MueLu_AggregationPhase2bAlgorithm_def.hpp

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#ifndef MUELU_AGGREGATIONPHASE2BALGORITHM_DEF_HPP_
#define MUELU_AGGREGATIONPHASE2BALGORITHM_DEF_HPP_

#include <Teuchos_Comm.hpp>
#include <Teuchos_CommHelpers.hpp>

#include <Xpetra_Vector.hpp>

#include "MueLu_AggregationPhase2bAlgorithm_decl.hpp"

#include "MueLu_Aggregates.hpp"
#include "MueLu_Exceptions.hpp"
#include "MueLu_GraphBase.hpp"
#include "MueLu_Monitor.hpp"

namespace MueLu {

  // Try to stick unaggregated nodes into a neighboring aggregate if they are
  // not already too big
  template <class LocalOrdinal, class GlobalOrdinal, class Node>
  void AggregationPhase2bAlgorithm<LocalOrdinal, GlobalOrdinal, Node>::BuildAggregates(const ParameterList& /* params */, const GraphBase& graph, Aggregates& aggregates, std::vector<unsigned>& aggStat, LO& numNonAggregatedNodes) const {
    Monitor m(*this, "BuildAggregates");

    const LO  numRows = graph.GetNodeNumVertices();
    const int myRank  = graph.GetComm()->getRank();

    ArrayRCP<LO> vertex2AggId = aggregates.GetVertex2AggId()->getDataNonConst(0);
    ArrayRCP<LO> procWinner   = aggregates.GetProcWinner()  ->getDataNonConst(0);

    LO numLocalAggregates = aggregates.GetNumAggregates();

    const int defaultConnectWeight = 100;
    const int penaltyConnectWeight = 10;

    std::vector<int> aggWeight    (numLocalAggregates, 0);
    std::vector<int> connectWeight(numRows, defaultConnectWeight);
    std::vector<int> aggPenalties (numRows, 0);

    // We do this cycle twice.
    // I don't know why, but ML does it too
    // taw: by running the aggregation routine more than once there is a chance that also
    // non-aggregated nodes with a node distance of two are added to existing aggregates.
    // Assuming that the aggregate size is 3 in each direction running the algorithm only twice
    // should be sufficient.
    for (int k = 0; k < 2; k++) {
      for (LO i = 0; i < numRows; i++) {
        if (aggStat[i] != READY)
          continue;

        ArrayView<const LocalOrdinal> neighOfINode = graph.getNeighborVertices(i);

        for (int j = 0; j < neighOfINode.size(); j++) {
          LO neigh = neighOfINode[j];

          // We don't check (neigh != i), as it is covered by checking (aggStat[neigh] == AGGREGATED)
          if (graph.isLocalNeighborVertex(neigh) && aggStat[neigh] == AGGREGATED)
            aggWeight[vertex2AggId[neigh]] += connectWeight[neigh];
        }

        int bestScore   = -100000;
        int bestAggId   = -1;
        int bestConnect = -1;

        for (int j = 0; j < neighOfINode.size(); j++) {
          LO neigh = neighOfINode[j];

          if (graph.isLocalNeighborVertex(neigh) && aggStat[neigh] == AGGREGATED) {
            int aggId = vertex2AggId[neigh];
            int score = aggWeight[aggId] - aggPenalties[aggId];

            if (score > bestScore) {
              bestAggId   = aggId;
              bestScore   = score;
              bestConnect = connectWeight[neigh];

            } else if (aggId == bestAggId && connectWeight[neigh] > bestConnect) {
              bestConnect = connectWeight[neigh];
            }

            // Reset the weights for the next loop
            aggWeight[aggId] = 0;
          }
        }

        if (bestScore >= 0) {
          aggStat     [i] = AGGREGATED;
          vertex2AggId[i] = bestAggId;
          procWinner  [i] = myRank;

          numNonAggregatedNodes--;

          aggPenalties[bestAggId]++;
          connectWeight[i] = bestConnect - penaltyConnectWeight;
        }
      }
    }
  }

} // end namespace

#endif /* MUELU_AGGREGATIONPHASE2BALGORITHM_DEF_HPP_ */