AbstractContinuumMechanicsSolver.hpp

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#ifndef ABSTRACTCONTINUUMMECHANICSSOLVER_HPP_
#define ABSTRACTCONTINUUMMECHANICSSOLVER_HPP_
 
#include "ContinuumMechanicsProblemDefinition.hpp"
#include "CompressibilityType.hpp"
#include "LinearSystem.hpp"
#include "OutputFileHandler.hpp"
#include "ReplicatableVector.hpp"
#include "AbstractTetrahedralMesh.hpp"
#include "QuadraticMesh.hpp"
#include "DistributedQuadraticMesh.hpp"
#include "Warnings.hpp"
#include "PetscException.hpp"
#include "GaussianQuadratureRule.hpp"
#include "PetscTools.hpp"
#include "MechanicsEventHandler.hpp"
#include "CommandLineArguments.hpp"
#include "VtkMeshWriter.hpp"
 
 
typedef enum _ApplyDirichletBcsType
{
    LINEAR_PROBLEM,
    NONLINEAR_PROBLEM_APPLY_TO_RESIDUAL_ONLY,
    NONLINEAR_PROBLEM_APPLY_TO_EVERYTHING
} ApplyDirichletBcsType;
 
//forward declarations
template<unsigned DIM> class StressRecoveror;
template<unsigned DIM> class VtkNonlinearElasticitySolutionWriter;
 
template<unsigned DIM>
class AbstractContinuumMechanicsSolver
{
    friend class StressRecoveror<DIM>;
    friend class VtkNonlinearElasticitySolutionWriter<DIM>;
 
protected:
    AbstractTetrahedralMesh<DIM, DIM>& mrQuadMesh;
 
    ContinuumMechanicsProblemDefinition<DIM>& mrProblemDefinition;
 
    bool mWriteOutput;
 
    std::string mOutputDirectory;
 
    OutputFileHandler* mpOutputFileHandler;
 
    std::vector<c_vector<double,DIM> > mSpatialSolution;
 
    std::vector<double> mPressureSolution;
 
 
    std::vector<double> mCurrentSolution;
 
    GaussianQuadratureRule<DIM>* mpQuadratureRule;
 
    GaussianQuadratureRule<DIM-1>* mpBoundaryQuadratureRule;
 
    CompressibilityType mCompressibilityType;
 
    unsigned mProblemDimension;
 
    unsigned mNumDofs;
 
    bool mVerbose;
 
    Vec mResidualVector;
 
    Vec mLinearSystemRhsVector;
 
    Mat mSystemLhsMatrix;
 
    Vec mDirichletBoundaryConditionsVector;
 
    Mat mPreconditionMatrix;
 
    void AllocateMatrixMemory();
 
 
    void ApplyDirichletBoundaryConditions(ApplyDirichletBcsType type, bool symmetricProblem);
 
    void AddIdentityBlockForDummyPressureVariables(ApplyDirichletBcsType type);
 
 
    void RemovePressureDummyValuesThroughLinearInterpolation();
 
public:
    AbstractContinuumMechanicsSolver(AbstractTetrahedralMesh<DIM, DIM>& rQuadMesh,
                                     ContinuumMechanicsProblemDefinition<DIM>& rProblemDefinition,
                                     std::string outputDirectory,
                                     CompressibilityType compressibilityType);
 
    virtual ~AbstractContinuumMechanicsSolver();
 
    void WriteCurrentSpatialSolution(std::string fileName, std::string fileExtension, int counterToAppend=-1);
 
    void WriteCurrentPressureSolution( int counterToAppend=-1);
 
    void SetWriteOutput(bool writeOutput=true);
 
    void CreateVtkOutput(std::string spatialSolutionName="Spatial solution");
 
    std::vector<double>& rGetCurrentSolution()
    {
        return mCurrentSolution;
    }
 
    virtual std::vector<c_vector<double,DIM> >& rGetSpatialSolution()=0;
 
    std::vector<double>& rGetPressures();
};
 
template<unsigned DIM>
AbstractContinuumMechanicsSolver<DIM>::AbstractContinuumMechanicsSolver(AbstractTetrahedralMesh<DIM, DIM>& rQuadMesh,
                                                                        ContinuumMechanicsProblemDefinition<DIM>& rProblemDefinition,
                                                                        std::string outputDirectory,
                                                                        CompressibilityType compressibilityType)
    : mrQuadMesh(rQuadMesh),
      mrProblemDefinition(rProblemDefinition),
      mOutputDirectory(outputDirectory),
      mpOutputFileHandler(nullptr),
      mpQuadratureRule(nullptr),
      mpBoundaryQuadratureRule(nullptr),
      mCompressibilityType(compressibilityType),
      mResidualVector(nullptr),
      mSystemLhsMatrix(nullptr),
      mPreconditionMatrix(nullptr)
{
    assert(DIM==2 || DIM==3);
 
    //Check that the mesh is Quadratic
    QuadraticMesh<DIM>* p_quad_mesh = dynamic_cast<QuadraticMesh<DIM>* >(&rQuadMesh);
    DistributedQuadraticMesh<DIM>* p_distributed_quad_mesh = dynamic_cast<DistributedQuadraticMesh<DIM>* >(&rQuadMesh);
 
    if ((p_quad_mesh == nullptr) && (p_distributed_quad_mesh == nullptr))
    {
        EXCEPTION("Continuum mechanics solvers require a quadratic mesh");
    }
 
 
    mVerbose = (mrProblemDefinition.GetVerboseDuringSolve() ||
                CommandLineArguments::Instance()->OptionExists("-mech_verbose") ||
                CommandLineArguments::Instance()->OptionExists("-mech_very_verbose") );
 
    mWriteOutput = (mOutputDirectory != "");
    if (mWriteOutput)
    {
        mpOutputFileHandler = new OutputFileHandler(mOutputDirectory);
    }
 
    // See dox for mProblemDimension
    mProblemDimension = mCompressibilityType==COMPRESSIBLE ? DIM : DIM+1;
    mNumDofs = mProblemDimension*mrQuadMesh.GetNumNodes();
 
    AllocateMatrixMemory();
 
    // In general the Jacobian for a mechanics problem is non-polynomial.
    // We therefore use the highest order integration rule available.
    mpQuadratureRule = new GaussianQuadratureRule<DIM>(3);
    // The boundary forcing terms (or tractions) are also non-polynomial in general.
    // Again, we use the highest order integration rule available.
    mpBoundaryQuadratureRule = new GaussianQuadratureRule<DIM-1>(3);
 
    mCurrentSolution.resize(mNumDofs, 0.0);
}
 
template<unsigned DIM>
AbstractContinuumMechanicsSolver<DIM>::~AbstractContinuumMechanicsSolver()
{
    if (mpOutputFileHandler)
    {
        delete mpOutputFileHandler;
    }
 
    if (mpQuadratureRule)
    {
        delete mpQuadratureRule;
        delete mpBoundaryQuadratureRule;
    }
 
    if (mResidualVector)
    {
        PetscTools::Destroy(mResidualVector);
        PetscTools::Destroy(mLinearSystemRhsVector);
        PetscTools::Destroy(mSystemLhsMatrix);
        PetscTools::Destroy(mPreconditionMatrix);
    }
 
    if (mDirichletBoundaryConditionsVector)
    {
        PetscTools::Destroy(mDirichletBoundaryConditionsVector);
    }
}
 
template<unsigned DIM>
void AbstractContinuumMechanicsSolver<DIM>::WriteCurrentSpatialSolution(std::string fileName,
                                                                        std::string fileExtension,
                                                                        int counterToAppend)
{
    // Only write output if the flag mWriteOutput has been set
    if (!mWriteOutput)
    {
        return;
    }
 
    if (PetscTools::AmMaster())
    {
        std::stringstream file_name;
 
        file_name << fileName;
        if (counterToAppend >= 0)
        {
            file_name << "_" << counterToAppend;
        }
        file_name << "." << fileExtension;
 
        out_stream p_file = mpOutputFileHandler->OpenOutputFile(file_name.str());
 
        std::vector<c_vector<double,DIM> >& r_spatial_solution = rGetSpatialSolution();
        for (unsigned i=0; i<r_spatial_solution.size(); i++)
        {
    //        for (unsigned j=0; j<DIM; j++)
    //        {
    //            *p_file << mrQuadMesh.GetNode(i)->rGetLocation()[j] << " ";
    //        }
            for (unsigned j=0; j<DIM; j++)
            {
                *p_file << r_spatial_solution[i](j) << " ";
            }
            *p_file << "\n";
        }
        p_file->close();
    }
    PetscTools::Barrier("WriteSpatial");
}
 
template<unsigned DIM>
void AbstractContinuumMechanicsSolver<DIM>::WriteCurrentPressureSolution(int counterToAppend)
{
    // Only write output if the flag mWriteOutput has been set
    if (mWriteOutput)
    {
        // Only the master writes
        if (PetscTools::AmMaster() && mWriteOutput)
        {
            std::stringstream file_name;
 
            file_name << "pressure";
            if (counterToAppend >= 0)
            {
                file_name << "_" << counterToAppend;
            }
            file_name << ".txt";
 
            out_stream p_file = mpOutputFileHandler->OpenOutputFile(file_name.str());
 
            std::vector<double> &r_pressure = rGetPressures();
            for (unsigned i = 0; i < r_pressure.size(); i++)
            {
                for (unsigned j = 0; j < DIM; j++)
                {
                    *p_file << mrQuadMesh.GetNode(i)->rGetLocation()[j] << " ";
                }
 
                *p_file << r_pressure[i] << "\n";
            }
            p_file->close();
        }
        PetscTools::Barrier("WritePressure");
    }
}
 
template<unsigned DIM>
void AbstractContinuumMechanicsSolver<DIM>::SetWriteOutput(bool writeOutput)
{
    if (writeOutput && (mOutputDirectory==""))
    {
        EXCEPTION("Can't write output if no output directory was given in constructor");
    }
    mWriteOutput = writeOutput;
}
 
// ** TO BE DEPRECATED - see #2321 **
template<unsigned DIM>
void AbstractContinuumMechanicsSolver<DIM>::CreateVtkOutput(std::string spatialSolutionName)
{
    if (this->mOutputDirectory=="")
    {
        EXCEPTION("No output directory was given so no output was written, cannot convert to VTK format");
    }
    VtkMeshWriter<DIM, DIM> mesh_writer(this->mOutputDirectory + "/vtk", "solution", true);
 
    mesh_writer.AddPointData(spatialSolutionName, this->rGetSpatialSolution());
 
    if (mCompressibilityType==INCOMPRESSIBLE)
    {
        mesh_writer.AddPointData("Pressure", rGetPressures());
    }
 
    //Output the element attribute as cell data.
    std::vector<double> element_attribute;
    for (typename QuadraticMesh<DIM>::ElementIterator iter = this->mrQuadMesh.GetElementIteratorBegin();
        iter != this->mrQuadMesh.GetElementIteratorEnd();
        ++iter)
    {
        element_attribute.push_back(iter->GetAttribute());
    }
    mesh_writer.AddCellData("Attribute", element_attribute);
 
    mesh_writer.WriteFilesUsingMesh(this->mrQuadMesh);
}
 
template<unsigned DIM>
std::vector<double>& AbstractContinuumMechanicsSolver<DIM>::rGetPressures()
{
    assert(mProblemDimension==DIM+1);
 
    mPressureSolution.clear();
    mPressureSolution.resize(mrQuadMesh.GetNumNodes());
 
    for (unsigned i=0; i<mrQuadMesh.GetNumNodes(); i++)
    {
        mPressureSolution[i] = mCurrentSolution[mProblemDimension*i + DIM];
    }
    return mPressureSolution;
}
 
template<unsigned DIM>
void AbstractContinuumMechanicsSolver<DIM>::RemovePressureDummyValuesThroughLinearInterpolation()
{
    assert(mProblemDimension==DIM+1);
 
    // For quadratic triangles, node 3 is between nodes 1 and 2, node 4 is between 0 and 2, etc
    unsigned internal_nodes_2d[3] = {3,4,5};
    unsigned neighbouring_vertices_2d[3][2] = { {1,2}, {2,0}, {0,1} };
 
    // ordering for quadratic tetrahedra
    unsigned internal_nodes_3d[6] = {4,5,6,7,8,9};
    unsigned neighbouring_vertices_3d[6][2] = { {0,1}, {1,2}, {0,2}, {0,3}, {1,3}, {2,3} };
 
    unsigned num_internal_nodes_per_element = DIM==2 ? 3 : 6;
 
    // loop over elements, then loop over edges.
    for (typename AbstractTetrahedralMesh<DIM,DIM>::ElementIterator iter = mrQuadMesh.GetElementIteratorBegin();
         iter != mrQuadMesh.GetElementIteratorEnd();
         ++iter)
    {
        for (unsigned i=0; i<num_internal_nodes_per_element; i++)
        {
            unsigned global_index;
            double left_val;
            double right_val;
 
            if (DIM == 2)
            {
                global_index = iter->GetNodeGlobalIndex( internal_nodes_2d[i] );
                unsigned vertex_0_global_index =iter->GetNodeGlobalIndex( neighbouring_vertices_2d[i][0] );
                unsigned vertex_1_global_index =iter->GetNodeGlobalIndex( neighbouring_vertices_2d[i][1] );
                left_val = mCurrentSolution[mProblemDimension*vertex_0_global_index + DIM];
                right_val = mCurrentSolution[mProblemDimension*vertex_1_global_index + DIM];
            }
            else
            {
                global_index = iter->GetNodeGlobalIndex( internal_nodes_3d[i] );
                unsigned vertex_0_global_index =iter->GetNodeGlobalIndex( neighbouring_vertices_3d[i][0] );
                unsigned vertex_1_global_index =iter->GetNodeGlobalIndex( neighbouring_vertices_3d[i][1] );
                left_val = mCurrentSolution[mProblemDimension*vertex_0_global_index + DIM];
                right_val = mCurrentSolution[mProblemDimension*vertex_1_global_index + DIM];
            }
 
            // this line assumes the internal node is midway between the two vertices
            mCurrentSolution[mProblemDimension*global_index + DIM] =  0.5 * (left_val + right_val);
        }
    }
}
 
/*
 * This method applies the appropriate BCs to the residual and/or linear system
 *
 * For the latter, and second input parameter==true, the BCs are imposed in such a way as
 * to ensure that a symmetric linear system remains symmetric. For each row with boundary condition
 * applied, both the row and column are zero'd and the RHS vector modified to take into account the
 * zero'd column.
 *
 * Suppose we have a matrix
 * [a b c] [x] = [ b1 ]
 * [d e f] [y]   [ b2 ]
 * [g h i] [z]   [ b3 ]
 * and we want to apply the boundary condition x=v without losing symmetry if the matrix is
 * symmetric. We apply the boundary condition
 * [1 0 0] [x] = [ v  ]
 * [d e f] [y]   [ b2 ]
 * [g h i] [z]   [ b3 ]
 * and then zero the column as well, adding a term to the RHS to take account for the
 * zero-matrix components
 * [1 0 0] [x] = [ v  ] - v[ 0 ]
 * [0 e f] [y]   [ b2 ]    [ d ]
 * [0 h i] [z]   [ b3 ]    [ g ]
 * Note the last term is the first column of the matrix, with one component zeroed, and
 * multiplied by the boundary condition value. This last term is then stored in
 * mDirichletBoundaryConditionsVector, and in general is equal to:
 * SUM_{d=1..D} v_d a'_d
 * where v_d is the boundary value of boundary condition d (d an index into the matrix),
 * and a'_d is the dth-column of the matrix but with the d-th component zeroed, and where
 * there are D boundary conditions
 */
template<unsigned DIM>
void AbstractContinuumMechanicsSolver<DIM>::ApplyDirichletBoundaryConditions(ApplyDirichletBcsType type, bool symmetricProblem)
{
    std::vector<unsigned> rows;
    std::vector<double> values;
 
    // Whether to apply symmetrically, ie alter columns as well as rows (see comment above)
    bool applySymmetrically = (type!=NONLINEAR_PROBLEM_APPLY_TO_RESIDUAL_ONLY) && symmetricProblem;
 
    if (applySymmetrically)
    {
        if (mDirichletBoundaryConditionsVector == nullptr)
        {
            VecDuplicate(mResidualVector, &mDirichletBoundaryConditionsVector);
        }
 
        PetscVecTools::Zero(mDirichletBoundaryConditionsVector);
        PetscMatTools::Finalise(mSystemLhsMatrix);
    }
 
    // collect the entries to be altered
 
    for (unsigned i=0; i<mrProblemDefinition.rGetDirichletNodes().size(); i++)
    {
        unsigned node_index = mrProblemDefinition.rGetDirichletNodes()[i];
 
        for (unsigned j=0; j<DIM; j++)
        {
            double dirichlet_val = mrProblemDefinition.rGetDirichletNodeValues()[i](j);
 
            if (dirichlet_val != ContinuumMechanicsProblemDefinition<DIM>::FREE)
            {
                double val;
                unsigned dof_index = mProblemDimension*node_index+j;
 
                if (type == LINEAR_PROBLEM)
                {
                    val = dirichlet_val;
                }
                else
                {
                    // The boundary conditions on the NONLINEAR SYSTEM are x=boundary_values
                    // on the boundary nodes. However:
                    // The boundary conditions on the LINEAR SYSTEM at each Newton step (Ju=f,
                    // where J is the Jacobian, u the negative update vector and f is the residual) is:
                    // u=current_soln-boundary_values on the boundary nodes
                    val = mCurrentSolution[dof_index] - dirichlet_val;
                }
                rows.push_back(dof_index);
                values.push_back(val);
            }
        }
    }
 
    // do the alterations
 
    if (applySymmetrically)
    {
        // Modify the matrix columns
        for (unsigned i=0; i<rows.size(); i++)
        {
            unsigned col = rows[i];
            double minus_value = -values[i];
 
            // Get a vector which will store the column of the matrix (column d, where d is
            // the index of the row (and column) to be altered for the boundary condition.
            // Since the matrix is symmetric when get row number "col" and treat it as a column.
            // PETSc uses compressed row format and therefore getting rows is far more efficient
            // than getting columns.
            Vec matrix_col = PetscMatTools::GetMatrixRowDistributed(mSystemLhsMatrix,col);
 
            // Zero the correct entry of the column
            PetscVecTools::SetElement(matrix_col, col, 0.0);
 
            // Set up the RHS Dirichlet boundary conditions vector
            // E.g. for a boundary node at the zeroth node (x_0 = value), this is equal to
            //   -value*[0 a_21 a_31 .. a_N1]
            // and will be added to the RHS.
            PetscVecTools::Finalise(matrix_col); //PETSc 3.19 doesn't like mixing "set" mode (above) with "add" mode
            PetscVecTools::AddScaledVector(mDirichletBoundaryConditionsVector, matrix_col, minus_value);
            PetscTools::Destroy(matrix_col);
        }
    }
 
    if (type!=NONLINEAR_PROBLEM_APPLY_TO_RESIDUAL_ONLY) // i.e. doing a whole linear system
    {
        // Now zero the appropriate rows and columns of the matrix. If the matrix is symmetric we apply the
        // boundary conditions in a way the symmetry isn't lost (rows and columns). If not only the row is
        // zeroed.
        if (applySymmetrically)
        {
            PetscMatTools::ZeroRowsAndColumnsWithValueOnDiagonal(mSystemLhsMatrix, rows, 1.0);
            PetscMatTools::ZeroRowsAndColumnsWithValueOnDiagonal(mPreconditionMatrix, rows, 1.0);
 
            // Apply the RHS boundary conditions modification if required.
            // (AddScaledVector now needs assembly to be finalised.)
            PetscVecTools::Finalise(mLinearSystemRhsVector);
            PetscVecTools::AddScaledVector(mLinearSystemRhsVector, mDirichletBoundaryConditionsVector, 1.0);
        }
        else
        {
            PetscMatTools::ZeroRowsWithValueOnDiagonal(mSystemLhsMatrix, rows, 1.0);
            PetscMatTools::ZeroRowsWithValueOnDiagonal(mPreconditionMatrix, rows, 1.0);
        }
    }
 
    if (type!=LINEAR_PROBLEM)
    {
        for (unsigned i=0; i<rows.size(); i++)
        {
            PetscVecTools::SetElement(mResidualVector, rows[i], values[i]);
        }
    }
 
    if (type!=NONLINEAR_PROBLEM_APPLY_TO_RESIDUAL_ONLY)
    {
        for (unsigned i=0; i<rows.size(); i++)
        {
            PetscVecTools::SetElement(mLinearSystemRhsVector, rows[i], values[i]);
        }
    }
}
 
template<unsigned DIM>
void AbstractContinuumMechanicsSolver<DIM>::AddIdentityBlockForDummyPressureVariables(ApplyDirichletBcsType type)
{
    assert(mCompressibilityType==INCOMPRESSIBLE);
 
    int lo, hi;
    VecGetOwnershipRange(mResidualVector, &lo, &hi);
 
    for (unsigned i=0; i<mrQuadMesh.GetNumNodes(); i++)
    {
        if (mrQuadMesh.GetNode(i)->IsInternal())
        {
            unsigned row = (DIM+1)*i + DIM; // DIM+1 is the problem dimension
            if (lo <= (int)row && (int)row < hi)
            {
                if (type!=LINEAR_PROBLEM)
                {
                    PetscVecTools::SetElement(mResidualVector, row, mCurrentSolution[row]-0.0);
                }
                if (type!=NONLINEAR_PROBLEM_APPLY_TO_RESIDUAL_ONLY) // ie doing a whole linear system
                {
                    double rhs_vector_val = type==LINEAR_PROBLEM ? 0.0 : mCurrentSolution[row]-0.0;
                    PetscVecTools::SetElement(mLinearSystemRhsVector, row, rhs_vector_val);
                    // This assumes the row is already zero, which is should be..
                    PetscMatTools::SetElement(mSystemLhsMatrix, row, row, 1.0);
                    PetscMatTools::SetElement(mPreconditionMatrix, row, row, 1.0);
                }
            }
        }
    }
}
 
template<unsigned DIM>
void AbstractContinuumMechanicsSolver<DIM>::AllocateMatrixMemory()
{
    Vec template_vec = mrQuadMesh.GetDistributedVectorFactory()->CreateVec(mProblemDimension);
 
    // three vectors
    VecDuplicate(template_vec, &mResidualVector);
    VecDuplicate(mResidualVector, &mLinearSystemRhsVector);
    // the one is only allocated if it will be needed (in ApplyDirichletBoundaryConditions),
    // depending on whether the matrix is kept symmetric.
    mDirichletBoundaryConditionsVector = nullptr;
    PetscTools::Destroy(template_vec);
 
    // two matrices
 
    int lo, hi;
    VecGetOwnershipRange(mResidualVector, &lo, &hi);
    PetscInt local_size = hi - lo;
 
 
    if (DIM==2)
    {
        // 2D: N elements around a point => 7N+3 non-zeros in that row? Assume N<=10 (structured mesh would have N_max=6) => 73.
        unsigned num_non_zeros = std::min(75u, mNumDofs);
 
        PetscTools::SetupMat(mSystemLhsMatrix, mNumDofs, mNumDofs, num_non_zeros, local_size, local_size);
        PetscTools::SetupMat(mPreconditionMatrix, mNumDofs, mNumDofs, num_non_zeros, local_size, local_size);
    }
    else
    {
        assert(DIM==3);
 
        // in 3d we get the number of containing elements for each node and use that to obtain an upper bound
        // for the number of non-zeros for each DOF associated with that node.
 
        int* num_non_zeros_each_row = new int[mNumDofs];
        for (unsigned i=0; i<mNumDofs; i++)
        {
            num_non_zeros_each_row[i] = 0;
        }
 
        for (typename AbstractMesh<DIM,DIM>::NodeIterator iter = mrQuadMesh.GetNodeIteratorBegin();
             iter != mrQuadMesh.GetNodeIteratorEnd();
             ++iter)
        {
            // this upper bound neglects the fact that two containing elements will share the same nodes..
            // 4 = max num dofs associated with this node
            // 30 = 3*9+3 = 3 dimensions x 9 other nodes on this element   +  3 vertices with a pressure unknown
            unsigned num_non_zeros_upper_bound = 4 + 30*iter->GetNumContainingElements();
 
            num_non_zeros_upper_bound = std::min(num_non_zeros_upper_bound, mNumDofs);
 
            unsigned i = iter->GetIndex();
 
            num_non_zeros_each_row[mProblemDimension*i + 0] = num_non_zeros_upper_bound;
            num_non_zeros_each_row[mProblemDimension*i + 1] = num_non_zeros_upper_bound;
            num_non_zeros_each_row[mProblemDimension*i + 2] = num_non_zeros_upper_bound;
 
            if (mCompressibilityType==INCOMPRESSIBLE)
            {
                if (!iter->IsInternal())
                {
                    num_non_zeros_each_row[mProblemDimension*i + 3] = num_non_zeros_upper_bound;
                }
                else
                {
                    num_non_zeros_each_row[mProblemDimension*i + 3] = 1;
                }
            }
        }
 
        // NOTE: PetscTools::SetupMat() or the below creates a MATAIJ matrix, which means the matrix will
        // be of type MATSEQAIJ if num_procs=1 and MATMPIAIJ otherwise. In the former case
        // MatSeqAIJSetPreallocation MUST be called [MatMPIAIJSetPreallocation will have
        // no effect (silently)], and vice versa in the latter case
 
        //PetscTools::SetupMat(mSystemLhsMatrix, mNumDofs, mNumDofs, 0, PETSC_DECIDE, PETSC_DECIDE);
        //PetscTools::SetupMat(mPreconditionMatrix, mNumDofs, mNumDofs, 0, PETSC_DECIDE, PETSC_DECIDE);
 
        // possible todo: create a PetscTools::SetupMatNoAllocation()
 
 
#if (PETSC_VERSION_MAJOR == 2 && PETSC_VERSION_MINOR == 2) //PETSc 2.2
        MatCreate(PETSC_COMM_WORLD,local_size,local_size,mNumDofs,mNumDofs,&mSystemLhsMatrix);
        MatCreate(PETSC_COMM_WORLD,local_size,local_size,mNumDofs,mNumDofs,&mPreconditionMatrix);
#else //New API
        MatCreate(PETSC_COMM_WORLD,&mSystemLhsMatrix);
        MatCreate(PETSC_COMM_WORLD,&mPreconditionMatrix);
        MatSetSizes(mSystemLhsMatrix,local_size,local_size,mNumDofs,mNumDofs);
        MatSetSizes(mPreconditionMatrix,local_size,local_size,mNumDofs,mNumDofs);
#endif
 
        if (PetscTools::IsSequential())
        {
            MatSetType(mSystemLhsMatrix, MATSEQAIJ);
            MatSetType(mPreconditionMatrix, MATSEQAIJ);
            MatSeqAIJSetPreallocation(mSystemLhsMatrix,    PETSC_DEFAULT, num_non_zeros_each_row);
            MatSeqAIJSetPreallocation(mPreconditionMatrix, PETSC_DEFAULT, num_non_zeros_each_row);
        }
        else
        {
            int* num_non_zeros_each_row_in_diag = new int[local_size];
            int* num_non_zeros_each_row_off_diag = new int[local_size];
            for (unsigned i=0; i<unsigned(local_size); i++)
            {
                num_non_zeros_each_row_in_diag[i] = num_non_zeros_each_row[lo+i];
                num_non_zeros_each_row_off_diag[i] = num_non_zeros_each_row[lo+i];
                // In the on process ("diagonal block") there cannot be more non-zero columns specified than there are rows
                if (num_non_zeros_each_row_in_diag[i] > local_size)
                {
                    num_non_zeros_each_row_in_diag[i] = local_size;
                }
            }
 
            MatSetType(mSystemLhsMatrix, MATMPIAIJ);
            MatSetType(mPreconditionMatrix, MATMPIAIJ);
            MatMPIAIJSetPreallocation(mSystemLhsMatrix,    PETSC_DEFAULT, num_non_zeros_each_row_in_diag, PETSC_DEFAULT, num_non_zeros_each_row_off_diag);
            MatMPIAIJSetPreallocation(mPreconditionMatrix, PETSC_DEFAULT, num_non_zeros_each_row_in_diag, PETSC_DEFAULT, num_non_zeros_each_row_off_diag);
        }
 
        MatSetFromOptions(mSystemLhsMatrix);
        MatSetFromOptions(mPreconditionMatrix);
#if (PETSC_VERSION_MAJOR == 3) //PETSc 3.x.x
        MatSetOption(mSystemLhsMatrix, MAT_IGNORE_OFF_PROC_ENTRIES, PETSC_TRUE);
        MatSetOption(mPreconditionMatrix, MAT_IGNORE_OFF_PROC_ENTRIES, PETSC_TRUE);
#else
        MatSetOption(mSystemLhsMatrix, MAT_IGNORE_OFF_PROC_ENTRIES);
        MatSetOption(mPreconditionMatrix, MAT_IGNORE_OFF_PROC_ENTRIES);
#endif
 
        //unsigned total_non_zeros = 0;
        //for (unsigned i=0; i<mNumDofs; i++)
        //{
        //   total_non_zeros += num_non_zeros_each_row[i];
        //}
        //std::cout << total_non_zeros << " versus " << 500*mNumDofs << "\n" << std::flush;
 
        delete [] num_non_zeros_each_row;
    }
}
#endif // ABSTRACTCONTINUUMMECHANICSSOLVER_HPP_