path: root/decoder/hg.cc

//TODO: lazily generate feature vectors for hyperarcs (because some of them will be pruned).  this means 1) storing ref to rule for those features 2) providing ff interface for regenerating its feature vector from hyperedge+states and probably 3) still caching feat. vect on hyperedge once it's been generated.  ff would normally just contribute its weighted score and result state, not component features.  however, the hypergraph drops the state used by ffs after rescoring is done, so recomputation would have to start at the leaves and work bottom up.  question: which takes more space, feature id+value, or state?

#include "hg.h"

#include <algorithm>
#include <cassert>
#include <numeric>
#include <set>
#include <map>
#include <iostream>
#include <sstream>

#include "viterbi.h"
#include "inside_outside.h"
#include "tdict.h"

using namespace std;

Hypergraph::Edge const* Hypergraph::ViterbiGoalEdge() const
{
  Edge const* r=0;
  for (unsigned i=0,e=edges_.size();i<e;++i) {
    Edge const& e=edges_[i];
    if (e.rule_ && e.rule_->IsGoal() && (!r || e.edge_prob_ > r->edge_prob_))
      r=&e;
  }
  return r;
}

std::string Hypergraph::stats(std::string const& name) const
{
  ostringstream o;
  o<<name<<" (nodes/edges): "<<nodes_.size()<<'/'<<edges_.size()<<endl;
  o<<name<<"       (paths): "<<NumberOfPaths()<<endl;
  return o.str();
}


double Hypergraph::NumberOfPaths() const {
  return Inside<double, TransitionCountWeightFunction>(*this);
}

struct ScaledTransitionEventWeightFunction {
  typedef SparseVector<prob_t> Result;
  ScaledTransitionEventWeightFunction(double alpha) : scale_(alpha) {}
  inline SparseVector<prob_t> operator()(const Hypergraph::Edge& e) const {
    SparseVector<prob_t> result;
    result.set_value(e.id_, e.edge_prob_.pow(scale_));
    return result;
  }
  const double scale_;
};

// safe to reinterpret a vector of these as a vector of prob_t (plain old data)
struct TropicalValue {
  TropicalValue() : v_() {}
  explicit TropicalValue(int v) {
    if (v == 0) v_ = prob_t::Zero();
    else if (v == 1) v_ = prob_t::One();
    else { cerr << "Bad value in TropicalValue(int).\n"; abort(); }
  }
  explicit TropicalValue(const prob_t& v) : v_(v) {}
  inline TropicalValue& operator+=(const TropicalValue& o) {
    if (v_ < o.v_) v_ = o.v_;
    return *this;
  }
  inline TropicalValue& operator*=(const TropicalValue& o) {
    v_ *= o.v_;
    return *this;
  }
  inline bool operator==(const TropicalValue& o) const { return v_ == o.v_; }
  prob_t v_;
};

struct ViterbiWeightFunction {
  typedef TropicalValue Weight;
  inline TropicalValue operator()(const Hypergraph::Edge& e) const {
    return TropicalValue(e.edge_prob_);
  }
};

struct ViterbiTransitionEventWeightFunction {
  typedef SparseVector<TropicalValue> Result;
  inline SparseVector<TropicalValue> operator()(const Hypergraph::Edge& e) const {
    SparseVector<TropicalValue> result;
    result.set_value(e.id_, TropicalValue(e.edge_prob_));
    return result;
  }
};


prob_t Hypergraph::ComputeEdgePosteriors(double scale, vector<prob_t>* posts) const {
  const ScaledEdgeProb weight(scale);
  const ScaledTransitionEventWeightFunction w2(scale);
  SparseVector<prob_t> pv;
  const double inside = InsideOutside<prob_t,
                  ScaledEdgeProb,
                  SparseVector<prob_t>,
                  ScaledTransitionEventWeightFunction>(*this, &pv, weight, w2);
  posts->resize(edges_.size());
  for (int i = 0; i < edges_.size(); ++i)
    (*posts)[i] = prob_t(pv.value(i));
  return prob_t(inside);
}

prob_t Hypergraph::ComputeBestPathThroughEdges(vector<prob_t>* post) const {
  // I don't like this - explicitly passing around counts of each edge.  It's clever but slow.
  SparseVector<TropicalValue> pv;
  const TropicalValue viterbi_weight = InsideOutside<TropicalValue,
                  ViterbiWeightFunction,
                  SparseVector<TropicalValue>,
                  ViterbiTransitionEventWeightFunction>(*this, &pv);
  post->resize(edges_.size());
  for (int i = 0; i < edges_.size(); ++i)
    (*post)[i] = pv.value(i).v_;
  return viterbi_weight.v_;
}

void Hypergraph::PushWeightsToSource(double scale) {
  vector<prob_t> posts;
  ComputeEdgePosteriors(scale, &posts);
  for (int i = 0; i < nodes_.size(); ++i) {
    const Hypergraph::Node& node = nodes_[i];
    prob_t z = prob_t::Zero();
    for (int j = 0; j < node.out_edges_.size(); ++j)
      z += posts[node.out_edges_[j]];
    for (int j = 0; j < node.out_edges_.size(); ++j) {
      edges_[node.out_edges_[j]].edge_prob_ = posts[node.out_edges_[j]] / z;
    }
  }
}

void Hypergraph::PushWeightsToGoal(double scale) {
  vector<prob_t> posts;
  ComputeEdgePosteriors(scale, &posts);
  for (int i = 0; i < nodes_.size(); ++i) {
    const Hypergraph::Node& node = nodes_[i];
    prob_t z = prob_t::Zero();
    for (int j = 0; j < node.in_edges_.size(); ++j)
      z += posts[node.in_edges_[j]];
    for (int j = 0; j < node.in_edges_.size(); ++j) {
      edges_[node.in_edges_[j]].edge_prob_ = posts[node.in_edges_[j]] / z;
    }
  }
}

struct EdgeExistsWeightFunction {
  EdgeExistsWeightFunction(const vector<bool>& prunes) : prunes_(prunes) {}
  bool operator()(const Hypergraph::Edge& edge) const {
    return !prunes_[edge.id_];
  }
 private:
  const vector<bool>& prunes_;
};

void Hypergraph::PruneEdges(const EdgeMask& prune_edge, bool run_inside_algorithm) {
  assert(prune_edge.size() == edges_.size());
  vector<bool> filtered = prune_edge;

  if (run_inside_algorithm) {
    const EdgeExistsWeightFunction wf(prune_edge);
    vector<Boolean> reachable;
    bool goal_derivable = Inside/* <Boolean, EdgeExistsWeightFunction> */(*this, &reachable, wf);
    if (!goal_derivable) {
      edges_.clear();
      nodes_.clear();
      nodes_.push_back(Node());
      return;
    }

    assert(reachable.size() == nodes_.size());
    for (int i = 0; i < edges_.size(); ++i) {
      bool prune = prune_edge[i];
      if (!prune) {
        const Edge& edge = edges_[i];
        for (int j = 0; j < edge.tail_nodes_.size(); ++j) {
          if (!reachable[edge.tail_nodes_[j]]) {
            prune = true;
            break;
          }
        }
      }
      filtered[i] = prune;
    }
  }
  TopologicallySortNodesAndEdges(nodes_.size() - 1, &filtered);
}

void Hypergraph::SetPromise(NodeProbs const& inside,NodeProbs const& outside,double power, bool normalize)
{
  int nn=nodes_.size();
  if (!nn) return;
  assert(inside.size()==nn);
  assert(outside.size()==nn);
  double sum; //TODO: prevent underflow by using prob_t?
  if (normalize)
  for (int i=0;i<nn;++i) {
    sum+=(nodes_[i].promise=pow(inside[i]*outside[i],power));
  }
  if (normalize) {
    double by=nn/sum; // so avg promise is 1
    for (int i=0;i<nn;++i)
      nodes_[i].promise*=by;
  }
}



void Hypergraph::MarginPrune(vector<prob_t> const& io,prob_t cutoff,vector<bool> const* preserve_mask,bool safe_inside,bool verbose)
{
  assert(io.size()==edges_.size());
  const prob_t BARELY_SMALLER(1e-6,false); // nearly 1; 1-epsilon
  //TODO: //FIXME: if EPSILON is 0, then remnants (useless edges that don't connect to top? or top-connected but not bottom-up buildable referneced?) are left in the hypergraph output that cause mr_vest_map to segfault.  adding EPSILON probably just covers up the symptom by making it far less frequent; I imagine any time threshold is set by DensityPrune, cutoff is exactly equal to the io of several nodes, but because of how it's computed, some round slightly down vs. slightly up.  probably the flaw is in PruneEdges.
  int ne=NumberOfEdges();
  cutoff*=BARELY_SMALLER;
  prob_t creep=BARELY_SMALLER.root(-(ne+1)); // start more permissive, then become less generous.  this is barely more than 1.  we want to do this because it's a disaster if something lower in a derivation tree is deleted, but the higher thing remains (unless safe_inside)

  vector<bool> prune(ne);
  if (verbose) {
    if (preserve_mask) cerr << preserve_mask->size() << " " << prune.size() << endl;
    cerr<<"Finishing prune for "<<prune.size()<<" edges; CUTOFF=" << cutoff << endl;
  }
  unsigned pc = 0;
  for (int i = 0; i < io.size(); ++i) {
    cutoff*=creep;
    const bool prune_edge = (io[i] < cutoff);
    if (prune_edge) {
      ++pc;
      prune[i] = !(preserve_mask && (*preserve_mask)[i]);
    }
  }
  if (verbose)
    cerr << "Finished pruning; removed " << pc << "/" << io.size() << " edges\n";
  PruneEdges(prune,safe_inside); // inside reachability check in case cutoff rounded down too much (probably redundant with EPSILON hack)
}

template <class V>
V nth_greatest(int n,vector<V> vs) {
  nth_element(vs.begin(),vs.begin()+n,vs.end(),greater<V>());
  return vs[n];
}

bool Hypergraph::PruneInsideOutside(double alpha,double density,const EdgeMask* preserve_mask,const bool use_sum_prod_semiring, const double scale,double promise_power,bool safe_inside)
{
  bool use_density=density!=0;
  bool use_beam=alpha!=0;
  assert(!use_beam||alpha>0);
  assert(!use_density||density>=1);
  assert(!use_sum_prod_semiring||scale>0);
  int rnum;
  if (use_density) {
    const int plen = ViterbiPathLength(*this);
    vector<WordID> bp;
    rnum = min(static_cast<int>(edges_.size()), static_cast<int>(density * static_cast<double>(plen)));
    cerr << "Density pruning: keep "<<rnum<<" of "<<edges_.size()<<" edges (viterbi = "<<plen<<" edges)"<<endl;
    if (rnum == edges_.size()) {
      cerr << "No pruning required: denisty already sufficient\n";
      if (!use_beam)
        return false;
      use_density=false;
    }
  }
  assert(use_density||use_beam);
  InsideOutsides<prob_t> io;
  OutsideNormalize<prob_t> norm;
  if (use_sum_prod_semiring)
    io.compute(*this,norm,ScaledEdgeProb(scale));
  else
    io.compute(*this,norm,ViterbiWeightFunction());  // the storage gets cast to Tropical from prob_t, scary - e.g. w/ specialized static allocator differences it could break.
  vector<prob_t> mm;
  io.compute_edge_marginals(*this,mm,EdgeProb()); // should be normalized to 1 for best edges in viterbi.  in sum, best is less than 1.

  prob_t cutoff=prob_t::One(); // we'll destroy everything smaller than this (note: nothing is bigger than 1).  so bigger cutoff = more pruning.
  bool density_won=false;
  if (use_density) {
    cutoff=nth_greatest(rnum,mm);
    density_won=true;
  }
  if (use_beam) {
    prob_t best=prob_t::One();
    if (use_sum_prod_semiring) {
      for (int i = 0; i < mm.size(); ++i)
        if (mm[i] > best) best = mm[i];
    }
    prob_t beam_cut=best*prob_t::exp(-alpha);
    if (!(use_density&&cutoff>beam_cut)) {
      density_won=false;
      cutoff=beam_cut;
    }
  }
  if (promise_power!=0)
    SetPromise(io.inside,io.outside,promise_power,true);
  MarginPrune(mm,cutoff,preserve_mask,safe_inside); // we do this last because otherwise indices in mm would be wrong for setting promise.
  return density_won;
}


void Hypergraph::PrintGraphviz() const {
  int ei = 0;
  cerr << "digraph G {\n  rankdir=LR;\n  nodesep=.05;\n";
  for (vector<Edge>::const_iterator i = edges_.begin();
       i != edges_.end(); ++i) {
    const Edge& edge=*i;
    ++ei;
    static const string none = "<null>";
    string rule = (edge.rule_ ? edge.rule_->AsString(false) : none);

    cerr << "   A_" << ei << " [label=\"" << rule << " p=" << edge.edge_prob_
         << " F:" << edge.feature_values_
         << "\" shape=\"rect\"];\n";
    Hypergraph::TailNodeVector indorder(edge.tail_nodes_.size(), 0);
    int ntc = 0;
    for (int i = 0; i < edge.rule_->e_.size(); ++i) {
      if (edge.rule_->e_[i] <= 0) indorder[ntc++] = 1 + (-1 * edge.rule_->e_[i]);
    }
    for (int i = 0; i < edge.tail_nodes_.size(); ++i) {
      cerr << "     " << edge.tail_nodes_[i] << " -> A_" << ei;
      if (edge.tail_nodes_.size() > 1) {
        cerr << " [label=\"" << indorder[i] << "\"]";
      }
      cerr << ";\n";
    }
    cerr << "     A_" << ei << " -> " << edge.head_node_ << ";\n";
  }
  for (vector<Node>::const_iterator ni = nodes_.begin();
      ni != nodes_.end(); ++ni) {
    cerr << "  " << ni->id_ << "[label=\"" << (ni->cat_ < 0 ? TD::Convert(ni->cat_ * -1) : "")
    //cerr << "  " << ni->id_ << "[label=\"" << ni->cat_
         << " n=" << ni->id_
//         << ",x=" << &*ni
//         << ",in=" << ni->in_edges_.size()
//         << ",out=" << ni->out_edges_.size()
         << "\"];\n";
  }
  cerr << "}\n";
}

void Hypergraph::Union(const Hypergraph& other) {
  if (&other == this) return;
  if (nodes_.empty()) { nodes_ = other.nodes_; edges_ = other.edges_; return; }
  int noff = nodes_.size();
  int eoff = edges_.size();
  int ogoal = other.nodes_.size() - 1;
  int cgoal = noff - 1;
  // keep a single goal node, so add nodes.size - 1
  nodes_.resize(nodes_.size() + ogoal);
  // add all edges
  edges_.resize(edges_.size() + other.edges_.size());

  for (int i = 0; i < ogoal; ++i) {
    const Node& on = other.nodes_[i];
    Node& cn = nodes_[i + noff];
    cn.id_ = i + noff;
    cn.in_edges_.resize(on.in_edges_.size());
    for (int j = 0; j < on.in_edges_.size(); ++j)
      cn.in_edges_[j] = on.in_edges_[j] + eoff;

    cn.out_edges_.resize(on.out_edges_.size());
    for (int j = 0; j < on.out_edges_.size(); ++j)
      cn.out_edges_[j] = on.out_edges_[j] + eoff;
  }

  for (int i = 0; i < other.edges_.size(); ++i) {
    const Edge& oe = other.edges_[i];
    Edge& ce = edges_[i + eoff];
    ce.id_ = i + eoff;
    ce.rule_ = oe.rule_;
    ce.feature_values_ = oe.feature_values_;
    if (oe.head_node_ == ogoal) {
      ce.head_node_ = cgoal;
      nodes_[cgoal].in_edges_.push_back(ce.id_);
    } else {
      ce.head_node_ = oe.head_node_ + noff;
    }
    ce.tail_nodes_.resize(oe.tail_nodes_.size());
    for (int j = 0; j < oe.tail_nodes_.size(); ++j)
      ce.tail_nodes_[j] = oe.tail_nodes_[j] + noff;
  }

  TopologicallySortNodesAndEdges(cgoal);
}

void Hypergraph::PruneUnreachable(int goal_node_id) {
  TopologicallySortNodesAndEdges(goal_node_id, NULL);
}

void Hypergraph::RemoveNoncoaccessibleStates(int goal_node_id) {
  if (goal_node_id < 0) goal_node_id += nodes_.size();
  assert(goal_node_id >= 0);
  assert(goal_node_id < nodes_.size());

  // TODO finish implementation
  abort();
}

struct DFSContext {
  int node;
  int edge_iter;
  int tail_iter;
  DFSContext(int n, int e, int t) : node(n), edge_iter(e), tail_iter(t) {}
};

enum ColorType { WHITE, GRAY, BLACK };

template <class T>
struct BadId {
  bool operator()(const T& obj) const { return obj.id_ == -1; }
};

template <class T>
struct IdCompare {
  bool operator()(const T& a, const T& b) { return a.id_ < b.id_; }
};

// this keeps the nodes' edge indices and edges' node indices in sync.  or do nodes not get removed when you prune_edges?  seems like they get reordered.
//TODO: if you had parallel arrays associating data w/ each node or edge, you'd want access to reloc_node and reloc_edge - expose in stateful object?
void Hypergraph::TopologicallySortNodesAndEdges(int goal_index,
                                                const vector<bool>* prune_edges) {
  // figure out which nodes are reachable from the goal
  vector<int> reloc_node(nodes_.size(), -1);
  vector<int> reloc_edge(edges_.size(), -1);
  vector<ColorType> color(nodes_.size(), WHITE);
  vector<DFSContext> stack;
  stack.reserve(nodes_.size());
  stack.push_back(DFSContext(goal_index, 0, 0));
  int node_count = 0;
  int edge_count = 0;
  while(!stack.empty()) {
    const DFSContext& p = stack.back();
    int cur_ni = p.node;
    int edge_i = p.edge_iter;
    int tail_i = p.tail_iter;
    stack.pop_back();
    const Node* cur_node = &nodes_[cur_ni];
    int edge_end = cur_node->in_edges_.size();
    while (edge_i != edge_end) {
      const Edge& cur_edge = edges_[cur_node->in_edges_[edge_i]];
      const int tail_end = cur_edge.tail_nodes_.size();
      if ((tail_end == tail_i) || (prune_edges && (*prune_edges)[cur_edge.id_])) {
        ++edge_i;
        tail_i = 0;
        continue;
      }
      const int tail_ni = cur_edge.tail_nodes_[tail_i];
      const int tail_color = color[tail_ni];
      if (tail_color == WHITE) {
        stack.push_back(DFSContext(cur_ni, edge_i, ++tail_i));
        cur_ni = tail_ni;
        cur_node = &nodes_[cur_ni];
        color[cur_ni] = GRAY;
        edge_i = 0;
        edge_end = cur_node->in_edges_.size();
        tail_i = 0;
      } else if (tail_color == BLACK) {
        ++tail_i;
      } else if (tail_color == GRAY) {
        // this can happen if, e.g., it is possible to rederive
        // a single cell in the CKY chart via a cycle.
        cerr << "Detected forbidden cycle in HG:\n";
        cerr << "  " << cur_edge.rule_->AsString() << endl;
        while(!stack.empty()) {
          const DFSContext& p = stack.back();
          cerr << "  " << edges_[nodes_[p.node].in_edges_[p.edge_iter]].rule_->AsString() << endl;
          stack.pop_back();
        }
        abort();
      }
    }
    color[cur_ni] = BLACK;
    reloc_node[cur_ni] = node_count++;
    if (prune_edges) {
      for (int i = 0; i < edge_end; ++i) {
        int ei = cur_node->in_edges_[i];
        if (!(*prune_edges)[ei])
          reloc_edge[cur_node->in_edges_[i]] = edge_count++;
      }
    } else {
      for (int i = 0; i < edge_end; ++i)
        reloc_edge[cur_node->in_edges_[i]] = edge_count++;
    }
  }
#ifndef HG_EDGES_TOPO_SORTED
  int ec = 0;
  for (int i = 0; i < reloc_edge.size(); ++i) {
    int& cp = reloc_edge[i];
    if (cp >= 0) { cp = ec++; }
  }
#endif

#if 0
  cerr << "TOPO:";
  for (int i = 0; i < reloc_node.size(); ++i)
    cerr << " " << reloc_node[i];
  cerr << endl;
  cerr << "EDGE:";
  for (int i = 0; i < reloc_edge.size(); ++i)
    cerr << " " << reloc_edge[i];
  cerr << endl;
#endif
  bool no_op = true;
  for (int i = 0; i < reloc_node.size() && no_op; ++i)
    if (reloc_node[i] != i) no_op = false;
  for (int i = 0; i < reloc_edge.size() && no_op; ++i)
    if (reloc_edge[i] != i) no_op = false;
  if (no_op) return;
  for (int i = 0; i < reloc_node.size(); ++i) {
    Node& node = nodes_[i];
    node.id_ = reloc_node[i];
    int c = 0;
    for (int j = 0; j < node.in_edges_.size(); ++j) {
      const int new_index = reloc_edge[node.in_edges_[j]];
      if (new_index >= 0)
        node.in_edges_[c++] = new_index;
    }
    node.in_edges_.resize(c);
    c = 0;
    for (int j = 0; j < node.out_edges_.size(); ++j) {
      const int new_index = reloc_edge[node.out_edges_[j]];
      if (new_index >= 0)
        node.out_edges_[c++] = new_index;
    }
    node.out_edges_.resize(c);
  }
  for (int i = 0; i < reloc_edge.size(); ++i) {
    Edge& edge = edges_[i];
    edge.id_ = reloc_edge[i];
    edge.head_node_ = reloc_node[edge.head_node_];
    for (int j = 0; j < edge.tail_nodes_.size(); ++j)
      edge.tail_nodes_[j] = reloc_node[edge.tail_nodes_[j]];
  }
  edges_.erase(remove_if(edges_.begin(), edges_.end(), BadId<Edge>()), edges_.end());
  nodes_.erase(remove_if(nodes_.begin(), nodes_.end(), BadId<Node>()), nodes_.end());
  sort(nodes_.begin(), nodes_.end(), IdCompare<Node>());
#ifndef HG_EDGES_TOPO_SORTED
  sort(edges_.begin(), edges_.end(), IdCompare<Edge>());
#endif
}

TRulePtr Hypergraph::kEPSRule;
TRulePtr Hypergraph::kUnaryRule;

void Hypergraph::EpsilonRemove(WordID eps) {
  if (!kEPSRule) {
    kEPSRule.reset(new TRule("[X] ||| <eps> ||| <eps>"));
    kUnaryRule.reset(new TRule("[X] ||| [X,1] ||| [X,1]"));
  }
  vector<bool> kill(edges_.size(), false);
  for (int i = 0; i < edges_.size(); ++i) {
    const Edge& edge = edges_[i];
    if (edge.tail_nodes_.empty() &&
        edge.rule_->f_.size() == 1 &&
        edge.rule_->f_[0] == eps) {
      kill[i] = true;
      if (!edge.feature_values_.empty()) {
        Node& node = nodes_[edge.head_node_];
        if (node.in_edges_.size() != 1) {
          cerr << "[WARNING] <eps> edge with features going into non-empty node - can't promote\n";
          // this *probably* means that there are multiple derivations of the
          // same sequence via different paths through the input forest
          // this needs to be investigated and fixed
        } else {
          for (int j = 0; j < node.out_edges_.size(); ++j)
            edges_[node.out_edges_[j]].feature_values_ += edge.feature_values_;
          // cerr << "PROMOTED " << edge.feature_values_ << endl;
	}
      }
    }
  }
  bool created_eps = false;
  PruneEdges(kill);
  for (int i = 0; i < nodes_.size(); ++i) {
    const Node& node = nodes_[i];
    if (node.in_edges_.empty()) {
      for (int j = 0; j < node.out_edges_.size(); ++j) {
        Edge& edge = edges_[node.out_edges_[j]];
        if (edge.rule_->Arity() == 2) {
          assert(edge.rule_->f_.size() == 2);
          assert(edge.rule_->e_.size() == 2);
          edge.rule_ = kUnaryRule;
          int cur = node.id_;
          int t = -1;
          assert(edge.tail_nodes_.size() == 2);
          for (int i = 0; i < 2; ++i) if (edge.tail_nodes_[i] != cur) { t = edge.tail_nodes_[i]; }
          assert(t != -1);
          edge.tail_nodes_.resize(1);
          edge.tail_nodes_[0] = t;
        } else {
          edge.rule_ = kEPSRule;
          edge.rule_->f_[0] = eps;
          edge.rule_->e_[0] = eps;
          edge.tail_nodes_.clear();
          created_eps = true;
        }
      }
    }
  }
  vector<bool> k2(edges_.size(), false);
  PruneEdges(k2);
  if (created_eps) EpsilonRemove(eps);
}

struct EdgeWeightSorter {
  const Hypergraph& hg;
  EdgeWeightSorter(const Hypergraph& h) : hg(h) {}
  bool operator()(int a, int b) const {
    return hg.edges_[a].edge_prob_ > hg.edges_[b].edge_prob_;
  }
};

void Hypergraph::SortInEdgesByEdgeWeights() {
  for (int i = 0; i < nodes_.size(); ++i) {
    Node& node = nodes_[i];
    sort(node.in_edges_.begin(), node.in_edges_.end(), EdgeWeightSorter(*this));
  }
}

Hypergraph* Hypergraph::CreateViterbiHypergraph(const vector<bool>* edges) const {
  typedef ViterbiPathTraversal::Result VE;
  VE vit_edges;
  if (edges) {
    assert(edges->size() == edges_.size());
    Viterbi(*this, &vit_edges, ViterbiPathTraversal(), EdgeSelectEdgeWeightFunction(*edges));
  } else {
    Viterbi(*this, &vit_edges, ViterbiPathTraversal() ,EdgeProb());
  }
  map<int, int> old2new_node;
  int num_new_nodes = 0;
  for (int i = 0; i < vit_edges.size(); ++i) {
    const Edge& edge = *vit_edges[i];
    for (int j = 0; j < edge.tail_nodes_.size(); ++j)
      assert(old2new_node.count(edge.tail_nodes_[j]) > 0);
    if (old2new_node.count(edge.head_node_) == 0) {
      old2new_node[edge.head_node_] = num_new_nodes;
      ++num_new_nodes;
    }
  }
  Hypergraph* out = new Hypergraph(num_new_nodes, vit_edges.size(), is_linear_chain_);
  for (map<int, int>::iterator it = old2new_node.begin();
       it != old2new_node.end(); ++it) {
    const Node& old_node = nodes_[it->first];
    Node& new_node = out->nodes_[it->second];
    new_node.cat_ = old_node.cat_;
    new_node.id_ = it->second;
  }

  for (int i = 0; i < vit_edges.size(); ++i) {
    const Edge& old_edge = *vit_edges[i];
    Edge& new_edge = out->edges_[i];
    new_edge = old_edge;
    new_edge.id_ = i;
    const int new_head_node = old2new_node[old_edge.head_node_];
    new_edge.head_node_ = new_head_node;
    out->nodes_[new_head_node].in_edges_.push_back(i);
    for (int j = 0; j < old_edge.tail_nodes_.size(); ++j) {
      const int new_tail_node = old2new_node[old_edge.tail_nodes_[j]];
      new_edge.tail_nodes_[j] = new_tail_node;
      out->nodes_[new_tail_node].out_edges_.push_back(i);
    }
  }
  return out;
}