include/fst/edit-fst.h

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// Copyright 2005-2010 Google, Inc.
// Author: dbikel@google.com (Dan Bikel)
//
// An \ref Fst implementation that allows non-destructive edit operations on an
// existing fst.

#ifndef FST_LIB_EDIT_FST_H_
#define FST_LIB_EDIT_FST_H_

#include <vector>
using std::vector;

#include <fst/cache.h>

#include <unordered_map>
using std::unordered_map;
using std::unordered_multimap;

namespace fst {

// The EditFst class enables non-destructive edit operations on a wrapped
// ExpandedFst. The implementation uses copy-on-write semantics at the node
// level: if a user has an underlying fst on which he or she wants to perform a
// relatively small number of edits (read: mutations), then this implementation
// will copy the edited node to an internal MutableFst and perform any edits in
// situ on that copied node. This class supports all the methods of MutableFst
// except for DeleteStates(const vector<StateId> &); thus, new nodes may also be
// added, and one may add transitions from existing nodes of the wrapped fst to
// new nodes.
//
// N.B.: The documentation for Fst::Copy(true) says that its behavior is
// undefined if invoked on an fst that has already been accessed.  This class
// requires that the Fst implementation it wraps provides consistent, reliable
// behavior when its Copy(true) method is invoked, where consistent means
// the graph structure, graph properties and state numbering and do not change.
// VectorFst and CompactFst, for example, are both well-behaved in this regard.

// The EditFstData class is a container for all mutable data for EditFstImpl;
// also, this class provides most of the actual implementation of what EditFst
// does (that is, most of EditFstImpl's methods delegate to methods in this, the
// EditFstData class).  Instances of this class are reference-counted and can be
// shared between otherwise independent EditFstImpl instances. This scheme
// allows EditFstImpl to implement the thread-safe, copy-on-write semantics
// required by Fst::Copy(true).
//
// template parameters:
//   A the type of arc to use
//   WrappedFstT the type of fst wrapped by the EditFst instance that
//     this EditFstData instance is backing
//   MutableFstT the type of mutable fst to use internally for edited states;
//     crucially, MutableFstT::Copy(false) *must* yield an fst that is
//     thread-safe for reading (VectorFst, for example, has this property)
template <typename A,
          typename WrappedFstT = ExpandedFst<A>,
          typename MutableFstT = VectorFst<A> >
class EditFstData {
 public:
  typedef A Arc;
  typedef typename A::Weight Weight;
  typedef typename A::StateId StateId;
  typedef typename unordered_map<StateId, StateId>::const_iterator
      IdMapIterator;
  typedef typename unordered_map<StateId, Weight>::const_iterator
      FinalWeightIterator;


  EditFstData() : num_new_states_(0) {
    SetEmptyAndDeleteKeysForInternalMaps();
  }

  EditFstData(const EditFstData &other) :
      edits_(other.edits_),
      external_to_internal_ids_(other.external_to_internal_ids_),
      edited_final_weights_(other.edited_final_weights_),
      num_new_states_(other.num_new_states_) {
  }

  ~EditFstData() {
  }

  static EditFstData<A, WrappedFstT, MutableFstT> *Read(istream &strm,
                                                        const FstReadOptions &opts);

  bool Write(ostream &strm, const FstWriteOptions &opts) const {
    // Serialize all private data members of this class.
    FstWriteOptions edits_opts(opts);
    edits_opts.write_header = true;  // Force writing contained header.
    edits_.Write(strm, edits_opts);
    WriteType(strm, external_to_internal_ids_);
    WriteType(strm, edited_final_weights_);
    WriteType(strm, num_new_states_);
    if (!strm) {
      LOG(ERROR) << "EditFstData::Write: write failed: " << opts.source;
      return false;
    }
    return true;
  }

  int RefCount() const { return ref_count_.count(); }
  int IncrRefCount() { return ref_count_.Incr(); }
  int DecrRefCount() { return ref_count_.Decr(); }

  StateId NumNewStates() const {
    return num_new_states_;
  }

  // accessor methods for the fst holding edited states
  StateId EditedStart() const {
    return edits_.Start();
  }

  Weight Final(StateId s, const WrappedFstT *wrapped) const {
    FinalWeightIterator final_weight_it = GetFinalWeightIterator(s);
    if (final_weight_it == NotInFinalWeightMap()) {
      IdMapIterator it = GetEditedIdMapIterator(s);
      return it == NotInEditedMap() ?
             wrapped->Final(s) : edits_.Final(it->second);
    }
    else {
      return final_weight_it->second;
    }
  }

  size_t NumArcs(StateId s, const WrappedFstT *wrapped) const {
    IdMapIterator it = GetEditedIdMapIterator(s);
    return it == NotInEditedMap() ?
           wrapped->NumArcs(s) : edits_.NumArcs(it->second);
  }

  size_t NumInputEpsilons(StateId s, const WrappedFstT *wrapped) const {
    IdMapIterator it = GetEditedIdMapIterator(s);
    return it == NotInEditedMap() ?
           wrapped->NumInputEpsilons(s) :
           edits_.NumInputEpsilons(it->second);
  }

  size_t NumOutputEpsilons(StateId s, const WrappedFstT *wrapped) const {
    IdMapIterator it = GetEditedIdMapIterator(s);
    return it == NotInEditedMap() ?
           wrapped->NumOutputEpsilons(s) :
           edits_.NumOutputEpsilons(it->second);
  }

  void SetEditedProperties(uint64 props, uint64 mask) {
    edits_.SetProperties(props, mask);
  }

  // non-const MutableFst operations

  // Sets the start state for this fst.
  void SetStart(StateId s) {
    edits_.SetStart(s);
  }

  // Sets the final state for this fst.
  Weight SetFinal(StateId s, Weight w, const WrappedFstT *wrapped) {
    Weight old_weight = Final(s, wrapped);
    IdMapIterator it = GetEditedIdMapIterator(s);
    // if we haven't already edited state s, don't add it to edited_ (which can
    // be expensive if s has many transitions); just use the
    // edited_final_weights_ map
    if (it == NotInEditedMap()) {
      edited_final_weights_[s] = w;
    }
    else {
      edits_.SetFinal(GetEditableInternalId(s, wrapped), w);
    }
    return old_weight;
  }

  // Adds a new state to this fst, initially with no arcs.
  StateId AddState(StateId curr_num_states) {
    StateId internal_state_id = edits_.AddState();
    StateId external_state_id = curr_num_states;
    external_to_internal_ids_[external_state_id] = internal_state_id;
    num_new_states_++;
    return external_state_id;
  }

  // Adds the specified arc to the specified state of this fst.
  const A *AddArc(StateId s, const Arc &arc, const WrappedFstT *wrapped) {
    StateId internal_id = GetEditableInternalId(s, wrapped);

    size_t num_arcs = edits_.NumArcs(internal_id);
    ArcIterator<MutableFstT> arc_it(edits_, internal_id);
    const A *prev_arc = NULL;
    if (num_arcs > 0) {
      // grab the final arc associated with this state in edits_
      arc_it.Seek(num_arcs - 1);
      prev_arc = &(arc_it.Value());
    }
    edits_.AddArc(internal_id, arc);
    return prev_arc;
  }

  void DeleteStates() {
    edits_.DeleteStates();
    num_new_states_ = 0;
    external_to_internal_ids_.clear();
    edited_final_weights_.clear();
  }

  // Removes all but the first n outgoing arcs of the specified state.
  void DeleteArcs(StateId s, size_t n, const WrappedFstT *wrapped) {
    edits_.DeleteArcs(GetEditableInternalId(s, wrapped), n);
  }

  // Removes all outgoing arcs from the specified state.
  void DeleteArcs(StateId s, const WrappedFstT *wrapped) {
    edits_.DeleteArcs(GetEditableInternalId(s, wrapped));
  }

  // end methods for non-const MutableFst operations

  // Provides information for the generic arc iterator.
  void InitArcIterator(StateId s, ArcIteratorData<Arc> *data,
                       const WrappedFstT *wrapped) const {
    IdMapIterator id_map_it = GetEditedIdMapIterator(s);
    if (id_map_it == NotInEditedMap()) {
      VLOG(3) << "EditFstData::InitArcIterator: iterating on state "
          << s << " of original fst";
      wrapped->InitArcIterator(s, data);
    } else {
      VLOG(2) << "EditFstData::InitArcIterator: iterating on edited state "
          << s << " (internal state id: " << id_map_it->second << ")";
      edits_.InitArcIterator(id_map_it->second, data);
    }
  }

    // Provides information for the generic mutable arc iterator.
  void InitMutableArcIterator(StateId s, MutableArcIteratorData<A> *data,
                              const WrappedFstT *wrapped) {
    data->base =
        new MutableArcIterator<MutableFstT>(&edits_,
                                            GetEditableInternalId(s, wrapped));
  }

  // Prints out the map from external to internal state id's (for debugging
  // purposes).
  void PrintMap() {
    for (IdMapIterator map_it = external_to_internal_ids_.begin();
        map_it != NotInEditedMap(); ++map_it) {
      LOG(INFO) << "(external,internal)=("
          << map_it->first << "," << map_it->second << ")";
    }
  }


 private:
  void SetEmptyAndDeleteKeysForInternalMaps() {
  }

  // Returns the iterator of the map from external to internal state id's
  // of edits_ for the specified external state id.
  IdMapIterator GetEditedIdMapIterator(StateId s) const {
    return external_to_internal_ids_.find(s);
  }
  IdMapIterator NotInEditedMap() const {
    return external_to_internal_ids_.end();
  }

  FinalWeightIterator GetFinalWeightIterator(StateId s) const {
    return edited_final_weights_.find(s);
  }
  FinalWeightIterator NotInFinalWeightMap() const {
    return edited_final_weights_.end();
  }

  // Returns the internal state id of the specified external id if the state has
  // already been made editable, or else copies the state from wrapped_
  // to edits_ and returns the state id of the newly editable state in edits_.
  //
  // \return makes the specified state editable if it isn't already and returns
  //         its state id in edits_
  StateId GetEditableInternalId(StateId s, const WrappedFstT *wrapped) {
    IdMapIterator id_map_it = GetEditedIdMapIterator(s);
    if (id_map_it == NotInEditedMap()) {
      StateId new_internal_id = edits_.AddState();
      VLOG(2) << "EditFstData::GetEditableInternalId: editing state " << s
          << " of original fst; new internal state id:" << new_internal_id;
      external_to_internal_ids_[s] = new_internal_id;
      for (ArcIterator< Fst<A> > arc_iterator(*wrapped, s);
          !arc_iterator.Done();
          arc_iterator.Next()) {
        edits_.AddArc(new_internal_id, arc_iterator.Value());
      }
      // copy the final weight
      FinalWeightIterator final_weight_it = GetFinalWeightIterator(s);
      if (final_weight_it == NotInFinalWeightMap()) {
        edits_.SetFinal(new_internal_id, wrapped->Final(s));
      } else {
        edits_.SetFinal(new_internal_id, final_weight_it->second);
        edited_final_weights_.erase(s);
      }
      return new_internal_id;
    } else {
      return id_map_it->second;
    }
  }

  // A mutable fst (by default, a VectorFst) to contain new states, and/or
  // copies of states from a wrapped ExpandedFst that have been modified in
  // some way.
  MutableFstT edits_;
  // A mapping from external state id's to the internal id's of states that
  // appear in edits_.
  unordered_map<StateId, StateId> external_to_internal_ids_;
  // A mapping from external state id's to final state weights assigned to
  // those states.  The states in this map are *only* those whose final weight
  // has been modified; if any other part of the state has been modified,
  // the entire state is copied to edits_, and all modifications reside there.
  unordered_map<StateId, Weight> edited_final_weights_;
  // The number of new states added to this mutable fst impl, which is <= the
  // number of states in edits_ (since edits_ contains both edited *and* new
  // states).
  StateId num_new_states_;
  RefCounter ref_count_;
};

// EditFstData method implementations: just the Read method.
template <typename A, typename WrappedFstT, typename MutableFstT>
EditFstData<A, WrappedFstT, MutableFstT> *
EditFstData<A, WrappedFstT, MutableFstT>::Read(istream &strm,
                                               const FstReadOptions &opts) {
  EditFstData<A, WrappedFstT, MutableFstT> *data =
      new EditFstData<A, WrappedFstT, MutableFstT>();
    // next read in MutabelFstT machine that stores edits
  FstReadOptions edits_opts(opts);
  edits_opts.header = 0;  // Contained header was written out, so read it in.

  // Because our internal representation of edited states is a solid object
  // of type MutableFstT (defaults to VectorFst<A>) and not a pointer,
  // and because the static Read method allocates a new object on the heap,
  // we need to call Read, check if there was a failure, use
  // MutableFstT::operator= to assign the object (not the pointer) to the
  // edits_ data member (which will increase the ref count by 1 on the impl)
  // and, finally, delete the heap-allocated object.
  MutableFstT *edits = MutableFstT::Read(strm, edits_opts);
  if (!edits) {
    return 0;
  }
  data->edits_ = *edits;
  delete edits;
  // finally, read in rest of private data members
  ReadType(strm, &data->external_to_internal_ids_);
  ReadType(strm, &data->edited_final_weights_);
  ReadType(strm, &data->num_new_states_);
  if (!strm) {
    LOG(ERROR) << "EditFst::Read: read failed: " << opts.source;
    return 0;
  }
  return data;
}

// This class enables non-destructive edit operations on a wrapped ExpandedFst.
// The implementation uses copy-on-write semantics at the node level: if a user
// has an underlying fst on which he or she wants to perform a relatively small
// number of edits (read: mutations), then this implementation will copy the
// edited node to an internal MutableFst and perform any edits in situ on that
// copied node. This class supports all the methods of MutableFst except for
// DeleteStates(const vector<StateId> &); thus, new nodes may also be added, and
// one may add transitions from existing nodes of the wrapped fst to new nodes.
//
// template parameters:
//   A the type of arc to use
//   WrappedFstT the type of fst wrapped by the EditFst instance that
//     this EditFstImpl instance is backing
//   MutableFstT the type of mutable fst to use internally for edited states;
//     crucially, MutableFstT::Copy(false) *must* yield an fst that is
//     thread-safe for reading (VectorFst, for example, has this property)
template <typename A,
          typename WrappedFstT = ExpandedFst<A>,
          typename MutableFstT = VectorFst<A> >
class EditFstImpl : public FstImpl<A> {
 public:
  using FstImpl<A>::SetProperties;
  using FstImpl<A>::SetInputSymbols;
  using FstImpl<A>::SetOutputSymbols;
  using FstImpl<A>::WriteHeader;

  typedef A Arc;
  typedef typename Arc::Weight Weight;
  typedef typename Arc::StateId StateId;

  // Constructs an editable fst implementation with no states.  Effectively,
  // this initially-empty fst will in every way mimic the behavior of
  // a VectorFst--more precisely, a VectorFstImpl instance--but with slightly
  // slower performance (by a constant factor), due to the fact that
  // this class maintains a mapping between external state id's and
  // their internal equivalents.
  EditFstImpl() {
    FstImpl<A>::SetType("edit");
    wrapped_ = new MutableFstT();
    InheritPropertiesFromWrapped();
    data_ = new EditFstData<A, WrappedFstT, MutableFstT>();
  }

  // Wraps the specified ExpandedFst. This constructor requires that the
  // specified Fst is an ExpandedFst instance. This requirement is only enforced
  // at runtime. (See below for the reason.)
  //
  // This library uses the pointer-to-implementation or "PIMPL" design pattern.
  // In particular, to make it convenient to bind an implementation class to its
  // interface, there are a pair of template "binder" classes, one for immutable
  // and one for mutable fst's (ImplToFst and ImplToMutableFst, respectively).
  // As it happens, the API for the ImplToMutableFst<I,F> class requires that
  // the implementation class--the template parameter "I"--have a constructor
  // taking a const Fst<A> reference.  Accordingly, the constructor here must
  // perform a static_cast to the WrappedFstT type required by EditFst and
  // therefore EditFstImpl.
  explicit EditFstImpl(const Fst<A> &wrapped)
      : wrapped_(static_cast<WrappedFstT *>(wrapped.Copy())) {
    FstImpl<A>::SetType("edit");

    data_ = new EditFstData<A, WrappedFstT, MutableFstT>();
    // have edits_ inherit all properties from wrapped_
    data_->SetEditedProperties(wrapped_->Properties(kFstProperties, false),
                               kFstProperties);
    InheritPropertiesFromWrapped();
  }

  // A copy constructor for this implementation class, used to implement
  // the Copy() method of the Fst interface.
  EditFstImpl(const EditFstImpl &impl)
      : FstImpl<A>(),
        wrapped_(static_cast<WrappedFstT *>(impl.wrapped_->Copy(true))),
        data_(impl.data_) {
    data_->IncrRefCount();
    SetProperties(impl.Properties());
  }

  ~EditFstImpl() {
    delete wrapped_;
    if (!data_->DecrRefCount()) {
      delete data_;
    }
  }

  // const Fst/ExpandedFst operations, declared in the Fst and ExpandedFst
  // interfaces
  StateId Start() const {
    StateId edited_start = data_->EditedStart();
    return edited_start == kNoStateId ? wrapped_->Start() : edited_start;
  }

  Weight Final(StateId s) const {
    return data_->Final(s, wrapped_);
  }

  size_t NumArcs(StateId s) const {
    return data_->NumArcs(s, wrapped_);
  }

  size_t NumInputEpsilons(StateId s) const {
    return data_->NumInputEpsilons(s, wrapped_);
  }

  size_t NumOutputEpsilons(StateId s) const {
    return data_->NumOutputEpsilons(s, wrapped_);
  }

  StateId NumStates() const {
    return wrapped_->NumStates() + data_->NumNewStates();
  }

  static EditFstImpl<A, WrappedFstT, MutableFstT> *
  Read(istream &strm,
       const FstReadOptions &opts);

  bool Write(ostream &strm, const FstWriteOptions &opts) const {
    FstHeader hdr;
    hdr.SetStart(Start());
    hdr.SetNumStates(NumStates());
    FstWriteOptions header_opts(opts);
    header_opts.write_isymbols = false;  // Let contained FST hold any symbols.
    header_opts.write_osymbols = false;
    WriteHeader(strm, header_opts, kFileVersion, &hdr);

    // First, serialize wrapped fst to stream.
    FstWriteOptions wrapped_opts(opts);
    wrapped_opts.write_header = true;  // Force writing contained header.
    wrapped_->Write(strm, wrapped_opts);

    data_->Write(strm, opts);

    strm.flush();
    if (!strm) {
      LOG(ERROR) << "EditFst::Write: write failed: " << opts.source;
      return false;
    }
    return true;
  }
  // end const Fst operations

  // non-const MutableFst operations

  // Sets the start state for this fst.
  void SetStart(StateId s) {
    MutateCheck();
    data_->SetStart(s);
    SetProperties(SetStartProperties(FstImpl<A>::Properties()));
  }

  // Sets the final state for this fst.
  void SetFinal(StateId s, Weight w) {
    MutateCheck();
    Weight old_weight = data_->SetFinal(s, w, wrapped_);
    SetProperties(SetFinalProperties(FstImpl<A>::Properties(), old_weight, w));
  }

  // Adds a new state to this fst, initially with no arcs.
  StateId AddState() {
    MutateCheck();
    SetProperties(AddStateProperties(FstImpl<A>::Properties()));
    return data_->AddState(NumStates());
  }

  // Adds the specified arc to the specified state of this fst.
  void AddArc(StateId s, const Arc &arc) {
    MutateCheck();
    const A *prev_arc = data_->AddArc(s, arc, wrapped_);
    SetProperties(AddArcProperties(FstImpl<A>::Properties(), s, arc, prev_arc));
  }

  void DeleteStates(const vector<StateId>& dstates) {
    FSTERROR() << ": EditFstImpl::DeleteStates(const std::vector<StateId>&): "
               << " not implemented";
    SetProperties(kError, kError);
  }

  // Deletes all states in this fst.
  void DeleteStates();

  // Removes all but the first n outgoing arcs of the specified state.
  void DeleteArcs(StateId s, size_t n) {
    MutateCheck();
    data_->DeleteArcs(s, n, wrapped_);
    SetProperties(DeleteArcsProperties(FstImpl<A>::Properties()));
  }

  // Removes all outgoing arcs from the specified state.
  void DeleteArcs(StateId s) {
    MutateCheck();
    data_->DeleteArcs(s, wrapped_);
    SetProperties(DeleteArcsProperties(FstImpl<A>::Properties()));
  }

  void ReserveStates(StateId s) {
  }

  void ReserveArcs(StateId s, size_t n) {
  }

  // end non-const MutableFst operations

  // Provides information for the generic state iterator.
  void InitStateIterator(StateIteratorData<Arc> *data) const {
    data->base = 0;
    data->nstates = NumStates();
  }

  // Provides information for the generic arc iterator.
  void InitArcIterator(StateId s, ArcIteratorData<Arc> *data) const {
    data_->InitArcIterator(s, data, wrapped_);
  }

  // Provides information for the generic mutable arc iterator.
  void InitMutableArcIterator(StateId s, MutableArcIteratorData<A> *data) {
    MutateCheck();
    data_->InitMutableArcIterator(s, data, wrapped_);
  }

 private:
  typedef typename unordered_map<StateId, StateId>::const_iterator
    IdMapIterator;
  typedef typename unordered_map<StateId, Weight>::const_iterator
    FinalWeightIterator;
  // Properties always true of this Fst class
  static const uint64 kStaticProperties = kExpanded | kMutable;
  // Current file format version
  static const int kFileVersion = 2;
  // Minimum file format version supported
  static const int kMinFileVersion = 2;

  // Causes this fst to inherit all the properties from its wrapped fst, except
  // for the two properties that always apply to EditFst instances: kExpanded
  // and kMutable.
  void InheritPropertiesFromWrapped() {
    SetProperties(wrapped_->Properties(kCopyProperties, false) |
                  kStaticProperties);
    SetInputSymbols(wrapped_->InputSymbols());
    SetOutputSymbols(wrapped_->OutputSymbols());
  }

  // This method ensures that any operations that alter the mutable data
  // portion of this EditFstImpl cause the data_ member to be copied when its
  // reference count is greater than 1.  Note that this method is distinct from
  // MutableFst::Mutate, which gets invoked whenever one of the basic mutation
  // methods defined in MutableFst is invoked, such as SetInputSymbols.
  // The MutateCheck here in EditFstImpl is invoked whenever one of the
  // mutating methods specifically related to the types of edits provided
  // by EditFst is performed, such as changing an arc of an existing state
  // of the wrapped fst via a MutableArcIterator, or adding a new state via
  // AddState().
  void MutateCheck() {
    if (data_->RefCount() > 1) {
      EditFstData<A, WrappedFstT, MutableFstT> *data_copy =
          new EditFstData<A, WrappedFstT, MutableFstT>(*data_);
      if (data_ && !data_->DecrRefCount()) {
        delete data_;
      }
      data_ = data_copy;
    }
  }

  // The fst that this fst wraps.  The purpose of this class is to enable
  // non-destructive edits on this wrapped fst.
  const WrappedFstT *wrapped_;
  // The mutable data for this EditFst instance, with delegates for all the
  // methods that can mutate data.
  EditFstData<A, WrappedFstT, MutableFstT> *data_;
};

template <typename A, typename WrappedFstT, typename MutableFstT>
const uint64 EditFstImpl<A, WrappedFstT, MutableFstT>::kStaticProperties;

// EditFstImpl IMPLEMENTATION STARTS HERE

template<typename A, typename WrappedFstT, typename MutableFstT>
inline void EditFstImpl<A, WrappedFstT, MutableFstT>::DeleteStates() {
  data_->DeleteStates();
  delete wrapped_;
  // we are deleting all states, so just forget about pointer to wrapped_
  // and do what default constructor does: set wrapped_ to a new VectorFst
  wrapped_ = new MutableFstT();
  uint64 newProps = DeleteAllStatesProperties(FstImpl<A>::Properties(),
                                              kStaticProperties);
  FstImpl<A>::SetProperties(newProps);
}

template <typename A, typename WrappedFstT, typename MutableFstT>
EditFstImpl<A, WrappedFstT, MutableFstT> *
EditFstImpl<A, WrappedFstT, MutableFstT>::Read(istream &strm,
                                               const FstReadOptions &opts) {
  EditFstImpl<A, WrappedFstT, MutableFstT> *impl = new EditFstImpl();
  FstHeader hdr;
  if (!impl->ReadHeader(strm, opts, kMinFileVersion, &hdr)) {
    return 0;
  }
  impl->SetStart(hdr.Start());

  // first, read in wrapped fst
  FstReadOptions wrapped_opts(opts);
  wrapped_opts.header = 0;  // Contained header was written out, so read it in.
  Fst<A> *wrapped_fst = Fst<A>::Read(strm, wrapped_opts);
  if (!wrapped_fst) {
    return 0;
  }
  impl->wrapped_ = static_cast<WrappedFstT *>(wrapped_fst);

  impl->data_ = EditFstData<A, WrappedFstT, MutableFstT>::Read(strm, opts);

  if (!impl->data_) {
    delete wrapped_fst;
    return 0;
  }

  return impl;
}

// END EditFstImpl IMPLEMENTATION

// Concrete, editable FST.  This class attaches interface to implementation.
template <typename A,
          typename WrappedFstT = ExpandedFst<A>,
          typename MutableFstT = VectorFst<A> >
class EditFst :
  public ImplToMutableFst< EditFstImpl<A, WrappedFstT, MutableFstT> > {
 public:
  friend class MutableArcIterator< EditFst<A, WrappedFstT, MutableFstT> >;

  typedef A Arc;
  typedef typename A::StateId StateId;
  typedef EditFstImpl<A, WrappedFstT, MutableFstT> Impl;

  EditFst() : ImplToMutableFst<Impl>(new Impl()) {}

  explicit EditFst(const Fst<A> &fst) :
      ImplToMutableFst<Impl>(new Impl(fst)) {}

  explicit EditFst(const WrappedFstT &fst) :
      ImplToMutableFst<Impl>(new Impl(fst)) {}

  // See Fst<>::Copy() for doc.
  EditFst(const EditFst<A, WrappedFstT, MutableFstT> &fst, bool safe = false) :
    ImplToMutableFst<Impl>(fst, safe) {}

  virtual ~EditFst() {}

  // Get a copy of this EditFst. See Fst<>::Copy() for further doc.
  virtual EditFst<A, WrappedFstT, MutableFstT> *Copy(bool safe = false) const {
    return new EditFst<A, WrappedFstT, MutableFstT>(*this, safe);
  }

  EditFst<A, WrappedFstT, MutableFstT> &
  operator=(const EditFst<A, WrappedFstT, MutableFstT> &fst) {
    SetImpl(fst.GetImpl(), false);
    return *this;
  }

  virtual EditFst<A, WrappedFstT, MutableFstT> &operator=(const Fst<A> &fst) {
    if (this != &fst) {
      SetImpl(new Impl(fst));
    }
    return *this;
  }

  // Read an EditFst from an input stream; return NULL on error.
  static EditFst<A, WrappedFstT, MutableFstT> *
  Read(istream &strm,
       const FstReadOptions &opts) {
    Impl* impl = Impl::Read(strm, opts);
    return impl ? new EditFst<A>(impl) : 0;
  }

  // Read an EditFst from a file; return NULL on error.
  // Empty filename reads from standard input.
  static EditFst<A, WrappedFstT, MutableFstT> *Read(const string &filename) {
    Impl* impl = ImplToExpandedFst<Impl, MutableFst<A> >::Read(filename);
    return impl ? new EditFst<A, WrappedFstT, MutableFstT>(impl) : 0;
  }

  virtual bool Write(ostream &strm, const FstWriteOptions &opts) const {
    return GetImpl()->Write(strm, opts);
  }

  virtual bool Write(const string &filename) const {
    return Fst<A>::WriteFile(filename);
  }

  virtual void InitStateIterator(StateIteratorData<Arc> *data) const {
    GetImpl()->InitStateIterator(data);
  }

  virtual void InitArcIterator(StateId s, ArcIteratorData<Arc> *data) const {
    GetImpl()->InitArcIterator(s, data);
  }

  virtual
  void InitMutableArcIterator(StateId s, MutableArcIteratorData<A> *data) {
    GetImpl()->InitMutableArcIterator(s, data);
  }
 private:
  explicit EditFst(Impl *impl) : ImplToMutableFst<Impl>(impl) {}

  // Makes visible to friends.
  Impl *GetImpl() const { return ImplToFst< Impl, MutableFst<A> >::GetImpl(); }

  void SetImpl(Impl *impl, bool own_impl = true) {
    ImplToFst< Impl, MutableFst<A> >::SetImpl(impl, own_impl);
  }
};

}  // namespace fst

#endif  // FST_LIB_EDIT_FST_H_