Tpetra_Import_def.hpp

// @HEADER
// *****************************************************************************
//          Tpetra: Templated Linear Algebra Services Package
//
// Copyright 2008 NTESS and the Tpetra contributors.
// SPDX-License-Identifier: BSD-3-Clause
// *****************************************************************************
// @HEADER
 
#ifndef TPETRA_IMPORT_DEF_HPP
#define TPETRA_IMPORT_DEF_HPP
 
#include "Tpetra_Distributor.hpp"
#include "Tpetra_Map.hpp"
#include "Tpetra_ImportExportData.hpp"
#include "Tpetra_Util.hpp"
#include "Tpetra_Import_Util.hpp"
#include "Tpetra_Export.hpp"
#include "Tpetra_Details_Behavior.hpp"
#include "Tpetra_Details_DualViewUtil.hpp"
#include "Tpetra_Details_gathervPrint.hpp"
#include "Tpetra_Details_Profiling.hpp"
#include "Teuchos_as.hpp"
#include <array>
#include <algorithm>
#include <numeric>
#include <vector>
#include "Tpetra_Details_iallreduce.hpp"
#include <memory>
 
namespace Teuchos {
template <class T>
std::string toString(const std::vector<T>& x) {
  std::ostringstream os;
  os << "[";
  const std::size_t N = x.size();
  for (std::size_t k = 0; k < N; ++k) {
    os << x[k];
    if (k + std::size_t(1) < N) {
      os << ",";
    }
  }
  os << "]";
  return os.str();
}
 
template <class ElementType, class DeviceType>
std::string toString(const Kokkos::View<const ElementType*, DeviceType>& x) {
  std::ostringstream os;
  os << "[";
  const std::size_t N = std::size_t(x.extent(0));
  for (std::size_t k = 0; k < N; ++k) {
    os << x[k];
    if (k + std::size_t(1) < N) {
      os << ",";
    }
  }
  os << "]";
  return os.str();
}
}  // namespace Teuchos
 
namespace Tpetra {
 
// head:  init(source, target, true, remotePIDs, Teuchos::null);
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
void Import<LocalOrdinal, GlobalOrdinal, Node>::
    init(const Teuchos::RCP<const map_type>& source,
         const Teuchos::RCP<const map_type>& /* target */,
         bool useRemotePIDs,
         Teuchos::Array<int>& remotePIDs,
         const Teuchos::RCP<Teuchos::ParameterList>& plist) {
  using std::endl;
  using Teuchos::Array;
  using Teuchos::null;
  using Teuchos::Ptr;
  using Teuchos::rcp;
  using ::Tpetra::Details::ProfilingRegion;
  ProfilingRegion regionImportInit("Tpetra::Import::init");
 
  std::unique_ptr<std::string> verbPrefix;
  if (this->verbose()) {
    std::ostringstream os;
    const int myRank = source->getComm()->getRank();
    os << "Proc " << myRank << ": Tpetra::Import::init: ";
    verbPrefix = std::unique_ptr<std::string>(new std::string(os.str()));
    os << endl;
    this->verboseOutputStream() << os.str();
  }
 
  Array<GlobalOrdinal> remoteGIDs;
 
  {
    Tpetra::Details::ProfilingRegion MM("Tpetra:iport_ctor:preIData");
    if (this->verbose()) {
      std::ostringstream os;
      os << *verbPrefix << "Call setupSamePermuteRemote" << endl;
      this->verboseOutputStream() << os.str();
    }
    setupSamePermuteRemote(remoteGIDs);
  }
  {
    Tpetra::Details::ProfilingRegion MM2("Tpetra:iport_ctor:preSetupExport");
    if (source->isDistributed()) {
      if (this->verbose()) {
        std::ostringstream os;
        os << *verbPrefix << "Call setupExport" << endl;
        this->verboseOutputStream() << os.str();
      }
      setupExport(remoteGIDs, useRemotePIDs, remotePIDs);
    } else if (this->verbose()) {
      std::ostringstream os;
      os << *verbPrefix << "Source Map not distributed; skip setupExport"
         << endl;
      this->verboseOutputStream() << os.str();
    }
  }
 
  TEUCHOS_ASSERT(!this->TransferData_->permuteFromLIDs_.need_sync_device());
  TEUCHOS_ASSERT(!this->TransferData_->permuteFromLIDs_.need_sync_host());
  TEUCHOS_ASSERT(!this->TransferData_->permuteToLIDs_.need_sync_device());
  TEUCHOS_ASSERT(!this->TransferData_->permuteToLIDs_.need_sync_host());
  TEUCHOS_ASSERT(!this->TransferData_->remoteLIDs_.need_sync_device());
  TEUCHOS_ASSERT(!this->TransferData_->remoteLIDs_.need_sync_host());
  TEUCHOS_ASSERT(!this->TransferData_->exportLIDs_.need_sync_device());
  TEUCHOS_ASSERT(!this->TransferData_->exportLIDs_.need_sync_host());
 
  this->detectRemoteExportLIDsContiguous();
 
  if (this->verbose()) {
    std::ostringstream os;
    os << *verbPrefix << "Done!" << endl;
    this->verboseOutputStream() << os.str();
  }
}
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
Import<LocalOrdinal, GlobalOrdinal, Node>::
    Import(const Teuchos::RCP<const map_type>& source,
           const Teuchos::RCP<const map_type>& target)
  : base_type(source, target, Teuchos::null, Teuchos::null, "Import") {
  Teuchos::Array<int> dummy;
  init(source, target, false, dummy, Teuchos::null);
}
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
Import<LocalOrdinal, GlobalOrdinal, Node>::
    Import(const Teuchos::RCP<const map_type>& source,
           const Teuchos::RCP<const map_type>& target,
           const Teuchos::RCP<Teuchos::FancyOStream>& out)
  : base_type(source, target, out, Teuchos::null, "Import") {
  Teuchos::Array<int> dummy;
  init(source, target, false, dummy, Teuchos::null);
}
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
Import<LocalOrdinal, GlobalOrdinal, Node>::
    Import(const Teuchos::RCP<const map_type>& source,
           const Teuchos::RCP<const map_type>& target,
           const Teuchos::RCP<Teuchos::ParameterList>& plist)
  : base_type(source, target, Teuchos::null, plist, "Import") {
  Teuchos::Array<int> dummy;
  init(source, target, false, dummy, plist);
}
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
Import<LocalOrdinal, GlobalOrdinal, Node>::
    Import(const Teuchos::RCP<const map_type>& source,
           const Teuchos::RCP<const map_type>& target,
           const Teuchos::RCP<Teuchos::FancyOStream>& out,
           const Teuchos::RCP<Teuchos::ParameterList>& plist)
  : base_type(source, target, out, plist, "Import") {
  Teuchos::Array<int> dummy;
  init(source, target, false, dummy, plist);
}
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
Import<LocalOrdinal, GlobalOrdinal, Node>::
    Import(const Teuchos::RCP<const map_type>& source,
           const Teuchos::RCP<const map_type>& target,
           Teuchos::Array<int>& remotePIDs,
           const Teuchos::RCP<Teuchos::ParameterList>& plist)
  : base_type(source, target, Teuchos::null, plist, "Import") {
  init(source, target, true, remotePIDs, plist);
}
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
Import<LocalOrdinal, GlobalOrdinal, Node>::
    Import(const Import<LocalOrdinal, GlobalOrdinal, Node>& rhs)
  : base_type(rhs) {}
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
Import<LocalOrdinal, GlobalOrdinal, Node>::
    Import(const Export<LocalOrdinal, GlobalOrdinal, Node>& exporter)
  : base_type(exporter, typename base_type::reverse_tag()) {}
 
// cblcbl
// This is the "createExpert" version of the constructor to be used with pid/gid pairs obtained from
// reverse communication
template <class LocalOrdinal, class GlobalOrdinal, class Node>
Import<LocalOrdinal, GlobalOrdinal, Node>::
    Import(const Teuchos::RCP<const Map<LocalOrdinal, GlobalOrdinal, Node>>& source,
           const Teuchos::RCP<const Map<LocalOrdinal, GlobalOrdinal, Node>>& target,
           const Teuchos::ArrayView<int>& userRemotePIDs,
           const Teuchos::ArrayView<const LocalOrdinal>& userExportLIDs,
           const Teuchos::ArrayView<const int>& userExportPIDs,
           const Teuchos::RCP<Teuchos::ParameterList>& plist,
           const Teuchos::RCP<Teuchos::FancyOStream>& out)
  : base_type(source, target, out, plist, "Import") {
  using std::endl;
  using Teuchos::arcp;
  using Teuchos::Array;
  using Teuchos::ArrayRCP;
  using Teuchos::ArrayView;
  using Teuchos::as;
  using Teuchos::null;
  using Teuchos::rcp;
  using ::Tpetra::Details::makeDualViewFromArrayView;
  using LO        = LocalOrdinal;
  using GO        = GlobalOrdinal;
  using size_type = Teuchos::Array<int>::size_type;
 
  std::unique_ptr<std::string> prefix;
  if (this->verbose()) {
    auto comm        = source.is_null() ? Teuchos::null : source->getComm();
    const int myRank = comm.is_null() ? -1 : comm->getRank();
    std::ostringstream os;
    os << "Proc " << myRank << ": Tpetra::Import createExpert ctor: ";
    prefix = std::unique_ptr<std::string>(new std::string(os.str()));
    os << "Start" << endl;
    this->verboseOutputStream() << os.str();
  }
 
  ArrayView<const GO> sourceGIDs = source->getLocalElementList();
  ArrayView<const GO> targetGIDs = target->getLocalElementList();
 
  Array<GO> tRemoteGIDs;
  if (this->verbose()) {
    std::ostringstream os;
    os << *prefix << "Call setupSamePermuteRemote" << endl;
    this->verboseOutputStream() << os.str();
  }
  setupSamePermuteRemote(tRemoteGIDs);
 
  if (this->verbose()) {
    std::ostringstream os;
    os << *prefix << "Sort & filter IDs" << endl;
    this->verboseOutputStream() << os.str();
  }
 
  auto tRemoteLIDs = this->TransferData_->remoteLIDs_.view_host();
  this->TransferData_->remoteLIDs_.modify_host();
  Teuchos::Array<int> tRemotePIDs(userRemotePIDs);
 
  if (this->verbose() && this->getNumRemoteIDs() > 0 && !source->isDistributed()) {
    std::ostringstream os;
    os << *prefix << "Target Map has remote LIDs but source Map is not "
                     "distributed.  Importing to a submap of the target Map."
       << endl;
    this->verboseOutputStream() << os.str();
  }
  // FIXME (mfh 03 Feb 2019) I don't see this as "abuse"; it's
  // perfectly valid Petra Object Model behavior.
  TPETRA_ABUSE_WARNING(getNumRemoteIDs() > 0 && !source->isDistributed(),
                       std::runtime_error,
                       "::constructExpert: Target Map has remote LIDs but source Map "
                       "is not distributed.  Importing to a submap of the target Map.");
  TEUCHOS_TEST_FOR_EXCEPTION(tRemotePIDs.size() != tRemoteGIDs.size() ||
                                 size_t(tRemoteGIDs.size()) != size_t(tRemoteLIDs.extent(0)),
                             std::runtime_error,
                             "Import::Import createExpert version: "
                             "Size mismatch on userRemotePIDs, remoteGIDs, and remoteLIDs "
                             "Array's to sort3.");
 
  sort3(tRemotePIDs.begin(),
        tRemotePIDs.end(),
        tRemoteGIDs.begin(),
        tRemoteLIDs.data());
 
  // Get rid of IDs that don't exist in SourceMap
  size_type cnt                 = 0;
  size_type indexIntoRemotePIDs = tRemotePIDs.size();
  for (size_type i = 0; i < indexIntoRemotePIDs; ++i) {
    if (tRemotePIDs[i] == -1) {
      ++cnt;
    }
  }
 
  if (cnt == 0) {  // done modifying remoteLIDs_
    this->TransferData_->remoteLIDs_.sync_device();
  } else {
    if (indexIntoRemotePIDs - cnt > 0) {
      Array<GO> newRemoteGIDs(indexIntoRemotePIDs - cnt);
      Array<LO> newRemoteLIDs(indexIntoRemotePIDs - cnt);
      Array<int> newRemotePIDs(indexIntoRemotePIDs - cnt);
      cnt = 0;
 
      for (size_type j = 0; j < indexIntoRemotePIDs; ++j)
        if (tRemotePIDs[j] != -1) {
          newRemoteGIDs[cnt] = tRemoteGIDs[j];
          newRemotePIDs[cnt] = tRemotePIDs[j];
          newRemoteLIDs[cnt] = target->getLocalElement(tRemoteGIDs[j]);
          ++cnt;
        }
      indexIntoRemotePIDs = cnt;
      tRemoteGIDs         = newRemoteGIDs;
      tRemotePIDs         = newRemotePIDs;
      makeDualViewFromArrayView(this->TransferData_->remoteLIDs_,
                                newRemoteLIDs().getConst(),
                                "remoteLIDs");
    } else {  // valid RemoteIDs empty
      indexIntoRemotePIDs = 0;
      tRemoteGIDs.clear();
      tRemotePIDs.clear();
      this->TransferData_->remoteLIDs_ = decltype(this->TransferData_->remoteLIDs_)();
    }
  }
 
  this->TransferData_->exportPIDs_ = Teuchos::Array<int>(userExportPIDs);
  makeDualViewFromArrayView(this->TransferData_->exportLIDs_,
                            userExportLIDs, "exportLIDs");
 
  bool locallyComplete = true;
  for (size_type i = 0; i < userExportPIDs.size() && locallyComplete; ++i) {
    if (userExportPIDs[i] == -1) {
      locallyComplete = false;
    }
  }
  this->TransferData_->isLocallyComplete_ = locallyComplete;
 
  if (this->verbose()) {
    std::ostringstream os;
    os << *prefix << "locallyComplete: "
       << (locallyComplete ? "true" : "false")
       << "; call createFromSendsAndRecvs" << endl;
    this->verboseOutputStream() << os.str();
  }
  {
    Tpetra::Details::ProfilingRegion MM3("Tpetra:iport_ctor:cFSAR ");
    Distributor& distributor = this->TransferData_->distributor_;
    distributor.createFromSendsAndRecvs(this->TransferData_->exportPIDs_, tRemotePIDs);
  }
 
  this->detectRemoteExportLIDsContiguous();
 
  TEUCHOS_ASSERT(!this->TransferData_->permuteFromLIDs_.need_sync_device());
  TEUCHOS_ASSERT(!this->TransferData_->permuteFromLIDs_.need_sync_host());
  TEUCHOS_ASSERT(!this->TransferData_->permuteToLIDs_.need_sync_device());
  TEUCHOS_ASSERT(!this->TransferData_->permuteToLIDs_.need_sync_host());
  TEUCHOS_ASSERT(!this->TransferData_->remoteLIDs_.need_sync_device());
  TEUCHOS_ASSERT(!this->TransferData_->remoteLIDs_.need_sync_host());
  TEUCHOS_ASSERT(!this->TransferData_->exportLIDs_.need_sync_device());
  TEUCHOS_ASSERT(!this->TransferData_->exportLIDs_.need_sync_host());
}
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
Import<LocalOrdinal, GlobalOrdinal, Node>::
    Import(const Teuchos::RCP<const map_type>& source,
           const Teuchos::RCP<const map_type>& target,
           const size_t numSameIDs,
           Teuchos::Array<LocalOrdinal>& permuteToLIDs,
           Teuchos::Array<LocalOrdinal>& permuteFromLIDs,
           Teuchos::Array<LocalOrdinal>& remoteLIDs,
           Teuchos::Array<LocalOrdinal>& exportLIDs,
           Teuchos::Array<int>& exportPIDs,
           Distributor& distributor,
           const Teuchos::RCP<Teuchos::FancyOStream>& out,
           const Teuchos::RCP<Teuchos::ParameterList>& plist)
  : base_type(source, target, out, plist, "Import") {
  using std::endl;
  using ::Tpetra::Details::makeDualViewFromArrayView;
 
  std::unique_ptr<std::string> prefix;
  if (this->verbose()) {
    auto comm        = source.is_null() ? Teuchos::null : source->getComm();
    const int myRank = comm.is_null() ? -1 : comm->getRank();
    std::ostringstream os;
    os << "Proc " << myRank << ": Tpetra::Import export ctor: ";
    prefix = std::unique_ptr<std::string>(new std::string(os.str()));
    os << "Start" << endl;
    this->verboseOutputStream() << os.str();
  }
 
  bool locallyComplete = true;
  for (Teuchos::Array<int>::size_type i = 0; i < exportPIDs.size(); ++i) {
    if (exportPIDs[i] == -1) {
      locallyComplete = false;
    }
  }
  if (this->verbose()) {
    std::ostringstream os;
    os << *prefix << "numSameIDs: " << numSameIDs << ", locallyComplete: "
       << (locallyComplete ? "true" : "false") << endl;
    this->verboseOutputStream() << os.str();
  }
 
  this->TransferData_->isLocallyComplete_ = locallyComplete;
  this->TransferData_->numSameIDs_        = numSameIDs;
 
  makeDualViewFromArrayView(this->TransferData_->permuteToLIDs_,
                            permuteToLIDs().getConst(),
                            "permuteToLIDs");
  TEUCHOS_ASSERT(size_t(this->TransferData_->permuteToLIDs_.extent(0)) ==
                 size_t(permuteToLIDs.size()));
  makeDualViewFromArrayView(this->TransferData_->permuteFromLIDs_,
                            permuteFromLIDs().getConst(),
                            "permuteFromLIDs");
  TEUCHOS_ASSERT(size_t(this->TransferData_->permuteFromLIDs_.extent(0)) ==
                 size_t(permuteFromLIDs.size()));
  makeDualViewFromArrayView(this->TransferData_->remoteLIDs_,
                            remoteLIDs().getConst(),
                            "remoteLIDs");
  TEUCHOS_ASSERT(size_t(this->TransferData_->remoteLIDs_.extent(0)) ==
                 size_t(remoteLIDs.size()));
  makeDualViewFromArrayView(this->TransferData_->exportLIDs_,
                            exportLIDs().getConst(),
                            "exportLIDs");
  TEUCHOS_ASSERT(size_t(this->TransferData_->exportLIDs_.extent(0)) ==
                 size_t(exportLIDs.size()));
  this->TransferData_->exportPIDs_.swap(exportPIDs);
  this->TransferData_->distributor_.swap(distributor);
 
  this->detectRemoteExportLIDsContiguous();
 
  TEUCHOS_ASSERT(!this->TransferData_->permuteFromLIDs_.need_sync_device());
  TEUCHOS_ASSERT(!this->TransferData_->permuteFromLIDs_.need_sync_host());
  TEUCHOS_ASSERT(!this->TransferData_->permuteToLIDs_.need_sync_device());
  TEUCHOS_ASSERT(!this->TransferData_->permuteToLIDs_.need_sync_host());
  TEUCHOS_ASSERT(!this->TransferData_->remoteLIDs_.need_sync_device());
  TEUCHOS_ASSERT(!this->TransferData_->remoteLIDs_.need_sync_host());
  TEUCHOS_ASSERT(!this->TransferData_->exportLIDs_.need_sync_device());
  TEUCHOS_ASSERT(!this->TransferData_->exportLIDs_.need_sync_host());
}
 
namespace {  // (anonymous)
 
template <class LO, class GO, class NT>
struct ImportLocalSetupResult {
  Teuchos::RCP<const ::Tpetra::Map<LO, GO, NT>> targetMap;
  LO numSameIDs;
  // std::vector<LO> permuteToLIDs; // users aren't supposed to have permutes
  // std::vector<LO> permuteFromLIDs; // users aren't suppoosed to have permutes
  std::vector<GO> remoteGIDs;
  std::vector<LO> remoteLIDs;
  std::vector<int> remotePIDs;
  LO numPermutes;  // users aren't supposed to have permutes
};
 
template <class T>
void printArray(std::ostream& out, const T x[], const std::size_t N) {
  out << "[";
  for (std::size_t k = 0; k < N; ++k) {
    out << x[k];
    if (k + 1 < N) {
      out << ", ";
    }
  }
  out << "]";
}
 
template <class LO, class GO, class NT>
ImportLocalSetupResult<LO, GO, NT>
setupSamePermuteRemoteFromUserGlobalIndexList(const ::Tpetra::Map<LO, GO, NT>& sourceMap,
                                              const GO targetMapRemoteOrPermuteGlobalIndices[],
                                              const int targetMapRemoteOrPermuteProcessRanks[],
                                              const LO numTargetMapRemoteOrPermuteGlobalIndices,
                                              const bool mayReorderTargetMapIndicesLocally,
                                              Teuchos::FancyOStream* out,        // only valid if verbose
                                              const std::string* verboseHeader,  // only valid if verbose
                                              const bool verbose,
                                              const bool debug) {
  using std::endl;
  const int myRank = sourceMap.getComm()->getRank();
  ImportLocalSetupResult<LO, GO, NT> result;
 
  if (verbose) {
    std::ostringstream os;
    os << *verboseHeader << "- Import::setupSPR w/ remote GIDs & PIDs: " << endl
       << *verboseHeader << "  Input GIDs: ";
    printArray(os, targetMapRemoteOrPermuteGlobalIndices, numTargetMapRemoteOrPermuteGlobalIndices);
    os << endl
       << "  Input PIDs: ";
    printArray(os, targetMapRemoteOrPermuteProcessRanks, numTargetMapRemoteOrPermuteGlobalIndices);
    os << endl;
    *out << os.str();
  }
 
  // In debug mode, check whether any of the input GIDs are
  // actually in the source Map on the calling process.  That's an
  // error, because it means duplicate GIDs on the calling
  // process.  Also check if any of the input PIDs are invalid.
  if (debug) {
    std::vector<GO> badGIDs;
    std::vector<int> badPIDs;
    const Teuchos::Comm<int>& comm = *(sourceMap.getComm());
    const int numProcs             = comm.getSize();
 
    for (LO k = 0; k < numTargetMapRemoteOrPermuteGlobalIndices; ++k) {
      const GO tgtGID = targetMapRemoteOrPermuteGlobalIndices[k];
      if (sourceMap.isNodeGlobalElement(tgtGID)) {
        badGIDs.push_back(tgtGID);
      }
      const int tgtPID = targetMapRemoteOrPermuteProcessRanks[k];
      if (tgtPID < 0 || tgtPID >= numProcs) {
        badPIDs.push_back(tgtPID);
      }
    }
 
    std::array<int, 2> lclStatus{{badGIDs.size() == 0 ? 1 : 0,
                                  badPIDs.size() == 0 ? 1 : 0}};
    std::array<int, 2> gblStatus{{0, 0}};  // output argument
    Teuchos::reduceAll<int, int>(comm, Teuchos::REDUCE_MIN, 2,
                                 lclStatus.data(), gblStatus.data());
    const bool good = gblStatus[0] == 1 && gblStatus[1] == 1;
    // Don't actually print all the "bad" GIDs and/or PIDs unless
    // in verbose mode, since there could be many of them.
    if (verbose && gblStatus[0] != 1) {
      std::ostringstream os;
      os << *verboseHeader << "- Some input GIDs are already in the source Map: ";
      printArray(os, badGIDs.data(), badGIDs.size());
      os << endl;
      ::Tpetra::Details::gathervPrint(*out, os.str(), comm);
    }
    if (verbose && gblStatus[0] != 1) {
      std::ostringstream os;
      os << *verboseHeader << "- Some input PIDs are invalid: ";
      printArray(os, badPIDs.data(), badPIDs.size());
      os << endl;
      ::Tpetra::Details::gathervPrint(*out, os.str(), comm);
    }
 
    if (!good) {
      std::ostringstream os;
      os << "Tpetra::Import constructor that takes remote GIDs and PIDs: ";
      if (gblStatus[0] != 1) {
        os << "Some input GIDs (global indices) are already in the source Map!  ";
      }
      if (gblStatus[1] != 1) {
        os << "Some input PIDs (process ranks) are invalid!  ";
      }
      os << "Rerun with the environment variable TPETRA_VERBOSE=Tpetra::Import "
            "to see what GIDs and/or PIDs are bad.";
      TEUCHOS_TEST_FOR_EXCEPTION(true, std::invalid_argument, os.str());
    }
  }
 
  // Create list of GIDs to go into target Map.  We need to copy
  // the GIDs into this list anyway, so once we have them, we can
  // sort the "remotes" in place.
  const LO numLclSrcIDs = static_cast<LO>(sourceMap.getLocalNumElements());
  const LO numLclTgtIDs = numLclSrcIDs + numTargetMapRemoteOrPermuteGlobalIndices;
  if (verbose) {
    std::ostringstream os;
    os << *verboseHeader << "- Copy source Map GIDs into target Map GID list: "
                            "numLclSrcIDs="
       << numLclSrcIDs
       << ", numTargetMapRemoteOrPermuteGlobalIndices="
       << numTargetMapRemoteOrPermuteGlobalIndices << endl;
    *out << os.str();
  }
  std::vector<GO> tgtGIDs(numLclTgtIDs);  // will go into target Map ctor
  if (sourceMap.isContiguous()) {
    GO curTgtGID = sourceMap.getMinGlobalIndex();
    for (LO k = 0; k < numLclSrcIDs; ++k, ++curTgtGID) {
      tgtGIDs[k] = curTgtGID;
    }
  } else {                                           // avoid calling getLocalElementList on a contiguous Map
    auto srcGIDs = sourceMap.getLocalElementList();  // Teuchos::ArrayView has a different
    for (LO k = 0; k < numLclSrcIDs; ++k) {          // iterator type, so can't std::copy
      tgtGIDs[k] = srcGIDs[k];
    }
  }
  std::copy(targetMapRemoteOrPermuteGlobalIndices,
            targetMapRemoteOrPermuteGlobalIndices + numTargetMapRemoteOrPermuteGlobalIndices,
            tgtGIDs.begin() + numLclSrcIDs);
 
  // Optionally, sort input by process rank, so that remotes
  // coming from the same process are grouped together.  Only sort
  // remote GIDs.  While doing so, detect permutes (input "remote"
  // GIDs whose rank is the same as that of the calling process).
  //
  // Permutes are actually an error.  We normally detect them in
  // debug mode, but if we sort, we have a nearly free opportunity
  // to do so.  We may also safely ignore permutes as duplicates.
  //
  // NOTE: tgtPIDs only includes remotes, not source Map entries.
  if (verbose) {
    std::ostringstream os;
    os << *verboseHeader << "- Sort by PID? "
       << (mayReorderTargetMapIndicesLocally ? "true" : "false") << endl;
    *out << os.str();
  }
  std::vector<int> tgtPIDs(targetMapRemoteOrPermuteProcessRanks,
                           targetMapRemoteOrPermuteProcessRanks + numTargetMapRemoteOrPermuteGlobalIndices);
  result.numPermutes = 0;
  if (mayReorderTargetMapIndicesLocally) {
    Tpetra::sort2(tgtPIDs.begin(), tgtPIDs.end(), tgtGIDs.begin() + numLclSrcIDs);
    auto range = std::equal_range(tgtPIDs.begin(), tgtPIDs.end(), myRank);  // binary search
    if (range.second > range.first) {
      result.numPermutes = static_cast<LO>(range.second - range.first);
    }
  } else {  // don't sort; linear search to count permutes
    result.numPermutes = static_cast<LO>(std::count(tgtPIDs.begin(), tgtPIDs.end(), myRank));
  }
  // The _actual_ number of remotes.
  const LO numRemotes = numTargetMapRemoteOrPermuteGlobalIndices - result.numPermutes;
  result.numSameIDs   = static_cast<LO>(sourceMap.getLocalNumElements());
 
  if (verbose) {
    std::ostringstream os;
    os << *verboseHeader << "- numSame=" << result.numSameIDs
       << ", numPermutes=" << result.numPermutes
       << ", numRemotes=" << numRemotes << endl;
    *out << os.str();
  }
 
  if (result.numPermutes == 0) {
    if (verbose) {
      std::ostringstream os;
      os << *verboseHeader << "- No permutes" << endl;
      *out << os.str();
    }
    result.remoteGIDs = std::vector<GO>(tgtGIDs.begin() + numLclSrcIDs, tgtGIDs.end());
    result.remotePIDs.swap(tgtPIDs);
    result.remoteLIDs.resize(numRemotes);
    for (LO k = 0; k < numRemotes; ++k) {
      const LO tgtLid      = result.numSameIDs + k;
      result.remoteLIDs[k] = tgtLid;
    }
    if (verbose) {
      std::ostringstream os;
      os << *verboseHeader << "- Remote GIDs: "
         << Teuchos::toString(result.remoteGIDs) << endl;
      os << *verboseHeader << "- Remote PIDs: "
         << Teuchos::toString(result.remotePIDs) << endl;
      os << *verboseHeader << "- Remote LIDs: "
         << Teuchos::toString(result.remoteLIDs) << endl;
      *out << os.str();
    }
  } else {  // separate permutes from remotes
    // This case doesn't need to be optimal; it just needs to be
    // correct.  Users really shouldn't give permutes to this
    // Import constructor.
    result.remoteGIDs.reserve(numRemotes);
    result.remoteLIDs.reserve(numRemotes);
    result.remotePIDs.reserve(numRemotes);
    for (LO k = 0; k < numTargetMapRemoteOrPermuteGlobalIndices; ++k) {
      const LO tgtLid  = result.numSameIDs + k;
      const GO tgtGid  = tgtGIDs[numLclSrcIDs + k];
      const int tgtPid = tgtPIDs[k];
 
      if (tgtPid != myRank) {  // it's a remote
        result.remoteGIDs.push_back(tgtGid);
        result.remoteLIDs.push_back(tgtLid);
        result.remotePIDs.push_back(tgtPid);
      }
    }
    if (verbose) {
      std::ostringstream os;
      os << *verboseHeader << "- Some permutes" << endl;
      *out << os.str();
    }
  }
 
  if (sourceMap.isDistributed()) {
    if (verbose) {
      std::ostringstream os;
      os << *verboseHeader << "- Sort remotes by PID, as Import always does"
         << endl
         << *verboseHeader << "-- remotePIDs before: "
         << Teuchos::toString(result.remotePIDs) << endl
         << *verboseHeader << "-- remoteGIDs before: "
         << Teuchos::toString(result.remoteGIDs) << endl
         << *verboseHeader << "-- remoteLIDs before: "
         << Teuchos::toString(result.remoteLIDs) << endl;
      *out << os.str();
    }
    // Import always sorts these, regardless of what the user wanted.
    sort3(result.remotePIDs.begin(),
          result.remotePIDs.end(),
          result.remoteGIDs.begin(),
          result.remoteLIDs.begin());
    if (verbose) {
      std::ostringstream os;
      os << *verboseHeader << "-- remotePIDs after: "
         << Teuchos::toString(result.remotePIDs) << endl
         << *verboseHeader << "-- remoteGIDs after: "
         << Teuchos::toString(result.remoteGIDs) << endl
         << *verboseHeader << "-- remoteLIDs after: "
         << Teuchos::toString(result.remoteLIDs) << endl;
      std::cerr << os.str();
    }
  }
 
  if (verbose) {
    std::ostringstream os;
    os << *verboseHeader << "- Make target Map" << endl;
    *out << os.str();
  }
  using ::Teuchos::rcp;
  typedef ::Tpetra::Map<LO, GO, NT> map_type;
  typedef ::Tpetra::global_size_t GST;
  const GST MAP_COMPUTES_GLOBAL_COUNT = ::Teuchos::OrdinalTraits<GST>::invalid();
  result.targetMap                    = rcp(new map_type(MAP_COMPUTES_GLOBAL_COUNT,
                                                         tgtGIDs.data(),
                                                         numLclTgtIDs,
                                                         sourceMap.getIndexBase(),
                                                         sourceMap.getComm()));
  if (verbose) {
    std::ostringstream os;
    os << *verboseHeader << "- Done with sameSPR..." << endl;
    *out << os.str();
  }
  return result;
}
}  // namespace
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
Import<LocalOrdinal, GlobalOrdinal, Node>::
    Import(const Teuchos::RCP<const Map<LocalOrdinal, GlobalOrdinal, Node>>& sourceMap,
           const GlobalOrdinal targetMapRemoteOrPermuteGlobalIndices[],
           const int targetMapRemoteOrPermuteProcessRanks[],
           const LocalOrdinal numTargetMapRemoteOrPermuteGlobalIndices,
           const bool mayReorderTargetMapIndicesLocally,
           const Teuchos::RCP<Teuchos::ParameterList>& plist,
           const Teuchos::RCP<Teuchos::FancyOStream>& debugOutput)
  :  // Special case: target Map is null on base_type construction.
     // It's worthwhile for invariants like out_ not being null.
     // We'll set TransferData_ again below.
  base_type(sourceMap, Teuchos::null, debugOutput, plist, "Import") {
  using std::endl;
  using Teuchos::ArrayView;
  using Teuchos::getFancyOStream;
  using Teuchos::RCP;
  using Teuchos::rcp;
  using Teuchos::rcpFromRef;
  using ::Tpetra::Details::Behavior;
  using ::Tpetra::Details::makeDualViewFromOwningHostView;
  using ::Tpetra::Details::makeDualViewFromVector;
  using ::Tpetra::Details::printDualView;
  using ::Tpetra::Details::view_alloc_no_init;
  typedef LocalOrdinal LO;
  typedef GlobalOrdinal GO;
  typedef Node NT;
 
  const bool debug = Behavior::debug("Import") ||
                     Behavior::debug("Tpetra::Import");
 
  std::unique_ptr<std::string> verbPfx;
  if (this->verbose()) {
    std::ostringstream os;
    const int myRank = sourceMap->getComm()->getRank();
    os << "Proc " << myRank << ": Tpetra::Import ctor from remotes: ";
    verbPfx = std::unique_ptr<std::string>(new std::string(os.str()));
    os << "mayReorder=" << (mayReorderTargetMapIndicesLocally ? "true" : "false")
       << endl;
    this->verboseOutputStream() << os.str();
  }
 
  TEUCHOS_ASSERT(!this->TransferData_.is_null());
  ImportLocalSetupResult<LO, GO, NT> localSetupResult =
      setupSamePermuteRemoteFromUserGlobalIndexList<LO, GO, NT>(*sourceMap,
                                                                targetMapRemoteOrPermuteGlobalIndices,
                                                                targetMapRemoteOrPermuteProcessRanks,
                                                                numTargetMapRemoteOrPermuteGlobalIndices,
                                                                mayReorderTargetMapIndicesLocally,
                                                                this->TransferData_->out_.getRawPtr(),
                                                                verbPfx.get(),
                                                                this->verbose(),
                                                                debug);
 
  // Since we invoked the base_type constructor above, we know that
  // out_ is nonnull, so we don't have to waste time creating it
  // again.
  using data_type = ImportExportData<LO, GO, NT>;
  TEUCHOS_ASSERT(!this->TransferData_.is_null());
  this->TransferData_              = rcp(new data_type(sourceMap,
                                                       localSetupResult.targetMap,
                                                       this->TransferData_->out_,
                                                       plist));
  this->TransferData_->numSameIDs_ = localSetupResult.numSameIDs;
  // Skip permutes; they are user error, because they duplicate
  // non-remote indices.
  makeDualViewFromVector(this->TransferData_->remoteLIDs_,
                         localSetupResult.remoteLIDs,
                         "remoteLIDs");
  // "Is locally complete" for an Import means that all target Map
  // indices on the calling process exist on at least one process
  // (not necessarily this one) in the source Map.  For this
  // constructor, this is true if and only if all input target PIDs
  // are valid PIDs in the communicator.
  //
  // FIXME (mfh 20 Feb 2018) For now, assume this is always true.
  this->TransferData_->isLocallyComplete_ = true;
 
  Teuchos::Array<GO> exportGIDs;
  if (sourceMap->isDistributed()) {
    if (this->verbose()) {
      std::ostringstream os;
      os << *verbPfx << "Make Distributor (createFromRecvs)" << endl;
      this->verboseOutputStream() << os.str();
    }
    ArrayView<const GO> remoteGIDs(localSetupResult.remoteGIDs.data(),
                                   localSetupResult.remoteGIDs.size());
    ArrayView<const int> remotePIDs(localSetupResult.remotePIDs.data(),
                                    localSetupResult.remotePIDs.size());
    // Call Distributor::createFromRecvs to turn the remote GIDs and
    // their owning PIDs into a send-and-receive communication plan.
    // remoteGIDs and remotePIDs are input; exportGIDs and
    // exportPIDs are output arrays that createFromRecvs allocates.
    Distributor& distributor = this->TransferData_->distributor_;
    distributor.createFromRecvs(remoteGIDs,
                                remotePIDs,
                                exportGIDs,
                                this->TransferData_->exportPIDs_);
    // Find the LIDs corresponding to the (outgoing) GIDs in
    // exportGIDs.  For sparse matrix-vector multiply, this tells
    // the calling process how to index into the source vector to
    // get the elements which it needs to send.
    //
    // NOTE (mfh 03 Mar 2014) This is now a candidate for a
    // thread-parallel kernel, but only if using the new thread-safe
    // Map implementation.
    if (this->verbose()) {
      std::ostringstream os;
      os << *verbPfx << "Compute exportLIDs" << endl;
      this->verboseOutputStream() << os.str();
    }
    using size_type              = typename Teuchos::Array<GO>::size_type;
    const size_type numExportIDs = exportGIDs.size();
 
    typename decltype(this->TransferData_->exportLIDs_)::t_host
        exportLIDs(view_alloc_no_init("exportLIDs"), numExportIDs);
 
    for (size_type k = 0; k < numExportIDs; ++k) {
      exportLIDs[k] = sourceMap->getLocalElement(exportGIDs[k]);
    }
    makeDualViewFromOwningHostView(this->TransferData_->exportLIDs_, exportLIDs);
  }
 
  if (this->verbose()) {
    std::ostringstream os;
    os << *verbPfx;
    printDualView(os, this->TransferData_->remoteLIDs_,
                  "ImportExportData::remoteLIDs_");
    os << endl;
    this->verboseOutputStream() << os.str();
  }
  if (this->verbose()) {
    std::ostringstream os;
    os << *verbPfx << "Done!" << endl;
    this->verboseOutputStream() << os.str();
  }
}
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
void Import<LocalOrdinal, GlobalOrdinal, Node>::
    describe(Teuchos::FancyOStream& out,
             const Teuchos::EVerbosityLevel verbLevel) const {
  // Call the base class' method.  It does all the work.
  this->describeImpl(out, "Tpetra::Import", verbLevel);
}
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
void Import<LocalOrdinal, GlobalOrdinal, Node>::
    print(std::ostream& os) const {
  auto out = Teuchos::getFancyOStream(Teuchos::rcpFromRef(os));
  // "Print" traditionally meant "everything."
  this->describe(*out, Teuchos::VERB_EXTREME);
}
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
void Import<LocalOrdinal, GlobalOrdinal, Node>::
    setupSamePermuteRemote(Teuchos::Array<GlobalOrdinal>& remoteGIDs) {
  using Teuchos::arcp;
  using Teuchos::Array;
  using Teuchos::ArrayRCP;
  using Teuchos::ArrayView;
  using Teuchos::as;
  using Teuchos::null;
  using ::Tpetra::Details::makeDualViewFromOwningHostView;
  using ::Tpetra::Details::ProfilingRegion;
  using ::Tpetra::Details::view_alloc_no_init;
  typedef LocalOrdinal LO;
  typedef GlobalOrdinal GO;
  typedef typename ArrayView<const GO>::size_type size_type;
  ProfilingRegion regionExport("Tpetra::Import::setupSamePermuteRemote");
 
  const map_type& source         = *(this->getSourceMap());
  const map_type& target         = *(this->getTargetMap());
  ArrayView<const GO> sourceGIDs = source.getLocalElementList();
  ArrayView<const GO> targetGIDs = target.getLocalElementList();
 
#ifdef HAVE_TPETRA_DEBUG
  ArrayView<const GO> rawSrcGids = sourceGIDs;
  ArrayView<const GO> rawTgtGids = targetGIDs;
#else
  const GO* const rawSrcGids = sourceGIDs.getRawPtr();
  const GO* const rawTgtGids = targetGIDs.getRawPtr();
#endif  // HAVE_TPETRA_DEBUG
  const size_type numSrcGids = sourceGIDs.size();
  const size_type numTgtGids = targetGIDs.size();
  const size_type numGids    = std::min(numSrcGids, numTgtGids);
 
  // Compute numSameIDs_: the number of initial GIDs that are the
  // same (and occur in the same order) in both Maps.  The point of
  // numSameIDs_ is for the common case of an Import where all the
  // overlapping GIDs are at the end of the target Map, but
  // otherwise the source and target Maps are the same.  This allows
  // a fast contiguous copy for the initial "same IDs."
  size_type numSameGids = 0;
  for (; numSameGids < numGids && rawSrcGids[numSameGids] == rawTgtGids[numSameGids]; ++numSameGids) {
  }  // third clause of 'for' does everything
  this->TransferData_->numSameIDs_ = numSameGids;
 
  // Compute permuteToLIDs_, permuteFromLIDs_, remoteGIDs, and
  // remoteLIDs_.  The first two arrays are IDs to be permuted, and
  // the latter two arrays are IDs to be received ("imported"),
  // called "remote" IDs.
  //
  // IDs to permute are in both the source and target Maps, which
  // means we don't have to send or receive them, but we do have to
  // rearrange (permute) them in general.  IDs to receive are in the
  // target Map, but not the source Map.
 
  // Iterate over the target Map's LIDs, since we only need to do
  // GID -> LID lookups for the source Map.
  const LO LINVALID   = Teuchos::OrdinalTraits<LO>::invalid();
  const LO numTgtLids = as<LO>(numTgtGids);
  LO numPermutes      = 0;
 
  for (LO tgtLid = numSameGids; tgtLid < numTgtLids; ++tgtLid) {
    const GO curTargetGid = rawTgtGids[tgtLid];
    // getLocalElement() returns LINVALID if the GID isn't in the
    // source Map.  This saves us a lookup (which
    // isNodeGlobalElement() would do).
    const LO srcLid = source.getLocalElement(curTargetGid);
    if (srcLid != LINVALID) {  // if source.isNodeGlobalElement (curTargetGid)
      ++numPermutes;
    }
  }
  const LO numRemotes = (numTgtLids - numSameGids) - numPermutes;
 
  using host_perm_type =
      typename decltype(this->TransferData_->permuteToLIDs_)::t_host;
  host_perm_type permuteToLIDs(view_alloc_no_init("permuteToLIDs"), numPermutes);
  host_perm_type permuteFromLIDs(view_alloc_no_init("permuteFromLIDs"), numPermutes);
  typename decltype(this->TransferData_->remoteLIDs_)::t_host remoteLIDs(view_alloc_no_init("permuteFromLIDs"), numRemotes);
 
  {
    LO numPermutes2 = 0;
    LO numRemotes2  = 0;
    for (LO tgtLid = numSameGids; tgtLid < numTgtLids; ++tgtLid) {
      const GO curTargetGid = rawTgtGids[tgtLid];
      const LO srcLid       = source.getLocalElement(curTargetGid);
      if (srcLid != LINVALID) {
        permuteToLIDs[numPermutes2]   = tgtLid;
        permuteFromLIDs[numPermutes2] = srcLid;
        ++numPermutes2;
      } else {
        remoteGIDs.push_back(curTargetGid);
        remoteLIDs[numRemotes2] = tgtLid;
        ++numRemotes2;
      }
    }
    TEUCHOS_ASSERT(numPermutes == numPermutes2);
    TEUCHOS_ASSERT(numRemotes == numRemotes2);
    TEUCHOS_ASSERT(size_t(numPermutes) + remoteGIDs.size() == size_t(numTgtLids - numSameGids));
  }
 
  makeDualViewFromOwningHostView(this->TransferData_->permuteToLIDs_, permuteToLIDs);
  makeDualViewFromOwningHostView(this->TransferData_->permuteFromLIDs_, permuteFromLIDs);
  makeDualViewFromOwningHostView(this->TransferData_->remoteLIDs_, remoteLIDs);
  if (remoteLIDs.extent(0) != 0 && !source.isDistributed()) {
    // This Import has remote LIDs, meaning that the target Map has
    // entries on this process that are not in the source Map on
    // this process.  However, the source Map is not distributed
    // globally.  This implies that this Import is not locally
    // complete on this process.
    this->TransferData_->isLocallyComplete_ = false;
    // mfh 12 Sep 2016: I disagree that this is "abuse"; it may be
    // correct behavior, depending on the circumstances.
    TPETRA_ABUSE_WARNING(true, std::runtime_error,
                         "::setupSamePermuteRemote(): Target has "
                         "remote LIDs but Source is not distributed globally.  Importing to a "
                         "submap of the target map.");
  }
}
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
void Import<LocalOrdinal, GlobalOrdinal, Node>::
    setupExport(Teuchos::Array<GlobalOrdinal>& remoteGIDs,
                bool useRemotePIDs,
                Teuchos::Array<int>& userRemotePIDs,
                const Teuchos::RCP<Teuchos::ParameterList>& plist) {
  using std::endl;
  using Teuchos::Array;
  using Teuchos::ArrayView;
  using ::Tpetra::Details::makeDualViewFromOwningHostView;
  using ::Tpetra::Details::view_alloc_no_init;
  using GO = GlobalOrdinal;
  typedef typename Array<int>::difference_type size_type;
  const char tfecfFuncName[] = "setupExport: ";
  const char suffix[]        = "  Please report this bug to the Tpetra developers.";
 
  std::unique_ptr<std::string> prefix;
  if (this->verbose()) {
    auto srcMap      = this->getSourceMap();
    auto comm        = srcMap.is_null() ? Teuchos::null : srcMap->getComm();
    const int myRank = comm.is_null() ? -1 : comm->getRank();
    std::ostringstream os;
    os << "Proc " << myRank << ": Tpetra::Import::setupExport: ";
    prefix = std::unique_ptr<std::string>(new std::string(os.str()));
    os << "Start" << std::endl;
    this->verboseOutputStream() << os.str();
  }
 
  TEUCHOS_TEST_FOR_EXCEPTION_CLASS_FUNC(this->getSourceMap().is_null(), std::logic_error,
                                        "Source Map is null.  " << suffix);
  const map_type& source = *(this->getSourceMap());
 
  TEUCHOS_TEST_FOR_EXCEPTION_CLASS_FUNC(!useRemotePIDs && (userRemotePIDs.size() > 0), std::invalid_argument,
                                        "remotePIDs are non-empty but their use has not been requested.");
  TEUCHOS_TEST_FOR_EXCEPTION_CLASS_FUNC(userRemotePIDs.size() > 0 && remoteGIDs.size() != userRemotePIDs.size(),
                                        std::invalid_argument,
                                        "remotePIDs must either be of size zero or match "
                                        "the size of remoteGIDs.");
 
  // For each entry remoteGIDs[i], remoteProcIDs[i] will contain
  // the process ID of the process that owns that GID.
  ArrayView<GO> remoteGIDsView = remoteGIDs();
  ArrayView<int> remoteProcIDsView;
 
  // lookup == IDNotPresent means that the source Map wasn't able to
  // figure out to which processes one or more of the GIDs in the
  // given list of remote GIDs belong.
  //
  // The previous abuse warning said "The target Map has GIDs not
  // found in the source Map."  This statement could be confusing,
  // because it doesn't refer to ownership by the current process,
  // but rather to ownership by _any_ process participating in the
  // Map.  (It could not possibly refer to ownership by the current
  // process, since remoteGIDs is exactly the list of GIDs owned by
  // the target Map but not owned by the source Map.  It was
  // constructed that way by setupSamePermuteRemote().)
  //
  // What this statement means is that the source and target Maps
  // don't contain the same set of GIDs globally (over all
  // processes).  That is, there is at least one GID owned by some
  // process in the target Map, which is not owned by _any_ process
  // in the source Map.
  Array<int> newRemotePIDs;
  LookupStatus lookup = AllIDsPresent;
 
  if (!useRemotePIDs) {
    newRemotePIDs.resize(remoteGIDsView.size());
    if (this->verbose()) {
      std::ostringstream os;
      os << *prefix << "Call sourceMap.getRemoteIndexList" << endl;
      this->verboseOutputStream() << os.str();
    }
    lookup = source.getRemoteIndexList(remoteGIDsView, newRemotePIDs());
  }
  Array<int>& remoteProcIDs = useRemotePIDs ? userRemotePIDs : newRemotePIDs;
 
  if (lookup == IDNotPresent) {
    Tpetra::Details::ProfilingRegion pr("Tpetra:iport_ctor:setupExport:1");
    // There is at least one GID owned by the calling process in the
    // target Map, which is not owned by any process in the source
    // Map.
    this->TransferData_->isLocallyComplete_ = false;
 
    // mfh 12 Sep 2016: I disagree that this is "abuse"; it may be
    // correct behavior, depending on the circumstances.
    TPETRA_ABUSE_WARNING(true, std::runtime_error,
                         "::setupExport(): the source Map wasn't "
                         "able to figure out which process owns one or more of the GIDs in the "
                         "list of remote GIDs.  This probably means that there is at least one "
                         "GID owned by some process in the target Map which is not owned by any"
                         " process in the source Map.  (That is, the source and target Maps do "
                         "not contain the same set of GIDs globally.)");
 
    // Ignore remote GIDs that aren't owned by any process in the
    // source Map.  getRemoteIndexList() gives each of these a
    // process ID of -1.
 
    const size_type numInvalidRemote =
        std::count_if(remoteProcIDs.begin(), remoteProcIDs.end(),
                      std::bind(std::equal_to<int>(), -1, std::placeholders::_1));
    TEUCHOS_TEST_FOR_EXCEPTION_CLASS_FUNC(numInvalidRemote == 0, std::logic_error,
                                          "Calling getRemoteIndexList "
                                          "on the source Map returned IDNotPresent, but none of the returned "
                                          "\"remote\" process ranks are -1.  Please report this bug to the "
                                          "Tpetra developers.");
 
    // If all of them are invalid, we can delete the whole array.
    const size_type totalNumRemote = this->getNumRemoteIDs();
    if (numInvalidRemote == totalNumRemote) {
      // all remotes are invalid; we have no remotes; we can delete the remotes
      remoteProcIDs.clear();
      remoteGIDs.clear();  // invalidates remoteGIDsView
      this->TransferData_->remoteLIDs_ =
          decltype(this->TransferData_->remoteLIDs_)();
    } else {
      // Some remotes are valid; we need to keep the valid ones.
      // Pack and resize remoteProcIDs, remoteGIDs, and remoteLIDs_.
      size_type numValidRemote = 0;
#ifdef HAVE_TPETRA_DEBUG
      ArrayView<GO> remoteGIDsPtr = remoteGIDsView;
#else
      GO* const remoteGIDsPtr = remoteGIDsView.getRawPtr();
#endif  // HAVE_TPETRA_DEBUG
 
      // Don't mark the DualView modified, since we'll reallocate it.
      auto remoteLIDs = this->TransferData_->remoteLIDs_.view_host();
 
      for (size_type r = 0; r < totalNumRemote; ++r) {
        // Pack in all the valid remote PIDs and GIDs.
        if (remoteProcIDs[r] != -1) {
          remoteProcIDs[numValidRemote] = remoteProcIDs[r];
          remoteGIDsPtr[numValidRemote] = remoteGIDsPtr[r];
          remoteLIDs[numValidRemote]    = remoteLIDs[r];
          ++numValidRemote;
        }
      }
      TEUCHOS_TEST_FOR_EXCEPTION_CLASS_FUNC(numValidRemote != totalNumRemote - numInvalidRemote,
                                            std::logic_error,
                                            "After removing invalid remote GIDs and packing "
                                            "the valid remote GIDs, numValidRemote = "
                                                << numValidRemote
                                                << " != totalNumRemote - numInvalidRemote = "
                                                << totalNumRemote - numInvalidRemote
                                                << ".  Please report this bug to the Tpetra developers.");
 
      remoteProcIDs.resize(numValidRemote);
      remoteGIDs.resize(numValidRemote);
 
      Kokkos::resize(remoteLIDs, numValidRemote);
      this->TransferData_->remoteLIDs_ = decltype(this->TransferData_->remoteLIDs_)();
      makeDualViewFromOwningHostView(this->TransferData_->remoteLIDs_, remoteLIDs);
    }
    // Revalidate the view after clear or resize.
    remoteGIDsView = remoteGIDs();
  }
 
  // Sort remoteProcIDs in ascending order, and apply the resulting
  // permutation to remoteGIDs and remoteLIDs_.  This ensures that
  // remoteProcIDs[i], remoteGIDs[i], and remoteLIDs_[i] all refer
  // to the same thing.
  {
    Tpetra::Details::ProfilingRegion pr("Tpetra:iport_ctor:setupExport:2");
    this->TransferData_->remoteLIDs_.modify_host();
    auto remoteLIDs = this->TransferData_->remoteLIDs_.view_host();
    sort3(remoteProcIDs.begin(),
          remoteProcIDs.end(),
          remoteGIDsView.getRawPtr(),
          remoteLIDs.data());
    this->TransferData_->remoteLIDs_.sync_device();
  }
 
  // Call the Distributor's createFromRecvs() method to turn the
  // remote GIDs and their owning processes into a send-and-receive
  // communication plan.  remoteGIDs and remoteProcIDs_ are input;
  // exportGIDs and exportProcIDs_ are output arrays which are
  // allocated by createFromRecvs().
  Array<GO> exportGIDs;
  {
    Tpetra::Details::ProfilingRegion pr("Tpetra:iport_ctor:setupExport:3");
 
    if (this->verbose()) {
      std::ostringstream os;
      os << *prefix << "Call createFromRecvs" << endl;
      this->verboseOutputStream() << endl;
    }
    this->TransferData_->distributor_.createFromRecvs(remoteGIDsView().getConst(),
                                                      remoteProcIDs, exportGIDs,
                                                      this->TransferData_->exportPIDs_);
  }
  // Find the LIDs corresponding to the (outgoing) GIDs in
  // exportGIDs.  For sparse matrix-vector multiply, this tells the
  // calling process how to index into the source vector to get the
  // elements which it needs to send.
  Tpetra::Details::ProfilingRegion pr("Tpetra:iport_ctor:setupExport:4");
 
  // NOTE (mfh 03 Mar 2014) This is now a candidate for a
  // thread-parallel kernel, but only if using the new thread-safe
  // Map implementation.
  const size_type numExportIDs = exportGIDs.size();
  if (numExportIDs > 0) {
    typename decltype(this->TransferData_->exportLIDs_)::t_host
        exportLIDs(view_alloc_no_init("exportLIDs"), numExportIDs);
    ArrayView<const GO> expGIDs = exportGIDs();
 
    for (size_type k = 0; k < numExportIDs; ++k) {
      exportLIDs[k] = source.getLocalElement(expGIDs[k]);
    }
    makeDualViewFromOwningHostView(this->TransferData_->exportLIDs_, exportLIDs);
  }
 
  if (this->verbose()) {
    std::ostringstream os;
    os << *prefix << "Done!" << endl;
    this->verboseOutputStream() << os.str();
  }
}
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
void Import<LocalOrdinal, GlobalOrdinal, Node>::
    findUnionTargetGIDs(Teuchos::Array<GlobalOrdinal>& unionTgtGIDs,
                        Teuchos::Array<std::pair<int, GlobalOrdinal>>& remotePGIDs,
                        typename Teuchos::Array<GlobalOrdinal>::size_type& numSameGIDs,
                        typename Teuchos::Array<GlobalOrdinal>::size_type& numPermuteGIDs,
                        typename Teuchos::Array<GlobalOrdinal>::size_type& numRemoteGIDs,
                        const Teuchos::ArrayView<const GlobalOrdinal>& sameGIDs1,
                        const Teuchos::ArrayView<const GlobalOrdinal>& sameGIDs2,
                        Teuchos::Array<GlobalOrdinal>& permuteGIDs1,
                        Teuchos::Array<GlobalOrdinal>& permuteGIDs2,
                        Teuchos::Array<GlobalOrdinal>& remoteGIDs1,
                        Teuchos::Array<GlobalOrdinal>& remoteGIDs2,
                        Teuchos::Array<int>& remotePIDs1,
                        Teuchos::Array<int>& remotePIDs2) const {
  typedef GlobalOrdinal GO;
  typedef typename Teuchos::Array<GO>::size_type size_type;
 
  const size_type numSameGIDs1 = sameGIDs1.size();
  const size_type numSameGIDs2 = sameGIDs2.size();
 
  // Sort the permute GIDs
  std::sort(permuteGIDs1.begin(), permuteGIDs1.end());
  std::sort(permuteGIDs2.begin(), permuteGIDs2.end());
 
  // Get the union of the two target maps
  // Reserve the maximum possible size to guard against reallocations from
  // push_back operations.
  unionTgtGIDs.reserve(numSameGIDs1 + numSameGIDs2 +
                       permuteGIDs1.size() + permuteGIDs2.size() +
                       remoteGIDs1.size() + remoteGIDs2.size());
 
  // Copy the same GIDs to unionTgtGIDs.  Cases for numSameGIDs1 !=
  // numSameGIDs2 must be treated separately.
  typename Teuchos::Array<GO>::iterator permuteGIDs1_end;
  typename Teuchos::Array<GO>::iterator permuteGIDs2_end;
  if (numSameGIDs2 > numSameGIDs1) {
    numSameGIDs      = numSameGIDs2;
    permuteGIDs2_end = permuteGIDs2.end();
 
    // Copy the same GIDs from tgtGIDs to the union
    std::copy(sameGIDs2.begin(), sameGIDs2.end(), std::back_inserter(unionTgtGIDs));
 
    // Remove GIDs from permuteGIDs1 that have already been copied in to unionTgtGIDs
    // set_difference allows the last (output) argument to alias the first.
    permuteGIDs1_end = std::set_difference(permuteGIDs1.begin(), permuteGIDs1.end(),
                                           unionTgtGIDs.begin() + numSameGIDs1, unionTgtGIDs.end(),
                                           permuteGIDs1.begin());
 
  } else {
    numSameGIDs      = numSameGIDs1;
    permuteGIDs1_end = permuteGIDs1.end();
 
    // Copy the same GIDs from tgtGIDs to the union
    std::copy(sameGIDs1.begin(), sameGIDs1.end(), std::back_inserter(unionTgtGIDs));
 
    // Remove GIDs from permuteGIDs2 that have already been copied in to unionTgtGIDs
    // set_difference allows the last (output) argument to alias the first.
    permuteGIDs2_end = std::set_difference(permuteGIDs2.begin(), permuteGIDs2.end(),
                                           unionTgtGIDs.begin() + numSameGIDs2, unionTgtGIDs.end(),
                                           permuteGIDs2.begin());
  }
 
  // Get the union of the permute GIDs and push it back on unionTgtGIDs
  std::set_union(permuteGIDs1.begin(), permuteGIDs1_end,
                 permuteGIDs2.begin(), permuteGIDs2_end,
                 std::back_inserter(unionTgtGIDs));
 
  // Sort the PID,GID pairs and find the unique set
  Teuchos::Array<std::pair<int, GO>> remotePGIDs1(remoteGIDs1.size());
  for (size_type k = 0; k < remoteGIDs1.size(); k++)
    remotePGIDs1[k] = std::make_pair(remotePIDs1[k], remoteGIDs1[k]);
  std::sort(remotePGIDs1.begin(), remotePGIDs1.end());
 
  Teuchos::Array<std::pair<int, GO>> remotePGIDs2(remoteGIDs2.size());
  for (size_type k = 0; k < remoteGIDs2.size(); k++)
    remotePGIDs2[k] = std::make_pair(remotePIDs2[k], remoteGIDs2[k]);
  std::sort(remotePGIDs2.begin(), remotePGIDs2.end());
 
  remotePGIDs.reserve(remotePGIDs1.size() + remotePGIDs2.size());
  std::merge(remotePGIDs1.begin(), remotePGIDs1.end(),
             remotePGIDs2.begin(), remotePGIDs2.end(),
             std::back_inserter(remotePGIDs));
  auto it = std::unique(remotePGIDs.begin(), remotePGIDs.end());
  remotePGIDs.resize(std::distance(remotePGIDs.begin(), it));
 
  // Finally, insert remote GIDs
  const size_type oldSize = unionTgtGIDs.size();
  unionTgtGIDs.resize(oldSize + remotePGIDs.size());
  for (size_type start = oldSize, k = 0; k < remotePGIDs.size(); k++)
    unionTgtGIDs[start + k] = remotePGIDs[k].second;
 
  // Compute output only quantities
  numRemoteGIDs  = remotePGIDs.size();
  numPermuteGIDs = unionTgtGIDs.size() - numSameGIDs - numRemoteGIDs;
 
  return;
}
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
Teuchos::RCP<const Import<LocalOrdinal, GlobalOrdinal, Node>>
Import<LocalOrdinal, GlobalOrdinal, Node>::
    setUnion(const Import<LocalOrdinal, GlobalOrdinal, Node>& rhs) const {
  using Teuchos::Array;
  using Teuchos::ArrayView;
  using Teuchos::as;
  using Teuchos::Comm;
  using Teuchos::outArg;
  using Teuchos::RCP;
  using Teuchos::rcp;
  using Teuchos::REDUCE_MIN;
  using Teuchos::reduceAll;
  using ::Tpetra::Details::Behavior;
  using LO          = LocalOrdinal;
  using GO          = GlobalOrdinal;
  using import_type = Import<LO, GO, Node>;
  using size_type   = typename Array<GO>::size_type;
 
  Tpetra::Details::ProfilingRegion pr("Tpetra::Import::setUnion(rhs)");
 
  RCP<const map_type> srcMap  = this->getSourceMap();
  RCP<const map_type> tgtMap1 = this->getTargetMap();
  RCP<const map_type> tgtMap2 = rhs.getTargetMap();
  RCP<const Comm<int>> comm   = srcMap->getComm();
 
  const bool debug = Behavior::debug("Import::setUnion") ||
                     Behavior::debug("Tpetra::Import::setUnion");
 
  if (debug) {
    TEUCHOS_TEST_FOR_EXCEPTION(!srcMap->isSameAs(*(rhs.getSourceMap())), std::invalid_argument,
                               "Tpetra::Import::setUnion: The source Map of the input Import must be the "
                               "same as (in the sense of Map::isSameAs) the source Map of this Import.");
    const Comm<int>& comm1 = *(tgtMap1->getComm());
    const Comm<int>& comm2 = *(tgtMap2->getComm());
    TEUCHOS_TEST_FOR_EXCEPTION(!::Tpetra::Details::congruent(comm1, comm2),
                               std::invalid_argument,
                               "Tpetra::Import::setUnion: "
                               "The target Maps must have congruent communicators.");
  }
 
  // It's probably worth the one all-reduce to check whether the two
  // Maps are the same.  If so, we can just return a copy of *this.
  // isSameAs() bypasses the all-reduce if the pointers are equal.
  if (tgtMap1->isSameAs(*tgtMap2)) {
    return rcp(new import_type(*this));
  }
 
  // Alas, the two target Maps are not the same.  That means we have
  // to compute their union, and the union Import object.
 
  Array<GO> unionTgtGIDs;
  Array<LO> remoteLIDsUnion;
  Array<GO> remoteGIDsUnion;
  Array<int> remotePIDsUnion;
  size_type numSameIDsUnion, numPermuteIDsUnion, numRemoteIDsUnion;
  Array<LO> permuteToLIDsUnion;
  Array<LO> permuteFromLIDsUnion;
  global_size_t unionNumLocalElements;
  global_size_t unionNumGlobalElements;
  std::shared_ptr<Details::CommRequest> req;
  {
    Tpetra::Details::ProfilingRegion prTgtMapLookups("Tpetra::Import::setUnion : Target Map lookups");
 
    // Get the same GIDs (same GIDs are a subview of the first numSame target
    // GIDs)
    const size_type numSameGIDs1  = this->getNumSameIDs();
    ArrayView<const GO> sameGIDs1 = (tgtMap1->getLocalElementList())(0, numSameGIDs1);
 
    const size_type numSameGIDs2  = rhs.getNumSameIDs();
    ArrayView<const GO> sameGIDs2 = (tgtMap2->getLocalElementList())(0, numSameGIDs2);
 
    // Get permute GIDs
    ArrayView<const LO> permuteToLIDs1 = this->getPermuteToLIDs();
    Array<GO> permuteGIDs1(permuteToLIDs1.size());
    for (size_type k = 0; k < permuteGIDs1.size(); k++)
      permuteGIDs1[k] = tgtMap1->getGlobalElement(permuteToLIDs1[k]);
 
    ArrayView<const LO> permuteToLIDs2 = rhs.getPermuteToLIDs();
    Array<GO> permuteGIDs2(permuteToLIDs2.size());
    for (size_type k = 0; k < permuteGIDs2.size(); k++)
      permuteGIDs2[k] = tgtMap2->getGlobalElement(permuteToLIDs2[k]);
 
    // Get remote GIDs
    ArrayView<const LO> remoteLIDs1 = this->getRemoteLIDs();
    Array<GO> remoteGIDs1(remoteLIDs1.size());
    for (size_type k = 0; k < remoteLIDs1.size(); k++)
      remoteGIDs1[k] = this->getTargetMap()->getGlobalElement(remoteLIDs1[k]);
 
    ArrayView<const LO> remoteLIDs2 = rhs.getRemoteLIDs();
    Array<GO> remoteGIDs2(remoteLIDs2.size());
    for (size_type k = 0; k < remoteLIDs2.size(); k++)
      remoteGIDs2[k] = rhs.getTargetMap()->getGlobalElement(remoteLIDs2[k]);
 
    // Get remote PIDs
    Array<int> remotePIDs1;
    Tpetra::Import_Util::getRemotePIDs(*this, remotePIDs1);
 
    Array<int> remotePIDs2;
    Tpetra::Import_Util::getRemotePIDs(rhs, remotePIDs2);
 
    // Get the union of the target GIDs
    Array<std::pair<int, GO>> remotePGIDs;
    findUnionTargetGIDs(unionTgtGIDs, remotePGIDs,
                        numSameIDsUnion, numPermuteIDsUnion, numRemoteIDsUnion,
                        sameGIDs1, sameGIDs2, permuteGIDs1, permuteGIDs2,
                        remoteGIDs1, remoteGIDs2, remotePIDs1, remotePIDs2);
    unionNumLocalElements = static_cast<global_size_t>(unionTgtGIDs.size());
    req                   = Details::iallreduce(unionNumLocalElements,
                                                unionNumGlobalElements, Teuchos::REDUCE_SUM, *comm);
 
    // Extract GIDs and compute LIDS, PIDs for the remotes in the union
    remoteLIDsUnion                     = Array<LO>(numRemoteIDsUnion);
    remoteGIDsUnion                     = Array<GO>(numRemoteIDsUnion);
    remotePIDsUnion                     = Array<int>(numRemoteIDsUnion);
    const size_type unionRemoteIDsStart = numSameIDsUnion + numPermuteIDsUnion;
    for (size_type k = 0; k < numRemoteIDsUnion; ++k) {
      remoteLIDsUnion[k] = unionRemoteIDsStart + k;
      remotePIDsUnion[k] = remotePGIDs[k].first;
      remoteGIDsUnion[k] = remotePGIDs[k].second;
    }
 
    // Compute the permute-to LIDs (in the union target Map).
    // Convert the permute GIDs to permute-from LIDs in the source Map.
    permuteToLIDsUnion   = Array<LO>(numPermuteIDsUnion);
    permuteFromLIDsUnion = Array<LO>(numPermuteIDsUnion);
 
    for (size_type k = 0; k < numPermuteIDsUnion; ++k) {
      size_type idx           = numSameIDsUnion + k;
      permuteToLIDsUnion[k]   = static_cast<LO>(idx);
      permuteFromLIDsUnion[k] = srcMap->getLocalElement(unionTgtGIDs[idx]);
    }
  }
 
  RCP<const map_type> unionTgtMap;
  {
    // Create the union target Map.
    Tpetra::Details::ProfilingRegion prTgtMap("Tpetra::Import::setUnion : Construct Target Map");
    const GO indexBaseUnion = std::min(tgtMap1->getIndexBase(), tgtMap2->getIndexBase());
    req->wait();
    unionTgtMap = rcp(new map_type(unionNumGlobalElements, unionTgtGIDs(), indexBaseUnion, comm));
  }
 
  // Thus far, we have computed the following in the union Import:
  //   - numSameIDs
  //   - numPermuteIDs and permuteFromLIDs
  //   - numRemoteIDs, remoteGIDs, remoteLIDs, and remotePIDs
  //
  // Now it's time to compute the export IDs and initialize the
  // Distributor.
 
  Array<LO> exportLIDsUnion;
  Array<int> exportPIDsUnion;
  Distributor distributor(comm, this->TransferData_->out_);
  {
    Tpetra::Details::ProfilingRegion prExportGIDs("Tpetra::Import::setUnion : Export GIDs");
 
    const size_type numExportIDs1 = this->getNumExportIDs();
    const size_type numExportIDs2 = rhs.getNumExportIDs();
 
    exportLIDsUnion = Array<LO>(numExportIDs1 + numExportIDs2);
    exportPIDsUnion = Array<int>(numExportIDs1 + numExportIDs2);
 
    if (numExportIDs1 + numExportIDs2 > 0) {
      auto exportLIDs1 = this->getExportLIDs();
      auto exportPIDs1 = this->getExportPIDs();
 
      auto exportLIDs2 = rhs.getExportLIDs();
      auto exportPIDs2 = rhs.getExportPIDs();
 
      std::copy(exportLIDs1.begin(), exportLIDs1.end(), exportLIDsUnion.begin());
      std::copy(exportLIDs2.begin(), exportLIDs2.end(), exportLIDsUnion.begin() + numExportIDs1);
 
      std::copy(exportPIDs1.begin(), exportPIDs1.end(), exportPIDsUnion.begin());
      std::copy(exportPIDs2.begin(), exportPIDs2.end(), exportPIDsUnion.begin() + numExportIDs1);
 
      std::vector<LO> idx(numExportIDs1 + numExportIDs2);
      std::iota(idx.begin(), idx.end(), 0);
      std::sort(idx.begin(), idx.end(), [&exportPIDsUnion, &exportLIDsUnion](const LO i1, const LO i2) { return (exportPIDsUnion[i1] == exportPIDsUnion[i2]) ? (exportLIDsUnion[i1] < exportLIDsUnion[i2]) : (exportPIDsUnion[i1] < exportPIDsUnion[i2]); });
 
      Tpetra::SortDetails::apply_permutation(exportLIDsUnion.begin(), exportLIDsUnion.end(), idx);
      Tpetra::SortDetails::apply_permutation(exportPIDsUnion.begin(), exportPIDsUnion.end(), idx);
 
      size_t knew = 0;
      for (size_type k = 1; k < numExportIDs1 + numExportIDs2; ++k) {
        if (!((exportPIDsUnion[knew] == exportPIDsUnion[k]) && (exportLIDsUnion[knew] == exportLIDsUnion[k]))) {
          ++knew;
          exportLIDsUnion[knew] = exportLIDsUnion[k];
          exportPIDsUnion[knew] = exportPIDsUnion[k];
        }
      }
      exportLIDsUnion.resize(knew + 1);
      exportPIDsUnion.resize(knew + 1);
    }
 
    distributor.createFromSendsAndRecvs(exportPIDsUnion, remotePIDsUnion);
  }
 
  RCP<const import_type> unionImport;
  {
    // Create and return the union Import. This uses the "expert" constructor
    Tpetra::Details::ProfilingRegion prImport("Tpetra::Import::setUnion : Construct Import");
    unionImport =
        rcp(new import_type(srcMap, unionTgtMap,
                            as<size_t>(numSameIDsUnion),
                            permuteToLIDsUnion, permuteFromLIDsUnion,
                            remoteLIDsUnion, exportLIDsUnion,
                            exportPIDsUnion, distributor,
                            this->TransferData_->out_));
  }
  return unionImport;
}
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
Teuchos::RCP<const Import<LocalOrdinal, GlobalOrdinal, Node>>
Import<LocalOrdinal, GlobalOrdinal, Node>::
    setUnion() const {
  using Teuchos::Array;
  using Teuchos::ArrayView;
  using Teuchos::as;
  using Teuchos::Comm;
  using Teuchos::outArg;
  using Teuchos::RCP;
  using Teuchos::rcp;
  using Teuchos::REDUCE_MIN;
  using Teuchos::reduceAll;
  typedef LocalOrdinal LO;
  typedef GlobalOrdinal GO;
 
  Tpetra::Details::ProfilingRegion pr("Tpetra::Import::setUnion()");
 
  Teuchos::RCP<const Import<LocalOrdinal, GlobalOrdinal, Node>> unionImport;
  RCP<const map_type> srcMap = this->getSourceMap();
  RCP<const map_type> tgtMap = this->getTargetMap();
  RCP<const Comm<int>> comm  = srcMap->getComm();
 
  ArrayView<const GO> srcGIDs = srcMap->getLocalElementList();
  ArrayView<const GO> tgtGIDs = tgtMap->getLocalElementList();
 
  // All elements in srcMap will be in the "new" target map, so...
  size_t numSameIDsNew   = srcMap->getLocalNumElements();
  size_t numRemoteIDsNew = this->getNumRemoteIDs();
  Array<LO> permuteToLIDsNew, permuteFromLIDsNew;  // empty on purpose
 
  // Grab some old data
  ArrayView<const LO> remoteLIDsOld = this->getRemoteLIDs();
  ArrayView<const LO> exportLIDsOld = this->getExportLIDs();
 
  // Build up the new map (same part)
  Array<GO> GIDs(numSameIDsNew + numRemoteIDsNew);
  for (size_t i = 0; i < numSameIDsNew; i++)
    GIDs[i] = srcGIDs[i];
 
  // Build up the new map (remote part) and remotes list
  Array<LO> remoteLIDsNew(numRemoteIDsNew);
  for (size_t i = 0; i < numRemoteIDsNew; i++) {
    GIDs[numSameIDsNew + i] = tgtGIDs[remoteLIDsOld[i]];
    remoteLIDsNew[i]        = numSameIDsNew + i;
  }
 
  // Build the new target map
  GO GO_INVALID = Teuchos::OrdinalTraits<GO>::invalid();
  RCP<const map_type> targetMapNew =
      rcp(new map_type(GO_INVALID, GIDs, tgtMap->getIndexBase(),
                       tgtMap->getComm()));
 
  // Exports are trivial (since the sourcemap doesn't change)
  Array<int> exportPIDsnew(this->getExportPIDs());
  Array<LO> exportLIDsnew(this->getExportLIDs());
 
  // Copy the Distributor (due to how the Import constructor works)
  Distributor D(this->getDistributor());
 
  // Build the importer using the "expert" constructor
  unionImport = rcp(new Import<LocalOrdinal, GlobalOrdinal, Node>(srcMap,
                                                                  targetMapNew,
                                                                  numSameIDsNew,
                                                                  permuteToLIDsNew,
                                                                  permuteFromLIDsNew,
                                                                  remoteLIDsNew,
                                                                  exportLIDsnew,
                                                                  exportPIDsnew, D));
 
  return unionImport;
}
 
template <class LocalOrdinal, class GlobalOrdinal, class Node>
Teuchos::RCP<const Import<LocalOrdinal, GlobalOrdinal, Node>>
Import<LocalOrdinal, GlobalOrdinal, Node>::
    createRemoteOnlyImport(const Teuchos::RCP<const map_type>& remoteTarget) const {
  using std::endl;
  using Teuchos::outArg;
  using Teuchos::rcp;
  using Teuchos::REDUCE_MIN;
  using Teuchos::reduceAll;
  using ::Tpetra::Details::Behavior;
  using ::Tpetra::Details::gathervPrint;
  using LO          = LocalOrdinal;
  using GO          = GlobalOrdinal;
  using import_type = Import<LocalOrdinal, GlobalOrdinal, Node>;
 
  const char funcPrefix[] = "Tpetra::createRemoteOnlyImport: ";
  int lclSuccess          = 1;
  int gblSuccess          = 1;
  const bool debug        = Behavior::debug();
 
  const size_t NumRemotes = this->getNumRemoteIDs();
  std::unique_ptr<std::string> procPrefix;
  Teuchos::RCP<const Teuchos::Comm<int>> comm;
  if (debug) {
    comm = remoteTarget.is_null() ? Teuchos::null : remoteTarget->getComm();
    std::ostringstream os;
    os << "Proc ";
    if (comm.is_null()) {
      os << "?";
    } else {
      os << comm->getRank();
    }
    os << ": ";
    procPrefix = std::unique_ptr<std::string>(new std::string(os.str()));
  }
 
  if (debug) {
    std::ostringstream lclErr;
    if (remoteTarget.is_null()) {
      lclSuccess = -1;
    } else if (NumRemotes != remoteTarget->getLocalNumElements()) {
      lclSuccess = 0;
      lclErr << *procPrefix << "getNumRemoteIDs() = " << NumRemotes
             << " != remoteTarget->getLocalNumElements() = "
             << remoteTarget->getLocalNumElements() << "." << endl;
    }
 
    if (comm.is_null()) {
      lclSuccess = gblSuccess;
    } else {
      reduceAll(*comm, REDUCE_MIN, lclSuccess, outArg(gblSuccess));
    }
    TEUCHOS_TEST_FOR_EXCEPTION(gblSuccess == -1, std::invalid_argument, funcPrefix << "Input target Map is null on at least one process.");
 
    if (gblSuccess != 1) {
      if (comm.is_null()) {
        TEUCHOS_TEST_FOR_EXCEPTION(true, std::runtime_error, lclErr.str());
      } else {
        std::ostringstream gblErr;
        gblErr << funcPrefix << endl;
        gathervPrint(gblErr, lclErr.str(), *comm);
        TEUCHOS_TEST_FOR_EXCEPTION(true, std::runtime_error, gblErr.str());
      }
    }
  }
 
  // Compute the new Remote LIDs
  Teuchos::ArrayView<const LO> oldRemoteLIDs = this->getRemoteLIDs();
  Teuchos::Array<LO> newRemoteLIDs(NumRemotes);
  const map_type& tgtMap = *(this->getTargetMap());
  size_t badCount        = 0;
 
  std::unique_ptr<std::vector<size_t>> badIndices;
  if (debug) {
    badIndices = std::unique_ptr<std::vector<size_t>>(new std::vector<size_t>);
  }
 
  for (size_t i = 0; i < NumRemotes; ++i) {
    const LO oldLclInd = oldRemoteLIDs[i];
    if (oldLclInd == Teuchos::OrdinalTraits<LO>::invalid()) {
      ++badCount;
      if (debug) {
        badIndices->push_back(i);
      }
      continue;
    }
    const GO gblInd = tgtMap.getGlobalElement(oldLclInd);
    if (gblInd == Teuchos::OrdinalTraits<GO>::invalid()) {
      ++badCount;
      if (debug) {
        badIndices->push_back(i);
      }
      continue;
    }
    const LO newLclInd = remoteTarget->getLocalElement(gblInd);
    if (newLclInd == Teuchos::OrdinalTraits<LO>::invalid()) {
      ++badCount;
      if (debug) {
        badIndices->push_back(i);
      }
      continue;
    }
    newRemoteLIDs[i] = newLclInd;
    // Now we make sure these guys are in sorted order (AztecOO-ML ordering)
    if (i > 0 && newRemoteLIDs[i] < newRemoteLIDs[i - 1]) {
      ++badCount;
      if (debug) {
        badIndices->push_back(i);
      }
    }
  }
 
  if (badCount != 0) {
    lclSuccess = 0;
  }
 
  if (debug) {
    if (comm.is_null()) {
      lclSuccess = gblSuccess;
    } else {
      reduceAll(*comm, REDUCE_MIN, lclSuccess, outArg(gblSuccess));
    }
    std::ostringstream lclErr;
    if (lclSuccess != 1) {
      lclErr << *procPrefix << "Count of bad indices: " << badCount
             << ", bad indices: [";
      // TODO (mfh 04 Sep 2019) Limit the maximum number of bad
      // indices to print.
      for (size_t k = 0; k < badCount; ++k) {
        const size_t badIndex = (*badIndices)[k];
        lclErr << "(" << badIndex << ","
               << oldRemoteLIDs[badIndex] << ")";
        if (k + size_t(1) < badCount) {
          lclErr << ", ";
        }
      }
      lclErr << "]" << endl;
    }
 
    if (gblSuccess != 1) {
      std::ostringstream gblErr;
      gblErr << funcPrefix << "this->getRemoteLIDs() has \"bad\" "
                              "indices on one or more processes.  \"Bad\" means that the "
                              "indices are invalid, they don't exist in the target Map, "
                              "they don't exist in remoteTarget, or they are not in "
                              "sorted order.  In what follows, I will show the \"bad\" "
                              "indices as (k, LID) pairs, where k is the zero-based "
                              "index of the LID in this->getRemoteLIDs()."
             << endl;
      if (comm.is_null()) {
        gblErr << lclErr.str();
      } else {
        gathervPrint(gblErr, lclErr.str(), *comm);
      }
      TEUCHOS_TEST_FOR_EXCEPTION(true, std::runtime_error, gblErr.str());
    }
  } else {  // not debug
    TEUCHOS_TEST_FOR_EXCEPTION(lclSuccess == 0, std::runtime_error, funcPrefix << "this->getRemoteLIDs() has " << badCount << "ind" << (badCount == 1 ? "ex" : "ices") << " \"bad\" indices on this process." << endl);
  }
 
  // Copy ExportPIDs and such
  // NOTE: Be careful: The "Expert" Import constructor we use does a "swap"
  // for most of the LID/PID lists and the Distributor, meaning it
  // ruins the existing object if we pass things in directly.  Hence
  // we copy them first.
  Teuchos::Array<int> newExportPIDs(this->getExportPIDs());
  Teuchos::Array<LO> newExportLIDs(this->getExportLIDs());
  Teuchos::Array<LO> dummy;
  Distributor newDistor(this->getDistributor());
 
  return rcp(new import_type(this->getSourceMap(), remoteTarget,
                             static_cast<size_t>(0), dummy, dummy,
                             newRemoteLIDs, newExportLIDs,
                             newExportPIDs, newDistor));
}
 
}  // namespace Tpetra
 
#define TPETRA_IMPORT_CLASS_INSTANT(LO, GO, NODE) \
  template class Import<LO, GO, NODE>;
 
// Explicit instantiation macro.
// Only invoke this when in the Tpetra namespace.
// Most users do not need to use this.
//
// LO: The local ordinal type.
// GO: The global ordinal type.
// NODE: The Kokkos Node type.
#define TPETRA_IMPORT_INSTANT(LO, GO, NODE) \
  TPETRA_IMPORT_CLASS_INSTANT(LO, GO, NODE)
 
#endif  // TPETRA_IMPORT_DEF_HPP