dSeparation_tpl.h

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namespace gum {


  // update a set of potentials, keeping only those d-connected with
  // query variables given evidence
  template < typename GUM_SCALAR, template < typename > class TABLE >
  void dSeparation::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();

    // mark the set of ancestors of the evidence
    NodeSet ev_ancestors(dag.size());
    {
      List< NodeId > anc_to_visit;
      for (const auto node : hardEvidence)
        anc_to_visit.insert(node);
      for (const auto node : softEvidence)
        anc_to_visit.insert(node);
      while (!anc_to_visit.empty()) {
        const NodeId node = anc_to_visit.front();
        anc_to_visit.popFront();

        if (!ev_ancestors.exists(node)) {
          ev_ancestors.insert(node);
          for (const auto par : dag.parents(node)) {
            anc_to_visit.insert(par);
          }
        }
      }
    }

    // create the marks indicating that we have visited a node
    NodeSet visited_from_child(dag.size());
    NodeSet visited_from_parent(dag.size());

    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 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
      const NodeId node = nodes_to_visit.front().first;
      const bool   direction = nodes_to_visit.front().second;
      nodes_to_visit.popFront();

      // check if the node has not already been visited in the same direction
      bool already_visited;
      if (direction) {
        already_visited = visited_from_child.exists(node);
        if (!already_visited) { visited_from_child.insert(node); }
      } else {
        already_visited = visited_from_parent.exists(node);
        if (!already_visited) { visited_from_parent.insert(node); }
      }

      // 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;
      }

      // if this is the first time we meet the node, then visit it
      if (!already_visited) {
        // mark the node as reachable if this is not a hard evidence
        const bool is_hard_evidence = hardEvidence.exists(node);

        // bounce the ball toward the neighbors
        if (direction && !is_hard_evidence) {   // visit from a child
          // visit the parents
          for (const auto par : dag.parents(node)) {
            nodes_to_visit.insert(std::pair< NodeId, bool >(par, true));
          }

          // visit the children
          for (const auto chi : dag.children(node)) {
            nodes_to_visit.insert(std::pair< NodeId, bool >(chi, false));
          }
        } else {   // visit from a parent
          if (!hardEvidence.exists(node)) {
            // visit the children
            for (const auto chi : dag.children(node)) {
              nodes_to_visit.insert(std::pair< NodeId, bool >(chi, false));
            }
          }
          if (ev_ancestors.exists(node)) {
            // visit the parents
            for (const auto par : dag.parents(node)) {
              nodes_to_visit.insert(std::pair< NodeId, bool >(par, true));
            }
          }
        }
      }
    }

    // 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 */