spanningForestPrim.cpp

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#include <agrum/agrum.h>

#include <agrum/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
    // potential 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 */