spanningForestPrim.cpp

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 *       - Pierre-Henri WUILLEMIN(_at_LIP6)                                 *
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#include <agrum/agrum.h>
 
#include <agrum/base/graphs/algorithms/spanningForestPrim.h>
 
namespace gum {

  SpanningForestPrim::SpanningForestPrim(const UndiGraph*             graph,
                                         const EdgeProperty< float >* cost) :
      SpanningForest(), _graph_(*graph), _costTable_(*cost), _spanning_tree_cost_(0),
      _require_computation_(true) {
    if (!graph || !cost) { GUM_ERROR(GraphError, "invalid null graph or edge cost pointer") }
 
    // for debugging purposes
    GUM_CONSTRUCTOR(SpanningForestPrim);
  }
 
  // copy constructor
  SpanningForestPrim::SpanningForestPrim(const SpanningForestPrim& from) :
      SpanningForest(), _graph_(from._graph_), _costTable_(from._costTable_),
      _edgesToExplore_(from._edgesToExplore_), _spanning_tree_(from._spanning_tree_),
      _spanning_tree_cost_(from._spanning_tree_cost_),
      _require_computation_(from._require_computation_) {
    // for debugging purposes
    GUM_CONS_CPY(SpanningForestPrim);
  }
 
  // destructor
  SpanningForestPrim::~SpanningForestPrim() {
    // for debugging purposes
    GUM_DESTRUCTOR(SpanningForestPrim);
  }

  float SpanningForestPrim::costOfSpanningForest() {
    if (_require_computation_) _compute_();
 
    return _spanning_tree_cost_;
  }

  const EdgeSet& SpanningForestPrim::edgesInSpanningForest() {
    if (_require_computation_) _compute_();
 
    return _spanning_tree_.edges();
  }

  const UndiGraph& SpanningForestPrim::spanningForest() {
    if (_require_computation_) _compute_();
 
    return _spanning_tree_;
  }

  void SpanningForestPrim::_compute_() {
    // compute a spanning tree in every connected component
    for (const auto node: _graph_.nodes()) {
      if (!_spanning_tree_.existsNode(node)) { _computeInAComponent_(node); }
    }
 
    // indicate that everything was computed
    _require_computation_ = false;
  }

  void SpanningForestPrim::_computeInAComponent_(const NodeId id) {
    // add the node to the spanning tree
    _spanning_tree_.addNodeWithId(id);
 
    // explore its neighborhood
    _exploreNode_(id);
 
    // get the next nodes to link to the current spanning tree nodes
 
    while (!_edgesToExplore_.empty()) {
      const Edge   edge   = _edgesToExplore_.pop();
      const NodeId first  = edge.first();
      const NodeId second = edge.second();
 
      // consider only the edges that have one extremal node not in the spanning
      // tree as those that can be added to the tree
 
      if (!_spanning_tree_.existsNode(first)) {
        // add the edge to the spanning tree
        _spanning_tree_.addNodeWithId(first);
        _spanning_tree_.addEdge(first, second);
        _spanning_tree_cost_ += _costTable_[edge];
 
        // We must explore the first node's neighborhood
        _exploreNode_(first);
      } else if (!_spanning_tree_.existsNode(second)) {
        // add the edge to the spanning tree
        _spanning_tree_.addNodeWithId(second);
        _spanning_tree_.addEdge(first, second);
        _spanning_tree_cost_ += _costTable_[edge];
 
        // We must explore the second node
        _exploreNode_(second);
      }
    }
  }

  void SpanningForestPrim::_exploreNode_(const NodeId id) {
    // add its neighbors  _edgesToExplore_ to indicate that they are
    // tensor next nodes to explore
    for (const auto node: _graph_.neighbours(id)) {
      if (!_spanning_tree_.existsNode(node)) {
        Edge edge(node, id);
        _edgesToExplore_.insert(edge, _costTable_[edge]);
      }
    }
  }
 
} /* namespace gum */