BayesBall_tpl.h

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namespace gum {


  // update a set of potentials, keeping only those d-connected with
  // query variables
  template < typename GUM_SCALAR, template < typename > class TABLE >
  void
     BayesBall::relevantPotentials(const IBayesNet< GUM_SCALAR >&     bn,
                                   const NodeSet&                     query,
                                   const NodeSet&                     hardEvidence,
                                   const NodeSet&                     softEvidence,
                                   Set< const TABLE< GUM_SCALAR >* >& potentials) {
    const DAG& dag = bn.dag();

    // create the marks (top = first and bottom = second)
    NodeProperty< std::pair< bool, bool > > marks;
    marks.resize(dag.size());
    const std::pair< bool, bool > empty_mark(false, false);

    HashTable< NodeId, Set< const TABLE< GUM_SCALAR >* > > node2potentials;
    for (const auto pot : potentials) {
      const Sequence< const DiscreteVariable* >& vars = pot->variablesSequence();
      for (const auto var : vars) {
        const NodeId id = bn.nodeId(*var);
        if (!node2potentials.exists(id)) {
          node2potentials.insert(id, Set< const TABLE< GUM_SCALAR >* >());
        }
        node2potentials[id].insert(pot);
      }
    }

    // indicate that we will send the ball to all the query nodes (as children):
    // in list nodes_to_visit, the first element is the next node to send the
    // ball to and the Boolean indicates whether we shall reach it from one of
    // its children (true) or from one parent (false)
    List< std::pair< NodeId, bool > > nodes_to_visit;
    for (const auto node : query) {
      nodes_to_visit.insert(std::pair< NodeId, bool >(node, true));
    }

    // perform the bouncing ball until __node2potentials becomes empty (which
    // means that we have reached all the potentials and, therefore, those
    // are d-connected to query) or until there is no node in the graph to send
    // the ball to
    while (!nodes_to_visit.empty() && !node2potentials.empty()) {
      // get the next node to visit
      NodeId node = nodes_to_visit.front().first;

      // if the marks of the node do not exist, create them
      if (!marks.exists(node)) marks.insert(node, empty_mark);

      // if the node belongs to the query, update __node2potentials: remove all
      // the potentials containing the node
      if (node2potentials.exists(node)) {
        auto& pot_set = node2potentials[node];
        for (const auto pot : pot_set) {
          const auto& vars = pot->variablesSequence();
          for (const auto var : vars) {
            const NodeId id = bn.nodeId(*var);
            if (id != node) {
              node2potentials[id].erase(pot);
              if (node2potentials[id].empty()) { node2potentials.erase(id); }
            }
          }
        }
        node2potentials.erase(node);

        // if __node2potentials is empty, no need to go on: all the potentials
        // are d-connected to the query
        if (node2potentials.empty()) return;
      }


      // bounce the ball toward the neighbors
      if (nodes_to_visit.front().second) {   // visit from a child
        nodes_to_visit.popFront();

        if (hardEvidence.exists(node)) { continue; }

        if (!marks[node].first) {
          marks[node].first = true;   // top marked
          for (const auto par : dag.parents(node)) {
            nodes_to_visit.insert(std::pair< NodeId, bool >(par, true));
          }
        }

        if (!marks[node].second) {
          marks[node].second = true;   // bottom marked
          for (const auto chi : dag.children(node)) {
            nodes_to_visit.insert(std::pair< NodeId, bool >(chi, false));
          }
        }
      } else {   // visit from a parent
        nodes_to_visit.popFront();

        const bool is_hard_evidence = hardEvidence.exists(node);
        const bool is_evidence = is_hard_evidence || softEvidence.exists(node);

        if (is_evidence && !marks[node].first) {
          marks[node].first = true;

          for (const auto par : dag.parents(node)) {
            nodes_to_visit.insert(std::pair< NodeId, bool >(par, true));
          }
        }

        if (!is_hard_evidence && !marks[node].second) {
          marks[node].second = true;

          for (const auto chi : dag.children(node)) {
            nodes_to_visit.insert(std::pair< NodeId, bool >(chi, false));
          }
        }
      }
    }


    // here, all the potentials that belong to __node2potentials are d-separated
    // from the query
    for (const auto elt : node2potentials) {
      for (const auto pot : elt.second) {
        potentials.erase(pot);
      }
    }
  }


} /* namespace gum */