text: // See www.openfst.org for extensive documentation on this weighted
  // finite-state transducer library.
  //
  // Functions and classes to invert an FST.
  
  #ifndef FST_LIB_INVERT_H_
  #define FST_LIB_INVERT_H_
  
  #include <fst/arc-map.h>
  #include <fst/mutable-fst.h>
  
  
  namespace fst {
  
  // Mapper to implement inversion of an arc.
  template <class A>
  struct InvertMapper {
    using FromArc = A;
    using ToArc = A;
  
    InvertMapper() {}
  
    ToArc operator()(const FromArc &arc) {
      return ToArc(arc.olabel, arc.ilabel, arc.weight, arc.nextstate);
    }
  
    MapFinalAction FinalAction() const { return MAP_NO_SUPERFINAL; }
  
    MapSymbolsAction InputSymbolsAction() const { return MAP_CLEAR_SYMBOLS; }
  
    MapSymbolsAction OutputSymbolsAction() const { return MAP_CLEAR_SYMBOLS; }
  
    uint64 Properties(uint64 props) { return InvertProperties(props); }
  };
  
  // Inverts the transduction corresponding to an FST by exchanging the
  // FST's input and output labels. This version modifies its input.
  //
  // Complexity:
  //
  //   Time: O(V + E)
  //   Space: O(1)
  //
  // where V is the number of states and E is the number of arcs.
  template <class Arc>
  inline void Invert(MutableFst<Arc> *fst) {
    std::unique_ptr<SymbolTable> input(
        fst->InputSymbols() ? fst->InputSymbols()->Copy() : nullptr);
    std::unique_ptr<SymbolTable> output(
        fst->OutputSymbols() ? fst->OutputSymbols()->Copy() : nullptr);
    ArcMap(fst, InvertMapper<Arc>());
    fst->SetInputSymbols(output.get());
    fst->SetOutputSymbols(input.get());
  }
  
  // Inverts the transduction corresponding to an FST by exchanging the
  // FST's input and output labels. This version is a delayed FST.
  //
  // Complexity:
  //
  //   Time: O(v + e)
  //   Space: O(1)
  //
  // where v is the number of states visited and e is the number of arcs visited.
  // Constant time and to visit an input state or arc is assumed and exclusive of
  // caching.
  template <class A>
  class InvertFst : public ArcMapFst<A, A, InvertMapper<A>> {
   public:
    using Arc = A;
  
    using Mapper = InvertMapper<Arc>;
    using Impl = internal::ArcMapFstImpl<A, A, InvertMapper<A>>;
  
    explicit InvertFst(const Fst<Arc> &fst)
        : ArcMapFst<Arc, Arc, Mapper>(fst, Mapper()) {
      GetMutableImpl()->SetOutputSymbols(fst.InputSymbols());
      GetMutableImpl()->SetInputSymbols(fst.OutputSymbols());
    }
  
    // See Fst<>::Copy() for doc.
    InvertFst(const InvertFst<Arc> &fst, bool safe = false)
        : ArcMapFst<Arc, Arc, Mapper>(fst, safe) {}
  
    // Get a copy of this InvertFst. See Fst<>::Copy() for further doc.
    InvertFst<Arc> *Copy(bool safe = false) const override {
      return new InvertFst(*this, safe);
    }
  
   private:
    using ImplToFst<Impl>::GetMutableImpl;
  };
  
  // Specialization for InvertFst.
  template <class Arc>
  class StateIterator<InvertFst<Arc>>
      : public StateIterator<ArcMapFst<Arc, Arc, InvertMapper<Arc>>> {
   public:
    explicit StateIterator(const InvertFst<Arc> &fst)
        : StateIterator<ArcMapFst<Arc, Arc, InvertMapper<Arc>>>(fst) {}
  };
  
  // Specialization for InvertFst.
  template <class Arc>
  class ArcIterator<InvertFst<Arc>>
      : public ArcIterator<ArcMapFst<Arc, Arc, InvertMapper<Arc>>> {
   public:
    using StateId = typename Arc::StateId;
  
    ArcIterator(const InvertFst<Arc> &fst, StateId s)
        : ArcIterator<ArcMapFst<Arc, Arc, InvertMapper<Arc>>>(fst, s) {}
  };
  
  // Useful alias when using StdArc.
  using StdInvertFst = InvertFst<StdArc>;
  
  }  // namespace fst
  
  #endif  // FST_LIB_INVERT_H_
  