Home | History | Annotate | Download | only in lib 
      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>
     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