DAGCycleDetector.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 A class for detecting directed cycles in DAGs when trying to apply
 * many changes to the graph
 *
 * When trying to assess whether multiple changes applied to a given DAG would
 * induce cycles, use class DAGCycleDetector instead of trying to apply the
 * modifications to the DAG itself and check whether and exception is raised:
 * the class is designed to be fast for such modifications. However, the number
 * of modifications checked should be higher than at least 3 for this class to
 * be competitive.
 * @author Christophe GONZALES(@AMU) and Pierre-Henri WUILLEMIN(@LIP6)
 */

#include <agrum/tools/core/sequence.h>
#include <agrum/tools/graphs/algorithms/DAGCycleDetector.h>

#ifdef GUM_NO_INLINE
#  include <agrum/tools/graphs/algorithms/DAGCycleDetector_inl.h>
#endif   // GUM_NOINLINE

namespace gum {

  /// sets the initial DAG from which changes shall be applied
  void DAGCycleDetector::setDAG(const DAG& dag) {
    // sets the dag
    _dag_ = dag;

    // get the set of roots and leaves of the dag
    List< NodeId >       roots, leaves;
    NodeProperty< Size > nb_parents, nb_children;

    for (const auto node: dag) {
      Size nb_ch = dag.children(node).size();
      nb_children.insert(node, nb_ch);

      if (!nb_ch) leaves.insert(node);

      Size nb_pa = dag.parents(node).size();
      nb_parents.insert(node, nb_pa);

      if (!nb_pa) roots.insert(node);
    }

    // recompute the set of ancestors
    NodeProperty< Size > empty_set;
    _ancestors_.clear();

    for (NodeId node: dag) {
      _ancestors_.insert(node, empty_set);
    }

    while (!roots.empty()) {
      // get a node and update the ancestors of its children
      NodeId               node           = roots.front();
      NodeProperty< Size > node_ancestors = _ancestors_[node];
      node_ancestors.insert(node, 1);
      const NodeSet& node_children = dag.children(node);
      roots.popFront();

      for (const auto ch: node_children) {
        _addWeightedSet_(_ancestors_[ch], node_ancestors, 1);
        --nb_parents[ch];

        if (!nb_parents[ch]) { roots.insert(ch); }
      }
    }

    // recompute the set of descendants
    _descendants_.clear();

    for (const auto node: dag) {
      _descendants_.insert(node, empty_set);
    }

    while (!leaves.empty()) {
      // get a node and update the descendants of its parents
      NodeId               node             = leaves.front();
      NodeProperty< Size > node_descendants = _descendants_[node];
      node_descendants.insert(node, 1);
      const NodeSet& node_parents = dag.parents(node);
      leaves.popFront();

      for (const auto pa: node_parents) {
        _addWeightedSet_(_descendants_[pa], node_descendants, 1);
        --nb_children[pa];

        if (!nb_children[pa]) { leaves.insert(pa); }
      }
    }
  }

  /// indicates whether a set of modifications would create a cycle
  bool DAGCycleDetector::hasCycleFromModifications(const std::vector< Change >& modifs) const {
    // first, we separate deletions and insertions (reversals are cut into
    // a deletion + an insertion) and we check that no insertion also exists
    // as a deletion (if so, we remove both operations). In addition, if
    // we try to add an arc (X,X) we return that it induces a cycle
    HashTable< Arc, Size > deletions(Size(modifs.size()));
    HashTable< Arc, Size > additions(Size(modifs.size()));

    for (const auto& modif: modifs) {
      Arc arc(modif.tail(), modif.head());

      switch (modif.type()) {
        case ChangeType::ARC_DELETION:
          if (deletions.exists(arc))
            ++deletions[arc];
          else
            deletions.insert(arc, 1);

          break;

        case ChangeType::ARC_ADDITION:
          if (modif.tail() == modif.head()) return true;

          if (additions.exists(arc))
            ++additions[arc];
          else
            additions.insert(arc, 1);

          break;

        case ChangeType::ARC_REVERSAL:
          if (deletions.exists(arc))
            ++deletions[arc];
          else
            deletions.insert(arc, 1);

          arc = Arc(modif.head(), modif.tail());

          if (additions.exists(arc))
            ++additions[arc];
          else
            additions.insert(arc, 1);

          break;

        default:
          GUM_ERROR(OperationNotAllowed, "undefined change type")
      }
    }

    for (auto iter = additions.beginSafe();   // safe iterator needed here
         iter != additions.endSafe();
         ++iter) {
      if (deletions.exists(iter.key())) {
        Size& nb_del = deletions[iter.key()];
        Size& nb_add = iter.val();

        if (nb_del > nb_add) {
          additions.erase(iter);
          nb_del -= nb_add;
        } else if (nb_del < nb_add) {
          deletions.erase(iter.key());
          nb_add -= nb_del;
        } else {
          deletions.erase(iter.key());
          additions.erase(iter);
        }
      }
    }

    // get the set of nodes involved in the modifications
    NodeSet extremities;

    for (const auto& modif: modifs) {
      extremities.insert(modif.tail());
      extremities.insert(modif.head());
    }

    // we now retrieve the set of ancestors and descendants of all the
    // extremities of the arcs involved in the modifications. We also
    // keep track of all the children and parents of these nodes
    NodeProperty< NodeProperty< Size > > ancestors, descendants;

    for (const auto& modif: modifs) {
      if (!ancestors.exists(modif.tail())) {
        NodeProperty< Size >& anc = ancestors.insert(modif.tail(), NodeProperty< Size >()).second;
        _restrictWeightedSet_(anc, _ancestors_[modif.tail()], extremities);

        NodeProperty< Size >& desc
           = descendants.insert(modif.tail(), NodeProperty< Size >()).second;
        _restrictWeightedSet_(desc, _descendants_[modif.tail()], extremities);
      }

      if (!ancestors.exists(modif.head())) {
        NodeProperty< Size >& anc = ancestors.insert(modif.head(), NodeProperty< Size >()).second;
        _restrictWeightedSet_(anc, _ancestors_[modif.head()], extremities);

        NodeProperty< Size >& desc
           = descendants.insert(modif.head(), NodeProperty< Size >()).second;
        _restrictWeightedSet_(desc, _descendants_[modif.head()], extremities);
      }
    }

    // we apply all the suppressions:
    // assume that the modif concerns arc (X,Y). Then, after removing
    // arc (X,Y), the sets of descendants of the nodes T in
    // ( X + ancestors (X) ) are updated by subtracting the number of paths
    // leading to Y and its set of descendants. These numbers are equal to
    // the numbers found in descendants[X] * the numbers of paths leading
    // to X, i.e., descendants[T][X].
    // By symmetry, the set of ancestors of nodes T in ( Y + descendants (Y) )
    // are
    // updated by subtracting the number of paths leading to X and its
    // ancestors.
    for (auto iter = deletions.cbegin(); iter != deletions.cend(); ++iter) {
      const Arc&                  arc       = iter.key();
      const NodeId                tail      = arc.tail();
      const NodeProperty< Size >& anc_tail  = ancestors[tail];
      const NodeId                head      = arc.head();
      const NodeProperty< Size >& desc_head = descendants[head];

      // update the set of descendants
      NodeProperty< Size > set_to_del = desc_head;
      set_to_del.insert(head, 1);
      _delWeightedSet_(descendants[tail], set_to_del, 1);

      for (auto iter = anc_tail.cbegin(); iter != anc_tail.cend(); ++iter) {
        _delWeightedSet_(descendants[iter.key()], set_to_del, descendants[iter.key()][tail]);
      }

      // update the set of ancestors
      set_to_del = anc_tail;
      set_to_del.insert(tail, 1);
      _delWeightedSet_(ancestors[head], set_to_del, 1);

      for (auto iter = desc_head.cbegin(); iter != desc_head.cend(); ++iter) {
        _delWeightedSet_(ancestors[iter.key()], set_to_del, ancestors[iter.key()][head]);
      }
    }

    // now we apply all the additions of arcs (including the additions after
    // arc reversals). After adding arc (X,Y), the set of ancestors T of Y and
    // its
    // descendants shall be updated by adding the number of paths leading to
    // X and its ancestors, i.e., ancestors[X] * the number of paths leading
    // to Y, i.e., ancestors[T][Y]. Similarly, the set of descendants of X and
    // its
    // ancestors should be updated by adding the number of paths leading to Y
    // and
    // its descendants. Finally, an arc (X,Y) can be added if and
    // only if Y does not belong to the ancestor set of X
    for (auto iter = additions.cbegin(); iter != additions.cend(); ++iter) {
      const Arc& arc  = iter.key();
      NodeId     tail = arc.tail();
      NodeId     head = arc.head();

      const NodeProperty< Size >& anc_tail = ancestors[tail];

      if (anc_tail.exists(head)) { return true; }

      const NodeProperty< Size >& desc_head = descendants[head];

      // update the set of ancestors
      NodeProperty< Size > set_to_add = anc_tail;
      set_to_add.insert(tail, 1);
      _addWeightedSet_(ancestors[head], set_to_add, 1);

      for (auto iter = desc_head.cbegin(); iter != desc_head.cend(); ++iter) {
        _addWeightedSet_(ancestors[iter.key()], set_to_add, ancestors[iter.key()][head]);
      }

      // update the set of descendants
      set_to_add = desc_head;
      set_to_add.insert(head, 1);
      _addWeightedSet_(descendants[tail], set_to_add, 1);

      for (auto iter = anc_tail.cbegin(); iter != anc_tail.cend(); ++iter) {
        _addWeightedSet_(descendants[iter.key()], set_to_add, descendants[iter.key()][tail]);
      }
    }

    return false;
  }

  /// adds a new arc to the current DAG
  void DAGCycleDetector::addArc(NodeId tail, NodeId head) {
    // check that the arc does not already exist
    if (_dag_.existsArc(tail, head)) return;

    // check that the arc would not create a cycle
    if (hasCycleFromAddition(tail, head)) {
      GUM_ERROR(InvalidDirectedCycle, "the arc would create a directed into a DAG")
    }

    _dag_.addArc(tail, head);

    // now we apply the addition of the arc as done in method
    // hasCycleFromModifications
    const NodeProperty< Size >& anc_tail  = _ancestors_[tail];
    const NodeProperty< Size >& desc_head = _descendants_[head];

    // update the set of ancestors
    NodeProperty< Size > set_to_add = anc_tail;
    set_to_add.insert(tail, 1);
    _addWeightedSet_(_ancestors_[head], set_to_add, 1);

    for (auto iter = desc_head.cbegin(); iter != desc_head.cend(); ++iter) {
      _addWeightedSet_(_ancestors_[iter.key()], set_to_add, _ancestors_[iter.key()][head]);
    }

    // update the set of descendants
    set_to_add = desc_head;
    set_to_add.insert(head, 1);
    _addWeightedSet_(_descendants_[tail], set_to_add, 1);

    for (auto iter = anc_tail.cbegin(); iter != anc_tail.cend(); ++iter) {
      _addWeightedSet_(_descendants_[iter.key()], set_to_add, _descendants_[iter.key()][tail]);
    }
  }

  /// removes an arc from the current DAG
  void DAGCycleDetector::eraseArc(NodeId tail, NodeId head) {
    // check that the arc exists
    if (!_dag_.existsArc(tail, head)) return;

    _dag_.eraseArc(Arc(tail, head));

    // we apply the deletion of the arc as done in method
    // hasCycleFromModifications
    const NodeProperty< Size >& anc_tail  = _ancestors_[tail];
    const NodeProperty< Size >& desc_head = _descendants_[head];

    // update the set of descendants
    NodeProperty< Size > set_to_del = desc_head;
    set_to_del.insert(head, 1);
    _delWeightedSet_(_descendants_[tail], set_to_del, 1);

    for (auto iter = anc_tail.cbegin(); iter != anc_tail.cend(); ++iter) {
      _delWeightedSet_(_descendants_[iter.key()], set_to_del, _descendants_[iter.key()][tail]);
    }

    // update the set of ancestors
    set_to_del = anc_tail;
    set_to_del.insert(tail, 1);
    _delWeightedSet_(_ancestors_[head], set_to_del, 1);

    for (auto iter = desc_head.cbegin(); iter != desc_head.cend(); ++iter) {
      _delWeightedSet_(_ancestors_[iter.key()], set_to_del, _ancestors_[iter.key()][head]);
    }
  }

} /* namespace gum */