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
27namespace fst {
28
29template<class A>
30void 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