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>
23 using __gnu_cxx::hash_set;
24
25 #include "fst/lib/connect.h"
26 #include "fst/lib/mutable-fst.h"
27
28 namespace fst {
29
30 template<class A>
RmFinalEpsilon(MutableFst<A> * fst)31 void 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