1 // reverse.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 // Functions and classes to sort arcs in an FST.
18
19 #ifndef FST_LIB_REVERSE_H__
20 #define FST_LIB_REVERSE_H__
21
22 #include "fst/lib/cache.h"
23
24 namespace fst {
25
26 // Reverses an FST. The reversed result is written to an output
27 // MutableFst. If A transduces string x to y with weight a, then the
28 // reverse of A transduces the reverse of x to the reverse of y with
29 // weight a.Reverse().
30 //
31 // Typically, a = a.Reverse() and Arc = RevArc (e.g. for
32 // TropicalWeight or LogWeight). In general, e.g. when the weights
33 // only form a left or right semiring, the output arc type must match
34 // the input arc type except having the reversed Weight type.
35 template<class Arc, class RevArc>
Reverse(const Fst<Arc> & ifst,MutableFst<RevArc> * ofst)36 void Reverse(const Fst<Arc> &ifst, MutableFst<RevArc> *ofst) {
37 typedef typename Arc::StateId StateId;
38 typedef typename Arc::Weight Weight;
39 typedef typename RevArc::Weight RevWeight;
40
41 ofst->DeleteStates();
42 ofst->SetInputSymbols(ifst.InputSymbols());
43 ofst->SetOutputSymbols(ifst.OutputSymbols());
44 StateId istart = ifst.Start();
45 StateId ostart = ofst->AddState();
46 ofst->SetStart(ostart);
47
48 for (StateIterator< Fst<Arc> > siter(ifst);
49 !siter.Done();
50 siter.Next()) {
51 StateId is = siter.Value();
52 StateId os = is + 1;
53 while (ofst->NumStates() <= os)
54 ofst->AddState();
55 if (is == istart)
56 ofst->SetFinal(os, RevWeight::One());
57
58 Weight final = ifst.Final(is);
59 if (final != Weight::Zero()) {
60 RevArc oarc(0, 0, final.Reverse(), os);
61 ofst->AddArc(0, oarc);
62 }
63
64 for (ArcIterator< Fst<Arc> > aiter(ifst, is);
65 !aiter.Done();
66 aiter.Next()) {
67 const Arc &iarc = aiter.Value();
68 RevArc oarc(iarc.ilabel, iarc.olabel, iarc.weight.Reverse(), os);
69 StateId nos = iarc.nextstate + 1;
70 while (ofst->NumStates() <= nos)
71 ofst->AddState();
72 ofst->AddArc(nos, oarc);
73 }
74 }
75 uint64 iprops = ifst.Properties(kCopyProperties, false);
76 uint64 oprops = ofst->Properties(kFstProperties, false);
77 ofst->SetProperties(ReverseProperties(iprops) | oprops, kFstProperties);
78 }
79
80 } // namespace fst
81
82 #endif // FST_LIB_REVERSE_H__
83