binaryJoinTreeConverterDefault.cpp

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/**
 *
 *   Copyright (c) 2005-2021 by Pierre-Henri WUILLEMIN(@LIP6) & Christophe GONZALES(@AMU)
 *   info_at_agrum_dot_org
 *
 *  This library is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU Lesser General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  This library is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU Lesser General Public License for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public License
 *  along with this library.  If not, see <http://www.gnu.org/licenses/>.
 *
 */


/** @file
 * @brief An algorithm for converting a join tree into a binary join tree
 *
 * @author Christophe GONZALES(@AMU) and Pierre-Henri WUILLEMIN(@LIP6)
 */

#include <agrum/agrum.h>

#include <agrum/tools/core/priorityQueue.h>
#include <agrum/tools/graphs/algorithms/binaryJoinTreeConverterDefault.h>

namespace gum {

  /// default constructor
  BinaryJoinTreeConverterDefault::BinaryJoinTreeConverterDefault() {
    GUM_CONSTRUCTOR(BinaryJoinTreeConverterDefault);
  }

  /// destructor
  BinaryJoinTreeConverterDefault::~BinaryJoinTreeConverterDefault() {
    GUM_DESTRUCTOR(BinaryJoinTreeConverterDefault);
  }

  /** @brief a function used to mark the nodes belonging to a given
   * connected component */
  void BinaryJoinTreeConverterDefault::_markConnectedComponent_(const CliqueGraph&    JT,
                                                                NodeId                root,
                                                                NodeProperty< bool >& mark) const {
    // we mark the nodes in a depth first search manner. To avoid a recursive
    // algorithm, use a vector to simulate a stack of nodes to inspect.
    // stack => depth first search
    std::vector< NodeId > nodes_to_inspect;
    nodes_to_inspect.reserve(JT.sizeNodes());

    // the idea to populate the marks is to use the stack: root is
    // put onto the stack. Then, while the stack is not empty, remove
    // the top of the stack and mark it and put into the stack its
    // adjacent nodes.
    nodes_to_inspect.push_back(root);

    while (!nodes_to_inspect.empty()) {
      // process the top of the stack
      NodeId current_node = nodes_to_inspect.back();
      nodes_to_inspect.pop_back();

      // only process the node if it has not been processed yet (actually,
      // this should not occur unless the clique graph is not singly connected)

      if (!mark[current_node]) {
        mark[current_node] = true;

        // put the neighbors onto the stack
        for (const auto neigh: JT.neighbours(current_node))
          if (!mark[neigh]) nodes_to_inspect.push_back(neigh);
      }
    }
  }

  /// returns the domain size of the union of two cliques
  float BinaryJoinTreeConverterDefault::_combinedSize_(
     const NodeSet&              nodes1,
     const NodeSet&              nodes2,
     const NodeProperty< Size >& domain_sizes) const {
    float result = 1;

    for (const auto node: nodes1)
      result *= domain_sizes[node];

    for (const auto node: nodes2)
      if (!nodes1.exists(node)) result *= domain_sizes[node];

    return result;
  }

  /// returns all the roots considered for all the connected components
  const NodeSet& BinaryJoinTreeConverterDefault::roots() const { return _roots_; }

  /// convert a clique and its adjacent cliques into a binary join tree
  void BinaryJoinTreeConverterDefault::_convertClique_(
     CliqueGraph&                JT,
     NodeId                      clique,
     NodeId                      from,
     const NodeProperty< Size >& domain_sizes) const {
    // get the neighbors of clique. If there are fewer than 3 neighbors,
    // there is nothing to do
    const NodeSet& neighbors = JT.neighbours(clique);

    if (neighbors.size() <= 2) return;

    if ((neighbors.size() == 3) && (clique != from)) return;

    // here we need to transform the neighbors into a binary tree
    // create a vector with all the ids of the cliques to combine
    std::vector< NodeId > cliques;
    cliques.reserve(neighbors.size());

    for (const auto nei: neighbors)
      if (nei != from) cliques.push_back(nei);

    // create a vector indicating wether the elements in cliques contain
    // relevant information or not (during the execution of the for
    // loop below, a cell of vector cliques may actually contain only
    // trash data)
    std::vector< bool > is_cliques_relevant(cliques.size(), true);

    // for each pair of cliques (i,j), compute the size of the clique that would
    // result from the combination of clique i with clique j and store the
    // result
    // into a priorityQueue
    std::pair< NodeId, NodeId > pair;

    PriorityQueue< std::pair< NodeId, NodeId >, float > queue;

    for (NodeId i = 0; i < cliques.size(); ++i) {
      pair.first            = i;
      const NodeSet& nodes1 = JT.separator(cliques[i], clique);

      for (NodeId j = i + 1; j < cliques.size(); ++j) {
        pair.second = j;
        queue.insert(pair, _combinedSize_(nodes1, JT.separator(cliques[j], clique), domain_sizes));
      }
    }

    // now parse the priority queue: the top element (i,j) gives the combination
    // to perform. When the result R has been computed, substitute i by R,
    // remove
    // table j and recompute all the priorities of all the pairs (R,k) still
    // available.
    for (NodeId k = 2; k < cliques.size(); ++k) {
      // get the combination to perform and do it
      pair      = queue.pop();
      NodeId ti = pair.first;
      NodeId tj = pair.second;

      // create a new clique that will become adjacent to ti and tj
      // and remove the edges between ti, tj and clique
      const NodeSet& nodes1   = JT.separator(cliques[ti], clique);
      const NodeSet& nodes2   = JT.separator(cliques[tj], clique);
      NodeId         new_node = JT.addNode(nodes1 + nodes2);
      JT.addEdge(cliques[ti], new_node);
      JT.addEdge(cliques[tj], new_node);
      JT.addEdge(clique, new_node);
      JT.eraseEdge(Edge(cliques[ti], clique));
      JT.eraseEdge(Edge(cliques[tj], clique));

      // substitute cliques[pair.first] by the result
      cliques[ti]             = new_node;
      is_cliques_relevant[tj] = false;   // now tj is no more a neighbor of clique

      // remove all the pairs involving tj in the priority queue

      for (NodeId ind = 0; ind < tj; ++ind) {
        if (is_cliques_relevant[ind]) {
          pair.first = ind;
          queue.erase(pair);
        }
      }

      pair.first = tj;

      for (NodeId ind = tj + 1; ind < cliques.size(); ++ind) {
        if (is_cliques_relevant[ind]) {
          pair.second = ind;
          queue.erase(pair);
        }
      }

      // update the "combined" size of all the pairs involving "new_node"
      {
        const NodeSet& nodes1 = JT.separator(cliques[ti], clique);
        pair.second           = ti;
        float newsize;

        for (NodeId ind = 0; ind < ti; ++ind) {
          if (is_cliques_relevant[ind]) {
            pair.first = ind;
            newsize    = _combinedSize_(nodes1, JT.separator(cliques[ind], clique), domain_sizes);
            queue.setPriority(pair, newsize);
          }
        }

        pair.first = ti;

        for (NodeId ind = ti + 1; ind < cliques.size(); ++ind) {
          if (is_cliques_relevant[ind]) {
            pair.second = ind;
            newsize     = _combinedSize_(nodes1, JT.separator(cliques[ind], clique), domain_sizes);
            queue.setPriority(pair, newsize);
          }
        }
      }
    }
  }

  /// convert a whole connected component into a binary join tree
  void BinaryJoinTreeConverterDefault::_convertConnectedComponent_(
     CliqueGraph&                JT,
     NodeId                      current_node,
     NodeId                      from,
     const NodeProperty< Size >& domain_sizes,
     NodeProperty< bool >&       mark) const {
    // first, indicate that the node has been marked (this avoids looping
    // if JT is not a tree
    mark[current_node] = true;

    // parse all the neighbors except nodes already converted and convert them
    for (const auto neigh: JT.neighbours(current_node)) {
      if (!mark[neigh]) {
        _convertConnectedComponent_(JT, neigh, current_node, domain_sizes, mark);
      }
    }

    // convert the current node
    _convertClique_(JT, current_node, from, domain_sizes);
  }

  /// computes the binary join tree
  CliqueGraph BinaryJoinTreeConverterDefault::convert(const CliqueGraph&          JT,
                                                      const NodeProperty< Size >& domain_sizes,
                                                      const NodeSet&              specified_roots) {
    // first, we copy the current clique graph. By default, this is what we
    // will return
    CliqueGraph binJT = JT;

    // check that there is no connected component without a root. In such a
    // case,
    // assign an arbitrary root to it
    _roots_ = specified_roots;

    NodeProperty< bool > mark = JT.nodesProperty(false, JT.sizeNodes());

    // for each specified root, populate its connected component
    for (const auto root: specified_roots) {
      // check that the root has not already been marked
      // in this case, this means that more than one root has been specified
      // for a given connected component
      if (mark[root])
        GUM_ERROR(InvalidNode,
                  "several roots have been specified for a given "
                  "connected component (last : "
                     << root << ")");

      _markConnectedComponent_(JT, root, mark);
    }

    // check that all nodes have been marked. If this is not the case, then
    // this means that we need to add new roots
    for (const auto& elt: mark)
      if (!elt.second) {
        _roots_ << elt.first;
        _markConnectedComponent_(JT, elt.first, mark);
      }

    // here, we know that each connected component has one and only one root.
    // Now we can apply a recursive collect algorithm starting from root
    // that transforms each clique with more than 3 neighbors into a set of
    // cliques having at most 3 neighbors.
    NodeProperty< bool > mark2 = JT.nodesProperty(false, JT.sizeNodes());

    for (const auto root: _roots_)
      _convertConnectedComponent_(binJT, root, root, domain_sizes, mark2);

    // binJT is now a binary join tree, so we can return it
    return binJT;
  }

} /* namespace gum */