MueLu_NotayAggregationFactory_def.hpp

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#ifndef MUELU_NOTAYAGGREGATIONFACTORY_DEF_HPP_
#define MUELU_NOTAYAGGREGATIONFACTORY_DEF_HPP_
 
#include <Xpetra_Map.hpp>
#include <Xpetra_Vector.hpp>
#include <Xpetra_MultiVectorFactory.hpp>
#include <Xpetra_MapFactory.hpp>
#include <Xpetra_VectorFactory.hpp>
 
#include "KokkosKernels_Handle.hpp"
#include "KokkosSparse_spgemm.hpp"
#include "KokkosSparse_spmv.hpp"
 
#include "MueLu_NotayAggregationFactory_decl.hpp"
 
#include "MueLu_Aggregates.hpp"
#include "MueLu_GraphBase.hpp"
#include "MueLu_Level.hpp"
#include "MueLu_MasterList.hpp"
#include "MueLu_Monitor.hpp"
#include "MueLu_Types.hpp"
#include "MueLu_Utilities.hpp"
 
 
namespace MueLu {
 
  namespace NotayUtils {
    template <class LocalOrdinal>
    LocalOrdinal RandomOrdinal(LocalOrdinal min, LocalOrdinal max) {
      return min + as<LocalOrdinal>((max-min+1) * (static_cast<double>(std::rand()) / (RAND_MAX + 1.0)));
    }
 
    template <class LocalOrdinal>
    void RandomReorder(Teuchos::Array<LocalOrdinal> & list) {
      typedef LocalOrdinal LO;
      LO n = Teuchos::as<LO>(list.size());
      for(LO i = 0; i < n-1; i++)
  std::swap(list[i], list[RandomOrdinal(i,n-1)]);
    }
  }
 
  template <class Scalar, class LocalOrdinal, class GlobalOrdinal, class Node>
  RCP<const ParameterList> NotayAggregationFactory<Scalar, LocalOrdinal, GlobalOrdinal, Node>::GetValidParameterList() const {
    RCP<ParameterList> validParamList = rcp(new ParameterList());
 
 
#define SET_VALID_ENTRY(name) validParamList->setEntry(name, MasterList::getEntry(name))
    SET_VALID_ENTRY("aggregation: pairwise: size");
    SET_VALID_ENTRY("aggregation: pairwise: tie threshold");
    SET_VALID_ENTRY("aggregation: compute aggregate qualities");
    SET_VALID_ENTRY("aggregation: Dirichlet threshold");
    SET_VALID_ENTRY("aggregation: ordering");
#undef SET_VALID_ENTRY
 
    // general variables needed in AggregationFactory
    validParamList->set< RCP<const FactoryBase> >("A",           null, "Generating factory of the matrix");
    validParamList->set< RCP<const FactoryBase> >("Graph",       null, "Generating factory of the graph");
    validParamList->set< RCP<const FactoryBase> >("DofsPerNode", null, "Generating factory for variable \'DofsPerNode\', usually the same as for \'Graph\'");
    validParamList->set< RCP<const FactoryBase> >("AggregateQualities", null, "Generating factory for variable \'AggregateQualities\'");
 
 
    return validParamList;
  }
 
  template <class Scalar, class LocalOrdinal, class GlobalOrdinal, class Node>
  void NotayAggregationFactory<Scalar, LocalOrdinal, GlobalOrdinal, Node>::DeclareInput(Level& currentLevel) const {
    const ParameterList& pL = GetParameterList();
 
    Input(currentLevel, "A");
    Input(currentLevel, "Graph");
    Input(currentLevel, "DofsPerNode");
    if (pL.get<bool>("aggregation: compute aggregate qualities")) {
        Input(currentLevel, "AggregateQualities");
    }
 
 
  }
 
 
  template <class Scalar, class LocalOrdinal, class GlobalOrdinal, class Node>
  void NotayAggregationFactory<Scalar, LocalOrdinal, GlobalOrdinal, Node>::Build(Level& currentLevel) const {
    FactoryMonitor m(*this, "Build", currentLevel);
    using STS = Teuchos::ScalarTraits<Scalar>;
    using MT  = typename STS::magnitudeType;
 
    const MT MT_TWO = Teuchos::ScalarTraits<MT>::one() + Teuchos::ScalarTraits<MT>::one();
 
    RCP<Teuchos::FancyOStream> out;
    if(const char* dbg = std::getenv("MUELU_PAIRWISEAGGREGATION_DEBUG")) {
      out = Teuchos::fancyOStream(Teuchos::rcpFromRef(std::cout));
      out->setShowAllFrontMatter(false).setShowProcRank(true);
    } else {
      out = Teuchos::getFancyOStream(rcp(new Teuchos::oblackholestream()));
    }
 
    const ParameterList& pL = GetParameterList();
 
    const MT kappa = static_cast<MT>(pL.get<double>("aggregation: Dirichlet threshold"));
    TEUCHOS_TEST_FOR_EXCEPTION(kappa <= MT_TWO,
                               Exceptions::RuntimeError,
                               "Pairwise requires kappa > 2"
                               " otherwise all rows are considered as Dirichlet rows.");
 
    // Parameters
    int maxNumIter = 3;
    if (pL.isParameter("aggregation: pairwise: size"))
      maxNumIter = pL.get<int>("aggregation: pairwise: size");
    TEUCHOS_TEST_FOR_EXCEPTION(maxNumIter < 1,
             Exceptions::RuntimeError,
             "NotayAggregationFactory::Build(): \"aggregation: pairwise: size\""
                               " must be a strictly positive integer");
 
 
    RCP<const GraphBase> graph = Get< RCP<GraphBase> >(currentLevel, "Graph");
    RCP<const Matrix> A = Get< RCP<Matrix> >(currentLevel, "A");
 
    // Setup aggregates & aggStat objects
    RCP<Aggregates> aggregates = rcp(new Aggregates(*graph));
    aggregates->setObjectLabel("PW");
 
    const LO numRows = graph->GetNodeNumVertices();
 
    // construct aggStat information
    std::vector<unsigned> aggStat(numRows, READY);
 
 
    const int DofsPerNode =  Get<int>(currentLevel,"DofsPerNode");
    TEUCHOS_TEST_FOR_EXCEPTION(DofsPerNode != 1, Exceptions::RuntimeError,
             "Pairwise only supports one dof per node");
 
    // This follows the paper:
    // Notay, "Aggregation-based algebraic multigrid for convection-diffusion equations",
    // SISC 34(3), pp. A2288-2316.
 
    // Handle Ordering
    std::string orderingStr = pL.get<std::string>("aggregation: ordering");
    enum {
      O_NATURAL,
      O_RANDOM,
      O_CUTHILL_MCKEE,
    } ordering;
 
    ordering = O_NATURAL;
    if (orderingStr == "random" ) ordering = O_RANDOM;
    else if(orderingStr == "natural") {}
    else if(orderingStr == "cuthill-mckee" || orderingStr == "cm") ordering = O_CUTHILL_MCKEE;
    else {
      TEUCHOS_TEST_FOR_EXCEPTION(1,Exceptions::RuntimeError,"Invalid ordering type");
    }
 
    // Get an ordering vector
    // NOTE: The orderingVector only orders *rows* of the matrix.  Off-proc columns
    // will get ignored in the aggregation phases, so we don't need to worry about
    // running off the end.
    Array<LO> orderingVector(numRows);
    for (LO i = 0; i < numRows; i++)
      orderingVector[i] = i;
    if (ordering == O_RANDOM)
      MueLu::NotayUtils::RandomReorder(orderingVector);
    else if (ordering == O_CUTHILL_MCKEE) {
      RCP<Xpetra::Vector<LO,LO,GO,NO> > rcmVector = MueLu::Utilities<SC,LO,GO,NO>::CuthillMcKee(*A);
      auto localVector = rcmVector->getData(0);
      for (LO i = 0; i < numRows; i++)
        orderingVector[i] = localVector[i];
    }
 
    // Get the party stated
    LO numNonAggregatedNodes = numRows, numDirichletNodes = 0;
    BuildInitialAggregates(pL, A, orderingVector(), kappa,
                           *aggregates, aggStat, numNonAggregatedNodes, numDirichletNodes);
    TEUCHOS_TEST_FOR_EXCEPTION(0 < numNonAggregatedNodes, Exceptions::RuntimeError,
                               "Initial pairwise aggregation failed to aggregate all nodes");
    LO numLocalAggregates = aggregates->GetNumAggregates();
    GetOStream(Statistics0) << "Init   : " << numLocalAggregates << " - "
                            << A->getLocalNumRows() / numLocalAggregates << std::endl;
 
    // Temporary data storage for further aggregation steps
    local_matrix_type intermediateP;
    local_matrix_type coarseLocalA;
 
    // Compute the on rank part of the local matrix
    // that the square submatrix that only contains
    // columns corresponding to local rows.
    LO numLocalDirichletNodes = numDirichletNodes;
    Array<LO> localVertex2AggId(aggregates->GetVertex2AggId()->getData(0).view(0, numRows));
    BuildOnRankLocalMatrix(A->getLocalMatrixDevice(), coarseLocalA);
    for(LO aggregationIter = 1; aggregationIter < maxNumIter; ++aggregationIter) {
      // Compute the intermediate prolongator
      BuildIntermediateProlongator(coarseLocalA.numRows(), numLocalDirichletNodes, numLocalAggregates,
                                   localVertex2AggId(), intermediateP);
 
      // Compute the coarse local matrix and coarse row sum
      BuildCoarseLocalMatrix(intermediateP, coarseLocalA);
 
      // Directly compute rowsum from A, rather than coarseA
      row_sum_type rowSum("rowSum", numLocalAggregates);
      {
        std::vector<std::vector<LO> > agg2vertex(numLocalAggregates);
        auto vertex2AggId = aggregates->GetVertex2AggId()->getData(0);
        for(LO i=0; i<(LO)numRows; i++) {
          if(aggStat[i] != AGGREGATED)
            continue;
          LO agg=vertex2AggId[i];
          agg2vertex[agg].push_back(i);
        }
        
        typename row_sum_type::HostMirror rowSum_h = Kokkos::create_mirror_view(rowSum);
        for(LO i = 0; i < numRows; i++) {
          // If not aggregated already, skip this guy
          if(aggStat[i] != AGGREGATED)
            continue;
          int agg = vertex2AggId[i];
          std::vector<LO> & myagg = agg2vertex[agg];
 
          size_t nnz = A->getNumEntriesInLocalRow(i);
          ArrayView<const LO> indices;
          ArrayView<const SC> vals;
          A->getLocalRowView(i, indices, vals);
 
          SC mysum = Teuchos::ScalarTraits<Scalar>::zero();
          for (LO colidx = 0; colidx < static_cast<LO>(nnz); colidx++) {
            bool found = false;
            if(indices[colidx] < numRows) {
              for(LO j=0; j<(LO)myagg.size(); j++)
                if (vertex2AggId[indices[colidx]] == agg)
                  found=true;
            }
            if(!found) {
              *out << "- ADDING col "<<indices[colidx]<<" = "<<vals[colidx] << std::endl;
                mysum += vals[colidx];
              }
              else {
              *out << "- NOT ADDING col "<<indices[colidx]<<" = "<<vals[colidx] << std::endl;
              }
          }
          
          rowSum_h[agg] = mysum;
        }
        Kokkos::deep_copy(rowSum, rowSum_h);
      }
 
      // Get local orderingVector
      Array<LO> localOrderingVector(numRows);
      for (LO i = 0; i < numRows; i++)
        localOrderingVector[i] = i;
      if (ordering == O_RANDOM)
        MueLu::NotayUtils::RandomReorder(localOrderingVector);
      else if (ordering == O_CUTHILL_MCKEE) {
        RCP<Xpetra::Vector<LO,LO,GO,NO> > rcmVector = MueLu::Utilities<SC,LO,GO,NO>::CuthillMcKee(*A);
        auto localVector = rcmVector->getData(0);
        for (LO i = 0; i < numRows; i++)
          localOrderingVector[i] = localVector[i];
      }
 
      // Compute new aggregates
      numLocalAggregates    = 0;
      numNonAggregatedNodes = static_cast<LO>(coarseLocalA.numRows());
      std::vector<LO> localAggStat(numNonAggregatedNodes, READY);
      localVertex2AggId.resize(numNonAggregatedNodes, -1);
      BuildFurtherAggregates(pL, A, localOrderingVector, coarseLocalA, kappa, rowSum,
                             localAggStat, localVertex2AggId,
                             numLocalAggregates, numNonAggregatedNodes);
 
      // After the first initial pairwise aggregation
      // the Dirichlet nodes have been removed.
      numLocalDirichletNodes = 0;
 
      // Update the aggregates
      RCP<LOMultiVector> vertex2AggIdMV = aggregates->GetVertex2AggId();
      ArrayRCP<LO>       vertex2AggId   = vertex2AggIdMV->getDataNonConst(0);
      for(size_t vertexIdx = 0; vertexIdx < A->getLocalNumRows(); ++vertexIdx) {
        LO oldAggIdx = vertex2AggId[vertexIdx];
        if(oldAggIdx != Teuchos::OrdinalTraits<LO>::invalid()) {
          vertex2AggId[vertexIdx] = localVertex2AggId[oldAggIdx];
        }
      }
 
      // We could probably print some better statistics at some point
      GetOStream(Statistics0) << "Iter " << aggregationIter << ": " << numLocalAggregates << " - "
                              << A->getLocalNumRows() / numLocalAggregates << std::endl;
    }
    aggregates->SetNumAggregates(numLocalAggregates);
    aggregates->AggregatesCrossProcessors(false);
    aggregates->ComputeAggregateSizes(true/*forceRecompute*/);
 
    // DO stuff
    Set(currentLevel, "Aggregates", aggregates);
    GetOStream(Statistics0) << aggregates->description() << std::endl;
  }
 
 
  template <class Scalar, class LocalOrdinal, class GlobalOrdinal, class Node>
  void NotayAggregationFactory<Scalar, LocalOrdinal, GlobalOrdinal, Node>::
  BuildInitialAggregates(const Teuchos::ParameterList& params,
                         const RCP<const Matrix>& A,
       const Teuchos::ArrayView<const LO> & orderingVector,
                         const typename Teuchos::ScalarTraits<Scalar>::magnitudeType kappa,
                         Aggregates& aggregates,
                         std::vector<unsigned>& aggStat,
                         LO& numNonAggregatedNodes,
                         LO& numDirichletNodes) const {
 
    Monitor m(*this, "BuildInitialAggregates");
    using STS = Teuchos::ScalarTraits<Scalar>;
    using MT  = typename STS::magnitudeType;
    using RealValuedVector = Xpetra::Vector<MT,LocalOrdinal,GlobalOrdinal,Node>;
 
    RCP<Teuchos::FancyOStream> out;
    if(const char* dbg = std::getenv("MUELU_PAIRWISEAGGREGATION_DEBUG")) {
      out = Teuchos::fancyOStream(Teuchos::rcpFromRef(std::cout));
      out->setShowAllFrontMatter(false).setShowProcRank(true);
    } else {
      out = Teuchos::getFancyOStream(rcp(new Teuchos::oblackholestream()));
    }
 
 
    const SC SC_ZERO    = Teuchos::ScalarTraits<SC>::zero();
    const MT MT_ZERO    = Teuchos::ScalarTraits<MT>::zero();
    const MT MT_ONE     = Teuchos::ScalarTraits<MT>::one();
    const MT MT_TWO     = MT_ONE + MT_ONE;
    const LO LO_INVALID = Teuchos::OrdinalTraits<LO>::invalid();
    const LO LO_ZERO    = Teuchos::OrdinalTraits<LO>::zero();
 
    const MT kappa_init = kappa / (kappa - MT_TWO);
    const LO numRows    = aggStat.size();
    const int myRank    = A->getMap()->getComm()->getRank();
 
    // For finding "ties" where we fall back to the ordering.  Making this larger than
    // hard zero substantially increases code robustness.
    double tie_criterion = params.get<double>("aggregation: pairwise: tie threshold");
    double tie_less = 1.0 - tie_criterion;
    double tie_more = 1.0 + tie_criterion;
 
    // NOTE: Assumes 1 dof per node.  This constraint is enforced in Build(),
    // and so we're not doing again here.
    // This should probably be fixed at some point.
 
    // Extract diagonal, rowsums, etc
    // NOTE: The ghostedRowSum vector here has has the sign flipped from Notay's S
    RCP<Vector> ghostedDiag = MueLu::Utilities<SC,LO,GO,NO>::GetMatrixOverlappedDiagonal(*A);
    RCP<Vector> ghostedRowSum = MueLu::Utilities<SC,LO,GO,NO>::GetMatrixOverlappedDeletedRowsum(*A);
    RCP<RealValuedVector> ghostedAbsRowSum = MueLu::Utilities<SC,LO,GO,NO>::GetMatrixOverlappedAbsDeletedRowsum(*A);
    const ArrayRCP<const SC> D     = ghostedDiag->getData(0);
    const ArrayRCP<const SC> S     = ghostedRowSum->getData(0);
    const ArrayRCP<const MT> AbsRs = ghostedAbsRowSum->getData(0);
 
    // Aggregates stuff
    ArrayRCP<LO>  vertex2AggId_rcp = aggregates.GetVertex2AggId()->getDataNonConst(0);
    ArrayRCP<LO>  procWinner_rcp   = aggregates.GetProcWinner()  ->getDataNonConst(0);
    ArrayView<LO> vertex2AggId     = vertex2AggId_rcp();
    ArrayView<LO> procWinner       = procWinner_rcp();
 
    // Algorithm 4.2
 
    // 0,1 : Initialize: Flag boundary conditions
    // Modification: We assume symmetry here aij = aji
    for (LO row = 0; row < Teuchos::as<LO>(A->getRowMap()->getLocalNumElements()); ++row) {
      MT aii = STS::magnitude(D[row]);
      MT rowsum = AbsRs[row];
 
      if(aii >= kappa_init * rowsum) {
        *out << "Flagging index " << row << " as dirichlet "
          "aii >= kappa*rowsum = " << aii << " >= " << kappa_init << " " << rowsum << std::endl;
  aggStat[row] = IGNORED;
  --numNonAggregatedNodes;
        ++numDirichletNodes;
      }
    }
 
 
    // 2 : Iteration
    LO aggIndex = LO_ZERO;
    for(LO i = 0; i < numRows; i++) {
      LO current_idx = orderingVector[i];
      // If we're aggregated already, skip this guy
      if(aggStat[current_idx] != READY)
        continue;
 
      MT best_mu = MT_ZERO;
      LO best_idx = LO_INVALID;
 
      size_t nnz = A->getNumEntriesInLocalRow(current_idx);
      ArrayView<const LO> indices;
      ArrayView<const SC> vals;
      A->getLocalRowView(current_idx, indices, vals);
 
      MT aii = STS::real(D[current_idx]);
      MT si  = STS::real(S[current_idx]);
      for (LO colidx = 0; colidx < static_cast<LO>(nnz); colidx++) {
        // Skip aggregated neighbors, off-rank neighbors, hard zeros and self
        LO col = indices[colidx];
        SC val = vals[colidx];
        if(current_idx == col || col >= numRows || aggStat[col] != READY  || val == SC_ZERO)
          continue;
 
        MT aij = STS::real(val);
        MT ajj = STS::real(D[col]);
        MT sj  =  - STS::real(S[col]);  // NOTE: The ghostedRowSum vector here has has the sign flipped from Notay's S
        if(aii - si + ajj - sj >= MT_ZERO) {
          // Modification: We assume symmetry here aij = aji
          MT mu_top    = MT_TWO / ( MT_ONE / aii + MT_ONE / ajj);
          MT mu_bottom =  - aij + MT_ONE / ( MT_ONE / (aii - si) + MT_ONE / (ajj - sj) );
          MT mu = mu_top / mu_bottom;
 
          // Modification: Explicitly check the tie criterion here
          if (mu > MT_ZERO && (best_idx == LO_INVALID || mu < best_mu * tie_less ||
                                    (mu < best_mu*tie_more && orderingVector[col] < orderingVector[best_idx]))) {
            best_mu  = mu;
            best_idx = col;
            *out << "[" << current_idx << "] Column     UPDATED " << col << ": "
                 << "aii - si + ajj - sj = " << aii << " - " << si << " + " << ajj << " - " << sj
                 << " = " << aii - si + ajj - sj<< ", aij = "<<aij << ", mu = " << mu << std::endl;
          }
          else {
          *out << "[" << current_idx << "] Column NOT UPDATED " << col << ": "
               << "aii - si + ajj - sj = " << aii << " - " << si << " + " << ajj << " - " << sj
               << " = " << aii - si + ajj - sj << ", aij = "<<aij<< ", mu = " << mu << std::endl;
          }
        }
        else {
          *out << "[" << current_idx << "] Column     FAILED " << col << ": "
               << "aii - si + ajj - sj = " << aii << " - " << si << " + " << ajj << " - " << sj
               << " = " << aii - si + ajj - sj << std::endl;
        }
      }// end column for loop
 
      if(best_idx == LO_INVALID) {
        *out << "No node buddy found for index " << current_idx
             << " [agg " << aggIndex << "]\n" << std::endl;
        // We found no potential node-buddy, so let's just make this a singleton
        // NOTE: The behavior of what to do if you have no un-aggregated neighbors is not specified in
        // the paper
 
        aggStat[current_idx] = ONEPT;
        vertex2AggId[current_idx] = aggIndex;
        procWinner[current_idx]   = myRank;
        numNonAggregatedNodes--;
        aggIndex++;
 
      } else {
        // We have a buddy, so aggregate, either as a singleton or as a pair, depending on mu
        if(best_mu <= kappa) {
          *out << "Node buddies (" << current_idx << "," << best_idx << ") [agg " << aggIndex << "]" << std::endl;
 
          aggStat[current_idx] = AGGREGATED;
          aggStat[best_idx]    = AGGREGATED;
          vertex2AggId[current_idx] = aggIndex;
          vertex2AggId[best_idx]    = aggIndex;
          procWinner[current_idx]   = myRank;
          procWinner[best_idx]      = myRank;
          numNonAggregatedNodes-=2;
          aggIndex++;
 
        } else {
          *out << "No buddy found for index " << current_idx << ","
            " but aggregating as singleton [agg " << aggIndex << "]" << std::endl;
 
          aggStat[current_idx] = ONEPT;
          vertex2AggId[current_idx] = aggIndex;
          procWinner[current_idx]   = myRank;
          numNonAggregatedNodes--;
          aggIndex++;
        } // best_mu
      } // best_idx
    }// end Algorithm 4.2
 
    *out << "vertex2aggid :";
    for(int i = 0; i < static_cast<int>(vertex2AggId.size()); ++i) {
      *out << i << "(" << vertex2AggId[i] << ")";
    }
    *out << std::endl;
 
    // update aggregate object
    aggregates.SetNumAggregates(aggIndex);
  } // BuildInitialAggregates
 
  template <class Scalar, class LocalOrdinal, class GlobalOrdinal, class Node>
  void NotayAggregationFactory<Scalar, LocalOrdinal, GlobalOrdinal, Node>::
  BuildFurtherAggregates(const Teuchos::ParameterList& params,
                         const RCP<const Matrix>& A,
                         const Teuchos::ArrayView<const LO> & orderingVector,
                         const typename Matrix::local_matrix_type& coarseA,
                         const typename Teuchos::ScalarTraits<Scalar>::magnitudeType kappa,
                         const Kokkos::View<typename Kokkos::ArithTraits<Scalar>::val_type*,
                                            Kokkos::LayoutLeft,
                                            typename Matrix::local_matrix_type::device_type>& rowSum,
                         std::vector<LocalOrdinal>& localAggStat,
                         Teuchos::Array<LocalOrdinal>& localVertex2AggID,
                         LO& numLocalAggregates,
                         LO& numNonAggregatedNodes) const {
    Monitor m(*this, "BuildFurtherAggregates");
 
    // Set debug outputs based on environment variable
    RCP<Teuchos::FancyOStream> out;
    if(const char* dbg = std::getenv("MUELU_PAIRWISEAGGREGATION_DEBUG")) {
      out = Teuchos::fancyOStream(Teuchos::rcpFromRef(std::cout));
      out->setShowAllFrontMatter(false).setShowProcRank(true);
    } else {
      out = Teuchos::getFancyOStream(rcp(new Teuchos::oblackholestream()));
    }
 
    using value_type = typename local_matrix_type::value_type;
    const value_type KAT_zero = Kokkos::ArithTraits<value_type>::zero();
    const magnitude_type MT_zero = Teuchos::ScalarTraits<magnitude_type>::zero();
    const magnitude_type MT_one  = Teuchos::ScalarTraits<magnitude_type>::one();
    const magnitude_type MT_two  = MT_one + MT_one;
    const LO LO_INVALID = Teuchos::OrdinalTraits<LO>::invalid() ;
 
    // For finding "ties" where we fall back to the ordering.  Making this larger than
    // hard zero substantially increases code robustness.
    double tie_criterion = params.get<double>("aggregation: pairwise: tie threshold");
    double tie_less = 1.0 - tie_criterion;
    double tie_more = 1.0 + tie_criterion;
 
    typename row_sum_type::HostMirror rowSum_h = Kokkos::create_mirror_view(rowSum);
    Kokkos::deep_copy(rowSum_h, rowSum);
 
    // Extracting the diagonal of a KokkosSparse::CrsMatrix
    // is not currently provided in kokkos-kernels so here
    // is an ugly way to get that done...
    const LO numRows = static_cast<LO>(coarseA.numRows());
    typename local_matrix_type::values_type::HostMirror diagA_h("diagA host", numRows);
    typename local_matrix_type::row_map_type::HostMirror row_map_h
      = Kokkos::create_mirror_view(coarseA.graph.row_map);
    Kokkos::deep_copy(row_map_h, coarseA.graph.row_map);
    typename local_matrix_type::index_type::HostMirror entries_h
      = Kokkos::create_mirror_view(coarseA.graph.entries);
    Kokkos::deep_copy(entries_h, coarseA.graph.entries);
    typename local_matrix_type::values_type::HostMirror values_h
      = Kokkos::create_mirror_view(coarseA.values);
    Kokkos::deep_copy(values_h, coarseA.values);
    for(LO rowIdx = 0; rowIdx < numRows; ++rowIdx) {
      for(LO entryIdx = static_cast<LO>(row_map_h(rowIdx));
          entryIdx < static_cast<LO>(row_map_h(rowIdx + 1));
          ++entryIdx) {
        if(rowIdx == static_cast<LO>(entries_h(entryIdx))) {
          diagA_h(rowIdx) = values_h(entryIdx);
        }
      }
    }
 
    for(LO currentIdx = 0; currentIdx < numRows; ++currentIdx) {
      if(localAggStat[currentIdx] != READY) {
        continue;
      }
 
      LO bestIdx = Teuchos::OrdinalTraits<LO>::invalid();
      magnitude_type best_mu = Teuchos::ScalarTraits<magnitude_type>::zero();
      const magnitude_type aii     = Teuchos::ScalarTraits<value_type>::real(diagA_h(currentIdx));
      const magnitude_type si      = Teuchos::ScalarTraits<value_type>::real(rowSum_h(currentIdx));
      for(auto entryIdx = row_map_h(currentIdx); entryIdx < row_map_h(currentIdx + 1); ++entryIdx) {
        const LO colIdx = static_cast<LO>(entries_h(entryIdx));
        if(currentIdx == colIdx || colIdx >= numRows || localAggStat[colIdx] != READY || values_h(entryIdx) == KAT_zero) {
          continue;
        }
 
        const magnitude_type aij = Teuchos::ScalarTraits<value_type>::real(values_h(entryIdx));
        const magnitude_type ajj = Teuchos::ScalarTraits<value_type>::real(diagA_h(colIdx));
        const magnitude_type sj  = - Teuchos::ScalarTraits<value_type>::real(rowSum_h(colIdx)); // NOTE: The ghostedRowSum vector here has has the sign flipped from Notay's S
        if(aii - si + ajj - sj >= MT_zero) {
          const magnitude_type mu_top    = MT_two / ( MT_one/aii + MT_one/ajj );
          const magnitude_type mu_bottom = -aij + MT_one / (MT_one / (aii - si) + MT_one / (ajj - sj));
          const magnitude_type mu = mu_top / mu_bottom;
 
          // Modification: Explicitly check the tie criterion here
    if (mu > MT_zero  && (bestIdx == LO_INVALID ||  mu < best_mu * tie_less ||
                                (mu < best_mu*tie_more && orderingVector[colIdx] < orderingVector[bestIdx]))) {
            best_mu = mu;
            bestIdx = colIdx;
            *out << "[" << currentIdx << "] Column     UPDATED " << colIdx << ": "
                 << "aii - si + ajj - sj = " << aii << " - " << si << " + " << ajj << " - " << sj
                 << " = " << aii - si + ajj - sj << ", aij = "<<aij<<" mu = " << mu << std::endl;
          }
          else {
            *out << "[" << currentIdx << "] Column NOT UPDATED " << colIdx << ": "
                 << "aii - si + ajj - sj = " << aii << " - " << si << " + " << ajj << " - " << sj
                 << " = " << aii - si + ajj - sj << ", aij = "<<aij<<", mu = " << mu << std::endl;
          }
        } else {
            *out << "[" << currentIdx << "] Column      FAILED " << colIdx << ": "
                 << "aii - si + ajj - sj = " << aii << " - " << si << " + " << ajj << " - " << sj
                 << " = " << aii - si + ajj - sj << std::endl;
        }
      } // end loop over row entries
 
      if(bestIdx == Teuchos::OrdinalTraits<LO>::invalid()) {
        localAggStat[currentIdx]      = ONEPT;
        localVertex2AggID[currentIdx] = numLocalAggregates;
        --numNonAggregatedNodes;
        ++numLocalAggregates;
      } else {
        if(best_mu <= kappa) {
          *out << "Node buddies (" << currentIdx << "," << bestIdx << ") [agg " << numLocalAggregates << "]" << std::endl;
 
          localAggStat[currentIdx]      = AGGREGATED;
          localVertex2AggID[currentIdx] = numLocalAggregates;
          --numNonAggregatedNodes;
 
          localAggStat[bestIdx]         = AGGREGATED;
          localVertex2AggID[bestIdx]    = numLocalAggregates;
          --numNonAggregatedNodes;
 
          ++numLocalAggregates;
        } else {
          *out << "No buddy found for index " << currentIdx << ","
            " but aggregating as singleton [agg " << numLocalAggregates << "]" << std::endl;
 
          localAggStat[currentIdx]      = ONEPT;
          localVertex2AggID[currentIdx] = numLocalAggregates;
          --numNonAggregatedNodes;
          ++numLocalAggregates;
        }
      }
    } // end loop over matrix rows
 
  } // BuildFurtherAggregates
 
  template <class Scalar, class LocalOrdinal, class GlobalOrdinal, class Node>
  void NotayAggregationFactory<Scalar, LocalOrdinal, GlobalOrdinal, Node>::
  BuildOnRankLocalMatrix(const typename Matrix::local_matrix_type& localA,
                         typename Matrix::local_matrix_type& onrankA) const {
    Monitor m(*this, "BuildOnRankLocalMatrix");
 
    // Set debug outputs based on environment variable
    RCP<Teuchos::FancyOStream> out;
    if(const char* dbg = std::getenv("MUELU_PAIRWISEAGGREGATION_DEBUG")) {
      out = Teuchos::fancyOStream(Teuchos::rcpFromRef(std::cout));
      out->setShowAllFrontMatter(false).setShowProcRank(true);
    } else {
      out = Teuchos::getFancyOStream(rcp(new Teuchos::oblackholestream()));
    }
 
    using local_graph_type  = typename local_matrix_type::staticcrsgraph_type;
    using values_type       = typename local_matrix_type::values_type;
    using size_type         = typename local_graph_type::size_type;
    using col_index_type    = typename local_graph_type::data_type;
    using array_layout      = typename local_graph_type::array_layout;
    using memory_traits     = typename local_graph_type::memory_traits;
    using row_pointer_type  = Kokkos::View<size_type*, array_layout, device_type, memory_traits>;
    using col_indices_type  = Kokkos::View<col_index_type*, array_layout, device_type, memory_traits>;
    // Extract on rank part of A
    // Simply check that the column index is less than the number of local rows
    // otherwise remove it.
 
    const int numRows = static_cast<int>(localA.numRows());
    row_pointer_type rowPtr("onrankA row pointer", numRows + 1);
    typename row_pointer_type::HostMirror rowPtr_h = Kokkos::create_mirror_view(rowPtr);
    typename local_graph_type::row_map_type::HostMirror origRowPtr_h
      = Kokkos::create_mirror_view(localA.graph.row_map);
    typename local_graph_type::entries_type::HostMirror origColind_h
      = Kokkos::create_mirror_view(localA.graph.entries);
    typename values_type::HostMirror origValues_h
      = Kokkos::create_mirror_view(localA.values);
    Kokkos::deep_copy(origRowPtr_h, localA.graph.row_map);
    Kokkos::deep_copy(origColind_h, localA.graph.entries);
    Kokkos::deep_copy(origValues_h, localA.values);
 
    // Compute the number of nnz entries per row
    rowPtr_h(0) = 0;
    for(int rowIdx = 0; rowIdx < numRows; ++rowIdx) {
      for(size_type entryIdx = origRowPtr_h(rowIdx); entryIdx < origRowPtr_h(rowIdx + 1); ++entryIdx) {
        if(origColind_h(entryIdx) < numRows) {rowPtr_h(rowIdx + 1) += 1;}
      }
      rowPtr_h(rowIdx + 1) = rowPtr_h(rowIdx + 1) + rowPtr_h(rowIdx);
    }
    Kokkos::deep_copy(rowPtr, rowPtr_h);
 
    const LO nnzOnrankA = rowPtr_h(numRows);
 
    // Now use nnz per row to allocate matrix views and store column indices and values
    col_indices_type colInd("onrankA column indices", rowPtr_h(numRows));
    values_type      values("onrankA values", rowPtr_h(numRows));
    typename col_indices_type::HostMirror colInd_h = Kokkos::create_mirror_view(colInd);
    typename values_type::HostMirror      values_h = Kokkos::create_mirror_view(values);
    int entriesInRow;
    for(int rowIdx = 0; rowIdx < numRows; ++rowIdx) {
      entriesInRow = 0;
      for(size_type entryIdx = origRowPtr_h(rowIdx); entryIdx < origRowPtr_h(rowIdx + 1); ++entryIdx) {
        if(origColind_h(entryIdx) < numRows) {
          colInd_h(rowPtr_h(rowIdx) + entriesInRow) = origColind_h(entryIdx);
          values_h(rowPtr_h(rowIdx) + entriesInRow) = origValues_h(entryIdx);
          ++entriesInRow;
        }
      }
    }
    Kokkos::deep_copy(colInd, colInd_h);
    Kokkos::deep_copy(values, values_h);
 
    onrankA = local_matrix_type("onrankA", numRows, numRows,
                                nnzOnrankA, values, rowPtr, colInd);
  }
 
  template <class Scalar, class LocalOrdinal, class GlobalOrdinal, class Node>
  void NotayAggregationFactory<Scalar, LocalOrdinal, GlobalOrdinal, Node>::
  BuildIntermediateProlongator(const LocalOrdinal numRows,
                               const LocalOrdinal numDirichletNodes,
                               const LocalOrdinal numLocalAggregates,
                               const Teuchos::ArrayView<const LocalOrdinal>& localVertex2AggID,
                               typename Matrix::local_matrix_type& intermediateP) const {
    Monitor m(*this, "BuildIntermediateProlongator");
 
    // Set debug outputs based on environment variable
    RCP<Teuchos::FancyOStream> out;
    if(const char* dbg = std::getenv("MUELU_PAIRWISEAGGREGATION_DEBUG")) {
      out = Teuchos::fancyOStream(Teuchos::rcpFromRef(std::cout));
      out->setShowAllFrontMatter(false).setShowProcRank(true);
    } else {
      out = Teuchos::getFancyOStream(rcp(new Teuchos::oblackholestream()));
    }
 
    using local_graph_type  = typename local_matrix_type::staticcrsgraph_type;
    using values_type       = typename local_matrix_type::values_type;
    using size_type         = typename local_graph_type::size_type;
    using col_index_type    = typename local_graph_type::data_type;
    using array_layout      = typename local_graph_type::array_layout;
    using memory_traits     = typename local_graph_type::memory_traits;
    using row_pointer_type  = Kokkos::View<size_type*, array_layout, device_type, memory_traits>;
    using col_indices_type  = Kokkos::View<col_index_type*, array_layout, device_type, memory_traits>;
 
    const LO LO_INVALID = Teuchos::OrdinalTraits<LO>::invalid();
 
    const int intermediatePnnz = numRows - numDirichletNodes;
    row_pointer_type rowPtr("intermediateP row pointer", numRows + 1);
    col_indices_type colInd("intermediateP column indices", intermediatePnnz);
    values_type      values("intermediateP values", intermediatePnnz);
    typename row_pointer_type::HostMirror rowPtr_h = Kokkos::create_mirror_view(rowPtr);
    typename col_indices_type::HostMirror colInd_h = Kokkos::create_mirror_view(colInd);
 
    rowPtr_h(0) = 0;
    for(int rowIdx = 0; rowIdx < numRows; ++rowIdx) {
      // Skip Dirichlet nodes
      if(localVertex2AggID[rowIdx] == LO_INVALID) {
        rowPtr_h(rowIdx + 1) = rowPtr_h(rowIdx);
      } else {
        rowPtr_h(rowIdx + 1) = rowPtr_h(rowIdx) + 1;
        colInd_h(rowPtr_h(rowIdx)) = localVertex2AggID[rowIdx];
      }
    }
 
    Kokkos::deep_copy(rowPtr, rowPtr_h);
    Kokkos::deep_copy(colInd, colInd_h);
    Kokkos::deep_copy(values, Kokkos::ArithTraits<typename values_type::value_type>::one());
 
    intermediateP = local_matrix_type("intermediateP",
                                      numRows, numLocalAggregates, intermediatePnnz,
                                      values, rowPtr, colInd);
  } // BuildIntermediateProlongator
 
  template <class Scalar, class LocalOrdinal, class GlobalOrdinal, class Node>
  void NotayAggregationFactory<Scalar, LocalOrdinal, GlobalOrdinal, Node>::
  BuildCoarseLocalMatrix(const typename Matrix::local_matrix_type& intermediateP,
                         typename Matrix::local_matrix_type& coarseA) const {
    Monitor m(*this, "BuildCoarseLocalMatrix");
 
    using local_graph_type  = typename local_matrix_type::staticcrsgraph_type;
    using values_type       = typename local_matrix_type::values_type;
    using size_type         = typename local_graph_type::size_type;
    using col_index_type    = typename local_graph_type::data_type;
    using array_layout      = typename local_graph_type::array_layout;
    using memory_traits     = typename local_graph_type::memory_traits;
    using row_pointer_type  = Kokkos::View<size_type*, array_layout, device_type, memory_traits>;
    using col_indices_type  = Kokkos::View<col_index_type*, array_layout, device_type, memory_traits>;
 
    local_matrix_type AP;
    localSpGEMM(coarseA, intermediateP, "AP", AP);
 
    // Note 03/11/20, lbv: does kh need to destroy and recreate the spgemm handle
    // I am not sure but doing it for safety in case it stashes data from the previous
    // spgemm computation...
 
    // Compute Ac = Pt * AP
    // Two steps needed:
    //   1. compute Pt
    //   2. perform multiplication
 
    // Step 1 compute Pt
    // Obviously this requires the same amount of storage as P except for the rowPtr
    row_pointer_type rowPtrPt(Kokkos::ViewAllocateWithoutInitializing("Pt row pointer"),
                              intermediateP.numCols() + 1);
    col_indices_type colIndPt(Kokkos::ViewAllocateWithoutInitializing("Pt column indices"),
                              intermediateP.nnz());
    values_type      valuesPt(Kokkos::ViewAllocateWithoutInitializing("Pt values"),
                              intermediateP.nnz());
 
    typename row_pointer_type::HostMirror rowPtrPt_h = Kokkos::create_mirror_view(rowPtrPt);
    typename col_indices_type::HostMirror entries_h = Kokkos::create_mirror_view(intermediateP.graph.entries);
    Kokkos::deep_copy(entries_h, intermediateP.graph.entries);
    Kokkos::deep_copy(rowPtrPt_h, 0);
    for(size_type entryIdx = 0; entryIdx < intermediateP.nnz(); ++entryIdx) {
      rowPtrPt_h(entries_h(entryIdx) + 1) += 1;
    }
    for(LO rowIdx = 0; rowIdx < intermediateP.numCols(); ++rowIdx) {
      rowPtrPt_h(rowIdx + 1) += rowPtrPt_h(rowIdx);
    }
    Kokkos::deep_copy(rowPtrPt, rowPtrPt_h);
 
    typename row_pointer_type::HostMirror rowPtrP_h = Kokkos::create_mirror_view(intermediateP.graph.row_map);
    Kokkos::deep_copy(rowPtrP_h, intermediateP.graph.row_map);
    typename col_indices_type::HostMirror colIndP_h = Kokkos::create_mirror_view(intermediateP.graph.entries);
    Kokkos::deep_copy(colIndP_h, intermediateP.graph.entries);
    typename values_type::HostMirror valuesP_h  = Kokkos::create_mirror_view(intermediateP.values);
    Kokkos::deep_copy(valuesP_h, intermediateP.values);
    typename col_indices_type::HostMirror colIndPt_h = Kokkos::create_mirror_view(colIndPt);
    typename values_type::HostMirror valuesPt_h = Kokkos::create_mirror_view(valuesPt);
    const col_index_type invalidColumnIndex = KokkosSparse::OrdinalTraits<col_index_type>::invalid();
    Kokkos::deep_copy(colIndPt_h, invalidColumnIndex);
 
    col_index_type colIdx = 0;
    for(LO rowIdx = 0; rowIdx < intermediateP.numRows(); ++rowIdx) {
      for(size_type entryIdxP = rowPtrP_h(rowIdx); entryIdxP < rowPtrP_h(rowIdx + 1); ++entryIdxP) {
        colIdx = entries_h(entryIdxP);
        for(size_type entryIdxPt = rowPtrPt_h(colIdx); entryIdxPt < rowPtrPt_h(colIdx + 1); ++entryIdxPt) {
          if(colIndPt_h(entryIdxPt) == invalidColumnIndex) {
            colIndPt_h(entryIdxPt) = rowIdx;
            valuesPt_h(entryIdxPt) = valuesP_h(entryIdxP);
            break;
          }
        } // Loop over entries in row of Pt
      } // Loop over entries in row of P
    } // Loop over rows of P
 
    Kokkos::deep_copy(colIndPt, colIndPt_h);
    Kokkos::deep_copy(valuesPt, valuesPt_h);
 
 
    local_matrix_type intermediatePt("intermediatePt",
                                     intermediateP.numCols(),
                                     intermediateP.numRows(),
                                     intermediateP.nnz(),
                                     valuesPt, rowPtrPt, colIndPt);
 
    // Create views for coarseA matrix
    localSpGEMM(intermediatePt, AP, "coarseA", coarseA);
  } // BuildCoarseLocalMatrix
 
  template <class Scalar, class LocalOrdinal, class GlobalOrdinal, class Node>
  void NotayAggregationFactory<Scalar, LocalOrdinal, GlobalOrdinal, Node>::
  localSpGEMM(const typename Matrix::local_matrix_type& A,
              const typename Matrix::local_matrix_type& B,
              const std::string matrixLabel,
              typename Matrix::local_matrix_type& C) const {
 
    using local_graph_type  = typename local_matrix_type::staticcrsgraph_type;
    using values_type       = typename local_matrix_type::values_type;
    using size_type         = typename local_graph_type::size_type;
    using col_index_type    = typename local_graph_type::data_type;
    using array_layout      = typename local_graph_type::array_layout;
    using memory_space      = typename device_type::memory_space;
    using memory_traits     = typename local_graph_type::memory_traits;
    using row_pointer_type  = Kokkos::View<size_type*, array_layout, device_type, memory_traits>;
    using col_indices_type  = Kokkos::View<col_index_type*, array_layout, device_type, memory_traits>;
 
    // Options
    int team_work_size = 16;
    std::string myalg("SPGEMM_KK_MEMORY");
    KokkosSparse::SPGEMMAlgorithm alg_enum = KokkosSparse::StringToSPGEMMAlgorithm(myalg);
    KokkosKernels::Experimental::KokkosKernelsHandle<typename row_pointer_type::const_value_type,
                                                     typename col_indices_type::const_value_type,
                                                     typename values_type::const_value_type,
                                                     execution_space,
                                                     memory_space,
                                                     memory_space> kh;
    kh.create_spgemm_handle(alg_enum);
    kh.set_team_work_size(team_work_size);
 
    // Create views for AP matrix
    row_pointer_type rowPtrC(Kokkos::ViewAllocateWithoutInitializing("C row pointer"),
                             A.numRows() + 1);
    col_indices_type colIndC;
    values_type      valuesC;
 
    // Symbolic multiplication
    KokkosSparse::Experimental::spgemm_symbolic(&kh, A.numRows(),
                                                B.numRows(), B.numCols(),
                                                A.graph.row_map, A.graph.entries, false,
                                                B.graph.row_map, B.graph.entries, false,
                                                rowPtrC);
 
    // allocate column indices and values of AP
    size_t nnzC = kh.get_spgemm_handle()->get_c_nnz();
    if (nnzC) {
      colIndC = col_indices_type(Kokkos::ViewAllocateWithoutInitializing("C column inds"), nnzC);
      valuesC = values_type(Kokkos::ViewAllocateWithoutInitializing("C values"), nnzC);
    }
 
    // Numeric multiplication
    KokkosSparse::Experimental::spgemm_numeric(&kh, A.numRows(),
                                               B.numRows(), B.numCols(),
                                               A.graph.row_map, A.graph.entries, A.values, false,
                                               B.graph.row_map, B.graph.entries, B.values, false,
                                               rowPtrC, colIndC, valuesC);
    kh.destroy_spgemm_handle();
 
    C = local_matrix_type(matrixLabel, A.numRows(), B.numCols(), nnzC, valuesC, rowPtrC, colIndC);
 
  } // localSpGEMM
 
} //namespace MueLu
 
#endif /* MUELU_NOTAYAGGREGATIONFACTORY_DEF_HPP_ */