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@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 <unordered_set>
25 using std::unordered_set;
26 using std::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>
RmFinalEpsilon(MutableFst<A> * fst)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