src/include/fst/fst.h

// 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: riley@google.com (Michael Riley)
//
// \file
// Finite-State Transducer (FST) - abstract base class definition,
// state and arc iterator interface, and suggested base implementation.
//

#ifndef FST_LIB_FST_H__
#define FST_LIB_FST_H__

#include <stddef.h>
#include <sys/types.h>
#include <cmath>
#include <string>

#include <fst/compat.h>
#include <fst/types.h>

#include <fst/arc.h>
#include <fst/properties.h>
#include <fst/register.h>
#include <iostream>
#include <fstream>
#include <fst/symbol-table.h>
#include <fst/util.h>


DECLARE_bool(fst_align);

namespace fst {

bool IsFstHeader(istream &, const string &);

class FstHeader;
template <class A> class StateIteratorData;
template <class A> class ArcIteratorData;
template <class A> class MatcherBase;

struct FstReadOptions {
  string source;                // Where you're reading from
  const FstHeader *header;      // Pointer to Fst header. If non-zero, use
                                // this info (don't read a stream header)
  const SymbolTable* isymbols;  // Pointer to input symbols. If non-zero, use
                                // this info (read and skip stream isymbols)
  const SymbolTable* osymbols;  // Pointer to output symbols. If non-zero, use
                                // this info (read and skip stream osymbols)

  explicit FstReadOptions(const string& src = "<unspecfied>",
                          const FstHeader *hdr = 0,
                          const SymbolTable* isym = 0,
                          const SymbolTable* osym = 0)
      : source(src), header(hdr), isymbols(isym), osymbols(osym) {}

  explicit FstReadOptions(const string& src,
                          const SymbolTable* isym,
                          const SymbolTable* osym = 0)
      : source(src), header(0), isymbols(isym), osymbols(osym) {}
};


struct FstWriteOptions {
  string source;                 // Where you're writing to
  bool write_header;             // Write the header?
  bool write_isymbols;           // Write input symbols?
  bool write_osymbols;           // Write output symbols?
  bool align;                    // Write data aligned where appropriate;
                                 // this may fail on pipes

  explicit FstWriteOptions(const string& src = "<unspecifed>",
                           bool hdr = true, bool isym = true,
                           bool osym = true, bool alig = FLAGS_fst_align)
      : source(src), write_header(hdr),
        write_isymbols(isym), write_osymbols(osym), align(alig) {}
};

//
// Fst HEADER CLASS
//
// This is the recommended Fst file header representation.
//
class FstHeader {
 public:
  enum {
    HAS_ISYMBOLS = 0x1,          // Has input symbol table
    HAS_OSYMBOLS = 0x2,          // Has output symbol table
    IS_ALIGNED   = 0x4,          // Memory-aligned (where appropriate)
  } Flags;

  FstHeader() : version_(0), flags_(0), properties_(0), start_(-1),
                numstates_(0), numarcs_(0) {}
  const string &FstType() const { return fsttype_; }
  const string &ArcType() const { return arctype_; }
  int32 Version() const { return version_; }
  int32 GetFlags() const { return flags_; }
  uint64 Properties() const { return properties_; }
  int64 Start() const { return start_; }
  int64 NumStates() const { return numstates_; }
  int64 NumArcs() const { return numarcs_; }

  void SetFstType(const string& type) { fsttype_ = type; }
  void SetArcType(const string& type) { arctype_ = type; }
  void SetVersion(int32 version) { version_ = version; }
  void SetFlags(int32 flags) { flags_ = flags; }
  void SetProperties(uint64 properties) { properties_ = properties; }
  void SetStart(int64 start) { start_ = start; }
  void SetNumStates(int64 numstates) { numstates_ = numstates; }
  void SetNumArcs(int64 numarcs) { numarcs_ = numarcs; }

  bool Read(istream &strm, const string &source, bool rewind = false);
  bool Write(ostream &strm, const string &source) const;

 private:

  string fsttype_;                   // E.g. "vector"
  string arctype_;                   // E.g. "standard"
  int32 version_;                    // Type version #
  int32 flags_;                      // File format bits
  uint64 properties_;                // FST property bits
  int64 start_;                      // Start state
  int64 numstates_;                  // # of states
  int64 numarcs_;                    // # of arcs
};


// Specifies matcher action.
enum MatchType { MATCH_INPUT,      // Match input label.
                 MATCH_OUTPUT,     // Match output label.
                 MATCH_BOTH,       // Match input or output label.
                 MATCH_NONE,       // Match nothing.
                 MATCH_UNKNOWN };  // Match type unknown.

//
// Fst INTERFACE CLASS DEFINITION
//

// A generic FST, templated on the arc definition, with
// common-demoninator methods (use StateIterator and ArcIterator to
// iterate over its states and arcs).
template <class A>
class Fst {
 public:
  typedef A Arc;
  typedef typename A::Weight Weight;
  typedef typename A::StateId StateId;

  virtual ~Fst() {}

  virtual StateId Start() const = 0;          // Initial state

  virtual Weight Final(StateId) const = 0;    // State's final weight

  virtual size_t NumArcs(StateId) const = 0;  // State's arc count

  virtual size_t NumInputEpsilons(StateId)
      const = 0;                              // State's input epsilon count

  virtual size_t NumOutputEpsilons(StateId)
      const = 0;                              // State's output epsilon count

  // If test=false, return stored properties bits for mask (some poss. unknown)
  // If test=true, return property bits for mask (computing o.w. unknown)
  virtual uint64 Properties(uint64 mask, bool test)
      const = 0;  // Property bits

  virtual const string& Type() const = 0;    // Fst type name

  // Get a copy of this Fst. The copying behaves as follows:
  //
  // (1) The copying is constant time if safe = false or if safe = true
  // and is on an otherwise unaccessed Fst.
  //
  // (2) If safe = true, the copy is thread-safe in that the original
  // and copy can be safely accessed (but not necessarily mutated) by
  // separate threads. For some Fst types, 'Copy(true)' should only be
  // called on an Fst that has not otherwise been accessed. Its behavior
  // is undefined otherwise.
  //
  // (3) If a MutableFst is copied and then mutated, then the original is
  // unmodified and vice versa (often by a copy-on-write on the initial
  // mutation, which may not be constant time).
  virtual Fst<A> *Copy(bool safe = false) const = 0;

  // Read an Fst from an input stream; returns NULL on error
  static Fst<A> *Read(istream &strm, const FstReadOptions &opts) {
    FstReadOptions ropts(opts);
    FstHeader hdr;
    if (ropts.header)
      hdr = *opts.header;
    else {
      if (!hdr.Read(strm, opts.source))
        return 0;
      ropts.header = &hdr;
    }
    FstRegister<A> *registr = FstRegister<A>::GetRegister();
    const typename FstRegister<A>::Reader reader =
      registr->GetReader(hdr.FstType());
    if (!reader) {
      LOG(ERROR) << "Fst::Read: Unknown FST type \"" << hdr.FstType()
                 << "\" (arc type = \"" << A::Type()
                 << "\"): " << ropts.source;
      return 0;
    }
    return reader(strm, ropts);
  };

  // Read an Fst from a file; return NULL on error
  // Empty filename reads from standard input
  static Fst<A> *Read(const string &filename) {
    if (!filename.empty()) {
      ifstream strm(filename.c_str(), ifstream::in | ifstream::binary);
      if (!strm) {
        LOG(ERROR) << "Fst::Read: Can't open file: " << filename;
        return 0;
      }
      return Read(strm, FstReadOptions(filename));
    } else {
      return Read(std::cin, FstReadOptions("standard input"));
    }
  }

  // Write an Fst to an output stream; return false on error
  virtual bool Write(ostream &strm, const FstWriteOptions &opts) const {
    LOG(ERROR) << "Fst::Write: No write stream method for " << Type()
               << " Fst type";
    return false;
  }

  // Write an Fst to a file; return false on error
  // Empty filename writes to standard output
  virtual bool Write(const string &filename) const {
    LOG(ERROR) << "Fst::Write: No write filename method for " << Type()
               << " Fst type";
    return false;
  }

  // Return input label symbol table; return NULL if not specified
  virtual const SymbolTable* InputSymbols() const = 0;

  // Return output label symbol table; return NULL if not specified
  virtual const SymbolTable* OutputSymbols() const = 0;

  // For generic state iterator construction; not normally called
  // directly by users.
  virtual void InitStateIterator(StateIteratorData<A> *) const = 0;

  // For generic arc iterator construction; not normally called
  // directly by users.
  virtual void InitArcIterator(StateId s, ArcIteratorData<A> *) const = 0;

  // For generic matcher construction; not normally called
  // directly by users.
  virtual MatcherBase<A> *InitMatcher(MatchType match_type) const;

 protected:

  bool WriteFile(const string &filename) const {
    if (!filename.empty()) {
      ofstream strm(filename.c_str(), ofstream::out | ofstream::binary);
      if (!strm) {
        LOG(ERROR) << "Fst::Write: Can't open file: " << filename;
        return false;
      }
      return Write(strm, FstWriteOptions(filename));
    } else {
      return Write(std::cout, FstWriteOptions("standard output"));
    }
  }
};


//
// STATE and ARC ITERATOR DEFINITIONS
//

// State iterator interface templated on the Arc definition; used
// for StateIterator specializations returned by the InitStateIterator
// Fst method.
template <class A>
class StateIteratorBase {
 public:
  typedef A Arc;
  typedef typename A::StateId StateId;

  virtual ~StateIteratorBase() {}

  bool Done() const { return Done_(); }       // End of iterator?
  StateId Value() const { return Value_(); }  // Current state (when !Done)
  void Next() { Next_(); }      // Advance to next state (when !Done)
  void Reset() { Reset_(); }    // Return to initial condition

 private:
  // This allows base class virtual access to non-virtual derived-
  // class members of the same name. It makes the derived class more
  // efficient to use but unsafe to further derive.
  virtual bool Done_() const = 0;
  virtual StateId Value_() const = 0;
  virtual void Next_() = 0;
  virtual void Reset_() = 0;
};


// StateIterator initialization data

template <class A> struct StateIteratorData {
  StateIteratorBase<A> *base;   // Specialized iterator if non-zero
  typename A::StateId nstates;  // O.w. total # of states
};


// Generic state iterator, templated on the FST definition
// - a wrapper around pointer to specific one.
// Here is a typical use: \code
//   for (StateIterator<StdFst> siter(fst);
//        !siter.Done();
//        siter.Next()) {
//     StateId s = siter.Value();
//     ...
//   } \endcode
template <class F>
class StateIterator {
 public:
  typedef F FST;
  typedef typename F::Arc Arc;
  typedef typename Arc::StateId StateId;

  explicit StateIterator(const F &fst) : s_(0) {
    fst.InitStateIterator(&data_);
  }

  ~StateIterator() { if (data_.base) delete data_.base; }

  bool Done() const {
    return data_.base ? data_.base->Done() : s_ >= data_.nstates;
  }

  StateId Value() const { return data_.base ? data_.base->Value() : s_; }

  void Next() {
    if (data_.base)
      data_.base->Next();
    else
      ++s_;
  }

  void Reset() {
    if (data_.base)
      data_.base->Reset();
    else
      s_ = 0;
  }

 private:
  StateIteratorData<Arc> data_;
  StateId s_;

  DISALLOW_COPY_AND_ASSIGN(StateIterator);
};


// Flags to control the behavior on an arc iterator:
static const uint32 kArcILabelValue    = 0x0001;  // Value() gives valid ilabel
static const uint32 kArcOLabelValue    = 0x0002;  //  "       "     "    olabel
static const uint32 kArcWeightValue    = 0x0004;  //  "       "     "    weight
static const uint32 kArcNextStateValue = 0x0008;  //  "       "     " nextstate
static const uint32 kArcNoCache   = 0x0010;       // No need to cache arcs

static const uint32 kArcValueFlags =
                  kArcILabelValue | kArcOLabelValue |
                  kArcWeightValue | kArcNextStateValue;

static const uint32 kArcFlags = kArcValueFlags | kArcNoCache;


// Arc iterator interface, templated on the Arc definition; used
// for Arc iterator specializations that are returned by the InitArcIterator
// Fst method.
template <class A>
class ArcIteratorBase {
 public:
  typedef A Arc;
  typedef typename A::StateId StateId;

  virtual ~ArcIteratorBase() {}

  bool Done() const { return Done_(); }            // End of iterator?
  const A& Value() const { return Value_(); }      // Current arc (when !Done)
  void Next() { Next_(); }           // Advance to next arc (when !Done)
  size_t Position() const { return Position_(); }  // Return current position
  void Reset() { Reset_(); }         // Return to initial condition
  void Seek(size_t a) { Seek_(a); }  // Random arc access by position
  uint32 Flags() const { return Flags_(); }  // Return current behavorial flags
  void SetFlags(uint32 flags, uint32 mask) {  // Set behavorial flags
    SetFlags_(flags, mask);
  }

 private:
  // This allows base class virtual access to non-virtual derived-
  // class members of the same name. It makes the derived class more
  // efficient to use but unsafe to further derive.
  virtual bool Done_() const = 0;
  virtual const A& Value_() const = 0;
  virtual void Next_() = 0;
  virtual size_t Position_() const = 0;
  virtual void Reset_() = 0;
  virtual void Seek_(size_t a) = 0;
  virtual uint32 Flags_() const = 0;
  virtual void SetFlags_(uint32 flags, uint32 mask) = 0;
};


// ArcIterator initialization data
template <class A> struct ArcIteratorData {
  ArcIteratorBase<A> *base;  // Specialized iterator if non-zero
  const A *arcs;             // O.w. arcs pointer
  size_t narcs;              // ... and arc count
  int *ref_count;            // ... and reference count if non-zero
};


// Generic arc iterator, templated on the FST definition
// - a wrapper around pointer to specific one.
// Here is a typical use: \code
//   for (ArcIterator<StdFst> aiter(fst, s));
//        !aiter.Done();
//         aiter.Next()) {
//     StdArc &arc = aiter.Value();
//     ...
//   } \endcode
template <class F>
class ArcIterator {
   public:
  typedef F FST;
  typedef typename F::Arc Arc;
  typedef typename Arc::StateId StateId;

  ArcIterator(const F &fst, StateId s) : i_(0) {
    fst.InitArcIterator(s, &data_);
  }

  explicit ArcIterator(const ArcIteratorData<Arc> &data) : data_(data), i_(0) {
    if (data_.ref_count)
      ++(*data_.ref_count);
  }

  ~ArcIterator() {
    if (data_.base)
      delete data_.base;
    else if (data_.ref_count)
      --(*data_.ref_count);
  }

  bool Done() const {
    return data_.base ?  data_.base->Done() : i_ >= data_.narcs;
  }

  const Arc& Value() const {
    return data_.base ? data_.base->Value() : data_.arcs[i_];
  }

  void Next() {
    if (data_.base)
      data_.base->Next();
    else
      ++i_;
  }

  void Reset() {
    if (data_.base)
      data_.base->Reset();
    else
      i_ = 0;
  }

  void Seek(size_t a) {
    if (data_.base)
      data_.base->Seek(a);
    else
      i_ = a;
  }

  size_t Position() const {
    return data_.base ? data_.base->Position() : i_;
  }

  uint32 Flags() const {
    if (data_.base)
      return data_.base->Flags();
    else
      return kArcValueFlags;
  }

  void SetFlags(uint32 flags, uint32 mask) {
    if (data_.base)
      data_.base->SetFlags(flags, mask);
  }

 private:
  ArcIteratorData<Arc> data_;
  size_t i_;
  DISALLOW_COPY_AND_ASSIGN(ArcIterator);
};

//
// MATCHER DEFINITIONS
//

template <class A>
MatcherBase<A> *Fst<A>::InitMatcher(MatchType match_type) const {
  return 0;  // Use the default matcher
}


//
// FST ACCESSORS - Useful functions in high-performance cases.
//

namespace internal {

// General case - requires non-abstract, 'final' methods. Use for inlining.
template <class F> inline
typename F::Arc::Weight Final(const F &fst, typename F::Arc::StateId s) {
  return fst.F::Final(s);
}

template <class F> inline
ssize_t NumArcs(const F &fst, typename F::Arc::StateId s) {
  return fst.F::NumArcs(s);
}

template <class F> inline
ssize_t NumInputEpsilons(const F &fst, typename F::Arc::StateId s) {
  return fst.F::NumInputEpsilons(s);
}

template <class F> inline
ssize_t NumOutputEpsilons(const F &fst, typename F::Arc::StateId s) {
  return fst.F::NumOutputEpsilons(s);
}


//  Fst<A> case - abstract methods.
template <class A> inline
typename A::Weight Final(const Fst<A> &fst, typename A::StateId s) {
  return fst.Final(s);
}

template <class A> inline
ssize_t NumArcs(const Fst<A> &fst, typename A::StateId s) {
  return fst.NumArcs(s);
}

template <class A> inline
ssize_t NumInputEpsilons(const Fst<A> &fst, typename A::StateId s) {
  return fst.NumInputEpsilons(s);
}

template <class A> inline
ssize_t NumOutputEpsilons(const Fst<A> &fst, typename A::StateId s) {
  return fst.NumOutputEpsilons(s);
}

}  // namespace internal

// A useful alias when using StdArc.
typedef Fst<StdArc> StdFst;


//
//  CONSTANT DEFINITIONS
//

const int kNoStateId   =  -1;  // Not a valid state ID
const int kNoLabel     =  -1;  // Not a valid label

//
// Fst IMPLEMENTATION BASE
//
// This is the recommended Fst implementation base class. It will
// handle reference counts, property bits, type information and symbols.
//

template <class A> class FstImpl {
 public:
  typedef typename A::Weight Weight;
  typedef typename A::StateId StateId;

  FstImpl()
      : properties_(0), type_("null"), isymbols_(0), osymbols_(0) {}

  FstImpl(const FstImpl<A> &impl)
      : properties_(impl.properties_), type_(impl.type_),
        isymbols_(impl.isymbols_ ? impl.isymbols_->Copy() : 0),
        osymbols_(impl.osymbols_ ? impl.osymbols_->Copy() : 0) {}

  virtual ~FstImpl() {
    delete isymbols_;
    delete osymbols_;
  }

  const string& Type() const { return type_; }

  void SetType(const string &type) { type_ = type; }

  virtual uint64 Properties() const { return properties_; }

  virtual uint64 Properties(uint64 mask) const { return properties_ & mask; }

  void SetProperties(uint64 props) {
    properties_ &= kError;          // kError can't be cleared
    properties_ |= props;
  }

  void SetProperties(uint64 props, uint64 mask) {
    properties_ &= ~mask | kError;  // kError can't be cleared
    properties_ |= props & mask;
  }

  // Allows (only) setting error bit on const FST impls
  void SetProperties(uint64 props, uint64 mask) const {
    if (mask != kError)
      FSTERROR() << "FstImpl::SetProperties() const: can only set kError";
    properties_ |= kError;
  }

  const SymbolTable* InputSymbols() const { return isymbols_; }

  const SymbolTable* OutputSymbols() const { return osymbols_; }

  SymbolTable* InputSymbols() { return isymbols_; }

  SymbolTable* OutputSymbols() { return osymbols_; }

  void SetInputSymbols(const SymbolTable* isyms) {
    if (isymbols_) delete isymbols_;
    isymbols_ = isyms ? isyms->Copy() : 0;
  }

  void SetOutputSymbols(const SymbolTable* osyms) {
    if (osymbols_) delete osymbols_;
    osymbols_ = osyms ? osyms->Copy() : 0;
  }

  int RefCount() const {
    return ref_count_.count();
  }

  int IncrRefCount() {
    return ref_count_.Incr();
  }

  int DecrRefCount() {
    return ref_count_.Decr();
  }

  // Read-in header and symbols from input stream, initialize Fst, and
  // return the header.  If opts.header is non-null, skip read-in and
  // use the option value.  If opts.[io]symbols is non-null, read-in
  // (if present), but use the option value.
  bool ReadHeader(istream &strm, const FstReadOptions& opts,
                  int min_version, FstHeader *hdr);

  // Write-out header and symbols from output stream.
  // If a opts.header is false, skip writing header.
  // If opts.[io]symbols is false, skip writing those symbols.
  // This method is needed for Impl's that implement Write methods.
  void WriteHeader(ostream &strm, const FstWriteOptions& opts,
                   int version, FstHeader *hdr) const {
    if (opts.write_header) {
      hdr->SetFstType(type_);
      hdr->SetArcType(A::Type());
      hdr->SetVersion(version);
      hdr->SetProperties(properties_);
      int32 file_flags = 0;
      if (isymbols_ && opts.write_isymbols)
        file_flags |= FstHeader::HAS_ISYMBOLS;
      if (osymbols_ && opts.write_osymbols)
        file_flags |= FstHeader::HAS_OSYMBOLS;
      if (opts.align)
        file_flags |= FstHeader::IS_ALIGNED;
      hdr->SetFlags(file_flags);
      hdr->Write(strm, opts.source);
    }
    if (isymbols_ && opts.write_isymbols) isymbols_->Write(strm);
    if (osymbols_ && opts.write_osymbols) osymbols_->Write(strm);
  }

  // Write-out header and symbols to output stream.
  // If a opts.header is false, skip writing header.
  // If opts.[io]symbols is false, skip writing those symbols.
  // type is the Fst type being written.
  // This method is used in the cross-type serialization methods Fst::WriteFst.
  static void WriteFstHeader(const Fst<A> &fst, ostream &strm,
                             const FstWriteOptions& opts, int version,
                             const string &type, FstHeader *hdr) {
    if (opts.write_header) {
      hdr->SetFstType(type);
      hdr->SetArcType(A::Type());
      hdr->SetVersion(version);
      hdr->SetProperties(fst.Properties(kFstProperties, false));
      int32 file_flags = 0;
      if (fst.InputSymbols() && opts.write_isymbols)
        file_flags |= FstHeader::HAS_ISYMBOLS;
      if (fst.OutputSymbols() && opts.write_osymbols)
        file_flags |= FstHeader::HAS_OSYMBOLS;
      if (opts.align)
        file_flags |= FstHeader::IS_ALIGNED;
      hdr->SetFlags(file_flags);
      hdr->Write(strm, opts.source);
    }
    if (fst.InputSymbols() && opts.write_isymbols) {
      fst.InputSymbols()->Write(strm);
    }
    if (fst.OutputSymbols() && opts.write_osymbols) {
      fst.OutputSymbols()->Write(strm);
    }
  }

  // In serialization routines where the header cannot be written until after
  // the machine has been serialized, this routine can be called to seek to
  // the beginning of the file an rewrite the header with updated fields.
  // It repositions the file pointer back at the end of the file.
  // returns true on success, false on failure.
  static bool UpdateFstHeader(const Fst<A> &fst, ostream &strm,
                              const FstWriteOptions& opts, int version,
                              const string &type, FstHeader *hdr,
                              size_t header_offset) {
    strm.seekp(header_offset);
    if (!strm) {
      LOG(ERROR) << "Fst::UpdateFstHeader: write failed: " << opts.source;
      return false;
    }
    WriteFstHeader(fst, strm, opts, version, type, hdr);
    if (!strm) {
      LOG(ERROR) << "Fst::UpdateFstHeader: write failed: " << opts.source;
      return false;
    }
    strm.seekp(0, ios_base::end);
    if (!strm) {
      LOG(ERROR) << "Fst::UpdateFstHeader: write failed: " << opts.source;
      return false;
    }
    return true;
  }

 protected:
  mutable uint64 properties_;           // Property bits

 private:
  string type_;                 // Unique name of Fst class
  SymbolTable *isymbols_;       // Ilabel symbol table
  SymbolTable *osymbols_;       // Olabel symbol table
  RefCounter ref_count_;        // Reference count

  void operator=(const FstImpl<A> &impl);  // disallow
};

template <class A> inline
bool FstImpl<A>::ReadHeader(istream &strm, const FstReadOptions& opts,
                            int min_version, FstHeader *hdr) {
  if (opts.header)
    *hdr = *opts.header;
  else if (!hdr->Read(strm, opts.source))
    return false;

  if (FLAGS_v >= 2) {
    LOG(INFO) << "FstImpl::ReadHeader: source: " << opts.source
              << ", fst_type: " << hdr->FstType()
              << ", arc_type: " << A::Type()
              << ", version: " << hdr->Version()
              << ", flags: " << hdr->GetFlags();
  }

  if (hdr->FstType() != type_) {
    LOG(ERROR) << "FstImpl::ReadHeader: Fst not of type \"" << type_
               << "\": " << opts.source;
    return false;
  }
  if (hdr->ArcType() != A::Type()) {
    LOG(ERROR) << "FstImpl::ReadHeader: Arc not of type \"" << A::Type()
               << "\": " << opts.source;
    return false;
  }
  if (hdr->Version() < min_version) {
    LOG(ERROR) << "FstImpl::ReadHeader: Obsolete " << type_
               << " Fst version: " << opts.source;
    return false;
  }
  properties_ = hdr->Properties();
  if (hdr->GetFlags() & FstHeader::HAS_ISYMBOLS)
    isymbols_ = SymbolTable::Read(strm, opts.source);
  if (hdr->GetFlags() & FstHeader::HAS_OSYMBOLS)
    osymbols_ =SymbolTable::Read(strm, opts.source);

  if (opts.isymbols) {
    delete isymbols_;
    isymbols_ = opts.isymbols->Copy();
  }
  if (opts.osymbols) {
    delete osymbols_;
    osymbols_ = opts.osymbols->Copy();
  }
  return true;
}


template<class Arc>
uint64 TestProperties(const Fst<Arc> &fst, uint64 mask, uint64 *known);


// This is a helper class template useful for attaching an Fst interface to
// its implementation, handling reference counting.
template < class I, class F = Fst<typename I::Arc> >
class ImplToFst : public F {
 public:
  typedef typename I::Arc Arc;
  typedef typename Arc::Weight Weight;
  typedef typename Arc::StateId StateId;

  virtual ~ImplToFst() { if (!impl_->DecrRefCount()) delete impl_;  }

  virtual StateId Start() const { return impl_->Start(); }

  virtual Weight Final(StateId s) const { return impl_->Final(s); }

  virtual size_t NumArcs(StateId s) const { return impl_->NumArcs(s); }

  virtual size_t NumInputEpsilons(StateId s) const {
    return impl_->NumInputEpsilons(s);
  }

  virtual size_t NumOutputEpsilons(StateId s) const {
    return impl_->NumOutputEpsilons(s);
  }

  virtual uint64 Properties(uint64 mask, bool test) const {
    if (test) {
      uint64 knownprops, testprops = TestProperties(*this, mask, &knownprops);
      impl_->SetProperties(testprops, knownprops);
      return testprops & mask;
    } else {
      return impl_->Properties(mask);
    }
  }

  virtual const string& Type() const { return impl_->Type(); }

  virtual const SymbolTable* InputSymbols() const {
    return impl_->InputSymbols();
  }

  virtual const SymbolTable* OutputSymbols() const {
    return impl_->OutputSymbols();
  }

 protected:
  ImplToFst() : impl_(0) {}

  ImplToFst(I *impl) : impl_(impl) {}

  ImplToFst(const ImplToFst<I, F> &fst) {
    impl_ = fst.impl_;
    impl_->IncrRefCount();
  }

  // This constructor presumes there is a copy constructor for the
  // implementation.
  ImplToFst(const ImplToFst<I, F> &fst, bool safe) {
    if (safe) {
      impl_ = new I(*(fst.impl_));
    } else {
      impl_ = fst.impl_;
      impl_->IncrRefCount();
    }
  }

  I *GetImpl() const { return impl_; }

  // Change Fst implementation pointer. If 'own_impl' is true,
  // ownership of the input implementation is given to this
  // object; otherwise, the input implementation's reference count
  // should be incremented.
  void SetImpl(I *impl, bool own_impl = true) {
    if (!own_impl)
      impl->IncrRefCount();
    if (impl_ && !impl_->DecrRefCount()) delete impl_;
    impl_ = impl;
  }

 private:
  // Disallow
  ImplToFst<I, F> &operator=(const ImplToFst<I, F> &fst);

  ImplToFst<I, F> &operator=(const Fst<Arc> &fst) {
    FSTERROR() << "ImplToFst: Assignment operator disallowed";
    GetImpl()->SetProperties(kError, kError);
    return *this;
  }

  I *impl_;
};


// Converts FSTs by casting their implementations, where this makes
// sense (which excludes implementations with weight-dependent virtual
// methods). Must be a friend of the Fst classes involved (currently
// the concrete Fsts: VectorFst, ConstFst, CompactFst).
template<class F, class G> void Cast(const F &ifst, G *ofst) {
  ofst->SetImpl(reinterpret_cast<typename G::Impl *>(ifst.GetImpl()), false);
}

// Fst Serialization
template <class A>
void FstToString(const Fst<A> &fst, string *result) {
  ostringstream ostrm;
  fst.Write(ostrm, FstWriteOptions("FstToString"));
  *result = ostrm.str();
}

template <class A>
Fst<A> *StringToFst(const string &s) {
  istringstream istrm(s);
  return Fst<A>::Read(istrm, FstReadOptions("StringToFst"));
}

}  // namespace fst

#endif  // FST_LIB_FST_H__