MueLu_AggregationPhase3Algorithm_def.hpp

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#ifndef MUELU_AGGREGATIONPHASE3ALGORITHM_DEF_HPP_
#define MUELU_AGGREGATIONPHASE3ALGORITHM_DEF_HPP_

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

#include <Xpetra_Vector.hpp>

#include "MueLu_AggregationPhase3Algorithm_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. Otherwise, make a new aggregate
  template <class LocalOrdinal, class GlobalOrdinal, class Node>
  void AggregationPhase3Algorithm<LocalOrdinal, GlobalOrdinal, Node>::BuildAggregates(const ParameterList& params, const GraphBase& graph, Aggregates& aggregates, std::vector<unsigned>& aggStat, LO& numNonAggregatedNodes) const {
    Monitor m(*this, "BuildAggregates");

    bool makeNonAdjAggs = false;
    bool error_on_isolated = false;
    if(params.isParameter("aggregation: error on nodes with no on-rank neighbors"))
      error_on_isolated = params.get<bool>("aggregation: error on nodes with no on-rank neighbors");
    if(params.isParameter("aggregation: phase3 avoid singletons"))
      makeNonAdjAggs = params.get<bool>("aggregation: phase3 avoid singletons");

    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();

    for (LO i = 0; i < numRows; i++) {
      if (aggStat[i] == AGGREGATED || aggStat[i] == IGNORED)
        continue;

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

      // We don't want a singleton. So lets see if there is an unaggregated
      // neighbor that we can also put with this point.
      bool isNewAggregate = false;
      bool failedToAggregate = true;
      for (int j = 0; j < neighOfINode.size(); j++) {
        LO neigh = neighOfINode[j];

        if (neigh != i && graph.isLocalNeighborVertex(neigh) && aggStat[neigh] == READY) {
          isNewAggregate = true;

          aggStat     [neigh] = AGGREGATED;
          vertex2AggId[neigh] = numLocalAggregates;
          procWinner  [neigh] = myRank;

          numNonAggregatedNodes--;
        }
      }

      if (isNewAggregate) {
        // Create new aggregate (not singleton)
        aggStat     [i] = AGGREGATED;
        procWinner  [i] = myRank;
        numNonAggregatedNodes--;
        aggregates.SetIsRoot(i);
        vertex2AggId[i] = numLocalAggregates++;

        failedToAggregate = false;
      } else {
        // We do not want a singleton, but there are no non-aggregated
        // neighbors. Lets see if we can connect to any other aggregates
        // NOTE: This is very similar to phase 2b, but simplier: we stop with
        // the first found aggregate
        int j = 0;
        for (; j < neighOfINode.size(); j++) {
          LO neigh = neighOfINode[j];

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

        if (j < neighOfINode.size()) {
          // Assign to an adjacent aggregate
          vertex2AggId[i] = vertex2AggId[neighOfINode[j]];
          numNonAggregatedNodes--;
          failedToAggregate = false;
        }
      }

      if (failedToAggregate && makeNonAdjAggs) {
        //  it we are still didn't find an aggregate home for i (i.e., we have
        //  a potential singleton), we are desperate. Basically, we seek to
        //  group i with any other local point to form an aggregate (even if
        //  it is not a neighbor of i. Either we find a vertex that is already
        //  aggregated or not aggregated.
        //    1) if found vertex is aggregated, then assign i to this aggregate
        //    2) if found vertex is not aggregated, create new aggregate


        for (LO ii = 0; ii < numRows; ii++) { // look for anyone else
          if ( (ii != i) && (aggStat[ii] != IGNORED) ) {
            failedToAggregate = false;       // found someone so start
            aggStat[i]   = AGGREGATED;  // marking i as aggregated
            procWinner[i]= myRank;

            if (aggStat[ii] == AGGREGATED)
              vertex2AggId[i] = vertex2AggId[ii];
            else {
              vertex2AggId[i]  = numLocalAggregates;
              vertex2AggId[ii] = numLocalAggregates;
              aggStat     [ii] = AGGREGATED;
              procWinner  [ii] = myRank;
              numNonAggregatedNodes--;   // acounts for ii now being aggregated
              aggregates.SetIsRoot(i);
              numLocalAggregates++;
            }
            numNonAggregatedNodes--;   // accounts for i now being aggregated
            break;
          } //if ( (ii != i) && (aggStat[ii] != IGNORED ...
        } //for (LO ii = 0; ...
      }
      if (failedToAggregate) {
        if (error_on_isolated) {
          // Error on this isolated node, as the user has requested
          std::ostringstream oss;
          oss<<"MueLu::AggregationPhase3Algorithm::BuildAggregates: MueLu has detected a non-Dirichlet node that has no on-rank neighbors and is terminating (by user request). "<<std::endl;
          oss<<"If this error is being generated at level 0, this is due to an initial partitioning problem in your matrix."<<std::endl;
          oss<<"If this error is being generated at any other level, try turning on repartitioning, which may fix this problem."<<std::endl;
          throw Exceptions::RuntimeError(oss.str());
        } else {
          // Create new aggregate (singleton)
          this->GetOStream(Warnings1) << "Found singleton: " << i << std::endl;

          aggregates.SetIsRoot(i);
          vertex2AggId[i] = numLocalAggregates++;
          numNonAggregatedNodes--;
        }
      }

      // One way or another, the node is aggregated (possibly into a singleton)
      aggStat   [i] = AGGREGATED;
      procWinner[i] = myRank;

    } // loop over numRows

    // update aggregate object
    aggregates.SetNumAggregates(numLocalAggregates);
  }

} // end namespace

#endif /* MUELU_AGGREGATIONPHASE3ALGORITHM_DEF_HPP_ */