DAGCycleDetector.cpp

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

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

namespace gum {

  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); }
      }
    }
  }

  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;
  }

  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]);
    }
  }

  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 */