From 96a32415ab43377cf1575bd3f4f2980f58028209 Mon Sep 17 00:00:00 2001
From: Determinant <ted.sybil@gmail.com>
Date: Fri, 14 Aug 2015 11:51:42 +0800
Subject: add implementation for kaldi io (by ymz)

---
 .../include/fst/extensions/ngram/ngram-fst.h       | 934 +++++++++++++++++++++
 1 file changed, 934 insertions(+)
 create mode 100644 kaldi_io/src/tools/openfst/include/fst/extensions/ngram/ngram-fst.h

(limited to 'kaldi_io/src/tools/openfst/include/fst/extensions/ngram/ngram-fst.h')

diff --git a/kaldi_io/src/tools/openfst/include/fst/extensions/ngram/ngram-fst.h b/kaldi_io/src/tools/openfst/include/fst/extensions/ngram/ngram-fst.h
new file mode 100644
index 0000000..d113fb3
--- /dev/null
+++ b/kaldi_io/src/tools/openfst/include/fst/extensions/ngram/ngram-fst.h
@@ -0,0 +1,934 @@
+
+// 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: sorenj@google.com (Jeffrey Sorensen)
+//
+#ifndef FST_EXTENSIONS_NGRAM_NGRAM_FST_H_
+#define FST_EXTENSIONS_NGRAM_NGRAM_FST_H_
+
+#include <stddef.h>
+#include <string.h>
+#include <algorithm>
+#include <string>
+#include <vector>
+using std::vector;
+
+#include <fst/compat.h>
+#include <fst/fstlib.h>
+#include <fst/mapped-file.h>
+#include <fst/extensions/ngram/bitmap-index.h>
+
+// NgramFst implements a n-gram language model based upon the LOUDS data
+// structure.  Please refer to "Unary Data Strucutres for Language Models"
+// http://research.google.com/pubs/archive/37218.pdf
+
+namespace fst {
+template <class A> class NGramFst;
+template <class A> class NGramFstMatcher;
+
+// Instance data containing mutable state for bookkeeping repeated access to
+// the same state.
+template <class A>
+struct NGramFstInst {
+  typedef typename A::Label Label;
+  typedef typename A::StateId StateId;
+  typedef typename A::Weight Weight;
+  StateId state_;
+  size_t num_futures_;
+  size_t offset_;
+  size_t node_;
+  StateId node_state_;
+  vector<Label> context_;
+  StateId context_state_;
+  NGramFstInst()
+      : state_(kNoStateId), node_state_(kNoStateId),
+        context_state_(kNoStateId) { }
+};
+
+// Implementation class for LOUDS based NgramFst interface
+template <class A>
+class NGramFstImpl : public FstImpl<A> {
+  using FstImpl<A>::SetInputSymbols;
+  using FstImpl<A>::SetOutputSymbols;
+  using FstImpl<A>::SetType;
+  using FstImpl<A>::WriteHeader;
+
+  friend class ArcIterator<NGramFst<A> >;
+  friend class NGramFstMatcher<A>;
+
+ public:
+  using FstImpl<A>::InputSymbols;
+  using FstImpl<A>::SetProperties;
+  using FstImpl<A>::Properties;
+
+  typedef A Arc;
+  typedef typename A::Label Label;
+  typedef typename A::StateId StateId;
+  typedef typename A::Weight Weight;
+
+  NGramFstImpl() : data_region_(0), data_(0), owned_(false) {
+    SetType("ngram");
+    SetInputSymbols(NULL);
+    SetOutputSymbols(NULL);
+    SetProperties(kStaticProperties);
+  }
+
+  NGramFstImpl(const Fst<A> &fst, vector<StateId>* order_out);
+
+  ~NGramFstImpl() {
+    if (owned_) {
+      delete [] data_;
+    }
+    delete data_region_;
+  }
+
+  static NGramFstImpl<A>* Read(istream &strm,  // NOLINT
+                               const FstReadOptions &opts) {
+    NGramFstImpl<A>* impl = new NGramFstImpl();
+    FstHeader hdr;
+    if (!impl->ReadHeader(strm, opts, kMinFileVersion, &hdr)) return 0;
+    uint64 num_states, num_futures, num_final;
+    const size_t offset = sizeof(num_states) + sizeof(num_futures) +
+        sizeof(num_final);
+    // Peek at num_states and num_futures to see how much more needs to be read.
+    strm.read(reinterpret_cast<char *>(&num_states), sizeof(num_states));
+    strm.read(reinterpret_cast<char *>(&num_futures), sizeof(num_futures));
+    strm.read(reinterpret_cast<char *>(&num_final), sizeof(num_final));
+    size_t size = Storage(num_states, num_futures, num_final);
+    MappedFile *data_region = MappedFile::Allocate(size);
+    char *data = reinterpret_cast<char *>(data_region->mutable_data());
+    // Copy num_states, num_futures and num_final back into data.
+    memcpy(data, reinterpret_cast<char *>(&num_states), sizeof(num_states));
+    memcpy(data + sizeof(num_states), reinterpret_cast<char *>(&num_futures),
+           sizeof(num_futures));
+    memcpy(data + sizeof(num_states) + sizeof(num_futures),
+           reinterpret_cast<char *>(&num_final), sizeof(num_final));
+    strm.read(data + offset, size - offset);
+    if (!strm) {
+      delete impl;
+      return NULL;
+    }
+    impl->Init(data, false, data_region);
+    return impl;
+  }
+
+  bool Write(ostream &strm,   // NOLINT
+             const FstWriteOptions &opts) const {
+    FstHeader hdr;
+    hdr.SetStart(Start());
+    hdr.SetNumStates(num_states_);
+    WriteHeader(strm, opts, kFileVersion, &hdr);
+    strm.write(data_, Storage(num_states_, num_futures_, num_final_));
+    return strm;
+  }
+
+  StateId Start() const {
+    return 1;
+  }
+
+  Weight Final(StateId state) const {
+    if (final_index_.Get(state)) {
+      return final_probs_[final_index_.Rank1(state)];
+    } else {
+      return Weight::Zero();
+    }
+  }
+
+  size_t NumArcs(StateId state, NGramFstInst<A> *inst = NULL) const {
+    if (inst == NULL) {
+      const size_t next_zero = future_index_.Select0(state + 1);
+      const size_t this_zero = future_index_.Select0(state);
+      return next_zero - this_zero - 1;
+    }
+    SetInstFuture(state, inst);
+    return inst->num_futures_ + ((state == 0) ? 0 : 1);
+  }
+
+  size_t NumInputEpsilons(StateId state) const {
+    // State 0 has no parent, thus no backoff.
+    if (state == 0) return 0;
+    return 1;
+  }
+
+  size_t NumOutputEpsilons(StateId state) const {
+    return NumInputEpsilons(state);
+  }
+
+  StateId NumStates() const {
+    return num_states_;
+  }
+
+  void InitStateIterator(StateIteratorData<A>* data) const {
+    data->base = 0;
+    data->nstates = num_states_;
+  }
+
+  static size_t Storage(uint64 num_states, uint64 num_futures,
+                        uint64 num_final) {
+    uint64 b64;
+    Weight weight;
+    Label label;
+    size_t offset = sizeof(num_states) + sizeof(num_futures) +
+        sizeof(num_final);
+    offset += sizeof(b64) * (
+        BitmapIndex::StorageSize(num_states * 2 + 1) +
+        BitmapIndex::StorageSize(num_futures + num_states + 1) +
+        BitmapIndex::StorageSize(num_states));
+    offset += (num_states + 1) * sizeof(label) + num_futures * sizeof(label);
+    // Pad for alignemnt, see
+    // http://en.wikipedia.org/wiki/Data_structure_alignment#Computing_padding
+    offset = (offset + sizeof(weight) - 1) & ~(sizeof(weight) - 1);
+    offset += (num_states + 1) * sizeof(weight) + num_final * sizeof(weight) +
+        (num_futures + 1) * sizeof(weight);
+    return offset;
+  }
+
+  void SetInstFuture(StateId state, NGramFstInst<A> *inst) const {
+    if (inst->state_ != state) {
+      inst->state_ = state;
+      const size_t next_zero = future_index_.Select0(state + 1);
+      const size_t this_zero = future_index_.Select0(state);
+      inst->num_futures_ = next_zero - this_zero - 1;
+      inst->offset_ = future_index_.Rank1(future_index_.Select0(state) + 1);
+    }
+  }
+
+  void SetInstNode(NGramFstInst<A> *inst) const {
+    if (inst->node_state_ != inst->state_) {
+      inst->node_state_ = inst->state_;
+      inst->node_ = context_index_.Select1(inst->state_);
+    }
+  }
+
+  void SetInstContext(NGramFstInst<A> *inst) const {
+    SetInstNode(inst);
+    if (inst->context_state_ != inst->state_) {
+      inst->context_state_ = inst->state_;
+      inst->context_.clear();
+      size_t node = inst->node_;
+      while (node != 0) {
+        inst->context_.push_back(context_words_[context_index_.Rank1(node)]);
+        node = context_index_.Select1(context_index_.Rank0(node) - 1);
+      }
+    }
+  }
+
+  // Access to the underlying representation
+  const char* GetData(size_t* data_size) const {
+    *data_size = Storage(num_states_, num_futures_, num_final_);
+    return data_;
+  }
+
+  void Init(const char* data, bool owned, MappedFile *file = 0);
+
+  const vector<Label> &GetContext(StateId s, NGramFstInst<A> *inst) const {
+    SetInstFuture(s, inst);
+    SetInstContext(inst);
+    return inst->context_;
+  }
+
+ private:
+  StateId Transition(const vector<Label> &context, Label future) const;
+
+  // Properties always true for this Fst class.
+  static const uint64 kStaticProperties = kAcceptor | kIDeterministic |
+      kODeterministic | kEpsilons | kIEpsilons | kOEpsilons | kILabelSorted |
+      kOLabelSorted | kWeighted | kCyclic | kInitialAcyclic | kNotTopSorted |
+      kAccessible | kCoAccessible | kNotString | kExpanded;
+  // Current file format version.
+  static const int kFileVersion = 4;
+  // Minimum file format version supported.
+  static const int kMinFileVersion = 4;
+
+  MappedFile *data_region_;
+  const char* data_;
+  bool owned_;  // True if we own data_
+  uint64 num_states_, num_futures_, num_final_;
+  size_t root_num_children_;
+  const Label *root_children_;
+  size_t root_first_child_;
+  // borrowed references
+  const uint64 *context_, *future_, *final_;
+  const Label *context_words_, *future_words_;
+  const Weight *backoff_, *final_probs_, *future_probs_;
+  BitmapIndex context_index_;
+  BitmapIndex future_index_;
+  BitmapIndex final_index_;
+
+  void operator=(const NGramFstImpl<A> &);  // Disallow
+};
+
+template<typename A>
+NGramFstImpl<A>::NGramFstImpl(const Fst<A> &fst, vector<StateId>* order_out)
+    : data_region_(0), data_(0), owned_(false) {
+  typedef A Arc;
+  typedef typename Arc::Label Label;
+  typedef typename Arc::Weight Weight;
+  typedef typename Arc::StateId StateId;
+  SetType("ngram");
+  SetInputSymbols(fst.InputSymbols());
+  SetOutputSymbols(fst.OutputSymbols());
+  SetProperties(kStaticProperties);
+
+  // Check basic requirements for an OpenGRM language model Fst.
+  int64 props = kAcceptor | kIDeterministic | kIEpsilons | kILabelSorted;
+  if (fst.Properties(props, true) != props) {
+    FSTERROR() << "NGramFst only accepts OpenGRM langauge models as input";
+    SetProperties(kError, kError);
+    return;
+  }
+
+  int64 num_states = CountStates(fst);
+  Label* context = new Label[num_states];
+
+  // Find the unigram state by starting from the start state, following
+  // epsilons.
+  StateId unigram = fst.Start();
+  while (1) {
+    if (unigram == kNoStateId) {
+      FSTERROR() << "Could not identify unigram state.";
+      SetProperties(kError, kError);
+      return;
+    }
+    ArcIterator<Fst<A> > aiter(fst, unigram);
+    if (aiter.Done()) {
+      LOG(WARNING) << "Unigram state " << unigram << " has no arcs.";
+      break;
+    }
+    if (aiter.Value().ilabel != 0) break;
+    unigram = aiter.Value().nextstate;
+  }
+
+  // Each state's context is determined by the subtree it is under from the
+  // unigram state.
+  queue<pair<StateId, Label> > label_queue;
+  vector<bool> visited(num_states);
+  // Force an epsilon link to the start state.
+  label_queue.push(make_pair(fst.Start(), 0));
+  for (ArcIterator<Fst<A> > aiter(fst, unigram);
+       !aiter.Done(); aiter.Next()) {
+    label_queue.push(make_pair(aiter.Value().nextstate, aiter.Value().ilabel));
+  }
+  // investigate states in breadth first fashion to assign context words.
+  while (!label_queue.empty()) {
+    pair<StateId, Label> &now = label_queue.front();
+    if (!visited[now.first]) {
+      context[now.first] = now.second;
+      visited[now.first] = true;
+      for (ArcIterator<Fst<A> > aiter(fst, now.first);
+           !aiter.Done(); aiter.Next()) {
+        const Arc &arc = aiter.Value();
+        if (arc.ilabel != 0) {
+          label_queue.push(make_pair(arc.nextstate, now.second));
+        }
+      }
+    }
+    label_queue.pop();
+  }
+  visited.clear();
+
+  // The arc from the start state should be assigned an epsilon to put it
+  // in front of the all other labels (which makes Start state 1 after
+  // unigram which is state 0).
+  context[fst.Start()] = 0;
+
+  // Build the tree of contexts fst by reversing the epsilon arcs from fst.
+  VectorFst<Arc> context_fst;
+  uint64 num_final = 0;
+  for (int i = 0; i < num_states; ++i) {
+    if (fst.Final(i) != Weight::Zero()) {
+      ++num_final;
+    }
+    context_fst.SetFinal(context_fst.AddState(), fst.Final(i));
+  }
+  context_fst.SetStart(unigram);
+  context_fst.SetInputSymbols(fst.InputSymbols());
+  context_fst.SetOutputSymbols(fst.OutputSymbols());
+  int64 num_context_arcs = 0;
+  int64 num_futures = 0;
+  for (StateIterator<Fst<A> > siter(fst); !siter.Done(); siter.Next()) {
+    const StateId &state = siter.Value();
+    num_futures += fst.NumArcs(state) - fst.NumInputEpsilons(state);
+    ArcIterator<Fst<A> > aiter(fst, state);
+    if (!aiter.Done()) {
+      const Arc &arc = aiter.Value();
+      // this arc goes from state to arc.nextstate, so create an arc from
+      // arc.nextstate to state to reverse it.
+      if (arc.ilabel == 0) {
+        context_fst.AddArc(arc.nextstate, Arc(context[state], context[state],
+                                              arc.weight, state));
+        num_context_arcs++;
+      }
+    }
+  }
+  if (num_context_arcs != context_fst.NumStates() - 1) {
+    FSTERROR() << "Number of contexts arcs != number of states - 1";
+    SetProperties(kError, kError);
+    return;
+  }
+  if (context_fst.NumStates() != num_states) {
+    FSTERROR() << "Number of contexts != number of states";
+    SetProperties(kError, kError);
+    return;
+  }
+  int64 context_props = context_fst.Properties(kIDeterministic |
+                                               kILabelSorted, true);
+  if (!(context_props & kIDeterministic)) {
+    FSTERROR() << "Input fst is not structured properly";
+    SetProperties(kError, kError);
+    return;
+  }
+  if (!(context_props & kILabelSorted)) {
+     ArcSort(&context_fst, ILabelCompare<Arc>());
+  }
+
+  delete [] context;
+
+  uint64 b64;
+  Weight weight;
+  Label label = kNoLabel;
+  const size_t storage = Storage(num_states, num_futures, num_final);
+  MappedFile *data_region = MappedFile::Allocate(storage);
+  char *data = reinterpret_cast<char *>(data_region->mutable_data());
+  memset(data, 0, storage);
+  size_t offset = 0;
+  memcpy(data + offset, reinterpret_cast<char *>(&num_states),
+         sizeof(num_states));
+  offset += sizeof(num_states);
+  memcpy(data + offset, reinterpret_cast<char *>(&num_futures),
+         sizeof(num_futures));
+  offset += sizeof(num_futures);
+  memcpy(data + offset, reinterpret_cast<char *>(&num_final),
+         sizeof(num_final));
+  offset += sizeof(num_final);
+  uint64* context_bits = reinterpret_cast<uint64*>(data + offset);
+  offset += BitmapIndex::StorageSize(num_states * 2 + 1) * sizeof(b64);
+  uint64* future_bits = reinterpret_cast<uint64*>(data + offset);
+  offset +=
+      BitmapIndex::StorageSize(num_futures + num_states + 1) * sizeof(b64);
+  uint64* final_bits = reinterpret_cast<uint64*>(data + offset);
+  offset += BitmapIndex::StorageSize(num_states) * sizeof(b64);
+  Label* context_words = reinterpret_cast<Label*>(data + offset);
+  offset += (num_states + 1) * sizeof(label);
+  Label* future_words = reinterpret_cast<Label*>(data + offset);
+  offset += num_futures * sizeof(label);
+  offset = (offset + sizeof(weight) - 1) & ~(sizeof(weight) - 1);
+  Weight* backoff = reinterpret_cast<Weight*>(data + offset);
+  offset += (num_states + 1) * sizeof(weight);
+  Weight* final_probs = reinterpret_cast<Weight*>(data + offset);
+  offset += num_final * sizeof(weight);
+  Weight* future_probs = reinterpret_cast<Weight*>(data + offset);
+  int64 context_arc = 0, future_arc = 0, context_bit = 0, future_bit = 0,
+        final_bit = 0;
+
+  // pseudo-root bits
+  BitmapIndex::Set(context_bits, context_bit++);
+  ++context_bit;
+  context_words[context_arc] = label;
+  backoff[context_arc] = Weight::Zero();
+  context_arc++;
+
+  ++future_bit;
+  if (order_out) {
+    order_out->clear();
+    order_out->resize(num_states);
+  }
+
+  queue<StateId> context_q;
+  context_q.push(context_fst.Start());
+  StateId state_number = 0;
+  while (!context_q.empty()) {
+    const StateId &state = context_q.front();
+    if (order_out) {
+      (*order_out)[state] = state_number;
+    }
+
+    const Weight &final = context_fst.Final(state);
+    if (final != Weight::Zero()) {
+      BitmapIndex::Set(final_bits, state_number);
+      final_probs[final_bit] = final;
+      ++final_bit;
+    }
+
+    for (ArcIterator<VectorFst<A> > aiter(context_fst, state);
+         !aiter.Done(); aiter.Next()) {
+      const Arc &arc = aiter.Value();
+      context_words[context_arc] = arc.ilabel;
+      backoff[context_arc] = arc.weight;
+      ++context_arc;
+      BitmapIndex::Set(context_bits, context_bit++);
+      context_q.push(arc.nextstate);
+    }
+    ++context_bit;
+
+    for (ArcIterator<Fst<A> > aiter(fst, state); !aiter.Done(); aiter.Next()) {
+      const Arc &arc = aiter.Value();
+      if (arc.ilabel != 0) {
+        future_words[future_arc] = arc.ilabel;
+        future_probs[future_arc] = arc.weight;
+        ++future_arc;
+        BitmapIndex::Set(future_bits, future_bit++);
+      }
+    }
+    ++future_bit;
+    ++state_number;
+    context_q.pop();
+  }
+
+  if ((state_number !=  num_states) ||
+      (context_bit != num_states * 2 + 1) ||
+      (context_arc != num_states) ||
+      (future_arc != num_futures) ||
+      (future_bit != num_futures + num_states + 1) ||
+      (final_bit != num_final)) {
+    FSTERROR() << "Structure problems detected during construction";
+    SetProperties(kError, kError);
+    return;
+  }
+
+  Init(data, false, data_region);
+}
+
+template<typename A>
+inline void NGramFstImpl<A>::Init(const char* data, bool owned,
+                                  MappedFile *data_region) {
+  if (owned_) {
+    delete [] data_;
+  }
+  delete data_region_;
+  data_region_ = data_region;
+  owned_ = owned;
+  data_ = data;
+  size_t offset = 0;
+  num_states_ = *(reinterpret_cast<const uint64*>(data_ + offset));
+  offset += sizeof(num_states_);
+  num_futures_ = *(reinterpret_cast<const uint64*>(data_ + offset));
+  offset += sizeof(num_futures_);
+  num_final_ = *(reinterpret_cast<const uint64*>(data_ + offset));
+  offset += sizeof(num_final_);
+  uint64 bits;
+  size_t context_bits = num_states_ * 2 + 1;
+  size_t future_bits = num_futures_ + num_states_ + 1;
+  context_ = reinterpret_cast<const uint64*>(data_ + offset);
+  offset += BitmapIndex::StorageSize(context_bits) * sizeof(bits);
+  future_ = reinterpret_cast<const uint64*>(data_ + offset);
+  offset += BitmapIndex::StorageSize(future_bits) * sizeof(bits);
+  final_ = reinterpret_cast<const uint64*>(data_ + offset);
+  offset += BitmapIndex::StorageSize(num_states_) * sizeof(bits);
+  context_words_ = reinterpret_cast<const Label*>(data_ + offset);
+  offset += (num_states_ + 1) * sizeof(*context_words_);
+  future_words_ = reinterpret_cast<const Label*>(data_ + offset);
+  offset += num_futures_ * sizeof(*future_words_);
+  offset = (offset + sizeof(*backoff_) - 1) & ~(sizeof(*backoff_) - 1);
+  backoff_ = reinterpret_cast<const Weight*>(data_ + offset);
+  offset += (num_states_ + 1) * sizeof(*backoff_);
+  final_probs_ = reinterpret_cast<const Weight*>(data_ + offset);
+  offset += num_final_ * sizeof(*final_probs_);
+  future_probs_ = reinterpret_cast<const Weight*>(data_ + offset);
+
+  context_index_.BuildIndex(context_, context_bits);
+  future_index_.BuildIndex(future_, future_bits);
+  final_index_.BuildIndex(final_, num_states_);
+
+  const size_t node_rank = context_index_.Rank1(0);
+  root_first_child_ = context_index_.Select0(node_rank) + 1;
+  if (context_index_.Get(root_first_child_) == false) {
+    FSTERROR() << "Missing unigrams";
+    SetProperties(kError, kError);
+    return;
+  }
+  const size_t last_child = context_index_.Select0(node_rank + 1) - 1;
+  root_num_children_ = last_child - root_first_child_ + 1;
+  root_children_ = context_words_ + context_index_.Rank1(root_first_child_);
+}
+
+template<typename A>
+inline typename A::StateId NGramFstImpl<A>::Transition(
+        const vector<Label> &context, Label future) const {
+  size_t num_children = root_num_children_;
+  const Label *children = root_children_;
+  const Label *loc = lower_bound(children, children + num_children, future);
+  if (loc == children + num_children || *loc != future) {
+    return context_index_.Rank1(0);
+  }
+  size_t node = root_first_child_ + loc - children;
+  size_t node_rank = context_index_.Rank1(node);
+  size_t first_child = context_index_.Select0(node_rank) + 1;
+  if (context_index_.Get(first_child) == false) {
+    return context_index_.Rank1(node);
+  }
+  size_t last_child = context_index_.Select0(node_rank + 1) - 1;
+  num_children = last_child - first_child + 1;
+  for (int word = context.size() - 1; word >= 0; --word) {
+    children = context_words_ + context_index_.Rank1(first_child);
+    loc = lower_bound(children, children + last_child - first_child + 1,
+                      context[word]);
+    if (loc == children + last_child - first_child + 1 ||
+        *loc != context[word]) {
+      break;
+    }
+    node = first_child + loc - children;
+    node_rank = context_index_.Rank1(node);
+    first_child = context_index_.Select0(node_rank) + 1;
+    if (context_index_.Get(first_child) == false) break;
+    last_child = context_index_.Select0(node_rank + 1) - 1;
+  }
+  return context_index_.Rank1(node);
+}
+
+/*****************************************************************************/
+template<class A>
+class NGramFst : public ImplToExpandedFst<NGramFstImpl<A> > {
+  friend class ArcIterator<NGramFst<A> >;
+  friend class NGramFstMatcher<A>;
+
+ public:
+  typedef A Arc;
+  typedef typename A::StateId StateId;
+  typedef typename A::Label Label;
+  typedef typename A::Weight Weight;
+  typedef NGramFstImpl<A> Impl;
+
+  explicit NGramFst(const Fst<A> &dst)
+      : ImplToExpandedFst<Impl>(new Impl(dst, NULL)) {}
+
+  NGramFst(const Fst<A> &fst, vector<StateId>* order_out)
+      : ImplToExpandedFst<Impl>(new Impl(fst, order_out)) {}
+
+  // Because the NGramFstImpl is a const stateless data structure, there
+  // is never a need to do anything beside copy the reference.
+  NGramFst(const NGramFst<A> &fst, bool safe = false)
+      : ImplToExpandedFst<Impl>(fst, false) {}
+
+  NGramFst() : ImplToExpandedFst<Impl>(new Impl()) {}
+
+  // Non-standard constructor to initialize NGramFst directly from data.
+  NGramFst(const char* data, bool owned) : ImplToExpandedFst<Impl>(new Impl()) {
+    GetImpl()->Init(data, owned, NULL);
+  }
+
+  // Get method that gets the data associated with Init().
+  const char* GetData(size_t* data_size) const {
+    return GetImpl()->GetData(data_size);
+  }
+
+  const vector<Label> GetContext(StateId s) const {
+    return GetImpl()->GetContext(s, &inst_);
+  }
+
+  virtual size_t NumArcs(StateId s) const {
+    return GetImpl()->NumArcs(s, &inst_);
+  }
+
+  virtual NGramFst<A>* Copy(bool safe = false) const {
+    return new NGramFst(*this, safe);
+  }
+
+  static NGramFst<A>* Read(istream &strm, const FstReadOptions &opts) {
+    Impl* impl = Impl::Read(strm, opts);
+    return impl ? new NGramFst<A>(impl) : 0;
+  }
+
+  static NGramFst<A>* Read(const string &filename) {
+    if (!filename.empty()) {
+      ifstream strm(filename.c_str(), ifstream::in | ifstream::binary);
+      if (!strm) {
+        LOG(ERROR) << "NGramFst::Read: Can't open file: " << filename;
+        return 0;
+      }
+      return Read(strm, FstReadOptions(filename));
+    } else {
+      return Read(cin, FstReadOptions("standard input"));
+    }
+  }
+
+  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 inline void InitStateIterator(StateIteratorData<A>* data) const {
+    GetImpl()->InitStateIterator(data);
+  }
+
+  virtual inline void InitArcIterator(
+      StateId s, ArcIteratorData<A>* data) const;
+
+  virtual MatcherBase<A>* InitMatcher(MatchType match_type) const {
+    return new NGramFstMatcher<A>(*this, match_type);
+  }
+
+ private:
+  explicit NGramFst(Impl* impl) : ImplToExpandedFst<Impl>(impl) {}
+
+  Impl* GetImpl() const {
+    return
+        ImplToExpandedFst<Impl, ExpandedFst<A> >::GetImpl();
+  }
+
+  void SetImpl(Impl* impl, bool own_impl = true) {
+    ImplToExpandedFst<Impl, Fst<A> >::SetImpl(impl, own_impl);
+  }
+
+  mutable NGramFstInst<A> inst_;
+};
+
+template <class A> inline void
+NGramFst<A>::InitArcIterator(StateId s, ArcIteratorData<A>* data) const {
+  GetImpl()->SetInstFuture(s, &inst_);
+  GetImpl()->SetInstNode(&inst_);
+  data->base = new ArcIterator<NGramFst<A> >(*this, s);
+}
+
+/*****************************************************************************/
+template <class A>
+class NGramFstMatcher : public MatcherBase<A> {
+ public:
+  typedef A Arc;
+  typedef typename A::Label Label;
+  typedef typename A::StateId StateId;
+  typedef typename A::Weight Weight;
+
+  NGramFstMatcher(const NGramFst<A> &fst, MatchType match_type)
+      : fst_(fst), inst_(fst.inst_), match_type_(match_type),
+        current_loop_(false),
+        loop_(kNoLabel, 0, A::Weight::One(), kNoStateId) {
+    if (match_type_ == MATCH_OUTPUT) {
+      swap(loop_.ilabel, loop_.olabel);
+    }
+  }
+
+  NGramFstMatcher(const NGramFstMatcher<A> &matcher, bool safe = false)
+      : fst_(matcher.fst_), inst_(matcher.inst_),
+        match_type_(matcher.match_type_), current_loop_(false),
+        loop_(kNoLabel, 0, A::Weight::One(), kNoStateId) {
+    if (match_type_ == MATCH_OUTPUT) {
+      swap(loop_.ilabel, loop_.olabel);
+    }
+  }
+
+  virtual NGramFstMatcher<A>* Copy(bool safe = false) const {
+    return new NGramFstMatcher<A>(*this, safe);
+  }
+
+  virtual MatchType Type(bool test) const {
+    return match_type_;
+  }
+
+  virtual const Fst<A> &GetFst() const {
+    return fst_;
+  }
+
+  virtual uint64 Properties(uint64 props) const {
+    return props;
+  }
+
+ private:
+  virtual void SetState_(StateId s) {
+    fst_.GetImpl()->SetInstFuture(s, &inst_);
+    current_loop_ = false;
+  }
+
+  virtual bool Find_(Label label) {
+    const Label nolabel = kNoLabel;
+    done_ = true;
+    if (label == 0 || label == nolabel) {
+      if (label == 0) {
+        current_loop_ = true;
+        loop_.nextstate = inst_.state_;
+      }
+      // The unigram state has no epsilon arc.
+      if (inst_.state_ != 0) {
+        arc_.ilabel = arc_.olabel = 0;
+        fst_.GetImpl()->SetInstNode(&inst_);
+        arc_.nextstate = fst_.GetImpl()->context_index_.Rank1(
+            fst_.GetImpl()->context_index_.Select1(
+                fst_.GetImpl()->context_index_.Rank0(inst_.node_) - 1));
+        arc_.weight = fst_.GetImpl()->backoff_[inst_.state_];
+        done_ = false;
+      }
+    } else {
+      const Label *start = fst_.GetImpl()->future_words_ + inst_.offset_;
+      const Label *end = start + inst_.num_futures_;
+      const Label* search = lower_bound(start, end, label);
+      if (search != end && *search == label) {
+        size_t state = search - start;
+        arc_.ilabel = arc_.olabel = label;
+        arc_.weight = fst_.GetImpl()->future_probs_[inst_.offset_ + state];
+        fst_.GetImpl()->SetInstContext(&inst_);
+        arc_.nextstate = fst_.GetImpl()->Transition(inst_.context_, label);
+        done_ = false;
+      }
+    }
+    return !Done_();
+  }
+
+  virtual bool Done_() const {
+    return !current_loop_ && done_;
+  }
+
+  virtual const Arc& Value_() const {
+    return (current_loop_) ? loop_ : arc_;
+  }
+
+  virtual void Next_() {
+    if (current_loop_) {
+      current_loop_ = false;
+    } else {
+      done_ = true;
+    }
+  }
+
+  const NGramFst<A>& fst_;
+  NGramFstInst<A> inst_;
+  MatchType match_type_;             // Supplied by caller
+  bool done_;
+  Arc arc_;
+  bool current_loop_;                // Current arc is the implicit loop
+  Arc loop_;
+};
+
+/*****************************************************************************/
+template<class A>
+class ArcIterator<NGramFst<A> > : public ArcIteratorBase<A> {
+ public:
+  typedef A Arc;
+  typedef typename A::Label Label;
+  typedef typename A::StateId StateId;
+  typedef typename A::Weight Weight;
+
+  ArcIterator(const NGramFst<A> &fst, StateId state)
+      : lazy_(~0), impl_(fst.GetImpl()), i_(0), flags_(kArcValueFlags) {
+    inst_ = fst.inst_;
+    impl_->SetInstFuture(state, &inst_);
+    impl_->SetInstNode(&inst_);
+  }
+
+  bool Done() const {
+    return i_ >= ((inst_.node_ == 0) ? inst_.num_futures_ :
+                  inst_.num_futures_ + 1);
+  }
+
+  const Arc &Value() const {
+    bool eps = (inst_.node_ != 0 && i_ == 0);
+    StateId state = (inst_.node_ == 0) ? i_ : i_ - 1;
+    if (flags_ & lazy_ & (kArcILabelValue | kArcOLabelValue)) {
+      arc_.ilabel =
+          arc_.olabel = eps ? 0 : impl_->future_words_[inst_.offset_ + state];
+      lazy_ &= ~(kArcILabelValue | kArcOLabelValue);
+    }
+    if (flags_ & lazy_ & kArcNextStateValue) {
+      if (eps) {
+        arc_.nextstate = impl_->context_index_.Rank1(
+            impl_->context_index_.Select1(
+                impl_->context_index_.Rank0(inst_.node_) - 1));
+      } else {
+        if (lazy_ & kArcNextStateValue) {
+          impl_->SetInstContext(&inst_);  // first time only.
+        }
+        arc_.nextstate =
+            impl_->Transition(inst_.context_,
+                              impl_->future_words_[inst_.offset_ + state]);
+      }
+      lazy_ &= ~kArcNextStateValue;
+    }
+    if (flags_ & lazy_ & kArcWeightValue) {
+      arc_.weight = eps ?  impl_->backoff_[inst_.state_] :
+          impl_->future_probs_[inst_.offset_ + state];
+      lazy_ &= ~kArcWeightValue;
+    }
+    return arc_;
+  }
+
+  void Next() {
+    ++i_;
+    lazy_ = ~0;
+  }
+
+  size_t Position() const { return i_; }
+
+  void Reset() {
+    i_ = 0;
+    lazy_ = ~0;
+  }
+
+  void Seek(size_t a) {
+    if (i_ != a) {
+      i_ = a;
+      lazy_ = ~0;
+    }
+  }
+
+  uint32 Flags() const {
+    return flags_;
+  }
+
+  void SetFlags(uint32 f, uint32 m) {
+    flags_ &= ~m;
+    flags_ |= (f & kArcValueFlags);
+  }
+
+ private:
+  virtual bool Done_() const { return Done(); }
+  virtual const Arc& Value_() const { return Value(); }
+  virtual void Next_() { Next(); }
+  virtual size_t Position_() const { return Position(); }
+  virtual void Reset_() { Reset(); }
+  virtual void Seek_(size_t a) { Seek(a); }
+  uint32 Flags_() const { return Flags(); }
+  void SetFlags_(uint32 f, uint32 m) { SetFlags(f, m); }
+
+  mutable Arc arc_;
+  mutable uint32 lazy_;
+  const NGramFstImpl<A> *impl_;
+  mutable NGramFstInst<A> inst_;
+
+  size_t i_;
+  uint32 flags_;
+
+  DISALLOW_COPY_AND_ASSIGN(ArcIterator);
+};
+
+/*****************************************************************************/
+// Specialization for NGramFst; see generic version in fst.h
+// for sample usage (but use the ProdLmFst type!). This version
+// should inline.
+template <class A>
+class StateIterator<NGramFst<A> > : public StateIteratorBase<A> {
+  public:
+  typedef typename A::StateId StateId;
+
+  explicit StateIterator(const NGramFst<A> &fst)
+    : s_(0), num_states_(fst.NumStates()) { }
+
+  bool Done() const { return s_ >= num_states_; }
+  StateId Value() const { return s_; }
+  void Next() { ++s_; }
+  void Reset() { s_ = 0; }
+
+ private:
+  virtual bool Done_() const { return Done(); }
+  virtual StateId Value_() const { return Value(); }
+  virtual void Next_() { Next(); }
+  virtual void Reset_() { Reset(); }
+
+  StateId s_, num_states_;
+
+  DISALLOW_COPY_AND_ASSIGN(StateIterator);
+};
+}  // namespace fst
+#endif  // FST_EXTENSIONS_NGRAM_NGRAM_FST_H_
-- 
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