Schur.cpp

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/**
 * @file Schur.cpp
 * @brief Implementation of Schur Complement
 * @date 2023-02-03
 *
 * @copyright Copyright (c) 2023
 *
 */
 
namespace MoFEM {
 
constexpr bool debug_schur = false;
 
struct OpSchurAssembleBaseImpl : public OpSchurAssembleBase {
  OpSchurAssembleBaseImpl() = delete;
 
protected:
  using OpSchurAssembleBase::OpSchurAssembleBase;
 
  /**
   * @brief Assemble Schur complement
   *
   */
  inline MoFEMErrorCode assembleSchurMat(
 
      Mat S,
 
      const UId &uid_row, VectorInt &row_ind,
 
      const UId &uid_col, VectorInt &col_ind,
 
      MatrixDouble &m, InsertMode iora
 
  );
};
 
// Assemble specialisations
MoFEMErrorCode OpSchurAssembleBaseImpl::assembleSchurMat(
 
    Mat S,
 
    const UId &uid_row, VectorInt &row_ind,
 
    const UId &uid_col, VectorInt &col_ind,
 
    MatrixDouble &m, InsertMode iora
 
) {
  return ::MatSetValues(
 
      S, row_ind.size(), &*row_ind.begin(), col_ind.size(), &*col_ind.begin(),
      &*m.data().begin(), iora
 
  );
}
 
/**
 * @brief Clear Schur complement internal data
 *
 */
struct OpSchurAssembleBegin : public OpSchurAssembleBaseImpl {
 
  OpSchurAssembleBegin();
 
  MoFEMErrorCode doWork(int side, EntityType type,
                        EntitiesFieldData::EntData &data);
};
 
struct OpSchurZeroRowsAndCols : public OpSchurAssembleBaseImpl {
 
  OpSchurZeroRowsAndCols(
      boost::shared_ptr<std::vector<unsigned char>> marker_ptr,
      double diag_val);
 
  MoFEMErrorCode doWork(int side, EntityType type,
                        EntitiesFieldData::EntData &data);
 
private:
  boost::shared_ptr<std::vector<unsigned char>> markerPtr;
  double diagVal;
};
 
/**
 * @brief Assemble Schur complement (Implementation)
 *
 */
template <typename OP_SCHUR_ASSEMBLE_BASE>
struct OpSchurAssembleEndImpl : public OP_SCHUR_ASSEMBLE_BASE {
 
  using OP = OP_SCHUR_ASSEMBLE_BASE;
 
  /**
   * @brief Construct a new Op Schur Assemble End object
   *
   * @param fields_name list of fields
   * @param field_ents list of entities on which schur complement is applied
   * (can be empty)
   * @param schur_ao map indices to schur matrix indices
   * @param schur_mat schur matrix
   * @param sym_schur true if schur is symmetric
   * @param symm_op true if block diagonal is symmetric
   */
  OpSchurAssembleEndImpl(
 
      std::vector<std::string> fields_name,
      std::vector<boost::shared_ptr<Range>> field_ents,
 
      SmartPetscObj<AO> schur_ao, SmartPetscObj<Mat> schur_mat,
 
      bool sym_schur, bool symm_op);
 
protected:
  template <typename I>
  MoFEMErrorCode doWorkImpl(
 
      int side, EntityType type, EntitiesFieldData::EntData &data
 
  );
 
  std::vector<std::string> fieldsName;
  std::vector<boost::shared_ptr<Range>> fieldEnts;
  SmartPetscObj<AO> schurAO;
  SmartPetscObj<Mat> schurMat;
  bool symSchur;
 
  MatrixDouble blockMat;
  MatrixDouble invMat;
  MatrixDouble bM, abM, abcM;
};
 
struct SchurDSYSV; ///< SY  symmetric
struct SchurDGESV; ///< GE  general (i.e., nonsymmetric, in some cases
                   ///< rectangular)
 
/**
 * @brief Assemble Schur complement
 *
 */
template <typename I> struct OpSchurAssembleEnd;
 
template <>
struct OpSchurAssembleEnd<SchurDSYSV>
    : public OpSchurAssembleEndImpl<OpSchurAssembleBaseImpl> {
  using OpSchurAssembleEndImpl<OpSchurAssembleBaseImpl>::OpSchurAssembleEndImpl;
  MoFEMErrorCode doWork(int side, EntityType type,
                        EntitiesFieldData::EntData &data);
};
 
template <>
struct OpSchurAssembleEnd<SchurDGESV>
    : public OpSchurAssembleEndImpl<OpSchurAssembleBaseImpl> {
  using OpSchurAssembleEndImpl<OpSchurAssembleBaseImpl>::OpSchurAssembleEndImpl;
  MoFEMErrorCode doWork(int side, EntityType type,
                        EntitiesFieldData::EntData &data);
};
 
/**
 * @brief Schur complement data storage
 *
 */
struct SchurElemMats : public boost::enable_shared_from_this<SchurElemMats> {
 
  SchurElemMats(const size_t idx, const UId uid_row, const UId uid_col);
  virtual ~SchurElemMats() = default;
 
  const size_t iDX;
  UId uidRow;
  UId uidCol;
 
  inline auto &getMat() const { return locMats[iDX]; }
  inline auto &getRowInd() const { return rowIndices[iDX]; }
  inline auto &getColInd() const { return colIndices[iDX]; }
 
  static MoFEMErrorCode MatSetValues(Mat M,
                                     const EntitiesFieldData::EntData &row_data,
                                     const EntitiesFieldData::EntData &col_data,
                                     const MatrixDouble &mat, InsertMode iora);
 
protected:
  static MoFEMErrorCode
  assembleStorage(const EntitiesFieldData::EntData &row_data,
                  const EntitiesFieldData::EntData &col_data,
                  const MatrixDouble &mat, InsertMode iora);
 
  friend struct OpSchurAssembleBegin;
  friend struct OpSchurZeroRowsAndCols;
  template <typename OP_SCHUR_ASSEMBLE_BASE>
  friend struct OpSchurAssembleEndImpl;
 
  struct uid_mi_tag {};
  struct col_mi_tag {};
 
  using SchurElemStorage = multi_index_container<
      const SchurElemMats *,
      indexed_by<
 
          ordered_unique<
              tag<uid_mi_tag>,
              composite_key<
                  SchurElemMats,
 
                  member<SchurElemMats, const UId, &SchurElemMats::uidRow>,
                  member<SchurElemMats, const UId, &SchurElemMats::uidCol>
 
                  >>,
 
          ordered_non_unique<tag<col_mi_tag>, member<SchurElemMats, const UId,
                                                     &SchurElemMats::uidCol>>
 
          >>;
 
  static boost::ptr_vector<MatrixDouble> locMats;
  static boost::ptr_vector<VectorInt> rowIndices;
  static boost::ptr_vector<VectorInt> colIndices;
  static boost::ptr_vector<SchurElemMats> schurElemMats;
  static size_t maxIndexCounter;
 
  static SchurElemStorage schurL2Storage;
};
 
struct DiagBlockIndex {
 
  virtual ~DiagBlockIndex() = default;
 
  /**
   * @brief block data indexes
   *
   */
  struct Indexes {
 
    Indexes(UId uid_row, UId uid_col, UId uid_fe, int row, int col, int nb_rows,
            int nb_cols, int loc_row, int loc_col, int mat_shift)
        : uid_row(uid_row), uid_col(uid_col), uid_fe(uid_fe), row(row),
          col(col), nb_rows(nb_rows), nb_cols(nb_cols), loc_row(loc_row),
          loc_col(loc_col), mat_shift(mat_shift) {}
 
    inline UId getRowUId() const { return uid_row; }
    inline UId getColUId() const { return uid_col; }
    inline UId getFEUId() const { return uid_fe; }
    inline int getRow() const { return row; }
    inline int getCol() const { return col; }
    inline int getNbRows() const { return nb_rows; }
    inline int getNbCols() const { return nb_cols; }
    inline int getLocRow() const { return loc_row; }
    inline int getLocCol() const { return loc_col; }
    inline int &getMatShift() const { return mat_shift; }
 
    inline int rowShift() const {
      return getRow() + getNbRows();
    } // shift such that lower bound is included
 
    inline int colShift() const {
      return getCol() + getNbCols();
    } // shift such that lower bound is included
 
  private:
    UId uid_row;
    UId uid_col;
    UId uid_fe;
    int row;
    int col;
    int nb_rows;
    int nb_cols;
    int loc_row;
    int loc_col;
    mutable int mat_shift;
  };
 
  using BlockIndex = multi_index_container<
 
      Indexes,
 
      indexed_by<
 
          ordered_unique<
 
              composite_key<Indexes,
 
                            const_mem_fun<Indexes, UId, &Indexes::getRowUId>,
                            const_mem_fun<Indexes, UId, &Indexes::getColUId>>
 
              >,
 
          ordered_non_unique<
 
              const_mem_fun<Indexes, UId, &Indexes::getFEUId>
 
              >,
 
          ordered_non_unique<
 
              const_mem_fun<Indexes, UId, &Indexes::getRowUId>
 
              >,
 
          ordered_non_unique<
 
              const_mem_fun<Indexes, UId, &Indexes::getColUId>
 
              >>>;
 
  BlockIndex blockIndex; ///< blocks indexes storage
};
 
struct BlockStructure : public DiagBlockIndex {
 
  SmartPetscObj<Vec> ghostX;
  SmartPetscObj<Vec> ghostY;
 
  boost::shared_ptr<std::vector<double>> dataBlocksPtr;
  boost::shared_ptr<std::vector<double>> preconditionerBlocksPtr;
  boost::shared_ptr<std::vector<double>> parentBlockStructurePtr;
 
  bool multiplyByPreconditioner = false;
};
 
PetscLogEvent SchurEvents::MOFEM_EVENT_schurMatSetValues;
PetscLogEvent SchurEvents::MOFEM_EVENT_opSchurAssembleEnd;
PetscLogEvent SchurEvents::MOFEM_EVENT_BlockStructureSetValues;
PetscLogEvent SchurEvents::MOFEM_EVENT_BlockStructureMult;
PetscLogEvent SchurEvents::MOFEM_EVENT_BlockStructureSolve;
PetscLogEvent SchurEvents::MOFEM_EVENT_zeroRowsAndCols;
PetscLogEvent SchurEvents::MOFEM_EVENT_AssembleSchurMat;
 
SchurEvents::SchurEvents() {
  PetscLogEventRegister("schurMatSetVal", 0, &MOFEM_EVENT_schurMatSetValues);
  PetscLogEventRegister("opSchurAsmEnd", 0, &MOFEM_EVENT_opSchurAssembleEnd);
  PetscLogEventRegister("blockSetVal", 0, &MOFEM_EVENT_BlockStructureSetValues);
  PetscLogEventRegister("blockMult", 0, &MOFEM_EVENT_BlockStructureMult);
  PetscLogEventRegister("blockSolve", 0, &MOFEM_EVENT_BlockStructureSolve);
  PetscLogEventRegister("schurZeroRandC", 0, &MOFEM_EVENT_zeroRowsAndCols);
  PetscLogEventRegister("assembleSchurMat", 0, &MOFEM_EVENT_AssembleSchurMat);
}
 
SchurElemMats::SchurElemStorage SchurElemMats::schurL2Storage;
boost::ptr_vector<MatrixDouble> SchurElemMats::locMats;
boost::ptr_vector<VectorInt> SchurElemMats::rowIndices;
boost::ptr_vector<VectorInt> SchurElemMats::colIndices;
boost::ptr_vector<SchurElemMats> SchurElemMats::schurElemMats;
size_t SchurElemMats::maxIndexCounter = 0;
 
SchurElemMats::SchurElemMats(const size_t idx, const UId uid_row,
                             const UId uid_col)
    : iDX(idx), uidRow(uid_row), uidCol(uid_col) {}
 
MoFEMErrorCode
SchurElemMats::assembleStorage(const EntitiesFieldData::EntData &row_data,
                               const EntitiesFieldData::EntData &col_data,
                               const MatrixDouble &mat, InsertMode iora) {
  MoFEMFunctionBegin;
 
#ifndef NDEBUG
  PetscLogEventBegin(SchurEvents::MOFEM_EVENT_schurMatSetValues, 0, 0, 0, 0);
#endif // NDEBUG
 
  // get row indices, in case of store, get indices from storage
  // storage keeps marked indices to manage boundary conditions
  auto get_row_indices = [&]() -> const VectorInt & {
    if (auto e_ptr = row_data.getFieldEntities()[0]) {
      if (auto stored_data_ptr =
              e_ptr->getSharedStoragePtr<EssentialBcStorage>()) {
        return stored_data_ptr->entityIndices;
      }
    }
    return row_data.getIndices();
  };
 
  const auto &row_ind = get_row_indices();
  const auto &col_ind = col_data.getIndices();
 
  const auto nb_rows = row_ind.size();
  const auto nb_cols = col_ind.size();
 
#ifndef NDEBUG
  if (mat.size1() != nb_rows) {
    SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
             "Wrong mat size %ld != %ld", mat.size1(), nb_rows);
  }
  if (mat.size2() != nb_cols) {
    SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
             "Wrong mat size %ld != %ld", mat.size2(), nb_cols);
  }
#endif // NDEBUG
 
  // get entity uid
  auto get_uid = [](auto &data) {
    if (data.getFieldEntities().size() == 1) {
 
      return data.getFieldEntities()[0]->getLocalUniqueId();
 
    } else {
 
      // Is assumed that sum of entities ids gives unique id, that is not true,
      // but corner case is improbable.
 
      // @todo: debug version should have test
 
      auto &uid0 = data.getFieldEntities()[0]->getLocalUniqueId();
      auto field_id0 = FieldEntity::getFieldBitNumberFromUniqueId(uid0);
      auto ent0 = FieldEntity::getHandleFromUniqueId(uid0);
      auto type0 = type_from_handle(ent0);
      auto id = id_from_handle(ent0);
 
      for (auto i = 1; i < data.getFieldEntities().size(); ++i) {
 
        // get entity id from ent
        id += id_from_handle(
 
            // get entity handle from unique uid
            FieldEntity::getHandleFromUniqueId(
                data.getFieldEntities()[i]->getLocalUniqueId())
 
        );
      }
 
      return FieldEntity::getLocalUniqueIdCalculate(
          field_id0,
 
          ent_form_type_and_id(type0, id)
 
      );
    }
  };
 
  auto uid_row = get_uid(row_data);
  auto uid_col = get_uid(col_data);
 
  auto it =
      SchurElemMats::schurL2Storage.template get<SchurElemMats::uid_mi_tag>()
          .find(boost::make_tuple(uid_row, uid_col));
 
  if (it ==
      SchurElemMats::schurL2Storage.template get<SchurElemMats::uid_mi_tag>()
          .end()) {
 
    // get size of arrays of matrices
    const auto size = SchurElemMats::locMats.size();
 
    // expand memory allocation
    if (SchurElemMats::maxIndexCounter == size) {
      SchurElemMats::locMats.push_back(new MatrixDouble());
      SchurElemMats::rowIndices.push_back(new VectorInt());
      SchurElemMats::colIndices.push_back(new VectorInt());
      SchurElemMats::schurElemMats.push_back(
          new SchurElemMats(SchurElemMats::maxIndexCounter, uid_row, uid_col));
    } else {
      SchurElemMats::schurElemMats[SchurElemMats::maxIndexCounter].uidRow =
          uid_row;
      SchurElemMats::schurElemMats[SchurElemMats::maxIndexCounter].uidCol =
          uid_col;
    }
 
    // add matrix to storage
    auto p = SchurElemMats::schurL2Storage.emplace(
        &SchurElemMats::schurElemMats[SchurElemMats::maxIndexCounter++]);
#ifndef NDEBUG
    if (!p.second) {
      SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY, "Failed to insert");
    }
#endif // NDEBUG
 
    auto asmb = [&](auto &sm) {
      sm.resize(nb_rows, nb_cols, false);
      noalias(sm) = mat;
    };
 
    asmb((*p.first)->getMat());
 
    auto add_indices = [](auto &storage, auto &ind) {
      storage.resize(ind.size(), false);
      noalias(storage) = ind;
    };
 
    add_indices((*p.first)->getRowInd(), row_ind);
    add_indices((*p.first)->getColInd(), col_ind);
 
  } else {
    // entry (submatrix) already exists
 
    auto asmb = [&](auto &sm) {
      MoFEMFunctionBeginHot;
 
#ifndef NDEBUG
      if (sm.size1() != nb_rows) {
        SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
                "Wrong mat or storage size %ld != %ld", sm.size1(), nb_rows);
      }
      if (sm.size2() != nb_cols) {
        SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
                "Wrong mat or storage size %ld != %ld", sm.size2(), nb_cols);
      }
#endif // NDEBUG
 
      switch (iora) {
      case ADD_VALUES:
        sm += mat;
        break;
      case INSERT_VALUES:
        noalias(sm) = mat;
        break;
      default:
        SETERRQ(PETSC_COMM_SELF, MOFEM_NOT_IMPLEMENTED,
                "Assembly type not implemented");
      }
      MoFEMFunctionReturnHot(0);
    };
 
    CHKERR asmb((*it)->getMat());
 
    // no need to set indices
  }
 
#ifndef NDEBUG
  PetscLogEventEnd(SchurEvents::MOFEM_EVENT_schurMatSetValues, 0, 0, 0, 0);
#endif // NDEBUG
 
  MoFEMFunctionReturn(0);
}
 
OpSchurAssembleBegin::OpSchurAssembleBegin()
    : OpSchurAssembleBaseImpl(NOSPACE, OPSPACE) {}
 
MoFEMErrorCode OpSchurAssembleBegin::doWork(int side, EntityType type,
                                            EntitiesFieldData::EntData &data) {
  MoFEMFunctionBegin;
#ifndef NDEBUG
  if constexpr (debug_schur)
    MOFEM_LOG("SELF", Sev::noisy) << "Schur assemble begin";
#endif
  SchurElemMats::schurL2Storage.clear();
  SchurElemMats::maxIndexCounter = 0;
 
  MoFEMFunctionReturn(0);
}
 
OpSchurZeroRowsAndCols::OpSchurZeroRowsAndCols(
    boost::shared_ptr<std::vector<unsigned char>> marker_ptr, double diag_val)
    : OpSchurAssembleBaseImpl(NOSPACE, OPSPACE), markerPtr(marker_ptr),
      diagVal(diag_val) {}
 
MoFEMErrorCode
OpSchurZeroRowsAndCols::doWork(int side, EntityType type,
                               EntitiesFieldData::EntData &data) {
  MoFEMFunctionBegin;
 
  auto fe_ptr = getFEMethod();
  auto prb_ptr = fe_ptr->problemPtr;
  auto &dof = prb_ptr->getNumeredRowDofsPtr()->get<PetscGlobalIdx_mi_tag>();
 
  auto find_loc_dof_index = [&](auto glob) {
    auto it = dof.find(glob);
    if (it != dof.end()) {
      return (*it)->getPetscLocalDofIdx();
    }
    MOFEM_LOG("SELF", Sev::error) << "Global dof " << glob;
    CHK_THROW_MESSAGE(
        MOFEM_DATA_INCONSISTENCY,
        "Global dof index not found in local numered dofs storage");
    return -1;
  };
 
  for (auto &s : SchurElemMats::schurL2Storage) {
 
    auto zero_ind = [&](auto &ind) {
      for (auto i = 0; i < ind.size(); ++i) {
        if (ind[i] >= 0) {
          if ((*markerPtr)[find_loc_dof_index(ind[i])]) {
            ind[i] = -1;
          }
        }
      }
    };
 
    zero_ind(s->getRowInd());
    zero_ind(s->getColInd());
 
  }
 
  MoFEMFunctionReturn(0);
};
 
template <typename OP_SCHUR_ASSEMBLE_BASE>
OpSchurAssembleEndImpl<OP_SCHUR_ASSEMBLE_BASE>::OpSchurAssembleEndImpl(
    std::vector<std::string> fields_name,
    std::vector<boost::shared_ptr<Range>> field_ents,
    SmartPetscObj<AO> schur_ao, SmartPetscObj<Mat> schur_mat, bool sym_schur,
    bool symm_op)
    : OP(NOSPACE, OP::OPSPACE, symm_op), fieldsName(fields_name),
      fieldEnts(field_ents), schurAO(schur_ao), schurMat(schur_mat),
      symSchur(sym_schur) {}
 
template <typename OP_SCHUR_ASSEMBLE_BASE>
template <typename I>
MoFEMErrorCode OpSchurAssembleEndImpl<OP_SCHUR_ASSEMBLE_BASE>::doWorkImpl(
    int side, EntityType type, EntitiesFieldData::EntData &data) {
 
  MoFEMFunctionBegin;
 
#ifndef NDEBUG
  PetscLogEventBegin(SchurEvents::MOFEM_EVENT_opSchurAssembleEnd, 0, 0, 0, 0);
#endif
 
#ifndef NDEBUG
  if constexpr (debug_schur)
    MOFEM_LOG("SELF", Sev::noisy) << "Schur assemble begin -> end";
#endif
 
  auto get_a00_uids = [&]() {
    auto get_field_bit = [&](auto &name) {
      return OP::getPtrFE()->mField.get_field_bit_number(name);
    };
 
    std::vector<std::pair<UId, UId>> a00_uids;
    a00_uids.reserve(fieldsName.size());
    for (auto ss = 0; ss != fieldsName.size(); ++ss) {
      auto field_bit = get_field_bit(fieldsName[ss]);
      auto row_ents = fieldEnts[ss];
      if (row_ents) {
        for (auto p = row_ents->pair_begin(); p != row_ents->pair_end(); ++p) {
          auto lo_uid =
              FieldEntity::getLoLocalEntityBitNumber(field_bit, p->first);
          auto hi_uid =
              FieldEntity::getHiLocalEntityBitNumber(field_bit, p->second);
          a00_uids.push_back(std::make_pair(lo_uid, hi_uid));
        }
      } else {
        auto lo_uid = FieldEntity::getLoLocalEntityBitNumber(
            field_bit, get_id_for_min_type<MBVERTEX>());
        auto hi_uid = FieldEntity::getHiLocalEntityBitNumber(
            field_bit, get_id_for_max_type<MBENTITYSET>());
        a00_uids.push_back(std::make_pair(lo_uid, hi_uid));
      }
    }
    return a00_uids;
  };
 
#ifndef NDEBUG
  auto get_field_name = [&](auto uid) {
    return OP::getPtrFE()->mField.get_field_name(field_bit_from_bit_number(
        FieldEntity::getFieldBitNumberFromUniqueId(uid)));
  };
  auto list_storage = [&](auto &storage) {
    MoFEMFunctionBegin;
    int i = 0;
    for (auto &p : storage) {
      MOFEM_LOG("SELF", Sev::noisy)
          << "List schur storage: " << i << " " << p->iDX << ": "
          << get_field_name(p->uidRow) << " " << get_field_name(p->uidCol)
          << " : " << p->getMat().size1() << " " << p->getMat().size2();
      ++i;
    }
    MoFEMFunctionReturn(0);
  };
#endif // NDEBUG
 
  auto assemble_dense_blocks = [&]() {
    using matrix_range = ublas::matrix_range<MatrixDouble>;
    using range = ublas::range;
    MoFEMFunctionBegin;
    auto &storage = SchurElemMats::schurL2Storage;
 
    auto assemble_schur = [this](auto &m, auto &uid_row, auto &uid_col,
                                 auto *row_ind_ptr, auto *col_ind_ptr) {
      MoFEMFunctionBegin;
 
      if (schurMat) {
 
        if (auto ierr = this->assembleSchurMat(
 
                schurMat, uid_row, *row_ind_ptr, uid_col, *col_ind_ptr, m,
                ADD_VALUES
 
                )) {
#ifndef NDEBUG
          auto field_ents = OP::getPtrFE()->mField.get_field_ents();
          auto row_ent_it = field_ents->find(uid_row);
          auto col_ent_it = field_ents->find(uid_col);
          MOFEM_LOG_CHANNEL("SELF");
          if (row_ent_it != field_ents->end())
            MOFEM_LOG("SELF", Sev::error)
                << "Assemble row entity: " << (*row_ent_it)->getName() << " "
                << (*col_ent_it)->getEntTypeName() << " side "
                << (*row_ent_it)->getSideNumber();
          if (col_ent_it != field_ents->end())
            MOFEM_LOG("SELF", Sev::error)
                << "Assemble col entity: " << (*col_ent_it)->getName() << " "
                << (*col_ent_it)->getEntTypeName() << " side "
                << (*col_ent_it)->getSideNumber();
#endif // NDEBUG
          CHK_THROW_MESSAGE(ierr, "MatSetValues");
        }
      }
 
      MoFEMFunctionReturn(0);
    };
 
    auto a00_uids = get_a00_uids();
 
    auto get_block_indexing = [&](auto &a00_uids) {
      // iterate over a00 uids and find blocks
      std::vector<const SchurElemMats *> block_list;
      block_list.reserve(storage.size());
      for (auto &rp_uid : a00_uids) {
        auto [rlo_uid, rhi_uid] = rp_uid;
        for (auto &cp_uid : a00_uids) {
          auto [clo_uid, chi_uid] = cp_uid;
 
          auto it =
              storage.template get<SchurElemMats::uid_mi_tag>().lower_bound(
                  boost::make_tuple(rlo_uid, clo_uid));
          auto hi_it =
              storage.template get<SchurElemMats::uid_mi_tag>().upper_bound(
                  boost::make_tuple(rhi_uid, chi_uid));
 
          for (; it != hi_it; ++it) {
            if ((*it)->uidRow >= rlo_uid && (*it)->uidRow < rhi_uid &&
                (*it)->uidCol >= clo_uid && (*it)->uidCol < chi_uid) {
              block_list.push_back(*it);
            }
          }
        }
      }
 
      // create block indexes map for blockMat
      std::map<UId, std::pair<size_t, const VectorInt *>>
          block_indexing; // uid block map
      for (auto d : block_list) {
        if (block_indexing.find(d->uidRow) == block_indexing.end()) {
          block_indexing[d->uidRow] =
              std::make_pair(d->getRowInd().size(), &(d->getRowInd()));
        }
      }
 
      // set indexes to block
      int mat_block_size = 0; // size of block matrix
      for (auto &p_uid : a00_uids) {
        auto [lo_uid, hi_uid] = p_uid;
        auto lo = block_indexing.lower_bound(lo_uid);
        auto up = block_indexing.upper_bound(hi_uid);
        for (; lo != up; ++lo) {
          lo->second.first = mat_block_size;
          mat_block_size += lo->second.second->size();
        }
      }
 
      return std::make_tuple(block_list, block_indexing, mat_block_size);
    };
 
    auto get_schur_block_list = [&](auto &block_indexing) {
      std::vector<const SchurElemMats *> block_list;
      block_list.reserve(storage.size());
      for (auto &s : storage) {
        if (block_indexing.find(s->uidRow) == block_indexing.end() &&
            block_indexing.find(s->uidCol) == block_indexing.end()) {
          block_list.push_back(s);
        }
      }
      return block_list;
    };
 
    auto bi = get_block_indexing(a00_uids);
    auto &[block_list, block_indexing, block_mat_size] = bi;
 
    if (block_mat_size == 0) {
 
      if (schurAO) {
        for (auto &s : storage) {
          CHKERR AOApplicationToPetsc(schurAO, s->getRowInd().size(),
                                      &*s->getRowInd().begin());
          CHKERR AOApplicationToPetsc(schurAO, s->getColInd().size(),
                                      &*s->getColInd().begin());
        }
      }
 
      for (auto &s : storage) {
        auto &m = s->getMat();
        CHKERR assemble_schur(m, s->uidRow, s->uidCol, &(s->getRowInd()),
                              &(s->getColInd()));
      }
      MoFEMFunctionReturnHot(0);
    }
 
    blockMat.resize(block_mat_size, block_mat_size, false);
    blockMat.clear();
 
    auto get_range = [](auto &bi) {
      return range(bi.first, bi.first + bi.second->size());
    };
 
    for (auto &s : block_list) {
      auto &m = s->getMat();
#ifndef NDEBUG
      if (block_indexing.find(s->uidRow) == block_indexing.end())
        SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY, "Wrong rlo_uid");
      if (block_indexing.find(s->uidCol) == block_indexing.end())
        SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY, "Wrong clo_uid");
#endif // NDEBUG
 
      auto &rbi = block_indexing.at(s->uidRow);
      auto &cbi = block_indexing.at(s->uidCol);
 
      auto sub_mat = matrix_range(blockMat, get_range(rbi), get_range(cbi));
      sub_mat = m;
    }
 
    auto get_zeroed_indices = [&](auto extractor_uid, auto extractor_ind) {
      auto &[block_list, block_indexing, block_mat_size] = bi;
      std::vector<int> zeroed_indices;
      zeroed_indices.reserve(block_mat_size);
      for (auto &s : block_list) {
        auto &bi = block_indexing.at(extractor_uid(s));
        auto &ind = extractor_ind(s);
        for (auto it = ind.begin(); it != ind.end(); ++it) {
          if (*it < 0) {
            auto idx = bi.first + std::distance(ind.begin(), it);
            zeroed_indices.push_back(idx);
          }
        }
      }
      std::sort(zeroed_indices.begin(), zeroed_indices.end());
      auto it = std::unique(zeroed_indices.begin(), zeroed_indices.end());
      zeroed_indices.resize(std::distance(zeroed_indices.begin(), it));
      return zeroed_indices;
    };
    auto zero_rows = get_zeroed_indices(
        [](auto &s) { return s->uidRow; },
        [](auto &s) -> VectorInt & { return s->getRowInd(); });
    auto zero_cols = get_zeroed_indices(
        [](auto &s) { return s->uidCol; },
        [](auto &s) -> VectorInt & { return s->getColInd(); });
 
    for (auto i : zero_rows) {
      for (auto j = 0; j != blockMat.size2(); ++j) {
        blockMat(i, j) = 0;
      }
    }
    for (auto j : zero_cols) {
      for (auto i = 0; i != blockMat.size1(); ++i) {
        blockMat(i, j) = 0;
      }
    }
    for (auto i : zero_rows) {
      blockMat(i, i) = 1;
    }
 
    CHKERR I::invertMat(blockMat, invMat);
 
    // clear storage and block list from a00 blocks, no more needed
    for (auto &s : block_list) {
      auto it = storage.template get<SchurElemMats::uid_mi_tag>().find(
          boost::make_tuple(s->uidRow, s->uidCol));
      storage.template get<SchurElemMats::uid_mi_tag>().erase(it);
    }
    block_list.clear();
 
    if (schurAO) {
      for (auto &s : storage) {
        if (block_indexing.find(s->uidRow) == block_indexing.end()) {
          CHKERR AOApplicationToPetsc(schurAO, s->getRowInd().size(),
                                      &*s->getRowInd().begin());
        }
        if (block_indexing.find(s->uidCol) == block_indexing.end()) {
          CHKERR AOApplicationToPetsc(schurAO, s->getColInd().size(),
                                      &*s->getColInd().begin());
        }
      }
    }
 
    auto schur_block_list = get_schur_block_list(block_indexing);
    for (auto &s : schur_block_list) {
      auto &m = s->getMat();
      CHKERR assemble_schur(m, s->uidRow, s->uidCol, &(s->getRowInd()),
                            &(s->getColInd()));
    }
    for (auto &s : schur_block_list) {
      auto it = storage.template get<SchurElemMats::uid_mi_tag>().find(
          boost::make_tuple(s->uidRow, s->uidCol));
      storage.template get<SchurElemMats::uid_mi_tag>().erase(it);
    }
    schur_block_list.clear();
 
    for (
 
        auto rp_uid_it = a00_uids.begin(); rp_uid_it != a00_uids.end();
        ++rp_uid_it
 
    ) {
      auto [rlo_uid, rhi_uid] = *rp_uid_it;
      for (auto rm = block_indexing.lower_bound(rlo_uid);
           rm != block_indexing.upper_bound(rhi_uid); ++rm) {
        auto &rbi = rm->second;
 
        auto a_lo_tmp =
            storage.template get<SchurElemMats::col_mi_tag>().lower_bound(
                rm->first);
        auto a_hi =
            storage.template get<SchurElemMats::col_mi_tag>().upper_bound(
                rm->first);
 
        for (
 
            auto cp_uid_it = (symSchur) ? rp_uid_it : a00_uids.begin();
            cp_uid_it != a00_uids.end(); ++cp_uid_it
 
        ) {
          auto [clo_uid, chi_uid] = *cp_uid_it;
          for (auto cm = block_indexing.lower_bound(clo_uid);
               cm != block_indexing.upper_bound(chi_uid); ++cm) {
            auto &cbi = cm->second;
 
            auto c_lo_tmp =
                storage.template get<SchurElemMats::uid_mi_tag>().lower_bound(
                    boost::make_tuple(cm->first, 0));
            auto c_hi =
                storage.template get<SchurElemMats::uid_mi_tag>().upper_bound(
                    boost::make_tuple(cm->first, FieldEntity::getHiBitNumberUId(
                                                     BITFIELDID_SIZE - 1)));
 
            auto sub_inv_mat =
                matrix_range(invMat, get_range(rbi), get_range(cbi));
            bM.resize(sub_inv_mat.size1(), sub_inv_mat.size2());
            noalias(bM) = sub_inv_mat;
 
            for (auto a_lo = a_lo_tmp; a_lo != a_hi; ++a_lo) {
#ifndef NDEBUG
              if (block_indexing.find((*a_lo)->uidRow) != block_indexing.end())
                SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
                        "Wrong a_lo->uidRow");
#endif
 
              auto &a = (*a_lo)->getMat();
              abM.resize(a.size1(), bM.size2(), false);
              abM.clear();
              cblas_dgemm(CblasRowMajor, CblasNoTrans, CblasNoTrans,
 
                          a.size1(), bM.size2(), a.size2(), 1.,
 
                          &*a.data().begin(), a.size2(),
 
                          &*bM.data().begin(), bM.size2(), 0.,
 
                          &*abM.data().begin(), abM.size2());
 
              for (auto c_lo = c_lo_tmp; c_lo != c_hi; ++c_lo) {
#ifndef NDEBUG
                if (block_indexing.find((*c_lo)->uidCol) !=
                    block_indexing.end())
                  SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
                          "Wrong a_lo->uidRow");
#endif
 
                auto &c = (*c_lo)->getMat();
 
                abcM.resize(abM.size1(), c.size2(), false);
                abcM.clear();
 
                cblas_dgemm(CblasRowMajor, CblasNoTrans, CblasNoTrans,
                            abM.size1(), c.size2(), abM.size2(), -1.,
 
                            &*abM.data().begin(), abM.size2(),
 
                            &*c.data().begin(), c.size2(), 0.,
 
                            &*abcM.data().begin(), abcM.size2());
 
                CHKERR assemble_schur(abcM, (*a_lo)->uidRow, (*c_lo)->uidCol,
                                      &((*a_lo)->getRowInd()),
                                      &((*c_lo)->getColInd()));
 
                if (symSchur && rp_uid_it != cp_uid_it) {
                  abcM = trans(abcM);
                  CHKERR assemble_schur(abcM, (*c_lo)->uidCol, (*a_lo)->uidRow,
                                        &((*c_lo)->getColInd()),
                                        &((*a_lo)->getRowInd()));
                }
              }
            }
          }
        }
      }
    }
 
    MoFEMFunctionReturn(0);
  };
 
  // Assemble Schur complements
  CHKERR assemble_dense_blocks();
 
#ifndef NDEBUG
  if constexpr (debug_schur)
    MOFEM_LOG("SELF", Sev::noisy) << "Schur assemble done";
#endif
 
#ifndef NDEBUG
  PetscLogEventEnd(SchurEvents::MOFEM_EVENT_opSchurAssembleEnd, 0, 0, 0, 0);
#endif
 
  MoFEMFunctionReturn(0);
}
 
struct SchurDSYSV {
 
  static auto invertMat(MatrixDouble &m, MatrixDouble &inv) {
    MoFEMFunctionBegin;
 
    const auto nb = m.size1();
#ifndef NDEBUG
    if (nb != m.size2()) {
      SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
              "It should be square matrix %ld != %ld", nb, m.size2());
    }
#endif
 
    inv.resize(nb, nb, false);
    inv.swap(m);
    m.resize(2 * nb, 2 * nb, false);
 
    VectorInt ipiv(nb);
    int info;
 
    // FIXME: add lapack_dsytrf and lapack_dsytri
 
    info = lapack_dgetrf(nb, nb, &*inv.data().begin(), nb, &*ipiv.begin());
    if (info)
      SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
               "lapack error info = %d", info);
    info = lapack_dgetri(nb, &*inv.data().begin(), nb, &*ipiv.begin(),
                         &*m.data().begin(), m.data().size());
    if (info)
      SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
               "lapack error info = %d", info);
 
    MoFEMFunctionReturn(0);
  }
};
 
struct SchurDGESV {
 
  static auto invertMat(MatrixDouble &m, MatrixDouble &inv) {
    MoFEMFunctionBeginHot;
 
    const auto nb = m.size1();
#ifndef NDEBUG
    if (nb != m.size2()) {
      SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
               "It should be square matrix %ld != %ld", nb, m.size2());
    }
#endif
 
    inv.resize(nb, nb, false);
    inv.swap(m);
 
    VectorInt ipiv(nb);
    int info;
 
    info = lapack_dgetrf(nb, nb, &*inv.data().begin(), nb, &*ipiv.begin());
    if (info)
      SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
               "lapack error info = %d", info);
    info = lapack_dgetri(nb, &*inv.data().begin(), nb, &*ipiv.begin(),
                         &*m.data().begin(), m.data().size());
    if (info)
      SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
               "lapack error info = %d", info);
 
    MoFEMFunctionReturnHot(0);
  }
};
 
MoFEMErrorCode
OpSchurAssembleEnd<SchurDSYSV>::doWork(int side, EntityType type,
                                       EntitiesFieldData::EntData &data) {
  return doWorkImpl<SchurDSYSV>(side, type, data);
}
 
MoFEMErrorCode
OpSchurAssembleEnd<SchurDGESV>::doWork(int side, EntityType type,
                                       EntitiesFieldData::EntData &data) {
  return doWorkImpl<SchurDGESV>(side, type, data);
}
 
boost::shared_ptr<BlockStructure> createBlockMatStructure(
 
    DM dm,                                //< dm
    SchurFEOpsFEandFields schur_fe_op_vec //< block elements
 
) {
 
  auto cmp_uid_lo = [](const boost::weak_ptr<FieldEntity> &a, const UId &b) {
    if (auto a_ptr = a.lock()) {
      if (a_ptr->getLocalUniqueId() < b)
        return true;
      else
        return false;
    } else {
      return false;
    }
  };
 
  auto cmp_uid_hi = [](const UId &b, const boost::weak_ptr<FieldEntity> &a) {
    if (auto a_ptr = a.lock()) {
      if (b < a_ptr->getLocalUniqueId())
        return true;
      else
        return false;
    } else {
      return true;
    }
  };
 
  // get uids for fields
  auto get_uid_pair = [](const auto &field_id) {
    auto lo_uid = FieldEntity::getLocalUniqueIdCalculate(
        field_id, get_id_for_min_type<MBVERTEX>());
    auto hi_uid = FieldEntity::getLocalUniqueIdCalculate(
        field_id, get_id_for_max_type<MBENTITYSET>());
    return std::make_pair(lo_uid, hi_uid);
  };
 
  // get uids pair
  auto get_it_pair = [cmp_uid_lo, cmp_uid_hi](auto &&field_ents, auto &&p_uid) {
    auto lo = std::lower_bound(field_ents.begin(), field_ents.end(),
                               p_uid.first, cmp_uid_lo);
    auto hi = std::upper_bound(field_ents.begin(), field_ents.end(),
                               p_uid.second, cmp_uid_hi);
    return std::make_pair(lo, hi);
  };
 
  // extract DOFs for rows/columns. DOFs are associated with fields entities
  // for given problem.
  auto row_extractor = [](auto &e) { return e->entityCacheRowDofs; };
  auto col_extractor = [](auto &e) { return e->entityCacheColDofs; };
 
  auto extract_data = [](auto &&its, auto extractor) {
    std::vector<std::tuple<UId, int, int, int>> data;
    data.reserve(std::distance(its.first, its.second));
    // iterate field dofs
    for (; its.first != its.second; ++its.first) {
      if (auto e = its.first->lock()) {
        if (auto cache = extractor(e).lock()) {
          int nb_dofs = 0;
          for (auto dof = cache->loHi[0]; dof != cache->loHi[1]; ++dof) {
            if ((*dof)->getPetscGlobalDofIdx() != -1)
              ++nb_dofs;
          }
          auto uid = e->getLocalUniqueId();
          if (nb_dofs) {
 
            auto glob = (*cache->loHi[0])->getPetscGlobalDofIdx();
            auto loc = (*cache->loHi[0])->getPetscLocalDofIdx();
            while (glob == -1 && cache->loHi[0] != cache->loHi[1]) {
              ++cache->loHi[0];
              glob = (*cache->loHi[0])->getPetscGlobalDofIdx();
              loc = (*cache->loHi[0])->getPetscLocalDofIdx();
            }
            data.emplace_back(uid, glob, nb_dofs, loc);
 
#ifndef NDEBUG
 
            for (auto lo = cache->loHi[0]; lo != cache->loHi[1]; ++lo) {
              auto glob = (*lo)->getPetscGlobalDofIdx();
              if (glob == -1) {
                CHK_THROW_MESSAGE(MOFEM_DATA_INCONSISTENCY,
                                  "Wrong global index");
              }
            }
 
#endif
          } else {
            data.emplace_back(uid, -1, 0, -1);
          }
        }
      }
    }
    return data;
  };
 
  auto data_ptr = boost::make_shared<BlockStructure>();
  auto m_field_ptr = getInterfacePtr(dm);
 
  // create element to extract data
  auto fe_method = boost::shared_ptr<MoFEM::FEMethod>(new MoFEM::FEMethod());
 
  for (auto &d : schur_fe_op_vec) {
 
    // extract bit numbers  for row and column
    auto get_bit_numbers = [&d](auto op) {
      std::vector<FieldBitNumber> bit_numbers(d.second.size());
      std::transform(d.second.begin(), d.second.end(), bit_numbers.begin(), op);
      return bit_numbers;
    };
 
    // extract bit numbers  for row
    auto row_bit_numbers = get_bit_numbers(
        [&](auto &p) { return m_field_ptr->get_field_bit_number(p.first); });
    // extract bit numbers  for row
    auto col_bit_numbers = get_bit_numbers(
        [&](auto &p) { return m_field_ptr->get_field_bit_number(p.second); });
 
    fe_method->preProcessHook = []() { return 0; };
    fe_method->postProcessHook = []() { return 0; };
    fe_method->operatorHook = [&]() {
      MoFEMFunctionBegin;
 
      auto fe_uid = fe_method->numeredEntFiniteElementPtr->getLocalUniqueId();
 
      for (auto f = 0; f != row_bit_numbers.size(); ++f) {
 
        auto row_data =
            extract_data(get_it_pair(fe_method->getRowFieldEnts(),
                                     get_uid_pair(row_bit_numbers[f])),
                         row_extractor);
        auto col_data =
            extract_data(get_it_pair(fe_method->getColFieldEnts(),
                                     get_uid_pair(col_bit_numbers[f])),
                         col_extractor);
 
        for (auto &r : row_data) {
          auto [r_uid, r_glob, r_nb_dofs, r_loc] = r;
          for (auto &c : col_data) {
            auto [c_uid, c_glob, c_nb_dofs, c_loc] = c;
            data_ptr->blockIndex.insert(BlockStructure::Indexes{
                r_uid, c_uid, fe_uid, r_glob, c_glob, r_nb_dofs, c_nb_dofs,
                r_loc, c_loc, -1});
          }
        }
      }
 
      MoFEMFunctionReturn(0);
    };
 
    CHKERR DMoFEMLoopFiniteElementsUpAndLowRank(dm, d.first, fe_method, 0,
                                                m_field_ptr->get_comm_size());
  };
 
  // order by column (that is for matrix multiplication)
  auto mem_size = 0;
  for (auto &v : data_ptr->blockIndex.get<0>()) {
    v.getMatShift() = mem_size;
    mem_size += v.getNbCols() * v.getNbRows();
  }
 
  std::vector<double> tmp;
  if (mem_size > tmp.max_size())
    CHK_THROW_MESSAGE(MOFEM_OPERATION_UNSUCCESSFUL, "Vector too big");
 
  data_ptr->dataBlocksPtr =
      boost::make_shared<std::vector<double>>(mem_size, 0);
 
  auto ghost_x = createDMVector(dm);
  auto ghost_y = createDMVector(dm);
  CHKERR VecSetDM(ghost_x, PETSC_NULLPTR);
  CHKERR VecSetDM(ghost_y, PETSC_NULLPTR);
 
  data_ptr->ghostX = ghost_x;
  data_ptr->ghostY = ghost_y;
 
  return data_ptr;
}
 
static MoFEMErrorCode mult_schur_block_shell(
    Mat mat, Vec x, Vec y, InsertMode iora,
    boost::function<
        int(DiagBlockIndex::BlockIndex::nth_index<0>::type::iterator)>
        shift_extractor,
    boost::shared_ptr<std::vector<double>> data_blocks_ptr,
    bool multiply_by_preconditioner, enum CBLAS_TRANSPOSE trans_A);
 
static MoFEMErrorCode solve_schur_block_shell(Mat mat, Vec y, Vec x,
                                              InsertMode iora,
                                              enum CBLAS_TRANSPOSE trans_A);
 
static PetscErrorCode mult(Mat mat, Vec x, Vec y) {
  BlockStructure *ctx;
  CHKERR MatShellGetContext(mat, (void **)&ctx);
  return mult_schur_block_shell(
      mat, x, y, INSERT_VALUES,
 
      [](DiagBlockIndex::BlockIndex::nth_index<0>::type::iterator it) {
        return it->getMatShift();
      },
 
      ctx->dataBlocksPtr, true, CblasNoTrans);
}
static PetscErrorCode mult_add(Mat mat, Vec x, Vec y) {
  BlockStructure *ctx;
  CHKERR MatShellGetContext(mat, (void **)&ctx);
  return mult_schur_block_shell(
      mat, x, y, ADD_VALUES,
 
      [](DiagBlockIndex::BlockIndex::nth_index<0>::type::iterator it) {
        return it->getMatShift();
      },
 
      ctx->dataBlocksPtr, true, CblasNoTrans);
}
static PetscErrorCode mult_transpose(Mat mat, Vec x, Vec y) {
  BlockStructure *ctx;
  CHKERR MatShellGetContext(mat, (void **)&ctx);
  return mult_schur_block_shell(
      mat, x, y, INSERT_VALUES,
 
      [](DiagBlockIndex::BlockIndex::nth_index<0>::type::iterator it) {
        return it->getMatShift();
      },
 
      ctx->dataBlocksPtr, true, CblasTrans);
}
static PetscErrorCode mult_transpose_add(Mat mat, Vec x, Vec y) {
  BlockStructure *ctx;
  CHKERR MatShellGetContext(mat, (void **)&ctx);
  return mult_schur_block_shell(
      mat, x, y, ADD_VALUES,
 
      [](DiagBlockIndex::BlockIndex::nth_index<0>::type::iterator it) {
        return it->getMatShift();
      },
 
      ctx->dataBlocksPtr, true, CblasTrans);
}
 
static PetscErrorCode solve(Mat mat, Vec x, Vec y) {
  return solve_schur_block_shell(mat, x, y, INSERT_VALUES, CblasNoTrans);
}
static PetscErrorCode solve_add(Mat mat, Vec x, Vec y) {
  return solve_schur_block_shell(mat, x, y, ADD_VALUES, CblasNoTrans);
}
static PetscErrorCode solve_transpose(Mat mat, Vec x, Vec y) {
  return solve_schur_block_shell(mat, x, y, INSERT_VALUES, CblasTrans);
}
static PetscErrorCode solve_transpose_add(Mat mat, Vec x, Vec y) {
  return solve_schur_block_shell(mat, x, y, ADD_VALUES, CblasTrans);
}
 
static PetscErrorCode zero_rows_columns(Mat A, PetscInt N,
                                        const PetscInt rows[], PetscScalar diag,
                                        Vec x, Vec b) {
 
  MoFEMFunctionBeginHot;
  BlockStructure *ctx;
  CHKERR MatShellGetContext(A, (void **)&ctx);
 
  PetscLogEventBegin(SchurEvents::MOFEM_EVENT_zeroRowsAndCols, 0, 0, 0, 0);
 
  using BlockIndexView = multi_index_container<
 
      const DiagBlockIndex::Indexes *,
 
      indexed_by<
 
          ordered_non_unique<
 
              const_mem_fun<DiagBlockIndex::Indexes, int,
                            &DiagBlockIndex::Indexes::rowShift>
 
              >,
 
          ordered_non_unique<
 
              const_mem_fun<DiagBlockIndex::Indexes, int,
                            &DiagBlockIndex::Indexes::colShift>
 
              >
 
          >>;
 
  BlockIndexView view;
  auto hint = view.get<0>().end();
  for (auto &v : ctx->blockIndex) {
    hint = view.insert(hint, &v);
  }
 
  const int *ptr = &rows[0];
  int is_nb_rows = N;
  SmartPetscObj<IS> is_local;
 
  MPI_Comm comm;
  CHKERR PetscObjectGetComm((PetscObject)A, &comm);
  int size;
  MPI_Comm_size(comm, &size);
  if (size > 1) {
    auto is = createISGeneral(comm, N, rows, PETSC_USE_POINTER);
    is_local = isAllGather(is);
  }
  if (is_local) {
    CHKERR ISGetSize(is_local, &is_nb_rows);
#ifndef NDEBUG
    if constexpr (debug_schur) {
      CHKERR ISView(is_local, PETSC_VIEWER_STDOUT_WORLD);
    }
#endif
    CHKERR ISGetIndices(is_local, &ptr);
  }
 
  int loc_m, loc_n;
  CHKERR MatGetLocalSize(A, &loc_m, &loc_n);
 
  for (auto n = 0; n != is_nb_rows; ++n) {
    auto row = ptr[n];
    auto rlo = view.get<0>().lower_bound(row);
    auto rhi = view.get<0>().end();
    for (; rlo != rhi; ++rlo) {
      auto r_shift = row - (*rlo)->getRow();
      if (r_shift >= 0 && r_shift < (*rlo)->getNbRows()) {
        auto *ptr = &(*ctx->dataBlocksPtr)[(*rlo)->getMatShift()];
        for (auto i = 0; i != (*rlo)->getNbCols(); ++i) {
          ptr[i + r_shift * (*rlo)->getNbCols()] = 0;
        }
      } else if ((*rlo)->getRow() + (*rlo)->getNbRows() > row) {
        break;
      }
    }
  }
 
  for (auto n = 0; n != is_nb_rows; ++n) {
    auto col = ptr[n];
    auto clo = view.get<1>().lower_bound(col);
    auto chi = view.get<1>().end();
    for (; clo != chi; ++clo) {
      auto c_shift = col - (*clo)->getCol();
      if (c_shift >= 0 && c_shift < (*clo)->getNbCols()) {
 
        auto *ptr = &(*ctx->dataBlocksPtr)[(*clo)->getMatShift()];
        for (auto i = 0; i != (*clo)->getNbRows(); ++i) {
          ptr[c_shift + i * (*clo)->getNbCols()] = 0;
        }
 
        // diagonal
        if (
 
            (*clo)->getRow() == (*clo)->getCol() &&
            (*clo)->getLocRow() < loc_m && (*clo)->getLocCol() < loc_n
 
        ) {
          auto r_shift = col - (*clo)->getCol();
          if (r_shift >= 0 && r_shift < (*clo)->getNbRows()) {
            ptr[c_shift + r_shift * (*clo)->getNbCols()] = diag;
          }
        }
      } else if ((*clo)->getCol() + (*clo)->getNbCols() > col) {
        break;
      }
    }
  }
 
  if (is_local) {
    CHKERR ISRestoreIndices(is_local, &ptr);
  }
 
  PetscLogEventEnd(SchurEvents::MOFEM_EVENT_zeroRowsAndCols, 0, 0, 0, 0);
 
  MoFEMFunctionReturnHot(0);
}
 
static PetscErrorCode mat_zero(Mat m) {
  MoFEMFunctionBegin;
  BlockStructure *ctx;
  CHKERR MatShellGetContext(m, (void **)&ctx);
  if (ctx->dataBlocksPtr)
    std::fill(ctx->dataBlocksPtr->begin(), ctx->dataBlocksPtr->end(), 0.);
  if (ctx->preconditionerBlocksPtr)
    std::fill(ctx->preconditionerBlocksPtr->begin(),
              ctx->preconditionerBlocksPtr->end(), 0.);
  MoFEMFunctionReturn(0);
}
 
static MoFEMErrorCode setSchurBlockMatOps(Mat mat_raw) {
  MoFEMFunctionBegin;
  CHKERR MatShellSetManageScalingShifts(mat_raw);
  CHKERR MatShellSetOperation(mat_raw, MATOP_MULT, (void (*)(void))mult);
  CHKERR MatShellSetOperation(mat_raw, MATOP_MULT_ADD,
                              (void (*)(void))mult_add);
  CHKERR MatShellSetOperation(mat_raw, MATOP_MULT_TRANSPOSE,
                              (void (*)(void))mult_transpose);
  CHKERR MatShellSetOperation(mat_raw, MATOP_MULT_TRANSPOSE_ADD,
                              (void (*)(void))mult_transpose_add);
  CHKERR MatShellSetOperation(mat_raw, MATOP_SOLVE, (void (*)(void))solve);
  CHKERR MatShellSetOperation(mat_raw, MATOP_SOLVE_ADD,
                              (void (*)(void))solve_add);
  CHKERR MatShellSetOperation(mat_raw, MATOP_SOLVE_TRANSPOSE,
                              (void (*)(void))solve_transpose);
  CHKERR MatShellSetOperation(mat_raw, MATOP_SOLVE_TRANSPOSE_ADD,
                              (void (*)(void))solve_transpose_add);
  CHKERR MatShellSetOperation(mat_raw, MATOP_ZERO_ENTRIES,
                              (void (*)(void))mat_zero);
  CHKERR MatShellSetOperation(mat_raw, MATOP_ZERO_ROWS_COLUMNS,
                              (void (*)(void))zero_rows_columns);
 
  MoFEMFunctionReturn(0);
};
 
SchurShellMatData createBlockMat(DM dm,
                                 boost::shared_ptr<BlockStructure> data) {
 
  auto problem_ptr = getProblemPtr(dm);
  auto nb_local = problem_ptr->nbLocDofsRow;
  auto nb_global = problem_ptr->nbDofsRow;
 
  // check in nb, rows is equal to nb. columns.
  if (nb_local != problem_ptr->nbLocDofsCol) {
    MOFEM_LOG("SELF", Sev::error)
        << "Wrong size " << nb_local << " != " << problem_ptr->nbLocDofsCol;
    CHK_THROW_MESSAGE(MOFEM_DATA_INCONSISTENCY,
                      "nb. cols is inconsistent with nb. rows");
  }
  if (nb_global != problem_ptr->nbDofsCol) {
    MOFEM_LOG("SELF", Sev::error)
        << "Wrong size " << nb_global << " != " << problem_ptr->nbDofsCol;
    CHK_THROW_MESSAGE(MOFEM_DATA_INCONSISTENCY,
                      "nb. cols is inconsistent with nb. rows");
  }
 
  // get comm from DM
  MPI_Comm comm;
  CHKERR PetscObjectGetComm((PetscObject)dm, &comm);
 
  Mat mat_raw;
  CHKERR MatCreateShell(comm, nb_local, nb_local, nb_global, nb_global,
                        (void *)data.get(), &mat_raw);
  CHKERR setSchurBlockMatOps(mat_raw);
  // CHKERR PetscObjectSetName((PetscObject)mat_raw, MoFEM_BLOCK_MAT);
 
  return std::make_pair(SmartPetscObj<Mat>(mat_raw), data);
}
 
static MoFEMErrorCode mult_schur_block_shell(
    Mat mat, Vec x, Vec y, InsertMode iora,
 
    boost::function<
        int(DiagBlockIndex::BlockIndex::nth_index<0>::type::iterator)>
        shift_extractor,
 
    boost::shared_ptr<std::vector<double>> data_blocks_ptr,
    bool multiply_by_preconditioner, enum CBLAS_TRANSPOSE trans_A) {
  MoFEMFunctionBegin;
  BlockStructure *ctx;
  CHKERR MatShellGetContext(mat, (void **)&ctx);
 
  PetscLogEventBegin(SchurEvents::MOFEM_EVENT_BlockStructureMult, 0, 0, 0, 0);
 
  int x_loc_size;
  CHKERR VecGetLocalSize(x, &x_loc_size);
  int y_loc_size;
  CHKERR VecGetLocalSize(y, &y_loc_size);
 
  auto ghost_vecs = std::make_pair(ctx->ghostX, ctx->ghostY);
  switch (trans_A) {
  case CblasNoTrans:
    break;
  case CblasTrans:
  case CblasConjTrans:
    std::swap(ghost_vecs.first, ghost_vecs.second);
    break;
  };
 
  Vec ghost_x = ghost_vecs.first;
  Vec ghost_y = ghost_vecs.second;
 
  CHKERR VecCopy(x, ghost_x);
 
  double *y_array;
  Vec loc_ghost_y;
  CHKERR VecGhostGetLocalForm(ghost_y, &loc_ghost_y);
  int nb_y;
  CHKERR VecGetLocalSize(loc_ghost_y, &nb_y);
  CHKERR VecGetArray(loc_ghost_y, &y_array);
  for (auto i = 0; i != nb_y; ++i)
    y_array[i] = 0.;
  CHKERR VecRestoreArray(loc_ghost_y, &y_array);
  CHKERR VecGhostRestoreLocalForm(ghost_y, &loc_ghost_y);
 
  auto mult = [&](int low_x, int hi_x, int low_y, int hi_y) {
    MoFEMFunctionBegin;
 
    const double *x_array;
    Vec loc_ghost_x;
    CHKERR VecGhostGetLocalForm(ghost_x, &loc_ghost_x);
    CHKERR VecGetArrayRead(loc_ghost_x, &x_array);
 
    double *y_array;
    Vec loc_ghost_y;
    CHKERR VecGhostGetLocalForm(ghost_y, &loc_ghost_y);
    int nb_y;
    CHKERR VecGetLocalSize(loc_ghost_y, &nb_y);
    CHKERR VecGetArray(loc_ghost_y, &y_array);
 
    double *block_ptr = &*data_blocks_ptr->begin();
    auto it = ctx->blockIndex.get<0>().begin();
    auto hi = ctx->blockIndex.get<0>().end();
 
    auto get_loc_row_no_trans = [](auto it) { return it->getLocRow(); };
    auto get_loc_col_no_trans = [](auto it) { return it->getLocCol(); };
    auto get_nb_rows_no_trans = [](auto it) { return it->getNbRows(); };
    auto get_nb_cols_no_trans = [](auto it) { return it->getNbCols(); };
    auto get_loc_row_trans = [](auto it) { return it->getLocCol(); };
    auto get_loc_col_trans = [](auto it) { return it->getLocRow(); };
 
    std::function<int(decltype(it))> get_loc_row;
    std::function<int(decltype(it))> get_loc_col;
    std::function<int(decltype(it))> get_nb_rows;
    std::function<int(decltype(it))> get_nb_cols;
 
    switch (trans_A) {
    case CblasNoTrans:
      get_loc_row = get_loc_row_no_trans;
      get_loc_col = get_loc_col_no_trans;
      break;
    case CblasTrans:
    case CblasConjTrans:
      get_loc_row = get_loc_row_trans;
      get_loc_col = get_loc_col_trans;
      break;
    };
 
    while (it != hi) {
 
      auto loc_row = get_loc_row(it);
      auto loc_col = get_loc_col(it);
 
      if (loc_row < low_y || loc_row >= hi_y ||
          loc_col < low_x || loc_col >= hi_x) {
        ++it;
        continue;
      }
 
      auto nb_rows = get_nb_rows_no_trans(it);
      auto nb_cols = get_nb_cols_no_trans(it);
      auto x_ptr = &x_array[loc_col];
      auto y_ptr = &y_array[loc_row];
      auto ptr = &block_ptr[shift_extractor(it)];
      cblas_dgemv(CblasRowMajor, trans_A, nb_rows, nb_cols, 1.0, ptr, nb_cols,
                  x_ptr, 1, 1.0, y_ptr, 1);
      ++it;
    }
 
    if (multiply_by_preconditioner && ctx->multiplyByPreconditioner) {
 
      if (!ctx->preconditionerBlocksPtr)
        SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
                "No parentBlockStructurePtr");
 
      auto preconditioner_ptr = &*ctx->preconditionerBlocksPtr->begin();
 
      auto it = ctx->blockIndex.get<0>().begin();
      auto hi = ctx->blockIndex.get<0>().end();
 
      while (it != hi) {
        auto loc_row = get_loc_row(it);
        auto loc_col = get_loc_col(it);
 
        if (loc_row < low_y || loc_row >= hi_y ||
            loc_col < low_x || loc_col >= hi_x) {
          ++it;
          continue;
        }
 
        if (it->getMatShift() != -1) {
          auto nb_rows = get_nb_rows_no_trans(it);
          auto nb_cols = get_nb_cols_no_trans(it);
          auto x_ptr = &x_array[loc_col];
          auto y_ptr = &y_array[loc_row];
          auto ptr = &preconditioner_ptr[it->getMatShift()];
          cblas_dgemv(CblasRowMajor, trans_A, nb_rows, nb_cols, 1.0, ptr,
                      nb_cols, x_ptr, 1, 1.0, y_ptr, 1);
        }
 
        ++it;
      }
    }
 
    CHKERR VecRestoreArrayRead(loc_ghost_x, &x_array);
    CHKERR VecRestoreArray(loc_ghost_y, &y_array);
    CHKERR VecGhostRestoreLocalForm(ghost_x, &loc_ghost_x);
    CHKERR VecGhostRestoreLocalForm(ghost_y, &loc_ghost_y);
    MoFEMFunctionReturn(0);
  };
 
  constexpr auto max_int = std::numeric_limits<int>::max();
  CHKERR VecGhostUpdateBegin(ghost_x, INSERT_VALUES, SCATTER_FORWARD);
  CHKERR mult(0, x_loc_size, 0, max_int);
  CHKERR VecGhostUpdateEnd(ghost_x, INSERT_VALUES, SCATTER_FORWARD);
  CHKERR mult(x_loc_size, max_int, 0, max_int);
 
  CHKERR VecGhostUpdateBegin(ghost_y, ADD_VALUES, SCATTER_REVERSE);
  CHKERR VecGhostUpdateEnd(ghost_y, ADD_VALUES, SCATTER_REVERSE);
 
  switch (iora) {
  case INSERT_VALUES:
    CHKERR VecCopy(ghost_y, y);
    break;
  case ADD_VALUES:
    CHKERR VecAXPY(y, 1., ghost_y);
    break;
  default:
    CHK_THROW_MESSAGE(MOFEM_NOT_IMPLEMENTED, "Wrong InsertMode");
  }
 
#ifndef NDEBUG
 
  auto print_norm = [&](auto msg, auto y) {
    MoFEMFunctionBegin;
    PetscReal norm;
    CHKERR VecNorm(y, NORM_2, &norm);
    int nb_loc_y;
    CHKERR VecGetLocalSize(y, &nb_loc_y);
    int nb_y;
    CHKERR VecGetSize(y, &nb_y);
    MOFEM_LOG("WORLD", Sev::noisy)
        << msg << " " << nb_y << " " << nb_loc_y << " norm " << norm;
    MoFEMFunctionReturn(0);
  };
 
  switch (iora) {
  case INSERT_VALUES:
    print_norm("mult_schur_block_shell insert x", x);
    print_norm("mult_schur_block_shell insert y", y);
    break;
  case ADD_VALUES:
    print_norm("mult_schur_block_shell add x", x);
    print_norm("mult_schur_block_shell add y", y);
    break;
  default:
    CHK_THROW_MESSAGE(MOFEM_NOT_IMPLEMENTED, "Wrong InsertMode");
  }
 
#endif // NDEBUG
 
  // PetscLogFlops(xxx)
  PetscLogEventEnd(SchurEvents::MOFEM_EVENT_BlockStructureMult, 0, 0, 0, 0);
 
  MoFEMFunctionReturn(0);
}
 
static MoFEMErrorCode solve_schur_block_shell(Mat mat, Vec y, Vec x,
                                              InsertMode iora,
                                              enum CBLAS_TRANSPOSE trans_A) {
  using matrix_range = ublas::matrix_range<MatrixDouble>;
  using range = ublas::range;
  MoFEMFunctionBegin;
  BlockStructure *ctx;
  CHKERR MatShellGetContext(mat, (void **)&ctx);
 
  PetscLogEventBegin(SchurEvents::MOFEM_EVENT_BlockStructureSolve, 0, 0, 0, 0);
 
  if (ctx->multiplyByPreconditioner) {
    if (!ctx->preconditionerBlocksPtr)
      SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
              "No preconditionerBlocksPtr");
  }
 
  if (iora == INSERT_VALUES) {
    CHKERR VecZeroEntries(x);
  }
 
  double *x_array;
  CHKERR VecGetArray(x, &x_array);
  const double *y_array;
  CHKERR VecGetArrayRead(y, &y_array);
 
  std::vector<const DiagBlockIndex::Indexes *> blocks;
  MatrixDouble block_mat;
  VectorDouble block_y;
  VectorInt ipiv;
 
  auto get_id = [](auto bi) { return bi->getRowUId(); };
  auto get_block_data = [](auto bi) {
    return std::make_tuple(bi->getNbRows(), bi->getNbRows(), bi->getLocRow());
  };
  auto block_sorter = [](auto a, auto b) {
    if (a->getRowUId() == b->getRowUId())
      return a->getColUId() < b->getColUId();
    else
      return (a->getRowUId() < b->getRowUId());
  };
 
  auto &block_index = ctx->blockIndex.get<1>();
  for (auto it = block_index.begin(); it != block_index.end();) {
 
    // get all blocks on finit element
    blocks.clear();
    auto last_uid = it->getFEUId();
    while (it != block_index.end() && it->getFEUId() == last_uid) {
      blocks.push_back(&*it);
      ++it;
    }
 
    // set local indices
    std::map<UId, std::tuple<int /*nb rows*/, int /*nb cols*/,
                             int /*loc index in vector*/>>
        block_indexing; // uid block map
    for (auto b : blocks) {
      // find all unique row uids (columns will be the same)
      if (block_indexing.find(get_id(b)) == block_indexing.end()) {
        block_indexing[get_id(b)] = get_block_data(b);
      }
    }
    std::sort(blocks.begin(), blocks.end(), block_sorter);
 
    // set indexes to block
    int mat_block_size = 0; // size of block matrix
    for (auto &b : block_indexing) {
      auto &index = std::get<0>(b.second);
      auto block_row_size = index;
      index = mat_block_size;
      mat_block_size += block_row_size;
    }
 
    std::vector<std::tuple<int, int, int, int, int>> block_data;
    block_data.reserve(blocks.size());
    for (auto s : blocks) {
      auto ruid = s->getRowUId();
      auto cuid = s->getColUId();
      auto &rbi = block_indexing.at(ruid);
      auto &cbi = block_indexing.at(cuid);
      if (auto shift = s->getMatShift(); shift != -1) {
        block_data.push_back(std::make_tuple(
 
            shift,
 
            s->getNbRows(), s->getNbCols(),
 
            get<0>(rbi), get<0>(cbi))
 
        );
      }
    }
 
    // fill block matrix for element
    block_mat.resize(mat_block_size, mat_block_size, false);
    block_mat.clear();
    for (auto &bd : block_data) {
      auto &[shift, nb_rows, nb_cols, ridx, cidx] = bd;
      auto ptr = &(*(ctx->dataBlocksPtr))[shift];
      for (auto i = 0; i != nb_rows; ++i, ptr += nb_cols) {
        auto sub_ptr = &block_mat(ridx + i, cidx);
        cblas_dcopy(nb_cols, ptr, 1, sub_ptr, 1);
      }
    }
    if (ctx->multiplyByPreconditioner) {
      for (auto &bd : block_data) {
        auto &[shift, nb_rows, nb_cols, ridx, cidx] = bd;
        auto ptr = &(*(ctx->preconditionerBlocksPtr))[shift];
        for (auto i = 0; i != nb_rows; ++i, ptr += nb_cols) {
          auto sub_ptr = &block_mat(ridx + i, cidx);
          cblas_daxpy(nb_cols, 1., ptr, 1, sub_ptr, 1);
        }
      }
    }
    if (trans_A == CblasNoTrans)
      block_mat = trans(block_mat);
 
    block_y.resize(mat_block_size, false);
    block_y.clear();
 
    for (auto &b : block_indexing) {
      // note that index for local block_y starts from 0, 
      // also is positon for rows, or columns depending on
      // trans_A
      auto [index, size, loc] = b.second;
      auto ptr = &y_array[loc];
      cblas_dcopy(size, ptr, 1, &block_y[index], 1);
    }
 
    // note: this not exploits symmetry, requires more implementation
    constexpr int nrhs = 1;
    ipiv.resize(mat_block_size, false);
    auto info = lapack_dgesv(mat_block_size, nrhs, &*block_mat.data().begin(),
                             mat_block_size, &*ipiv.data().begin(),
                             &*block_y.data().begin(), mat_block_size);
    if (info != 0) {
      MOFEM_LOG("SELF", Sev::error)
          << "error lapack solve dgesv info = " << info;
      MOFEM_LOG("SELF", Sev::error) << block_mat;
      SETERRQ(PETSC_COMM_SELF, MOFEM_OPERATION_UNSUCCESSFUL,
               "error lapack solve dgesv info = %d", info);
    }
 
    for (auto &b : block_indexing) {
      auto [index, size, loc] = b.second;
      auto ptr = &x_array[loc];
      cblas_daxpy(size, 1.0, &block_y[index], 1, ptr, 1);
    }
  }
 
  CHKERR VecRestoreArray(x, &x_array);
  CHKERR VecRestoreArrayRead(y, &y_array);
 
  // PetscLogFlops(xxx)
  PetscLogEventEnd(SchurEvents::MOFEM_EVENT_BlockStructureSolve, 0, 0, 0, 0);
 
  MoFEMFunctionReturn(0);
}
 
MoFEMErrorCode
assembleBlockMatSchur(MoFEM::Interface &m_field, Mat B, Mat S,
                      std::vector<std::string> fields_name,
                      std::vector<boost::shared_ptr<Range>> field_ents,
                      SmartPetscObj<AO> ao) {
  using matrix_range = ublas::matrix_range<MatrixDouble>;
  using range = ublas::range;
  MoFEMFunctionBegin;
  BlockStructure *ctx;
  CHKERR MatShellGetContext(B, (void **)&ctx);
 
  constexpr bool debug = false;
 
  PetscLogEventBegin(SchurEvents::MOFEM_EVENT_AssembleSchurMat, 0, 0, 0, 0);
 
  if (ao.use_count() == 0) {
    SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY, "No AO set");
  }
 
  if (ctx->multiplyByPreconditioner) {
    if (!ctx->preconditionerBlocksPtr)
      SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
              "No preconditionerBlocksPtr");
  }
 
  std::vector<std::pair<UId, UId>> a00_fields_uids;
  a00_fields_uids.reserve(fields_name.size());
  auto it_field_ents = field_ents.begin();
  if (fields_name.size() != field_ents.size()) {
    SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
            "fields_name.size() != field_ents.size()");
  }
  for (auto &f : fields_name) {
    int bn = m_field.get_field_bit_number(f);
    if (*it_field_ents) {
      for (auto p = (*it_field_ents)->pair_begin();
           p != (*it_field_ents)->pair_end(); ++p) {
        a00_fields_uids.emplace_back(
            DofEntity::getLoFieldEntityUId(bn, p->first),
            DofEntity::getHiFieldEntityUId(bn, p->second));
      }
    } else {
      a00_fields_uids.emplace_back(FieldEntity::getLoBitNumberUId(bn),
                                   FieldEntity::getHiBitNumberUId(bn));
    }
    ++it_field_ents;
  }
 
  std::vector<const DiagBlockIndex::Indexes *> fe_blocks;
  std::vector<const DiagBlockIndex::Indexes *> a00_blocks;
  std::vector<const DiagBlockIndex::Indexes *> a01_blocks;
  std::vector<const DiagBlockIndex::Indexes *> a10_blocks;
  std::vector<const DiagBlockIndex::Indexes *> a11_blocks;
  MatrixDouble block_mat_a00;
  MatrixDouble block_mat_a01;
  MatrixDouble block_mat_a10;
  MatrixDouble block_mat_a11;
  VectorInt ipiv;
  std::vector<int> glob_idx;
  std::vector<int> row_glob_idx, col_glob_idx;
 
  auto &block_index = ctx->blockIndex.get<1>();
  for (auto it = block_index.begin(); it != block_index.end();) {
 
    // get blocks on finit element
    fe_blocks.clear();
    a00_blocks.clear();
    a01_blocks.clear();
    a10_blocks.clear();
    a11_blocks.clear();
 
    auto last_uid = it->getFEUId();
    while (it != block_index.end() && it->getFEUId() == last_uid) {
      fe_blocks.push_back(&*it);
      ++it;
    }
 
    for (auto b_ptr : fe_blocks) {
      auto check_id = [&](auto uid) {
        for (auto &uid_pair : a00_fields_uids) {
          if (uid >= uid_pair.first && uid < uid_pair.second) {
            return true;
          }
        }
        return false;
      };
      auto r00 = check_id(b_ptr->getRowUId());
      auto c00 = check_id(b_ptr->getColUId());
      if (r00 && c00) {
        a00_blocks.push_back(b_ptr);
      } else if (r00 && !c00) {
        a01_blocks.push_back(b_ptr);
      } else if (!r00 && c00) {
        a10_blocks.push_back(b_ptr);
      } else {
        a11_blocks.push_back(b_ptr);
      }
    }
 
    for (auto b : a11_blocks) {
 
      row_glob_idx.resize(b->getNbRows());
      col_glob_idx.resize(b->getNbCols());
 
      std::iota(row_glob_idx.begin(), row_glob_idx.end(), b->getRow());
      std::iota(col_glob_idx.begin(), col_glob_idx.end(), b->getCol());
      CHKERR AOApplicationToPetsc(ao, col_glob_idx.size(), col_glob_idx.data());
      CHKERR AOApplicationToPetsc(ao, row_glob_idx.size(), row_glob_idx.data());
 
      auto shift = b->getMatShift();
      if (shift != -1) {
        auto ptr = &(*(ctx->dataBlocksPtr))[shift];
        CHKERR MatSetValues(S, row_glob_idx.size(), row_glob_idx.data(),
                            col_glob_idx.size(), col_glob_idx.data(), ptr,
                            ADD_VALUES);
        if (ctx->multiplyByPreconditioner) {
          auto ptr = &(*(ctx->preconditionerBlocksPtr))[shift];
          CHKERR MatSetValues(S, row_glob_idx.size(), row_glob_idx.data(),
                              col_glob_idx.size(), col_glob_idx.data(), ptr,
                              ADD_VALUES);
        }
      }
    }
 
    if (debug) {
      MOFEM_LOG("WORLD", Sev::warning)
          << "a00_blocks.size() " << a00_blocks.size();
      MOFEM_LOG("WORLD", Sev::warning)
          << "a01_blocks.size() " << a01_blocks.size();
      MOFEM_LOG("WORLD", Sev::warning)
          << "a10_blocks.size() " << a10_blocks.size();
      MOFEM_LOG("WORLD", Sev::warning)
          << "a11_blocks.size() " << a11_blocks.size();
    }
 
    if(a00_blocks.size()) {
 
      for (auto r : a00_blocks) {
        auto r_uid = r->getRowUId();
        auto range = ctx->blockIndex.get<2>().equal_range(r_uid);
        for (auto it = range.first; it != range.second; ++it) {
          if (it->getFEUId() != last_uid && it->getColUId() != r_uid) {
            a01_blocks.push_back(&*it);
          }
        }
      }
 
      for (auto c : a00_blocks) {
        auto c_uid = c->getColUId();
        auto range = ctx->blockIndex.get<3>().equal_range(c_uid);
        for (auto it = range.first; it != range.second; ++it) {
          if (it->getFEUId() != last_uid && it->getRowUId() != c_uid) {
            a10_blocks.push_back(&*it);
          }
        }
      }
 
      auto sort = [](auto &blocks) {
        std::sort(blocks.begin(), blocks.end(), [](auto a, auto b) {
          if (a->getRowUId() == b->getRowUId())
            return a->getColUId() < b->getColUId();
          else
            return a->getRowUId() < b->getRowUId();
        });
      };
 
      sort(a00_blocks);
      sort(a01_blocks);
      sort(a10_blocks);
 
      if (debug) {
        MOFEM_LOG("WORLD", Sev::warning)
            << "a01_blocks.size() " << a01_blocks.size();
        MOFEM_LOG("WORLD", Sev::warning)
            << "a10_blocks.size() " << a10_blocks.size();
      }
 
      // set local indices
      auto set_local_indices = [](auto &row_blocks, auto &col_blocks) {
        // index, size, global index
        std::map<UId, std::tuple<int, int, int>>
            block_indexing; // uid block map
        for (auto b : row_blocks) {
          if (block_indexing.find(b->getRowUId()) == block_indexing.end()) {
            block_indexing[b->getRowUId()] =
                std::make_tuple(b->getNbRows(), b->getNbRows(), b->getRow());
          }
        }
        for (auto b : col_blocks) {
          if (block_indexing.find(b->getColUId()) == block_indexing.end()) {
            block_indexing[b->getColUId()] =
                std::make_tuple(b->getNbCols(), b->getNbCols(), b->getCol());
          }
        }
        // set indexes to block
        int mat_block_size = 0; // size of block matrix
        for (auto &b : block_indexing) {
          // index, size, global index
          auto &[index, size, glob] = b.second;
          index = mat_block_size;
          mat_block_size += size;
        }
        return std::make_pair(block_indexing, mat_block_size);
      };
 
      auto set_block = [&](auto &blocks, auto &row_indexing, auto &col_indexing,
                           auto &block_mat, int row_block_size,
                           int col_block_size) {
        std::vector<std::tuple<int, int, int, int, int>> block_data;
        block_data.reserve(blocks.size());
        for (auto s : blocks) {
          auto ruid = s->getRowUId();
          auto cuid = s->getColUId();
          auto &rbi = row_indexing.at(ruid);
          auto &cbi = col_indexing.at(cuid);
          if (auto shift = s->getMatShift(); shift != -1) {
            block_data.push_back(std::make_tuple(
 
                shift,
 
                s->getNbRows(), s->getNbCols(),
 
                get<0>(rbi), get<0>(cbi))
 
            );
          }
        }
        block_mat.resize(row_block_size, col_block_size, false);
        block_mat.clear();
        for (auto &bd : block_data) {
          auto &[shift, nb_rows, nb_cols, ridx, cidx] = bd;
          auto ptr = &(*(ctx->dataBlocksPtr))[shift];
          for (auto i = 0; i != nb_rows; ++i, ptr += nb_cols) {
            auto sub_ptr = &block_mat(ridx + i, cidx);
            cblas_dcopy(nb_cols, ptr, 1, sub_ptr, 1);
          }
        }
        if (ctx->multiplyByPreconditioner) {
          for (auto &bd : block_data) {
            auto &[shift, nb_rows, nb_cols, ridx, cidx] = bd;
            auto ptr = &(*(ctx->preconditionerBlocksPtr))[shift];
            for (auto i = 0; i != nb_rows; ++i, ptr += nb_cols) {
              auto sub_ptr = &block_mat(ridx + i, cidx);
              cblas_daxpy(nb_cols, 1., ptr, 1, sub_ptr, 1);
            }
          }
        }
      };
 
      auto [a00_indexing, a00_size] = set_local_indices(a00_blocks, a00_blocks);
      auto [a11_indexing, a11_size] = set_local_indices(a10_blocks, a01_blocks);
 
      if (debug) {
        MOFEM_LOG("WORLD", Sev::warning)
            << "a00_indexing.size() " << a00_indexing.size() << " a00_size "
            << a00_size;
        MOFEM_LOG("WORLD", Sev::warning)
            << "a11_indexing.size() " << a11_indexing.size() << " a11_size "
            << a11_size;
      }
 
      set_block(a00_blocks, a00_indexing, a00_indexing, block_mat_a00, a00_size,
                a00_size);
      set_block(a01_blocks, a00_indexing, a11_indexing, block_mat_a01, a00_size,
                a11_size);
      set_block(a10_blocks, a11_indexing, a00_indexing, block_mat_a10, a11_size,
                a00_size);
      block_mat_a11.resize(a11_size, a11_size, false);
      block_mat_a11.clear();
 
      block_mat_a00 = trans(block_mat_a00);
      block_mat_a01 = trans(block_mat_a01);
 
      ipiv.resize(block_mat_a00.size1(), false);
      auto info =
          lapack_dgesv(block_mat_a00.size1(), block_mat_a01.size1(),
                       block_mat_a00.data().data(), block_mat_a00.size2(),
                       &*ipiv.data().begin(), block_mat_a01.data().data(),
                       block_mat_a01.size2());
      if (info)
        SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
                 "lapack error info = %d", info);
 
      // block_mat_a01 = trans(block_mat_a01);
      cblas_dgemm(CblasRowMajor, CblasNoTrans, CblasTrans,
                  block_mat_a11.size1(), block_mat_a11.size2(),
                  block_mat_a00.size1(), -1.,
 
                  block_mat_a10.data().data(), block_mat_a10.size2(),
 
                  block_mat_a01.data().data(), block_mat_a01.size2(),
 
                  0., block_mat_a11.data().data(), block_mat_a11.size2()
 
      );
 
      int idx = 0;
      std::vector<int> glob_idx(block_mat_a11.size1());
      for (auto &r : a11_indexing) {
        auto [r_index, r_size, r_glob] = r.second;
        std::iota(&glob_idx[idx], &glob_idx[idx + r_size], r_glob);
        idx += r_size;
      }
      CHKERR AOApplicationToPetsc(ao, glob_idx.size(), glob_idx.data());
      CHKERR MatSetValues(S, glob_idx.size(), glob_idx.data(), glob_idx.size(),
                          glob_idx.data(), block_mat_a11.data().data(),
                          ADD_VALUES);
    }
  }
 
  // PetscLogFlops(xxx)
  PetscLogEventEnd(SchurEvents::MOFEM_EVENT_AssembleSchurMat, 0, 0, 0, 0);
 
  MoFEMFunctionReturn(0);
}
 
boost::shared_ptr<std::vector<double>> getBlockMatStorageMat(Mat B) {
  BlockStructure *ctx;
  CHKERR MatShellGetContext(B, (void **)&ctx);
  return ctx->dataBlocksPtr;
}
 
boost::shared_ptr<std::vector<double>>
getBlockMatPrconditionerStorageMat(Mat B) {
  BlockStructure *ctx;
  CHKERR MatShellGetContext(B, (void **)&ctx);
  return ctx->dataBlocksPtr;
}
 
inline MoFEMErrorCode shell_block_mat_asmb_wrap_impl(
    BlockStructure *ctx, const EntitiesFieldData::EntData &row_data,
    const EntitiesFieldData::EntData &col_data, const MatrixDouble &mat,
    InsertMode iora,
    boost::function<int(const DiagBlockIndex::Indexes *)> shift_extractor,
    boost::shared_ptr<std::vector<double>> data_blocks_ptr) {
 
  MoFEMFunctionBegin;
 
  if (row_data.getIndices().empty())
    MoFEMFunctionReturnHot(0);
  if (col_data.getIndices().empty())
    MoFEMFunctionReturnHot(0);
 
#ifndef NDEBUG
 
  PetscLogEventBegin(SchurEvents::MOFEM_EVENT_BlockStructureSetValues, 0, 0, 0,
                     0);
 
#endif // NDEBUG
 
  switch (iora) {
  case ADD_VALUES:
  case INSERT_VALUES:
    break;
  default:
    SETERRQ(PETSC_COMM_SELF, MOFEM_NOT_IMPLEMENTED, "Wrong InsertMode");
  }
 
  auto get_row_data = [&]() -> std::pair<bool, VectorInt> {
    if (auto e_ptr = row_data.getFieldEntities()[0]) {
      if (auto stored_data_ptr =
              e_ptr->getSharedStoragePtr<EssentialBcStorage>()) {
        MOFEM_LOG("SELF", Sev::warning) << "Can lead to unhandled behaviour";
        return std::make_pair(true, stored_data_ptr->entityIndices);
      }
    }
    return std::make_pair(false, row_data.getIndices());
  };
 
  auto row_indices = get_row_data();
 
  std::vector<int> ent_row_indices;
  std::vector<int> ent_col_indices;
 
  for (auto &rent : row_data.getFieldEntities()) {
    if (auto r_cache = rent->entityCacheRowDofs.lock()) {
 
      auto &row_uid = rent->getLocalUniqueId();
      auto &row_ind = row_indices.second;
 
      for (auto &cent : col_data.getFieldEntities()) {
        if (auto c_cache = cent->entityCacheColDofs.lock()) {
 
          auto &col_uid = cent->getLocalUniqueId();
          auto &col_ind = col_data.getIndices();
 
          auto it = ctx->blockIndex.get<0>().find(
 
              boost::make_tuple(row_uid, col_uid)
 
          );
 
#ifndef NDEBUG
 
          if (it == ctx->blockIndex.get<0>().end()) {
            MOFEM_LOG_CHANNEL("SELF");
            MOFEM_TAG_AND_LOG("SELF", Sev::error, "BlockMat")
                << "missing block: row "
                << row_data.getFieldDofs()[0]->getName() << " col "
                << col_data.getFieldDofs()[0]->getName();
            SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
                    "Block not allocated");
          }
 
#endif
 
          auto ri = row_ind.begin();
          auto rie = row_ind.end();
 
          auto shift = shift_extractor(&*it);
          auto s_mat = &(*data_blocks_ptr)[shift];
 
          auto get_ent_indices = [](auto &cache, auto &ind) {
            ind.clear();
            ind.reserve(std::distance(cache->loHi[0], cache->loHi[1]));
            for (auto e = cache->loHi[0]; e != cache->loHi[1]; ++e) {
              auto glob = (*e)->getPetscGlobalDofIdx();
              if (glob != -1)
                ind.push_back(glob);
            }
          };
 
          get_ent_indices(r_cache, ent_row_indices);
          if (ent_row_indices.empty())
            continue;
          get_ent_indices(c_cache, ent_col_indices);
          if (ent_col_indices.empty())
            continue;
 
          if (mat.size1() == ent_row_indices.size() &&
              mat.size2() == ent_col_indices.size()) {
 
            if (iora == ADD_VALUES) {
              cblas_daxpy(mat.data().size(), 1.0, mat.data().data(), 1, s_mat,
                          1);
            } else {
              cblas_dcopy(mat.data().size(), mat.data().data(), 1, s_mat, 1);
            }
 
          } else {
 
            int row = 0;
            for (auto re : ent_row_indices) {
              ri = std::find(ri, rie, re);
              if (!(ri == rie && *ri != -1)) {
 
                auto ci = col_ind.begin();
                auto cie = col_ind.end();
 
                int col = 0;
                for (auto ce : ent_col_indices) {
                  ci = std::find(ci, cie, ce);
                  if (!(ci == cie && *ci != -1)) {
                    auto &m = s_mat[row * ent_col_indices.size() + col];
                    if (iora == ADD_VALUES) {
                      m += mat(std::distance(row_ind.begin(), ri),
                               std::distance(col_ind.begin(), ci));
                    } else {
                      m = mat(std::distance(row_ind.begin(), ri),
                              std::distance(col_ind.begin(), ci));
                    }
                  }
                  ++col;
                } // cols
              }
              ++row;
            } // rows
          }
 
        } // iterate entities
      }
    }
  }
 
#ifndef NDEBUG
  PetscLogEventEnd(SchurEvents::MOFEM_EVENT_BlockStructureSetValues, 0, 0, 0,
                   0);
#endif // NDEBUG
 
  MoFEMFunctionReturn(0);
}
 
MoFEMErrorCode
shell_block_mat_asmb_wrap(Mat M, const EntitiesFieldData::EntData &row_data,
                          const EntitiesFieldData::EntData &col_data,
                          const MatrixDouble &mat, InsertMode iora) {
  MoFEMFunctionBegin;
  PetscBool is_mat_shell = PETSC_FALSE;
  PetscObjectTypeCompare((PetscObject)M, MATSHELL, &is_mat_shell);
  if (is_mat_shell) {
    BlockStructure *ctx;
    CHKERR MatShellGetContext(M, (void **)&ctx);
    CHKERR shell_block_mat_asmb_wrap_impl(
        ctx, row_data, col_data, mat, iora,
        [](const DiagBlockIndex::Indexes *idx) { return idx->getMatShift(); },
        ctx->dataBlocksPtr);
  } else {
    CHKERR MatSetValues<AssemblyTypeSelector<PETSC>>(M, row_data, col_data, mat,
                                                     iora);
  }
  MoFEMFunctionReturn(0);
}
 
MoFEMErrorCode shell_block_preconditioner_mat_asmb_wrap(
    Mat M, const EntitiesFieldData::EntData &row_data,
    const EntitiesFieldData::EntData &col_data, const MatrixDouble &mat,
    InsertMode iora) {
  MoFEMFunctionBegin;
  PetscBool is_mat_shell = PETSC_FALSE;
  PetscObjectTypeCompare((PetscObject)M, MATSHELL, &is_mat_shell);
  if (is_mat_shell) {
    BlockStructure *ctx;
    CHKERR MatShellGetContext(M, (void **)&ctx);
    if (!ctx->preconditionerBlocksPtr)
      SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
              "No preconditionerBlocksPtr");
    CHKERR shell_block_mat_asmb_wrap_impl(
        ctx, row_data, col_data, mat, iora,
        [](const DiagBlockIndex::Indexes *idx) { return idx->getMatShift(); },
        ctx->preconditionerBlocksPtr);
  } else {
    CHKERR MatSetValues<AssemblyTypeSelector<PETSC>>(M, row_data, col_data, mat,
                                                     iora);
  }
  MoFEMFunctionReturn(0);
}
 
boost::shared_ptr<NestSchurData> createSchurNestedMatrixStruture(
 
    std::pair<SmartPetscObj<DM>, SmartPetscObj<DM>> dms,
    boost::shared_ptr<BlockStructure> block_mat_data_ptr,
 
    std::vector<std::string> fields_names, //< a00 fields
    std::vector<boost::shared_ptr<Range>>
        field_ents, //< a00 ranges (can be null),
    bool add_preconditioner_block) {
 
  if (!block_mat_data_ptr) {
    CHK_THROW_MESSAGE(MOFEM_DATA_INCONSISTENCY, "Block data not set");
  }
 
  if (fields_names.size() != field_ents.size())
    CHK_THROW_MESSAGE(MOFEM_DATA_INCONSISTENCY,
                      "fields_names.size() != field_ents.size()");
 
  auto [schur_dm, block_dm] = dms;
  auto schur_prb = getProblemPtr(schur_dm);
  auto block_prb = getProblemPtr(block_dm);
  // auto m_field_ptr = getInterfacePtr(block_dm);
 
  auto schur_dofs_row = schur_prb->getNumeredRowDofsPtr();
  auto schur_dofs_col = schur_prb->getNumeredColDofsPtr();
  auto block_dofs_row = block_prb->getNumeredRowDofsPtr();
  auto block_dofs_col = block_prb->getNumeredColDofsPtr();
 
  auto ao_schur_row = schur_prb->getSubData()->getSmartRowMap();
  auto ao_schur_col = schur_prb->getSubData()->getSmartColMap();
  auto ao_block_row = block_prb->getSubData()->getSmartRowMap();
  auto ao_block_col = block_prb->getSubData()->getSmartColMap();
 
  auto schur_vec_x = createDMVector(schur_dm);
  auto block_vec_x = createDMVector(block_dm);
  auto schur_vec_y = vectorDuplicate(schur_vec_x);
  auto block_vec_y = vectorDuplicate(block_vec_x);
  CHKERR VecSetDM(schur_vec_x, PETSC_NULLPTR);
  CHKERR VecSetDM(block_vec_x, PETSC_NULLPTR);
  CHKERR VecSetDM(schur_vec_y, PETSC_NULLPTR);
  CHKERR VecSetDM(block_vec_y, PETSC_NULLPTR);
 
  auto find_field_ent = [&](auto uid, auto prb, auto rc) {
    boost::shared_ptr<NumeredDofEntity_multiIndex> dofs;
 
    switch (rc) {
    case ROW:
      dofs = prb->getNumeredRowDofsPtr();
      break;
    case COL:
      dofs = prb->getNumeredColDofsPtr();
      break;
    default:
      CHK_THROW_MESSAGE(MOFEM_NOT_IMPLEMENTED, "Wrong RowCol");
      break;
    }
 
    auto lo = dofs->get<Unique_mi_tag>().lower_bound(uid);
    if (lo == dofs->get<Unique_mi_tag>().end())
      return boost::shared_ptr<NumeredDofEntity>();
    auto hi = dofs->get<Unique_mi_tag>().upper_bound(
        DofEntity::getUniqueIdCalculate(MAX_DOFS_ON_ENTITY - 1, uid));
    if (lo != hi)
      return *lo;
 
    return boost::shared_ptr<NumeredDofEntity>();
  };
 
  std::array<boost::shared_ptr<BlockStructure>, 4> data_ptrs;
 
  for (auto r = 0; r != 3; ++r) {
    data_ptrs[r] = boost::make_shared<BlockStructure>();
    data_ptrs[r]->dataBlocksPtr = block_mat_data_ptr->dataBlocksPtr;
  }
  data_ptrs[3] = boost::make_shared<BlockStructure>();
  data_ptrs[3]->dataBlocksPtr = block_mat_data_ptr->dataBlocksPtr;
 
  data_ptrs[0]->ghostX = schur_vec_x;
  data_ptrs[0]->ghostY = schur_vec_y;
  data_ptrs[1]->ghostX = block_vec_x;
  data_ptrs[1]->ghostY = schur_vec_y;
  data_ptrs[2]->ghostX = schur_vec_x;
  data_ptrs[2]->ghostY = block_vec_y;
  data_ptrs[3]->ghostX = block_vec_x;
  data_ptrs[3]->ghostY = block_vec_y;
 
  int idx = 0;
  for (auto &d : block_mat_data_ptr->blockIndex.get<0>()) {
 
    auto insert = [&](auto &m, auto &dof_r, auto &dof_c, auto &d) {
      m.insert(
 
          BlockStructure::Indexes{
              d.getRowUId(), d.getColUId(), d.getFEUId(),
 
              dof_r->getPetscGlobalDofIdx(), dof_c->getPetscGlobalDofIdx(),
 
              d.getNbRows(), d.getNbCols(),
 
              dof_r->getPetscLocalDofIdx(), dof_c->getPetscLocalDofIdx(),
 
              d.getMatShift()}
 
      );
    };
 
    auto dof_schur_row = find_field_ent(d.getRowUId(), schur_prb, ROW);
    auto dof_schur_col = find_field_ent(d.getColUId(), schur_prb, COL);
    auto dof_block_row = find_field_ent(d.getRowUId(), block_prb, ROW);
    auto dof_block_col = find_field_ent(d.getColUId(), block_prb, COL);
 
    if (dof_schur_row && dof_schur_col) {
      insert(data_ptrs[0]->blockIndex, dof_schur_row, dof_schur_col, d);
    }
 
    if (dof_schur_row && dof_block_col) {
      insert(data_ptrs[1]->blockIndex, dof_schur_row, dof_block_col, d);
    }
 
    if (dof_block_row && dof_schur_col) {
      insert(data_ptrs[2]->blockIndex, dof_block_row, dof_schur_col, d);
    }
 
    if (dof_block_row && dof_block_col) {
      insert(data_ptrs[3]->blockIndex, dof_block_row, dof_block_col, d);
    }
 
    ++idx;
  }
 
  // set data for a00 solve (inverse blocks)
  auto set_up_a00_data = [&](auto inv_block_data) {
    MoFEMFunctionBegin;
 
    if (add_preconditioner_block) {
      auto preconditioned_block = boost::make_shared<std::vector<double>>(
          inv_block_data->dataBlocksPtr->size(), 0);
      inv_block_data->preconditionerBlocksPtr = preconditioned_block;
      inv_block_data->multiplyByPreconditioner = true;
      block_mat_data_ptr->preconditionerBlocksPtr =
          inv_block_data->preconditionerBlocksPtr;
      block_mat_data_ptr->multiplyByPreconditioner = false;
    }
 
    MoFEMFunctionReturn(0);
  };
 
  CHKERR set_up_a00_data(data_ptrs[3]);
 
  MPI_Comm comm;
  CHKERR PetscObjectGetComm((PetscObject)schur_dm, &comm);
 
  auto create_shell_mat = [&](auto nb_r_loc, auto nb_c_loc, auto nb_r_glob,
                              auto nb_c_glob, auto data_ptr) {
    Mat mat_raw;
    CHKERR MatCreateShell(comm, nb_r_loc, nb_c_loc, nb_r_glob, nb_c_glob,
                          (void *)data_ptr.get(), &mat_raw);
    CHKERR setSchurBlockMatOps(mat_raw);
    return SmartPetscObj<Mat>(mat_raw);
  };
 
  auto schur_nb_global = schur_prb->getNbDofsRow();
  auto block_nb_global = block_prb->getNbDofsRow();
  auto schur_nb_local = schur_prb->getNbLocalDofsRow();
  auto block_nb_local = block_prb->getNbLocalDofsRow();
 
  std::array<SmartPetscObj<Mat>, 4> mats_array;
  mats_array[0] =
      create_shell_mat(schur_nb_local, schur_nb_local, schur_nb_global,
                       schur_nb_global, data_ptrs[0]);
  mats_array[1] =
      create_shell_mat(schur_nb_local, block_nb_local, schur_nb_global,
                       block_nb_global, data_ptrs[1]);
  mats_array[2] =
      create_shell_mat(block_nb_local, schur_nb_local, block_nb_global,
                       schur_nb_global, data_ptrs[2]);
  mats_array[3] =
      create_shell_mat(block_nb_local, block_nb_local, block_nb_global,
                       block_nb_global, data_ptrs[3]);
 
  MOFEM_TAG_AND_LOG("SYNC", Sev::verbose, "NestedSchur")
      << "(0, 0) " << schur_nb_local << " " << schur_nb_global << " "
      << data_ptrs[0]->blockIndex.size();
  MOFEM_TAG_AND_LOG("SYNC", Sev::verbose, "NestedSchur")
      << "(0, 1) " << schur_nb_local << " " << block_nb_local << " "
      << schur_nb_global << " " << block_nb_global << " "
      << data_ptrs[1]->blockIndex.size();
  MOFEM_TAG_AND_LOG("SYNC", Sev::verbose, "NestedSchur")
      << "(1, 0) " << block_nb_local << " " << schur_nb_local << " "
      << block_nb_global << " " << schur_nb_global << " "
      << data_ptrs[2]->blockIndex.size();
  MOFEM_TAG_AND_LOG("SYNC", Sev::verbose, "NestedSchur")
      << "(1, 1) " << block_nb_local << " " << block_nb_global << " "
      << data_ptrs[3]->blockIndex.size();
 
  MOFEM_LOG_SEVERITY_SYNC(comm, Sev::verbose);
 
  auto schur_is = schur_prb->getSubData()->getSmartRowIs();
  auto block_is = block_prb->getSubData()->getSmartRowIs();
 
  return boost::make_shared<NestSchurData>(
 
      mats_array, data_ptrs, block_mat_data_ptr,
      std::make_pair(schur_is, block_is)
 
  );
}
 
std::pair<SmartPetscObj<Mat>, boost::shared_ptr<NestSchurData>>
createSchurNestedMatrix(boost::shared_ptr<NestSchurData> schur_net_data_ptr) {
 
  if (!schur_net_data_ptr)
    CHK_THROW_MESSAGE(MOFEM_DATA_INCONSISTENCY, "No data");
 
  auto [mat_arrays, data_ptrs, block_mat_data_ptr, iss] = *schur_net_data_ptr;
  auto [schur_is, block_is] = iss;
 
  std::array<IS, 2> is_a = {schur_is, block_is};
  std::array<Mat, 4> mats_a = {
 
      mat_arrays[0], mat_arrays[1], mat_arrays[2], mat_arrays[3]
 
  };
 
  MPI_Comm comm;
  CHKERR PetscObjectGetComm((PetscObject)mat_arrays[0], &comm);
 
  Mat mat_raw;
  CHKERR MatCreateNest(
 
      comm, 2, is_a.data(), 2, is_a.data(), mats_a.data(), &mat_raw
 
  );
 
  return std::make_pair(SmartPetscObj<Mat>(mat_raw), schur_net_data_ptr);
}
 
OpSchurAssembleBase *createOpSchurAssembleBegin() {
  return new OpSchurAssembleBegin();
}
 
OpSchurAssembleBase *
createOpSchurAssembleEnd(std::vector<std::string> fields_name,
                         std::vector<boost::shared_ptr<Range>> field_ents,
                         SmartPetscObj<AO> ao, SmartPetscObj<Mat> schur,
                         bool sym_schur, bool symm_op) {
  if (symm_op)
    return new OpSchurAssembleEnd<SchurDSYSV>(fields_name, field_ents, ao,
                                              schur, sym_schur, symm_op);
  else
    return new OpSchurAssembleEnd<SchurDGESV>(fields_name, field_ents, ao,
                                              schur, sym_schur, symm_op);
}
 
OpSchurAssembleBase *createOpSchurZeroRowsAndCols(
    boost::shared_ptr<std::vector<unsigned char>> marker_ptr, double diag_val) {
  return new OpSchurZeroRowsAndCols(marker_ptr, diag_val);
}
 
OpSchurAssembleBase *
createOpSchurAssembleEnd(std::vector<std::string> fields_name,
                         std::vector<boost::shared_ptr<Range>> field_ents,
                         std::vector<SmartPetscObj<AO>> sequence_of_aos,
                         std::vector<SmartPetscObj<Mat>> sequence_of_mats,
                         std::vector<bool> sym_schur, bool symm_op,
                         boost::shared_ptr<BlockStructure> diag_blocks) {
  return createOpSchurAssembleEnd(
      fields_name, field_ents, sequence_of_aos.back(), sequence_of_mats.back(),
      sym_schur.back(), symm_op);
}
 
OpSchurAssembleBase *
createOpSchurAssembleEnd(std::vector<std::string> fields_name,
                         std::vector<boost::shared_ptr<Range>> field_ents,
                         std::vector<SmartPetscObj<AO>> sequence_of_aos,
                         std::vector<SmartPetscObj<Mat>> sequence_of_mats,
                         std::vector<bool> sym_schur,
                         std::vector<double> diag_eps, bool symm_op,
                         boost::shared_ptr<BlockStructure> diag_blocks) {
  return createOpSchurAssembleEnd(
      fields_name, field_ents, sequence_of_aos.back(), sequence_of_mats.back(),
      sym_schur.back(), symm_op);
}
 
MoFEMErrorCode setSchurA00MatSolvePC(SmartPetscObj<PC> pc) {
  MoFEMFunctionBegin;
 
  auto apply = [](PC pc, Vec f, Vec x) {
    MoFEMFunctionBeginHot;
    Mat A, P;
    CHKERR PCGetOperators(pc, &A, &P);
    CHKERR MatSolve(P, f, x);
    MoFEMFunctionReturnHot(0);
  };
 
  auto apply_transpose = [](PC pc, Vec f, Vec x) {
    MoFEMFunctionBeginHot;
    Mat A, P;
    CHKERR PCGetOperators(pc, &A, &P);
    CHKERR MatSolveTranspose(P, f, x);
    MoFEMFunctionReturnHot(0);
  };
 
  CHKERR PCSetType(pc, PCSHELL);
  CHKERR PCShellSetApply(pc, apply);
  CHKERR PCShellSetApplyTranspose(pc, apply_transpose);
  CHKERR PCShellSetName(pc, "MoFEMSchurBlockPC");
 
  MoFEMFunctionReturn(0);
}
 
// This is used to assemble local element matrices for Schur complement
// and matrix for KSP
template <>
MoFEMErrorCode
MatSetValues<SchurElemMats>(Mat M, const EntitiesFieldData::EntData &row_data,
                            const EntitiesFieldData::EntData &col_data,
                            const MatrixDouble &mat, InsertMode iora) {
  return SchurElemMats::MatSetValues(M, row_data, col_data, mat, iora);
}
 
MoFEMErrorCode
schur_mat_set_values_wrap(Mat M, const EntitiesFieldData::EntData &row_data,
                          const EntitiesFieldData::EntData &col_data,
                          const MatrixDouble &mat, InsertMode iora) {
  return MatSetValues<AssemblyTypeSelector<PETSC>>(M, row_data, col_data, mat,
                                                   iora);
}
 
// Is assumed that standard PETSc assembly works for matrices used by KSP
SchurBackendMatSetValuesPtr::MatSetValuesPtr
    SchurBackendMatSetValuesPtr::matSetValuesPtr = schur_mat_set_values_wrap;
 
// We assemble matrix for KSP and store local matrices for Schur complement.
// Schur complement is calculated and assembled in OpSchurAssembleEnd.
MoFEMErrorCode
SchurElemMats::MatSetValues(Mat M, const EntitiesFieldData::EntData &row_data,
                            const EntitiesFieldData::EntData &col_data,
                            const MatrixDouble &mat, InsertMode iora) {
  MoFEMFunctionBegin;
  CHKERR assembleStorage(row_data, col_data, mat, iora);
  CHKERR SchurBackendMatSetValuesPtr::matSetValuesPtr(M, row_data, col_data,
                                                      mat, iora);
  MoFEMFunctionReturn(0);
}
 
// All is now wrapped in specialisation of
// MatSetValues<AssemblyTypeSelector<SCHUR>>
template <>
MoFEMErrorCode MatSetValues<AssemblyTypeSelector<SCHUR>>(
    Mat M, const EntitiesFieldData::EntData &row_data,
    const EntitiesFieldData::EntData &col_data, const MatrixDouble &mat,
    InsertMode iora) {
  return MatSetValues<SchurElemMats>(M, row_data, col_data, mat, iora);
}
 
// Standard (PETSc) assembly block matrices do noe work
template <>
MoFEMErrorCode
MatSetValues<BlockStructure>(Mat M, const EntitiesFieldData::EntData &row_data,
                             const EntitiesFieldData::EntData &col_data,
                             const MatrixDouble &mat, InsertMode iora) {
  return shell_block_mat_asmb_wrap(M, row_data, col_data, mat, iora);
}
 
struct SchurElemMatsBlock : public SchurElemMats {
 
  static MoFEMErrorCode MatSetValues(Mat M,
                                     const EntitiesFieldData::EntData &row_data,
                                     const EntitiesFieldData::EntData &col_data,
                                     const MatrixDouble &mat, InsertMode iora);
};
 
SchurBackendMatSetValuesPtr::MatSetValuesPtr
    SchurBackendMatSetValuesPtr::matSetValuesBlockPtr =
        shell_block_mat_asmb_wrap;
 
MoFEMErrorCode
SchurElemMatsBlock::MatSetValues(Mat M,
                                 const EntitiesFieldData::EntData &row_data,
                                 const EntitiesFieldData::EntData &col_data,
                                 const MatrixDouble &mat, InsertMode iora) {
  MoFEMFunctionBegin;
  CHKERR assembleStorage(row_data, col_data, mat, iora);
  CHKERR SchurBackendMatSetValuesPtr::matSetValuesBlockPtr(M, row_data,
                                                           col_data, mat, iora);
  MoFEMFunctionReturn(0);
}
 
template <>
MoFEMErrorCode
MatSetValues<SchurElemMatsBlock>(Mat M,
                                 const EntitiesFieldData::EntData &row_data,
                                 const EntitiesFieldData::EntData &col_data,
                                 const MatrixDouble &mat, InsertMode iora) {
  return SchurElemMatsBlock::MatSetValues(M, row_data, col_data, mat, iora);
}
 
struct SchurElemMatsPreconditionedBlock : public SchurElemMats {
 
  static MoFEMErrorCode MatSetValues(Mat M,
                                     const EntitiesFieldData::EntData &row_data,
                                     const EntitiesFieldData::EntData &col_data,
                                     const MatrixDouble &mat, InsertMode iora);
};
 
SchurBackendMatSetValuesPtr::MatSetValuesPtr
    SchurBackendMatSetValuesPtr::matSetValuesPreconditionedBlockPtr =
        shell_block_preconditioner_mat_asmb_wrap;
 
MoFEMErrorCode SchurElemMatsPreconditionedBlock::MatSetValues(
    Mat M, const EntitiesFieldData::EntData &row_data,
    const EntitiesFieldData::EntData &col_data, const MatrixDouble &mat,
    InsertMode iora) {
  MoFEMFunctionBegin;
  CHKERR assembleStorage(row_data, col_data, mat, iora);
  CHKERR SchurBackendMatSetValuesPtr::matSetValuesPreconditionedBlockPtr(
      M, row_data, col_data, mat, iora);
  MoFEMFunctionReturn(0);
}
 
template <>
MoFEMErrorCode MatSetValues<SchurElemMatsPreconditionedBlock>(
    Mat M, const EntitiesFieldData::EntData &row_data,
    const EntitiesFieldData::EntData &col_data, const MatrixDouble &mat,
    InsertMode iora) {
  return SchurElemMatsPreconditionedBlock::MatSetValues(M, row_data, col_data,
                                                        mat, iora);
}
 
MoFEMErrorCode
schurSwitchPreconditioner(boost::shared_ptr<BlockStructure> block_mat_data) {
  MoFEMFunctionBegin;
  if (block_mat_data->multiplyByPreconditioner) {
    block_mat_data->multiplyByPreconditioner = false;
  } else {
    if (!block_mat_data->preconditionerBlocksPtr) {
      SETERRQ(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
              "preconditionerBlocksPtr not set");
    }
    block_mat_data->multiplyByPreconditioner = true;
  }
  MoFEMFunctionReturn(0);
}
 
MoFEMErrorCode
schurSaveBlockMesh(boost::shared_ptr<BlockStructure> block_mat_data,
                   std::string filename) {
  MoFEMFunctionBegin;
 
  moab::Core core;
  moab::Interface &moab = core;
 
  ReadUtilIface *iface;
  CHKERR moab.query_interface(iface);
 
  auto size = block_mat_data->blockIndex.size();
 
  EntityHandle starting_vertex;
  vector<double *> arrays_coord;
  CHKERR iface->get_node_coords(3, 4 * size, 0, starting_vertex, arrays_coord);
  EntityHandle starting_handle;
  EntityHandle *array;
  CHKERR iface->get_element_connect(size, 4, MBQUAD, 0, starting_handle, array);
 
  double def_val_nrm2 = 0;
  Tag th_nrm2;
  CHKERR moab.tag_get_handle("nrm2", 1, MB_TYPE_DOUBLE, th_nrm2,
                             MB_TAG_CREAT | MB_TAG_DENSE, &def_val_nrm2);
 
  int def_val_mat_shift = 0;
  Tag th_mat_shift;
  CHKERR moab.tag_get_handle("mat_shift", 1, MB_TYPE_INTEGER, th_mat_shift,
                             MB_TAG_CREAT | MB_TAG_DENSE, &def_val_mat_shift);
 
  int i = 0;
  for (auto &d : block_mat_data->blockIndex) {
    auto row = d.getRow();
    auto col = d.getCol();
    auto nb_rows = d.getNbRows();
    auto nb_cols = d.getNbCols();
    auto mat_shift = d.getMatShift();
 
    // q0
    arrays_coord[0][4 * i + 0] = row;
    arrays_coord[1][4 * i + 0] = col;
    arrays_coord[2][4 * i + 0] = 0;
 
    // q1
    arrays_coord[0][4 * i + 1] = row + nb_rows;
    arrays_coord[1][4 * i + 1] = col;
    arrays_coord[2][4 * i + 1] = 0;
 
    // q2
    arrays_coord[0][4 * i + 2] = row + nb_rows;
    arrays_coord[1][4 * i + 2] = col + nb_cols;
    arrays_coord[2][4 * i + 2] = 0;
 
    // q3
    arrays_coord[0][4 * i + 3] = row;
    arrays_coord[1][4 * i + 3] = col + nb_cols;
    arrays_coord[2][4 * i + 3] = 0;
 
    // ele conn
    array[4 * i + 0] = starting_vertex + 4 * i + 0;
    array[4 * i + 1] = starting_vertex + 4 * i + 1;
    array[4 * i + 2] = starting_vertex + 4 * i + 2;
    array[4 * i + 3] = starting_vertex + 4 * i + 3;
 
    auto prt = &(*block_mat_data->dataBlocksPtr)[d.getMatShift()];
    auto nrm2 = cblas_dnrm2(nb_rows * nb_cols, prt, 1);
    EntityHandle ele = starting_handle + i;
    CHKERR moab.tag_set_data(th_nrm2, &ele, 1, &nrm2);
    CHKERR moab.tag_set_data(th_mat_shift, &ele, 1, &mat_shift);
 
    ++i;
  }
 
  CHKERR iface->update_adjacencies(starting_handle, size, 4, array);
  CHKERR moab.write_file(filename.c_str(), "VTK", "");
 
  MoFEMFunctionReturn(0);
}
 
} // namespace MoFEM