ImmersedBoundaryCellPopulation.cpp

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*/
 
#include "ImmersedBoundaryCellPopulation.hpp"
 
#include <iomanip>
 
#include "ApoptoticCellProperty.hpp"
#include "CellPopulationElementWriter.hpp"
#include "ImmersedBoundaryMeshWriter.hpp"
#include "ImmersedBoundaryBoundaryCellWriter.hpp"
#include "ShortAxisImmersedBoundaryDivisionRule.hpp"
#include "Warnings.hpp"
 
template <unsigned DIM>
ImmersedBoundaryCellPopulation<DIM>::ImmersedBoundaryCellPopulation(
    ImmersedBoundaryMesh<DIM, DIM>& rMesh,
    std::vector<CellPtr>& rCells,
    bool deleteMesh,
    bool validate,
    const std::vector<unsigned> locationIndices)
    : AbstractOffLatticeCellPopulation<DIM>(rMesh, rCells, locationIndices),
      mDeleteMesh(deleteMesh),
      mIntrinsicSpacing(0.01),
      mPopulationHasActiveSources(false),
      mOutputNodeRegionToVtk(false),
      mReMeshFrequency(UINT_MAX)
{
    mpImmersedBoundaryMesh = static_cast<ImmersedBoundaryMesh<DIM, DIM>*>(&(this->mrMesh));
    mpImmersedBoundaryDivisionRule.reset(new ShortAxisImmersedBoundaryDivisionRule<DIM>());
 
    /*
     * If no location indices are specified, associate with elements from the
     * mesh (assumed to be sequentially ordered).
     */
    std::list<CellPtr>::iterator it = this->mCells.begin();
    for (unsigned i = 0; it != this->mCells.end(); ++it, ++i)
    {
        // Assume that the ordering matches
        unsigned index = locationIndices.empty() ? i : locationIndices[i];
        AbstractCellPopulation<DIM, DIM>::AddCellUsingLocationIndex(index, *it);
    }
 
    // Check each element has only one cell attached
    if (validate)
    {
        Validate();
    }
 
    mInteractionDistance = 0.05 * CalculateIntrinsicCellSize();
 
    // Set the mesh division spacing distance
    rMesh.SetElementDivisionSpacing(0.25 * mInteractionDistance);
    rMesh.SetNeighbourDist(0.5 * mInteractionDistance);
}
 
template <unsigned DIM>
ImmersedBoundaryCellPopulation<DIM>::ImmersedBoundaryCellPopulation(
    ImmersedBoundaryMesh<DIM, DIM>& rMesh)
    : AbstractOffLatticeCellPopulation<DIM>(rMesh),
      mDeleteMesh(true),
      mIntrinsicSpacing(0.01),
      mPopulationHasActiveSources(false),
      mOutputNodeRegionToVtk(false),
      mReMeshFrequency(UINT_MAX)
{
    mpImmersedBoundaryMesh = static_cast<ImmersedBoundaryMesh<DIM, DIM>*>(&(this->mrMesh));
}
 
template <unsigned DIM>
ImmersedBoundaryCellPopulation<DIM>::~ImmersedBoundaryCellPopulation()
{
    if (mDeleteMesh)
    {
        delete &this->mrMesh;
    }
}
 
template <unsigned DIM>
double ImmersedBoundaryCellPopulation<DIM>::CalculateIntrinsicCellSize()
{
    double average_intrinsic_size = 0.0;
 
    for (auto elem_iter = mpImmersedBoundaryMesh->GetElementIteratorBegin();
         elem_iter != mpImmersedBoundaryMesh->GetElementIteratorEnd();
         ++elem_iter)
    {
        average_intrinsic_size += mpImmersedBoundaryMesh->GetVolumeOfElement(elem_iter->GetIndex());
    }
 
    return sqrt(average_intrinsic_size / mpImmersedBoundaryMesh->GetNumElements());
}
 
template <unsigned DIM>
double ImmersedBoundaryCellPopulation<DIM>::GetDampingConstant(unsigned nodeIndex)
{
    return 0.0;
}
 
template <unsigned DIM>
ImmersedBoundaryMesh<DIM, DIM>& ImmersedBoundaryCellPopulation<DIM>::rGetMesh()
{
    return *mpImmersedBoundaryMesh;
}
 
template <unsigned DIM>
const ImmersedBoundaryMesh<DIM, DIM>& ImmersedBoundaryCellPopulation<DIM>::rGetMesh() const
{
    return *mpImmersedBoundaryMesh;
}
 
template <unsigned DIM>
ImmersedBoundaryElement<DIM, DIM>* ImmersedBoundaryCellPopulation<DIM>::GetElement(unsigned elementIndex)
{
    return mpImmersedBoundaryMesh->GetElement(elementIndex);
}
 
template <unsigned DIM>
ImmersedBoundaryElement<DIM - 1, DIM>* ImmersedBoundaryCellPopulation<DIM>::GetLamina(unsigned laminaIndex)
{
    return mpImmersedBoundaryMesh->GetLamina(laminaIndex);
}
 
template <unsigned DIM>
unsigned ImmersedBoundaryCellPopulation<DIM>::GetNumNodes()
{
    return this->mrMesh.GetNumNodes();
}
 
template <unsigned DIM>
c_vector<double, DIM> ImmersedBoundaryCellPopulation<DIM>::GetLocationOfCellCentre(CellPtr pCell)
{
    return mpImmersedBoundaryMesh->GetCentroidOfElement(this->mCellLocationMap[pCell.get()]);
}
 
template <unsigned DIM>
Node<DIM>* ImmersedBoundaryCellPopulation<DIM>::GetNode(unsigned index)
{
    return this->mrMesh.GetNode(index);
}
 
template <unsigned DIM>
void ImmersedBoundaryCellPopulation<DIM>::SetInteractionDistance(double newDistance)
{
    assert(newDistance >= 0.0);
    mInteractionDistance = newDistance;
}
 
template <unsigned DIM>
double ImmersedBoundaryCellPopulation<DIM>::GetInteractionDistance() const
{
    return mInteractionDistance;
}
 
template <unsigned DIM>
void ImmersedBoundaryCellPopulation<DIM>::SetReMeshFrequency(unsigned newFrequency)
{
    mReMeshFrequency = newFrequency;
}
 
template <unsigned DIM>
unsigned ImmersedBoundaryCellPopulation<DIM>::GetReMeshFrequency() const
{
    return mReMeshFrequency;
}
 
template <unsigned DIM>
double ImmersedBoundaryCellPopulation<DIM>::GetIntrinsicSpacing() const
{
    return mIntrinsicSpacing;
}
 
template <unsigned DIM>
std::set<unsigned> ImmersedBoundaryCellPopulation<DIM>::GetNeighbouringLocationIndices(
    CellPtr pCell)
{
    return mpImmersedBoundaryMesh->GetNeighbouringElementIndices(this->GetLocationIndexUsingCell(pCell));
}
 
template <unsigned DIM>
unsigned ImmersedBoundaryCellPopulation<DIM>::AddNode(Node<DIM>* pNewNode)
{
    if (pNewNode == nullptr)
    {
        return 0;
    }
    return mpImmersedBoundaryMesh->AddNode(pNewNode);
}
 
template <unsigned DIM>
void ImmersedBoundaryCellPopulation<DIM>::SetNode(unsigned nodeIndex, ChastePoint<DIM>& rNewLocation)
{
    mpImmersedBoundaryMesh->SetNode(nodeIndex, rNewLocation);
}
 
template <unsigned DIM>
ImmersedBoundaryElement<DIM, DIM>* ImmersedBoundaryCellPopulation<DIM>::GetElementCorrespondingToCell(
    CellPtr pCell)
{
    return mpImmersedBoundaryMesh->GetElement(this->GetLocationIndexUsingCell(pCell));
}
 
template <unsigned DIM>
unsigned ImmersedBoundaryCellPopulation<DIM>::GetNumElements()
{
    return mpImmersedBoundaryMesh->GetNumElements();
}
 
template <unsigned DIM>
unsigned ImmersedBoundaryCellPopulation<DIM>::GetNumLaminas()
{
    return mpImmersedBoundaryMesh->GetNumLaminas();
}
 
template <unsigned DIM>
CellPtr ImmersedBoundaryCellPopulation<DIM>::AddCell(
    CellPtr pNewCell,
    CellPtr pParentCell)
{
    // Get the element associated with this cell
    ImmersedBoundaryElement<DIM, DIM>* p_element = GetElementCorrespondingToCell(pParentCell);
 
    // Get the orientation of division
    c_vector<double, DIM> division_vector = mpImmersedBoundaryDivisionRule->CalculateCellDivisionVector(pParentCell, *this);
 
    // Divide the element
    unsigned new_elem_idx = mpImmersedBoundaryMesh->DivideElementAlongGivenAxis(p_element, division_vector, true);
 
    // Associate the new cell with the element
    this->mCells.push_back(pNewCell);
 
    // Update location cell map
    CellPtr p_created_cell = this->mCells.back();
    this->SetCellUsingLocationIndex(new_elem_idx, p_created_cell);
    this->mCellLocationMap[p_created_cell.get()] = new_elem_idx;
 
    return p_created_cell;
}
 
template <unsigned DIM>
unsigned ImmersedBoundaryCellPopulation<DIM>::RemoveDeadCells()
{
    unsigned num_removed = 0;
 
    for (std::list<CellPtr>::iterator it = this->mCells.begin();
         it != this->mCells.end();
         )
    {
        if ((*it)->IsDead())
        {
            // Count the cell as dead
            num_removed++;
 
            if (!(this->GetElement(this->GetLocationIndexUsingCell((*it)))->IsDeleted()))
            {
                this->GetElement(this->GetLocationIndexUsingCell(*it))->MarkAsDeleted();
            }
 
            // Delete the cell
            it = this->mCells.erase(it);
        }
        else
        {
            ++it;
        }
    }
    return num_removed;
}
 
template <unsigned DIM>
void ImmersedBoundaryCellPopulation<DIM>::UpdateNodeLocations(
    [[maybe_unused]] double dt) // [[maybe_unused]] due to unused-but-set-parameter warning in GCC 7,8,9
{
    if constexpr (DIM == 2)
    {
        // Helper variables, pre-declared for efficiency
        unsigned num_grid_pts_x = this->rGetMesh().GetNumGridPtsX();
        unsigned num_grid_pts_y = this->rGetMesh().GetNumGridPtsY();
 
        double characteristic_spacing = this->rGetMesh().GetCharacteristicNodeSpacing();
        double grid_spacing_x = 1.0 / (double)num_grid_pts_x;
        double grid_spacing_y = 1.0 / (double)num_grid_pts_y;
 
        unsigned first_idx_x;
        unsigned first_idx_y;
 
        std::vector<unsigned> x_indices(4);
        std::vector<unsigned> y_indices(4);
 
        std::vector<double> x_deltas(4);
        std::vector<double> y_deltas(4);
 
        double delta;
 
        c_vector<double, DIM> displacement = zero_vector<double>(DIM);
 
        // Get references to the fluid velocity grid
        const multi_array<double, 3>& vel_grids = this->rGetMesh().rGet2dVelocityGrids();
 
        // Iterate over all nodes
        c_vector<double, DIM> node_location;
        for (auto node_iter = this->rGetMesh().GetNodeIteratorBegin(false);
            node_iter != this->rGetMesh().GetNodeIteratorEnd();
            ++node_iter)
        {
            // Get location of current node
            node_location = node_iter->rGetLocation();
 
            // Get first grid index in each dimension, taking account of possible wrap-around
            first_idx_x = unsigned(floor(node_location[0] / grid_spacing_x)) + num_grid_pts_x - 1;
            first_idx_y = unsigned(floor(node_location[1] / grid_spacing_y)) + num_grid_pts_y - 1;
 
            // Calculate all four indices and deltas in each dimension
            for (unsigned i = 0; i < 4; i++)
            {
                x_indices[i] = (first_idx_x + i) % num_grid_pts_x;
                y_indices[i] = (first_idx_y + i) % num_grid_pts_y;
 
                x_deltas[i] = Delta1D(fabs(x_indices[i] * grid_spacing_x - node_location[0]), grid_spacing_x);
                y_deltas[i] = Delta1D(fabs(y_indices[i] * grid_spacing_x - node_location[1]), grid_spacing_y);
            }
 
            // Loop over the 4x4 grid which will influence the displacement of the current node
            for (unsigned x_idx = 0; x_idx < 4; ++x_idx)
            {
                for (unsigned y_idx = 0; y_idx < 4; ++y_idx)
                {
                    // The applied velocity is weighted by the delta function
                    delta = x_deltas[x_idx] * y_deltas[y_idx];
                    displacement[0] += vel_grids[0][x_indices[x_idx]][y_indices[y_idx]] * delta;
                    displacement[1] += vel_grids[1][x_indices[x_idx]][y_indices[y_idx]] * delta;
                }
            }
 
            // Normalise by timestep
            displacement *= dt;
 
            // If the displacement is too big, warn the user once and scale it back
            if (norm_2(displacement) > characteristic_spacing)
            {
                if (norm_2(displacement) > 10.0 * characteristic_spacing)
                {
                    EXCEPTION("Nodes are moving more than 10x CharacteristicNodeSpacing. Aborting.");
                }
 
                WARN_ONCE_ONLY("Nodes are moving more than the CharacteristicNodeSpacing. This could cause elements to become inverted so the motion has been restricted. Use a smaller timestep to avoid these warnings.");
                displacement *= characteristic_spacing / norm_2(displacement);
            }
 
            // Get new node location
            node_location += displacement;
 
            // Account for periodic boundary
            for (unsigned i = 0; i < DIM; ++i)
            {
                node_location[i] = fmod(node_location[i] + 1.0, 1.0);
            }
 
            // Create ChastePoint for new node location
            ChastePoint<DIM> new_point(node_location);
 
            // Move the node
            this->SetNode(node_iter->GetIndex(), new_point);
        }
 
        // If active sources, we need to update those location as well
        if (this->DoesPopulationHaveActiveSources())
        {
            std::vector<std::shared_ptr<FluidSource<DIM>>>& r_element_sources = this->rGetMesh().rGetElementFluidSources();
            std::vector<std::shared_ptr<FluidSource<DIM>>>& r_balance_sources = this->rGetMesh().rGetBalancingFluidSources();
 
            // Construct a vector of all sources combined
            std::vector<std::shared_ptr<FluidSource<DIM>>> combined_sources;
            combined_sources.insert(combined_sources.end(), r_element_sources.begin(), r_element_sources.end());
            combined_sources.insert(combined_sources.end(), r_balance_sources.begin(), r_balance_sources.end());
 
            c_vector<double, DIM> source_location;
 
            // Iterate over all sources and update their locations
            for (unsigned source_idx = 0; source_idx < combined_sources.size(); source_idx++)
            {
                // Get location of current node
                source_location = combined_sources[source_idx]->rGetLocation();
 
                // Get first grid index in each dimension, taking account of possible wrap-around
                first_idx_x = unsigned(floor(source_location[0] / grid_spacing_x)) + num_grid_pts_x - 1;
                first_idx_y = unsigned(floor(source_location[1] / grid_spacing_y)) + num_grid_pts_y - 1;
 
                // Calculate all four indices and deltas in each dimension
                for (unsigned i = 0; i < 4; ++i)
                {
                    x_indices[i] = (first_idx_x + i) % num_grid_pts_x;
                    y_indices[i] = (first_idx_y + i) % num_grid_pts_y;
 
                    x_deltas[i] = Delta1D(fabs(x_indices[i] * grid_spacing_x - source_location[0]), grid_spacing_x);
                    y_deltas[i] = Delta1D(fabs(y_indices[i] * grid_spacing_x - source_location[1]), grid_spacing_y);
                }
 
                // Loop over the 4x4 grid which will influence the displacement of the current node
                for (unsigned x_idx = 0; x_idx < 4; ++x_idx)
                {
                    for (unsigned y_idx = 0; y_idx < 4; ++y_idx)
                    {
                        // The applied velocity is weighted by the delta function
                        delta = x_deltas[x_idx] * y_deltas[y_idx];
                        displacement[0] += vel_grids[0][x_indices[x_idx]][y_indices[y_idx]] * delta;
                        displacement[1] += vel_grids[1][x_indices[x_idx]][y_indices[y_idx]] * delta;
                    }
                }
 
                // Normalise by timestep
                displacement *= dt;
 
                // If the displacement is too big, warn the user once and scale it back
                if (norm_2(displacement) > characteristic_spacing)
                {
                    if (norm_2(displacement) > 10.0 * characteristic_spacing)
                    {
                        EXCEPTION("Sources are moving more than 10x CharacteristicNodeSpacing. Aborting.");
                    }
 
                    WARN_ONCE_ONLY("Sources are moving more than the CharacteristicNodeSpacing. This could cause elements to become inverted so the motion has been restricted. Use a smaller timestep to avoid these warnings.");
                    displacement *= characteristic_spacing / norm_2(displacement);
                }
 
                // Get new node location
                source_location += displacement;
 
                // Account for periodic boundary
                for (unsigned i = 0; i < DIM; ++i)
                {
                    source_location[i] = fmod(source_location[i] + 1.0, 1.0);
                }
 
                // Move the node
                combined_sources[source_idx]->rGetModifiableLocation() = source_location;
            }
        }
 
        // Finally, call ReMesh if required
        const auto num_time_steps = SimulationTime::Instance()->GetTimeStepsElapsed();
        if (num_time_steps > 0 && num_time_steps % mReMeshFrequency == 0)
        {
            mpImmersedBoundaryMesh->ReMesh();
        }
    }
    else
    {
        NEVER_REACHED;
    }
}
 
template <unsigned DIM>
double ImmersedBoundaryCellPopulation<DIM>::Delta1D(double dist, double spacing)
{
    return (0.25 * (1.0 + cos(M_PI * dist / (2 * spacing))));
}
 
template <unsigned DIM>
bool ImmersedBoundaryCellPopulation<DIM>::IsCellAssociatedWithADeletedLocation(CellPtr pCell)
{
    return GetElementCorrespondingToCell(pCell)->IsDeleted();
}
 
template <unsigned DIM>
void ImmersedBoundaryCellPopulation<DIM>::Update(bool hasHadBirthsOrDeaths)
{
    // If the first cell has target atea property, assume there is a target area modifier in place
    if (this->Begin()->GetCellData()->HasItem("target area"))
    {
        for (auto cell_iter = this->Begin();
             cell_iter != this->End();
             ++cell_iter)
        {
            double target_area = cell_iter->GetCellData()->GetItem("target area");
            double actual_area = this->GetVolumeOfCell(*cell_iter);
 
            double strength = 1e-2 * (target_area - actual_area) / target_area;
 
            this->GetElementCorrespondingToCell(*cell_iter)->GetFluidSource()->SetStrength(strength);
        }
    }
}
 
template <unsigned DIM>
void ImmersedBoundaryCellPopulation<DIM>::Validate()
{
    // Check each element has only one cell attached
    std::vector<unsigned> validated_element = std::vector<unsigned>(this->GetNumElements(), 0);
    for (auto cell_iter = this->Begin();
         cell_iter != this->End();
         ++cell_iter)
    {
        unsigned elem_index = this->GetLocationIndexUsingCell(*cell_iter);
        validated_element[elem_index]++;
    }
 
    for (unsigned i = 0; i < validated_element.size(); ++i)
    {
        if (validated_element[i] == 0)
        {
            EXCEPTION("At time " << SimulationTime::Instance()->GetTime() << ", Element " << i << " does not appear to have a cell associated with it");
        }
 
        if (validated_element[i] > 1)
        {
            // This should never be reached as you can only set one cell per element index
            NEVER_REACHED;
            EXCEPTION("At time " << SimulationTime::Instance()->GetTime() << ", Element " << i << " appears to have " << validated_element[i] << " cells associated with it"); //LCOV_EXCL_LINE
        }
    }
}
 
template <unsigned DIM>
void ImmersedBoundaryCellPopulation<DIM>::AcceptPopulationWriter(
    boost::shared_ptr<AbstractCellPopulationWriter<DIM, DIM> > pPopulationWriter)
{
    pPopulationWriter->Visit(this);
}
 
template <unsigned DIM>
void ImmersedBoundaryCellPopulation<DIM>::AcceptPopulationEventWriter(
    boost::shared_ptr<AbstractCellPopulationEventWriter<DIM, DIM> > pPopulationEventWriter)
{
    pPopulationEventWriter->Visit(this);
}
 
template <unsigned DIM>
void ImmersedBoundaryCellPopulation<DIM>::AcceptPopulationCountWriter(
    boost::shared_ptr<AbstractCellPopulationCountWriter<DIM, DIM> > pPopulationCountWriter)
{
    pPopulationCountWriter->Visit(this);
}
 
template <unsigned DIM>
void ImmersedBoundaryCellPopulation<DIM>::AcceptCellWriter(
    boost::shared_ptr<AbstractCellWriter<DIM, DIM> > pCellWriter, CellPtr pCell)
{
    pCellWriter->VisitCell(pCell, this);
}
 
template <unsigned DIM>
double ImmersedBoundaryCellPopulation<DIM>::GetVolumeOfCell(CellPtr pCell)
{
    // Get the element index corresponding to this cell
    unsigned elem_index = this->GetLocationIndexUsingCell(pCell);
 
    // Get the cell's volume from the immersed boundary mesh
    double cell_volume = mpImmersedBoundaryMesh->GetVolumeOfElement(elem_index);
 
    return cell_volume;
}
 
template <unsigned DIM>
void ImmersedBoundaryCellPopulation<DIM>::WriteVtkResultsToFile(
    const std::string& rDirectory)
{
    // Create mesh writer for VTK output
    ImmersedBoundaryMeshWriter<DIM, DIM> mesh_writer(rDirectory, "results", false);
 
    // Find the cell overlap information, and get the number of cell parts needed for each element
    mesh_writer.FindElementOverlaps(*mpImmersedBoundaryMesh);
    const std::vector<std::vector<unsigned>>& r_elem_parts = mesh_writer.rGetElementParts();
 
    // Iterate over any cell writers that are present
    for (auto cell_writer_iter = this->mCellWriters.begin();
         cell_writer_iter != this->mCellWriters.end();
         ++cell_writer_iter)
    {
        // Create vector to store VTK cell data
        std::vector<double> vtk_cell_data;
 
        // Iterate over immersed boundary elements
        for (auto elem_iter = mpImmersedBoundaryMesh->GetElementIteratorBegin();
             elem_iter != mpImmersedBoundaryMesh->GetElementIteratorEnd();
             ++elem_iter)
        {
            /*
             * Get index of this element in the mesh, and the number of parts it
             * is broken into for visualisation.
             */
            const unsigned elem_index = elem_iter->GetIndex();
            const auto num_elem_parts = r_elem_parts[elem_index].empty() ? 1 : r_elem_parts[elem_index].size();
 
            // Get the cell corresponding to this element
            CellPtr p_cell = this->GetCellUsingLocationIndex(elem_index);
            assert(p_cell);
 
            /*
             * Populate the vector of VTK cell data. We loop over the number of
             * output cells as this takes into account that some elements will
             * be broken into pieces for visualisation.
             */
            for (unsigned elem_part = 0; elem_part < num_elem_parts; ++elem_part)
            {
                vtk_cell_data.push_back((*cell_writer_iter)->GetCellDataForVtkOutput(p_cell, this));
            }
        }
 
        /*
         * Iterate over immersed boundary laminas (no associated cell) to ensure
         * vtk_cell_data is the correct size.
         */
        for (auto lam_iter = mpImmersedBoundaryMesh->GetLaminaIteratorBegin();
             lam_iter != mpImmersedBoundaryMesh->GetLaminaIteratorEnd();
             ++lam_iter)
        {
            vtk_cell_data.push_back(-1.0);
        }
 
        mesh_writer.AddCellData((*cell_writer_iter)->GetVtkCellDataName(), vtk_cell_data);
    }
 
    /*
     * When outputting any CellData, we assume that the first cell is
     * representative of all cells.
     */
    const unsigned num_cell_data_items = this->Begin()->GetCellData()->GetNumItems();
    std::vector<std::string> cell_data_names = this->Begin()->GetCellData()->GetKeys();
 
    std::vector<std::vector<double>> cell_data;
    for (unsigned var = 0; var < num_cell_data_items; ++var)
    {
        std::vector<double> cell_data_var;
        cell_data.push_back(cell_data_var);
    }
 
    // Iterate over immersed boundary elements
    for (auto elem_iter = mpImmersedBoundaryMesh->GetElementIteratorBegin();
         elem_iter != mpImmersedBoundaryMesh->GetElementIteratorEnd();
         ++elem_iter)
    {
        /*
         * Get index of this element in the mesh, and the number of parts it is
         * broken into for visualisation.
         */
        const unsigned elem_index = elem_iter->GetIndex();
        const auto num_elem_parts = r_elem_parts[elem_index].empty() ? 1 : r_elem_parts[elem_index].size();
 
        // Get the cell corresponding to this element
        CellPtr p_cell = this->GetCellUsingLocationIndex(elem_index);
        assert(p_cell);
 
        for (unsigned var = 0; var < num_cell_data_items; var++)
        {
            /*
             * Populate the vector of VTK cell data. We loop over the number of
             * output cells as this takes into account that some elements will
             * be broken into pieces for visualisation.
             */
            for (unsigned elem_part = 0; elem_part < num_elem_parts; ++elem_part)
            {
                cell_data[var].push_back(p_cell->GetCellData()->GetItem(cell_data_names[var]));
            }
        }
    }
 
    /*
     * Iterate over immersed boundary laminas (no associated cell) to ensure
     * cell_data is the correct size.
     */
    for (auto lam_iter = mpImmersedBoundaryMesh->GetLaminaIteratorBegin();
         lam_iter != mpImmersedBoundaryMesh->GetLaminaIteratorEnd();
         ++lam_iter)
    {
        for (unsigned var = 0; var < num_cell_data_items; ++var)
        {
            cell_data[var].push_back(DOUBLE_UNSET);
        }
    }
 
    for (unsigned var = 0; var < num_cell_data_items; ++var)
    {
        mesh_writer.AddCellData(cell_data_names[var], cell_data[var]);
    }
 
    // Write node regions
    if (mOutputNodeRegionToVtk)
    {
        std::vector<double> node_regions;
        for (auto node_iter = mpImmersedBoundaryMesh->GetNodeIteratorBegin();
             node_iter != mpImmersedBoundaryMesh->GetNodeIteratorEnd();
             ++node_iter)
        {
            node_regions.push_back(static_cast<double>(node_iter->GetRegion()));
        }
        mesh_writer.AddPointData("Node Regions", node_regions);
    }
 
    unsigned num_timesteps = SimulationTime::Instance()->GetTimeStepsElapsed();
    std::stringstream time;
    time << num_timesteps;
 
    mesh_writer.WriteVtkUsingMesh(*mpImmersedBoundaryMesh, time.str());
 
    *(this->mpVtkMetaFile) << "        <DataSet timestep=\"";
    *(this->mpVtkMetaFile) << num_timesteps;
    *(this->mpVtkMetaFile) << "\" group=\"\" part=\"0\" file=\"results_";
    *(this->mpVtkMetaFile) << num_timesteps;
    *(this->mpVtkMetaFile) << ".vtu\"/>\n";
}
 
template <unsigned DIM>
void ImmersedBoundaryCellPopulation<DIM>::OpenWritersFiles(
    OutputFileHandler& rOutputFileHandler)
{
    if (this->mOutputResultsForChasteVisualizer)
    {
        if (!this->template HasWriter<CellPopulationElementWriter>())
        {
            this->template AddPopulationWriter<CellPopulationElementWriter>();
        }
    }
 
    AbstractCellPopulation<DIM>::OpenWritersFiles(rOutputFileHandler);
}
 
template <unsigned DIM>
void ImmersedBoundaryCellPopulation<DIM>::OutputCellPopulationParameters(
    out_stream& rParamsFile)
{
    // Add the division rule parameters
    *rParamsFile << "\t\t<ImmersedBoundaryDivisionRule>\n";
    mpImmersedBoundaryDivisionRule->OutputCellImmersedBoundaryDivisionRuleInfo(rParamsFile);
    *rParamsFile << "\t\t</ImmersedBoundaryDivisionRule>\n";
 
    // Call method on direct parent class
    AbstractOffLatticeCellPopulation<DIM>::OutputCellPopulationParameters(rParamsFile);
}
 
template <unsigned DIM>
double ImmersedBoundaryCellPopulation<DIM>::GetWidth(const unsigned& rDimension)
{
    double width = this->mrMesh.GetWidth(rDimension);
    return width;
}
 
template <unsigned DIM>
std::set<unsigned> ImmersedBoundaryCellPopulation<DIM>::GetNeighbouringNodeIndices(
    unsigned index)
{
    return mpImmersedBoundaryMesh->GetNeighbouringNodeIndices(index);
}
 
template <unsigned DIM>
TetrahedralMesh<DIM, DIM>* ImmersedBoundaryCellPopulation<DIM>::GetTetrahedralMeshForPdeModifier()
{
    // This method only works in 2D sequential
    assert(PetscTools::IsSequential());
    if constexpr (DIM == 2)
    {
        unsigned num_vertex_nodes = mpImmersedBoundaryMesh->GetNumNodes();
        unsigned num_vertex_elements = mpImmersedBoundaryMesh->GetNumElements();
 
        std::string mesh_file_name = "mesh";
 
        // Get a unique temporary foldername
        std::stringstream pid;
        pid << getpid();
        OutputFileHandler output_file_handler("2D_temporary_tetrahedral_mesh_" + pid.str());
        std::string output_dir = output_file_handler.GetOutputDirectoryFullPath();
 
        // Compute the number of nodes in the TetrahedralMesh
        unsigned num_tetrahedral_nodes = num_vertex_nodes + num_vertex_elements;
 
        // Write node file
        out_stream p_node_file = output_file_handler.OpenOutputFile(mesh_file_name+".node");
        (*p_node_file) << std::scientific;
        (*p_node_file) << std::setprecision(20);
        (*p_node_file) << num_tetrahedral_nodes << "\t2\t0\t1" << std::endl;
 
        // Begin by writing each node in the VertexMesh
        auto nodes = mpImmersedBoundaryMesh->rGetNodes();
        for (auto p_node : nodes)
        {
            unsigned index = p_node->GetIndex();
            const c_vector<double, DIM>& r_location = p_node->rGetLocation();
            unsigned is_boundary_node = p_node->IsBoundaryNode() ? 1 : 0;
 
            (*p_node_file) << index << "\t" << r_location[0] << "\t" << r_location[1] << "\t" << is_boundary_node << std::endl;
        }
 
        // Now write an additional node at each ImmersedBoundaryElement's centroid
        unsigned num_tetrahedral_elements = 0;
        for (unsigned vertex_elem_index = 0;
             vertex_elem_index < num_vertex_elements;
             ++vertex_elem_index)
        {
            unsigned index = num_vertex_nodes + vertex_elem_index;
 
            c_vector<double, DIM> location = mpImmersedBoundaryMesh->GetCentroidOfElement(vertex_elem_index);
 
            // Any node located at a ImmersedBoundaryElement's centroid will not be a boundary node
            unsigned is_boundary_node = 0;
            (*p_node_file) << index << "\t" << location[0] << "\t" << location[1] << "\t" << is_boundary_node << std::endl;
 
            // Also keep track of how many tetrahedral elements there will be
            num_tetrahedral_elements += mpImmersedBoundaryMesh->GetElement(vertex_elem_index)->GetNumNodes();
        }
        p_node_file->close();
 
        // Write element file
        out_stream p_elem_file = output_file_handler.OpenOutputFile(mesh_file_name+".ele");
        (*p_elem_file) << std::scientific;
        (*p_elem_file) << num_tetrahedral_elements << "\t3\t0" << std::endl;
 
        std::set<std::pair<unsigned, unsigned> > tetrahedral_edges;
 
        unsigned tetrahedral_elem_index = 0;
        for (unsigned vertex_elem_index = 0;
             vertex_elem_index < num_vertex_elements;
             ++vertex_elem_index)
        {
            ImmersedBoundaryElement<DIM, DIM>* p_vertex_element = mpImmersedBoundaryMesh->GetElement(vertex_elem_index);
 
            // Iterate over nodes owned by this ImmersedBoundaryElement
            unsigned num_nodes_in_vertex_element = p_vertex_element->GetNumNodes();
            for (unsigned local_index = 0;
                 local_index < num_nodes_in_vertex_element;
                 ++local_index)
            {
                unsigned node_0_index = p_vertex_element->GetNodeGlobalIndex(local_index);
                unsigned node_1_index = p_vertex_element->GetNodeGlobalIndex((local_index+1)%num_nodes_in_vertex_element);
                unsigned node_2_index = num_vertex_nodes + vertex_elem_index;
 
                (*p_elem_file) << tetrahedral_elem_index++ << "\t" << node_0_index << "\t" << node_1_index << "\t" << node_2_index << std::endl;
 
                // Add edges to the set if they are not already present
                std::pair<unsigned, unsigned> edge_0 = this->CreateOrderedPair(node_0_index, node_1_index);
                std::pair<unsigned, unsigned> edge_1 = this->CreateOrderedPair(node_1_index, node_2_index);
                std::pair<unsigned, unsigned> edge_2 = this->CreateOrderedPair(node_2_index, node_0_index);
 
                tetrahedral_edges.insert(edge_0);
                tetrahedral_edges.insert(edge_1);
                tetrahedral_edges.insert(edge_2);
            }
        }
        p_elem_file->close();
 
        // Write edge file
        out_stream p_edge_file = output_file_handler.OpenOutputFile(mesh_file_name+".edge");
        (*p_edge_file) << std::scientific;
        (*p_edge_file) << tetrahedral_edges.size() << "\t1" << std::endl;
 
        unsigned edge_index = 0;
        for (auto edge_iter = tetrahedral_edges.begin();
             edge_iter != tetrahedral_edges.end();
             ++edge_iter)
        {
            std::pair<unsigned, unsigned> this_edge = *edge_iter;
 
            // To be a boundary edge both nodes need to be boundary nodes.
            bool is_boundary_edge = false;
            if (this_edge.first < mpImmersedBoundaryMesh->GetNumNodes() &&
                this_edge.second < mpImmersedBoundaryMesh->GetNumNodes())
            {
                is_boundary_edge = (mpImmersedBoundaryMesh->GetNode(this_edge.first)->IsBoundaryNode() &&
                                    mpImmersedBoundaryMesh->GetNode(this_edge.second)->IsBoundaryNode() );
            }
            unsigned is_boundary_edge_unsigned = is_boundary_edge ? 1 : 0;
 
            (*p_edge_file) << edge_index++ << "\t" << this_edge.first << "\t" << this_edge.second << "\t" << is_boundary_edge_unsigned << std::endl;
        }
        p_edge_file->close();
 
        // Having written the mesh to file, now construct it using TrianglesMeshReader
        TetrahedralMesh<DIM, DIM>* p_mesh = new TetrahedralMesh<DIM, DIM>;
 
        // Nested scope so reader is destroyed before we remove the temporary files
        {
            TrianglesMeshReader<DIM, DIM> mesh_reader(output_dir + mesh_file_name);
            p_mesh->ConstructFromMeshReader(mesh_reader);
        }
 
        // Delete the temporary files
        output_file_handler.FindFile("").Remove();
 
        /*
         * The original files have been deleted, it is better if the mesh object
         * forgets about them.
         */
        p_mesh->SetMeshHasChangedSinceLoading();
 
        return p_mesh;
    }
    else
    {
        EXCEPTION("ImmersedBoundaryCellPopulation::GetTetrahedralMeshForPDEModifier is only implemented in 2D");
    }
}
 
template <unsigned DIM>
bool ImmersedBoundaryCellPopulation<DIM>::IsPdeNodeAssociatedWithNonApoptoticCell(unsigned pdeNodeIndex)
{
    bool non_apoptotic_cell_present = true;
 
    if (pdeNodeIndex < this->GetNumNodes())
    {
        std::set<unsigned> containing_element_indices = this->GetNode(pdeNodeIndex)->rGetContainingElementIndices();
 
        for (auto iter = containing_element_indices.begin();
             iter != containing_element_indices.end();
             iter++) //LCOV_EXCL_LINE
        {
            if (this->GetCellUsingLocationIndex(*iter)->template HasCellProperty<ApoptoticCellProperty>() )
            {
                non_apoptotic_cell_present = false;
                break;
            }
        }
    }
    else
    {
        /*
         * This node of the tetrahedral finite element mesh is in the centre of
         * the element of the immersed boundary-based cell population, so we can use an
         * offset to compute which cell to interrogate.
         */
        non_apoptotic_cell_present = !(this->GetCellUsingLocationIndex(pdeNodeIndex - this->GetNumNodes())->template HasCellProperty<ApoptoticCellProperty>());
    }
 
    return non_apoptotic_cell_present;
}
 
template <unsigned DIM>
double ImmersedBoundaryCellPopulation<DIM>::GetCellDataItemAtPdeNode(
    unsigned pdeNodeIndex,
    std::string& rVariableName,
    bool dirichletBoundaryConditionApplies,
    double dirichletBoundaryValue)
{
    unsigned num_nodes = this->GetNumNodes();
    double value = 0.0;
 
    /*
     * Cells correspond to nodes in the centre of the vertex element; nodes on
     * vertices have averaged values from containing cells.
     */
    if (pdeNodeIndex >= num_nodes)
    {
        // Offset to relate elements in vertex mesh to nodes in tetrahedral mesh
        assert(pdeNodeIndex-num_nodes < num_nodes);
 
        CellPtr p_cell = this->GetCellUsingLocationIndex(pdeNodeIndex - num_nodes);
        value = p_cell->GetCellData()->GetItem(rVariableName);
    }
    else
    {
        if (dirichletBoundaryConditionApplies)
        {
            // We need to impose the Dirichlet boundaries again here as not represented in cell data
            value = dirichletBoundaryValue;
        }
        else
        {
            assert(pdeNodeIndex < num_nodes);
            Node<DIM>* p_node = this->GetNode(pdeNodeIndex);
 
            // Average over data from containing elements (cells)
            std::set<unsigned> containing_elements = p_node->rGetContainingElementIndices();
            for (auto index_iter = containing_elements.begin();
                 index_iter != containing_elements.end();
                 ++index_iter)
            {
                assert(*index_iter < num_nodes);
                CellPtr p_cell = this->GetCellUsingLocationIndex(*index_iter);
                value += p_cell->GetCellData()->GetItem(rVariableName);
            }
            value /= containing_elements.size();
        }
    }
 
    return value;
}
 
template <unsigned DIM>
boost::shared_ptr<AbstractImmersedBoundaryDivisionRule<DIM> > ImmersedBoundaryCellPopulation<DIM>::GetImmersedBoundaryDivisionRule()
{
    return mpImmersedBoundaryDivisionRule;
}
 
template <unsigned DIM>
void ImmersedBoundaryCellPopulation<DIM>::SetImmersedBoundaryDivisionRule(
    boost::shared_ptr<AbstractImmersedBoundaryDivisionRule<DIM> > pImmersedBoundaryDivisionRule)
{
    mpImmersedBoundaryDivisionRule = pImmersedBoundaryDivisionRule;
}
 
template <unsigned DIM>
bool ImmersedBoundaryCellPopulation<DIM>::DoesPopulationHaveActiveSources() const
{
    return mPopulationHasActiveSources;
}
 
template <unsigned DIM>
bool ImmersedBoundaryCellPopulation<DIM>::IsCellOnBoundary(CellPtr pCell)
{
    return this->GetElementCorrespondingToCell(pCell)->IsElementOnBoundary();
}
 
template <unsigned DIM>
void ImmersedBoundaryCellPopulation<DIM>::SetIfPopulationHasActiveSources(
    bool hasActiveSources)
{
    mPopulationHasActiveSources = hasActiveSources;
}
 
template <unsigned DIM>
void ImmersedBoundaryCellPopulation<DIM>::SetOutputNodeRegionToVtk(
    bool outputNodeRegionsToVtk)
{
    mOutputNodeRegionToVtk = outputNodeRegionsToVtk;
}
 
template <unsigned DIM>
double ImmersedBoundaryCellPopulation<DIM>::GetDefaultTimeStep()
{
    return 0.002;
}
 
// Explicit instantiation
template class ImmersedBoundaryCellPopulation<1>;
template class ImmersedBoundaryCellPopulation<2>;
template class ImmersedBoundaryCellPopulation<3>;
 
// Serialization for Boost >= 1.36
#include "SerializationExportWrapperForCpp.hpp"
EXPORT_TEMPLATE_CLASS_SAME_DIMS(ImmersedBoundaryCellPopulation)