rmfinalepsilon.h revision 4a68b3365c8c50aa93505e99ead2565ab73dcdb0 
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 <ext/hash_set>
23using __gnu_cxx::hash_set;
24
25#include "fst/lib/connect.h"
26#include "fst/lib/mutable-fst.h"
27
28namespace fst {
29
30template<class A>
31void RmFinalEpsilon(MutableFst<A>* fst) {
32  typedef typename A::StateId StateId;
33  typedef typename A::Weight Weight;
34
35  // Determine the coaccesibility of states.
36  vector<bool> access;
37  vector<bool> coaccess;
38  uint64 props = 0;
39  SccVisitor<A> scc_visitor(0, &access, &coaccess, &props);
40  DfsVisit(*fst, &scc_visitor);
41
42  // Find potential list of removable final states. These are final states
43  // that have no outgoing transitions or final states that have a
44  // non-coaccessible future. Complexity O(S)
45  hash_set<StateId> finals;
46  for (StateIterator<Fst<A> > siter(*fst); !siter.Done(); siter.Next()) {
47    StateId s = siter.Value();
48    if (fst->Final(s) != Weight::Zero()) {
49      bool future_coaccess = false;
50      for (ArcIterator<Fst<A> > aiter(*fst, s); !aiter.Done(); aiter.Next()) {
51        const A& arc = aiter.Value();
52        if (coaccess[arc.nextstate]) {
53          future_coaccess = true;
54          break;
55        }
56      }
57      if (!future_coaccess) {
58        finals.insert(s);
59      }
60    }
61  }
62
63  // Move the final weight. Complexity O(E)
64  vector<A> arcs;
65  for (StateIterator<Fst<A> > siter(*fst); !siter.Done(); siter.Next()) {
66    StateId s = siter.Value();
67    Weight w(fst->Final(s));
68
69    arcs.clear();
70    for (ArcIterator<Fst<A> > aiter(*fst, s); !aiter.Done(); aiter.Next()) {
71      const A& arc = aiter.Value();
72      // is next state in the list of finals
73      if (finals.find(arc.nextstate) != finals.end()) {
74        // sum up all epsilon arcs
75        if (arc.ilabel == 0 && arc.olabel == 0) {
76          w = Plus(Times(fst->Final(arc.nextstate), arc.weight), w);
77        } else {
78          arcs.push_back(arc);
79        }
80      } else {
81        arcs.push_back(arc);
82      }
83    }
84
85    // If some arcs (epsilon arcs) were deleted, delete all
86    // arcs and add back only the non epsilon arcs
87    if (arcs.size() < fst->NumArcs(s)) {
88      fst->DeleteArcs(s);
89      fst->SetFinal(s, w);
90      for (size_t i = 0; i < arcs.size(); ++i) {
91        fst->AddArc(s, arcs[i]);
92      }
93    }
94  }
95
96  Connect(fst);
97}
98
99}
100
101#endif  // FST_LIB_RMFINALEPSILON_H__
102