       // is next state in the list of finals
     79       if (finals.find(arc.nextstate) != finals.end()) {
     80         // sum up all epsilon arcs
     81         if (arc.ilabel == 0 && arc.olabel == 0) {
     82           w = Plus(Times(fst->Final(arc.nextstate), arc.weight), w);
     83         } else {
     84           arcs.push_back(arc);
     85         }
     86       } else {
     87         arcs.push_back(arc);
     88       }
     89     }
     90 
     91     // If some arcs (epsilon arcs) were deleted, delete all
     92     // arcs and add back only the non epsilon arcs
     93     if (arcs.size() < fst->NumArcs(s)) {
     94       fst->DeleteArcs(s);
     95       fst->SetFinal(s, w);
     96       for (size_t i = 0; i < arcs.size(); ++i) {
     97         fst->AddArc(s, arcs[i]);
     98       }
     99     }
    100   }
    101 
    102   Connect(fst);
    103 }
    104 
    105 }  // namespace fst
    106 
    107 #endif  // FST_LIB_RMFINALEPSILON_H__
    108