ImmersedBoundaryMesh.cpp

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*/
 
#include "ImmersedBoundaryMesh.hpp"
 
#include <algorithm>
#include <cmath>
 
#include "ImmersedBoundaryEnumerations.hpp"
#include "MeshUtilityFunctions.hpp"
#include "RandomNumberGenerator.hpp"
#include "UblasCustomFunctions.hpp"
#include "Warnings.hpp"
 
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::ImmersedBoundaryMesh(std::vector<Node<SPACE_DIM>*> nodes,
                                                                   std::vector<ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>*> elements,
                                                                   std::vector<ImmersedBoundaryElement<ELEMENT_DIM - 1, SPACE_DIM>*> laminas,
                                                                   unsigned numGridPtsX,
                                                                   unsigned numGridPtsY)
        : mNumGridPtsX(numGridPtsX),
          mNumGridPtsY(numGridPtsY),
          mCharacteristicNodeSpacing(DOUBLE_UNSET),
          mElementDivisionSpacing(DOUBLE_UNSET),
          mNeighbourDist(0.1),
          mSummaryOfNodeLocations(0.0),
          mCellRearrangementThreshold(0.05)
{
    // Clear mNodes and mElements
    Clear();
 
    switch (SPACE_DIM)
    {
        case 2:
            m2dVelocityGrids.resize(extents[2][mNumGridPtsX][mNumGridPtsY]);
            break;
 
        case 3:
            EXCEPTION("Not implemented yet in 3D");
            break;
 
        //LCOV_EXCL_START
        default:
            NEVER_REACHED;
            break;
        //LCOV_EXCL_STOP
    }
 
    // Populate mNodes, mElements, and mLaminas
    for (unsigned node_it = 0; node_it < nodes.size(); node_it++)
    {
        Node<SPACE_DIM>* p_temp_node = nodes[node_it];
        this->mNodes.push_back(p_temp_node);
    }
    for (unsigned elem_it = 0; elem_it < elements.size(); elem_it++)
    {
        ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>* p_temp_element = elements[elem_it];
        mElements.push_back(p_temp_element);
    }
    for (unsigned lam_it = 0; lam_it < laminas.size(); lam_it++)
    {
        ImmersedBoundaryElement<ELEMENT_DIM - 1, SPACE_DIM>* p_temp_lamina = laminas[lam_it];
        mLaminas.push_back(p_temp_lamina);
    }
 
    // Register elements with nodes
    for (unsigned elem_it = 0; elem_it < mElements.size(); elem_it++)
    {
        ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>* p_element = mElements[elem_it];
 
        unsigned element_index = p_element->GetIndex();
        unsigned num_nodes_in_element = p_element->GetNumNodes();
 
        for (unsigned node_idx = 0; node_idx < num_nodes_in_element; node_idx++)
        {
            p_element->GetNode(node_idx)->AddElement(element_index);
        }
    }
 
    // Register laminas with nodes
    //\todo is there a way we can register laminas with nodes?
 
    // Set characteristic node spacing to the average distance between nodes in elements
    double total_perimeter = 0.0;
    unsigned total_nodes = 0;
    for (unsigned elem_it = 0; elem_it < mElements.size(); elem_it++)
    {
        total_perimeter += this->GetSurfaceAreaOfElement(elem_it);
        total_nodes += mElements[elem_it]->GetNumNodes();
    }
    mCharacteristicNodeSpacing = total_perimeter / double(total_nodes);
 
    // Position fluid sources at the centroid of each element, and set strength to zero
    for (unsigned elem_it = 0; elem_it < elements.size(); elem_it++)
    {
        unsigned elem_idx = mElements[elem_it]->GetIndex();
 
        // Create a new fluid source at the correct location for each element
        unsigned source_idx = static_cast<unsigned>(mElementFluidSources.size());
        c_vector<double, SPACE_DIM> source_location = this->GetCentroidOfElement(elem_idx);
        mElementFluidSources.push_back(std::make_shared<FluidSource<SPACE_DIM>>(source_idx, source_location));
 
        // Set source parameters
        mElementFluidSources.back()->SetAssociatedElementIndex(elem_idx);
        mElementFluidSources.back()->SetStrength(0.0);
 
        // Associate source with element
        mElements[elem_it]->SetFluidSource(mElementFluidSources.back());
    }
 
    //Set up a number of sources to balance any active sources associated with elements
    double balancing_source_spacing = 2.0 * mCharacteristicNodeSpacing;
 
    // We start at the characteristic spacing in from the left-hand end, and place a source every 2 spacings
    double current_location = mCharacteristicNodeSpacing;
 
    while (current_location < 1.0)
    {
        // Create a new fluid source at the current x-location and zero y-location
        unsigned source_idx = static_cast<unsigned>(mBalancingFluidSources.size());
        mBalancingFluidSources.push_back(std::make_shared<FluidSource<SPACE_DIM>>(source_idx, current_location));
 
        mBalancingFluidSources.back()->SetStrength(0.0);
 
        // Increment the current location
        current_location += balancing_source_spacing;
    }
 
    // Calculate a default neighbour dist, as half the root of the average element volume
    constexpr double power = 1.0 / SPACE_DIM;
    const double total_volume_of_elems = std::accumulate(mElements.begin(), mElements.end(), 0.0,
                                                         [this](double d, ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>* a)
                                                         {
                                                             return d + this->GetVolumeOfElement(a->GetIndex());
                                                         });
    mNeighbourDist = 0.5 * std::pow(total_volume_of_elems / mElements.size(), power);
 
    this->mMeshChangesDuringSimulation = true;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetElongationShapeFactorOfElement(
    [[maybe_unused]] unsigned index) // [[maybe_unused]] due to unused-but-set-parameter warning in GCC 7,8,9
{
    if constexpr (SPACE_DIM == 2)
    {
        c_vector<double, 3> moments = CalculateMomentsOfElement(index);
 
        double discriminant = sqrt((moments(0) - moments(1)) * (moments(0) - moments(1)) + 4.0 * moments(2) * moments(2));
 
        // Note that as the matrix of second moments of area is symmetric, both its eigenvalues are real
        double largest_eigenvalue = (moments(0) + moments(1) + discriminant) * 0.5;
        double smallest_eigenvalue = (moments(0) + moments(1) - discriminant) * 0.5;
 
        double elongation_shape_factor = sqrt(largest_eigenvalue / smallest_eigenvalue);
        return elongation_shape_factor;
    }
    else
    {
        NEVER_REACHED;
    }
}
 
bool CustomComparisonForVectorX(c_vector<double, 2> vecA, c_vector<double, 2> vecB)
{
    return vecA[0] < vecB[0];
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetTortuosityOfMesh()
{
    if constexpr (SPACE_DIM == 2)
    {
        double total_length = 0.0;
 
        // Get the current elements
        std::vector<c_vector<double, 2> > centroids(mElements.size());
        for (unsigned elem_it = 0; elem_it < mElements.size(); elem_it++)
        {
            centroids[elem_it] = this->GetCentroidOfElement(mElements[elem_it]->GetIndex());
        }
 
        // Sort centroids by X
        std::sort(centroids.begin(), centroids.end(), CustomComparisonForVectorX);
 
        // Calculate piecewise linear length connecting centroids
        for (unsigned cent_it = 1; cent_it < centroids.size(); cent_it++)
        {
            total_length += norm_2(centroids[cent_it - 1] - centroids[cent_it]);
        }
 
        double straight_line_length = norm_2(centroids[0] - centroids[centroids.size() - 1]);
 
        // Avoid division by zero
        if (straight_line_length < 0.00001)
        {
            return 0;
        }
 
        return total_length / straight_line_length;
    }
    else
    {
        NEVER_REACHED;
    }
}
 
bool CustomComparisonForSkewnessMeasure(std::pair<unsigned, c_vector<double, 2> > pairA, std::pair<unsigned, c_vector<double, 2> > pairB)
{
    return pairA.second[0] < pairB.second[0];
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetSkewnessOfElementMassDistributionAboutAxis(
    [[maybe_unused]] unsigned elemIndex, // [[maybe_unused]] due to unused-but-set-parameter warning in GCC 7,8,9
    c_vector<double, SPACE_DIM> axis)
{
    /*
     * Method outline:
     *
     * Given an arbitrary axis and a closed polygon, we calculate the skewness of the mass distribution of the polygon
     * perpendicular to the axis.  This is used as a measure of asymmetry.
     *
     * To simplify calculating the mass distribution, we translate the centroid of the element to the origin and rotate
     * about the centroid so the axis is vertical; then we sort all the nodes in ascending order of their x-coordinate.
     *
     * For each node in order, we need to know the length of the intersection through the node of the vertical line with
     * the polygon.  Once calculated, we have a piecewise-linear PDF for the mass distribution, which can be normalised
     * by the surface area of the polygon.
     *
     * By integrating the pdf directly, we can calculate exactly the necessary moments of the distribution needed for
     * the skewness.
     *
     * This method does not work for concave shapes, and emits a warning but continues anyway. Results will be close
     * to correct for mildly concave shapes or may be correct depending on the alignment of the element.
     */
 
    // This method only works in 2D
    if constexpr (SPACE_DIM == 2 && ELEMENT_DIM == 2)
    {
        // Get relevant info about the element
        ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>* p_elem = this->GetElement(elemIndex);
        unsigned num_nodes = p_elem->GetNumNodes();
        double area_of_elem = this->GetVolumeOfElement(elemIndex);
        c_vector<double, SPACE_DIM> centroid = this->GetCentroidOfElement(elemIndex);
 
        // Get the unit axis and trig terms for rotation
        c_vector<double, SPACE_DIM> unit_axis = axis / norm_2(axis);
        double sin_theta = unit_axis[0];
        double cos_theta = unit_axis[1];
 
        // We need the (rotated) node locations in two orders - original and ordered left-to-right.
        // For the latter we need to keep track of index, so we store that as part of a pair.
        std::vector<c_vector<double, SPACE_DIM> > node_locations_original_order;
        std::vector<std::pair<unsigned, c_vector<double, SPACE_DIM> > > ordered_locations;
 
        // Get the node locations of the current element relative to its centroid, and rotate them
        for (unsigned node_idx = 0; node_idx < num_nodes; node_idx++)
        {
            const c_vector<double, SPACE_DIM>& node_location = p_elem->GetNode(node_idx)->rGetLocation();
 
            c_vector<double, SPACE_DIM> displacement = this->GetVectorFromAtoB(centroid, node_location);
 
            c_vector<double, SPACE_DIM> rotated_location;
            rotated_location[0] = cos_theta * displacement[0] - sin_theta * displacement[1];
            rotated_location[1] = sin_theta * displacement[0] + cos_theta * displacement[1];
 
            node_locations_original_order.push_back(rotated_location);
        }
 
        // Fill up a vector of identical points, and sort it so nodes are ordered in ascending x value
        for (unsigned i = 0; i < node_locations_original_order.size(); i++)
        {
            ordered_locations.push_back(std::pair<unsigned, c_vector<double, SPACE_DIM> >(i, node_locations_original_order[i]));
        }
 
        std::sort(ordered_locations.begin(), ordered_locations.end(), CustomComparisonForSkewnessMeasure);
 
        /*
        * For each node, we must find every place where the axis (now rotated to be vertical) intersects the polygon:
        *
        *       |
        *     __|______
        *    /  |      \
        *   /   |       \
        *  /____|___    |
        *       |  |    |
        *  _____|__|    |
        *  \    |       |
        *   \   |      /
        *    \__|_____/
        *       |
        *       |
        *       ^
        * For instance, the number of times the vertical intersects the polygon above is 4 and, for each node, we need to
        * find all such intersections.  We can do this by checking where the dot product of the the vector a with the unit
        * x direction changes sign as we iterate over the original node locations, where a is the vector from the current
        * node to the test node.
        */
 
        // For each node, we keep track of all the y-locations where the vertical through the node meets the polygon
        std::vector<std::vector<double> > knots(num_nodes);
 
        // Iterate over ordered locations from left to right
        for (unsigned location = 0; location < num_nodes; location++)
        {
            // Get the two parts of the pair
            unsigned this_idx = ordered_locations[location].first;
            // this_location is the location of the node relative to centroid
            c_vector<double, SPACE_DIM> this_location = ordered_locations[location].second;
 
            // The y-coordinate of the current location is always a knot
            // because we are passing the vertical through this node
            knots[location].push_back(this_location[1]);
 
            // To calculate all the intersection points, we need to iterate over every other location and see, sequentially,
            // if the x-coordinate of location i+1 and i+2 crosses the x-coordinate of the current location.
            // i.e. check whether each other edge crosses the vertical through this_location/current node
            unsigned next_idx = (this_idx + 1) % num_nodes;
            c_vector<double, SPACE_DIM> to_previous = node_locations_original_order[next_idx] - this_location;
            c_vector<double, SPACE_DIM> to_next;
 
            for (unsigned node_idx = this_idx + 2; node_idx < this_idx + num_nodes; node_idx++)
            {
                unsigned idx = node_idx % num_nodes;
 
                to_next = node_locations_original_order[idx] - this_location;
 
                // If the segment between to_previous and to_next intersects the vertical through this_location, the clause
                // in the if statement below will be triggered
                if (to_previous[0] * to_next[0] <= 0.0)
                {
                    // Find how far between to_previous and to_next the point of intersection is
                    double interp = 0.5;
                    if (to_previous[0] - to_next[0] != 0.0)
                    {
                        interp = to_previous[0] / (to_previous[0] - to_next[0]);
                    }
 
                    assert(interp >= 0.0 && interp <= 1.0);
 
                    // Record the y-value of the intersection point
                    double new_intersection = this_location[1] + to_previous[1] + interp * (to_next[1] - to_previous[1]);
                    knots[location].push_back(new_intersection);
                }
 
                to_previous = to_next;
            }
 
            if (knots[location].size() > 2)
            {
                WARN_ONCE_ONLY("Axis intersects polygon more than 2 times (concavity) - check element is fairly convex.");
            }
        }
 
        // For ease, construct a vector of the x-locations of all the nodes, in order
        std::vector<double> ordered_x(num_nodes);
        for (unsigned location = 0; location < num_nodes; location++)
        {
            ordered_x[location] = ordered_locations[location].second[0];
        }
 
        // Calculate the mass contributions at each x-location - this is the length of the intersection of the vertical
        // through each location
        std::vector<double> mass_contributions(num_nodes);
        for (unsigned i = 0; i < num_nodes; i++)
        {
            std::sort(knots[i].begin(), knots[i].end());
 
            switch (knots[i].size())
            {
                case 1:
                    mass_contributions[i] = 0.0;
                    break;
 
                case 2:
                    mass_contributions[i] = knots[i][1] - knots[i][0];
                    break;
 
                default:
                    mass_contributions[i] += knots[i][knots[i].size()-1] - knots[i][0];
            }
 
            // Normalise, so that these lengths define a pdf
            mass_contributions[i] /= area_of_elem;
        }
 
        // Calculate moments. Because we just have a bunch of linear segments, we can integrate the pdf exactly
        double e_x0 = 0.0;
        double e_x1 = 0.0;
        double e_x2 = 0.0;
        double e_x3 = 0.0;
 
        for (unsigned i = 1; i < num_nodes; i++)
        {
            double x0 = ordered_x[i - 1];
            double x1 = ordered_x[i];
 
            double fx0 = mass_contributions[i - 1];
            double fx1 = mass_contributions[i];
 
            // We need squared, cubed, ..., order 5 for each x
            double x0_2 = x0 * x0;
            double x0_3 = x0_2 * x0;
            double x0_4 = x0_3 * x0;
            double x0_5 = x0_4 * x0;
 
            double x1_2 = x1 * x1;
            double x1_3 = x1_2 * x1;
            double x1_4 = x1_3 * x1;
            double x1_5 = x1_4 * x1;
 
            if (x1 - x0 > 0)
            {
                // Calculate y = mx + c for this section of the pdf
                double m = (fx1 - fx0) / (x1 - x0);
                double c = fx0 - m * x0;
 
                e_x0 += m * (x1_2 - x0_2) / 2.0 + c * (x1 - x0);
                e_x1 += m * (x1_3 - x0_3) / 3.0 + c * (x1_2 - x0_2) / 2.0;
                e_x2 += m * (x1_4 - x0_4) / 4.0 + c * (x1_3 - x0_3) / 3.0;
                e_x3 += m * (x1_5 - x0_5) / 5.0 + c * (x1_4 - x0_4) / 4.0;
            }
        }
 
        // Check that we have correctly defined a pdf
        if (fabs(e_x0 - 1.0) < 1e-6)
        {
            WARN_ONCE_ONLY("Mass distribution of element calculated incorrectly due to element concavity. Skewness may not be correct!");
        }
 
        // Calculate the standard deviation, and return the skewness
        double sd = sqrt(e_x2 - e_x1 * e_x1);
        return (e_x3 - 3.0 * e_x1 * sd * sd - e_x1 * e_x1 * e_x1) / (sd * sd * sd);
    }
    else
    {
        NEVER_REACHED;
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
ChasteCuboid<SPACE_DIM> ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::CalculateBoundingBoxOfElement(unsigned index)
{
    ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>* p_elem = this->GetElement(index);
 
    // Get the location of node zero as a reference point
    c_vector<double, SPACE_DIM> ref_point = p_elem->GetNode(0)->rGetLocation();
 
    // Vector to represent the n-dimensional 'bottom left'-most node
    c_vector<double, SPACE_DIM> bottom_left = zero_vector<double>(SPACE_DIM);
 
    // Vector to represent the n-dimensional 'top right'-most node
    c_vector<double, SPACE_DIM> top_right = zero_vector<double>(SPACE_DIM);
 
    // Loop over all nodes in the element and update bottom_left and top_right, relative to node zero to account for periodicity
    for (unsigned node_idx = 0; node_idx < p_elem->GetNumNodes(); node_idx++)
    {
        c_vector<double, SPACE_DIM> vec_to_node = this->GetVectorFromAtoB(ref_point, p_elem->GetNode(node_idx)->rGetLocation());
 
        for (unsigned dim = 0; dim < SPACE_DIM; dim++)
        {
            if (vec_to_node[dim] < bottom_left[dim])
            {
                bottom_left[dim] = vec_to_node[dim];
            }
            else if (vec_to_node[dim] > top_right[dim])
            {
                top_right[dim] = vec_to_node[dim];
            }
        }
    }
 
    // Create Chaste points, rescaled by the location of node zero
    ChastePoint<SPACE_DIM> min(bottom_left + ref_point);
    ChastePoint<SPACE_DIM> max(top_right + ref_point);
 
    return ChasteCuboid<SPACE_DIM>(min, max);
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::ImmersedBoundaryMesh()
{
    this->mMeshChangesDuringSimulation = false;
    Clear();
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::~ImmersedBoundaryMesh()
{
    Clear();
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
std::set<unsigned> ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetNeighbouringNodeIndices(unsigned nodeIndex)
{
    // Create a set of neighbouring node indices
    std::set<unsigned> neighbouring_node_indices;
 
    // Find the indices of the elements owned by this node
    std::set<unsigned> containing_elem_indices = this->GetNode(nodeIndex)->rGetContainingElementIndices();
 
    // Iterate over these elements
    for (std::set<unsigned>::iterator elem_iter = containing_elem_indices.begin();
         elem_iter != containing_elem_indices.end();
         ++elem_iter)
    {
        // Find the local index of this node in this element
        unsigned local_index = GetElement(*elem_iter)->GetNodeLocalIndex(nodeIndex);
 
        // Find the global indices of the preceding and successive nodes in this element
        unsigned num_nodes = GetElement(*elem_iter)->GetNumNodes();
        unsigned previous_local_index = (local_index + num_nodes - 1) % num_nodes;
        unsigned next_local_index = (local_index + 1) % num_nodes;
 
        // Add the global indices of these two nodes to the set of neighbouring node indices
        neighbouring_node_indices.insert(GetElement(*elem_iter)->GetNodeGlobalIndex(previous_local_index));
        neighbouring_node_indices.insert(GetElement(*elem_iter)->GetNodeGlobalIndex(next_local_index));
    }
 
    return neighbouring_node_indices;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::SolveNodeMapping(unsigned index) const
{
    assert(index < this->mNodes.size());
    return index;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::SolveElementMapping(unsigned index) const
{
    assert(index < this->mElements.size());
    return index;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::SolveBoundaryElementMapping(unsigned index) const
{
    return index;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::Clear()
{
    // Delete elements
    for (unsigned i = 0; i < mElements.size(); i++)
    {
        delete mElements[i];
    }
    mElements.clear();
 
    // Delete laminas
    for (auto lamina : mLaminas)
    {
        delete(lamina);
    }
    mLaminas.clear();
 
    // Delete nodes
    for (unsigned i = 0; i < this->mNodes.size(); i++)
    {
        delete this->mNodes[i];
    }
    this->mNodes.clear();
 
    mBalancingFluidSources.clear();
    mElementFluidSources.clear();
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetCharacteristicNodeSpacing() const
{
    return mCharacteristicNodeSpacing;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetSpacingRatio() const
{
    return mCharacteristicNodeSpacing / (1.0 / double(mNumGridPtsX));
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
const std::vector<Node<SPACE_DIM>*>& ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::rGetNodes() const
{
    return this->mNodes;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetNumGridPtsX() const
{
    return mNumGridPtsX;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetNumGridPtsY() const
{
    return mNumGridPtsY;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::SetNumGridPtsX(unsigned meshPointsX)
{
    mNumGridPtsX = meshPointsX;
    m2dVelocityGrids.resize(extents[2][mNumGridPtsX][mNumGridPtsY]);
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::SetNumGridPtsY(unsigned meshPointsY)
{
    mNumGridPtsY = meshPointsY;
    m2dVelocityGrids.resize(extents[2][mNumGridPtsX][mNumGridPtsY]);
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::SetNumGridPtsXAndY(unsigned numGridPts)
{
    mNumGridPtsX = numGridPts;
    mNumGridPtsY = numGridPts;
    m2dVelocityGrids.resize(extents[2][mNumGridPtsX][mNumGridPtsY]);
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::SetCharacteristicNodeSpacing(double nodeSpacing)
{
    mCharacteristicNodeSpacing = nodeSpacing;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
std::vector<std::shared_ptr<FluidSource<SPACE_DIM>>>& ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::rGetElementFluidSources()
{
    mElementFluidSources.clear();
    for (const auto& element : mElements) {
        if (element->GetFluidSource() != nullptr) {
            mElementFluidSources.push_back(element->GetFluidSource());
        }
    }
    return mElementFluidSources;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
std::vector<std::shared_ptr<FluidSource<SPACE_DIM>>>& ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::rGetBalancingFluidSources()
{
    return mBalancingFluidSources;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
const multi_array<double, 3>& ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::rGet2dVelocityGrids() const
{
    return m2dVelocityGrids;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
multi_array<double, 3>& ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::rGetModifiable2dVelocityGrids()
{
    return m2dVelocityGrids;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::AddNode(
    Node<SPACE_DIM>* pNewNode)
{
    pNewNode->SetIndex(this->mNodes.size());
    this->mNodes.push_back(pNewNode);
    return this->mNodes.size() - 1;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::SetCellRearrangementThreshold(double cellRearrangementThreshold)
{
    mCellRearrangementThreshold = cellRearrangementThreshold;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetCellRearrangementThreshold()
{
    return mCellRearrangementThreshold;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
c_vector<double, SPACE_DIM> ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetVectorFromAtoB(const c_vector<double, SPACE_DIM>& rLocation1, const c_vector<double, SPACE_DIM>& rLocation2)
{
    // This code currently assumes the grid is precisely [0,1)x[0,1)
    c_vector<double, SPACE_DIM> vector = rLocation2 - rLocation1;
 
    /*
     * Handle the periodic condition here: if the points are more
     * than 0.5 apart in any direction, choose -(1.0-dist).
     */
    for (unsigned dim = 0; dim < SPACE_DIM; dim++)
    {
        if (fabs(vector[dim]) > 0.5)
        {
            vector[dim] = copysign(fabs(vector[dim]) - 1.0, -vector[dim]);
        }
    }
 
    return vector;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::SetNode(unsigned nodeIndex, ChastePoint<SPACE_DIM> point)
{
    this->mNodes[nodeIndex]->SetPoint(point);
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetNumNodes() const
{
    return this->mNodes.size();
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetNumElements() const
{
    return mElements.size();
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetNumAllElements() const
{
    return mElements.size();
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetNumLaminas() const
{
    return mLaminas.size();
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>* ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetElement(unsigned index) const
{
    assert(index < mElements.size());
    return mElements[index];
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
ImmersedBoundaryElement<ELEMENT_DIM - 1, SPACE_DIM>* ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetLamina(unsigned index) const
{
    assert(index < mLaminas.size());
    return mLaminas[index];
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetNeighbourDist() const
{
    return mNeighbourDist;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::SetNeighbourDist(double neighbourDist)
{
    mNeighbourDist = neighbourDist;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
c_vector<double, SPACE_DIM> ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetCentroidOfElement(
    [[maybe_unused]] unsigned index) // [[maybe_unused]] due to unused-but-set-parameter warning in GCC 7,8,9
{
    // Only implemented in 2D
    if constexpr (SPACE_DIM == 2)
    {
        ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>* p_element = GetElement(index);
 
        unsigned num_nodes = p_element->GetNumNodes();
        c_vector<double, SPACE_DIM> centroid = zero_vector<double>(SPACE_DIM);
 
        double centroid_x = 0;
        double centroid_y = 0;
 
        // Note that we cannot use GetVolumeOfElement() below as it returns the absolute, rather than signed, area
        double element_signed_area = 0.0;
 
        // Map the first vertex to the origin and employ GetVectorFromAtoB() to allow for periodicity
        c_vector<double, SPACE_DIM> first_node_location = p_element->GetNodeLocation(0);
        c_vector<double, SPACE_DIM> pos_1 = zero_vector<double>(SPACE_DIM);
 
        // Loop over vertices
        for (unsigned local_index = 0; local_index < num_nodes; local_index++)
        {
            c_vector<double, SPACE_DIM> next_node_location = p_element->GetNodeLocation((local_index + 1) % num_nodes);
            c_vector<double, SPACE_DIM> pos_2 = GetVectorFromAtoB(first_node_location, next_node_location);
 
            double this_x = pos_1[0];
            double this_y = pos_1[1];
            double next_x = pos_2[0];
            double next_y = pos_2[1];
 
            double signed_area_term = this_x * next_y - this_y * next_x;
 
            centroid_x += (this_x + next_x) * signed_area_term;
            centroid_y += (this_y + next_y) * signed_area_term;
            element_signed_area += 0.5 * signed_area_term;
 
            pos_1 = pos_2;
        }
 
        assert(element_signed_area != 0.0);
 
        // Finally, map back and employ GetVectorFromAtoB() to allow for periodicity
        centroid = first_node_location;
        centroid[0] += centroid_x / (6.0 * element_signed_area);
        centroid[1] += centroid_y / (6.0 * element_signed_area);
 
        centroid[0] = centroid[0] < 0 ? centroid[0] + 1.0 : fmod(centroid[0], 1.0);
        centroid[1] = centroid[1] < 0 ? centroid[1] + 1.0 : fmod(centroid[1], 1.0);
 
        return centroid;
    }
    else
    {
        NEVER_REACHED;
    }
}
 
template <>
void ImmersedBoundaryMesh<1, 1>::ConstructFromMeshReader(AbstractMeshReader<1, 1>& rMeshReader)
{
    EXCEPTION("ImmersedBoundaryMesh not yet supported for the specified dimensions");
}
 
template <>
void ImmersedBoundaryMesh<1, 2>::ConstructFromMeshReader(AbstractMeshReader<1, 2>& rMeshReader)
{
    EXCEPTION("ImmersedBoundaryMesh not yet supported for the specified dimensions");
}
 
template <>
void ImmersedBoundaryMesh<1, 3>::ConstructFromMeshReader(AbstractMeshReader<1, 3>& rMeshReader)
{
    EXCEPTION("ImmersedBoundaryMesh not yet supported for the specified dimensions");
}
 
template <>
void ImmersedBoundaryMesh<2, 3>::ConstructFromMeshReader(AbstractMeshReader<2, 3>& rMeshReader)
{
    EXCEPTION("ImmersedBoundaryMesh not yet supported for the specified dimensions");
}
 
template <>
void ImmersedBoundaryMesh<2, 2>::ConstructFromMeshReader(AbstractMeshReader<2, 2>& rMeshReader)
{
    ImmersedBoundaryMeshReader<2, 2>& rIBMeshReader = dynamic_cast<ImmersedBoundaryMeshReader<2, 2>&>(rMeshReader);
 
    assert(!rIBMeshReader.HasNodePermutation());
 
    // Store numbers of nodes and elements
    unsigned num_nodes = rIBMeshReader.GetNumNodes();
    unsigned num_elements = rIBMeshReader.GetNumElements();
    unsigned num_laminas = rIBMeshReader.GetNumLaminas();
    this->mCharacteristicNodeSpacing = rIBMeshReader.GetCharacteristicNodeSpacing();
 
    // Add nodes
    rIBMeshReader.Reset();
    mNodes.reserve(num_nodes);
    std::vector<double> node_data;
    for (unsigned node_idx = 0; node_idx < num_nodes; node_idx++)
    {
        node_data = rIBMeshReader.GetNextNode();
        unsigned is_boundary_node = (bool)node_data[2];
        node_data.pop_back();
        this->mNodes.push_back(new Node<2>(node_idx, node_data, is_boundary_node));
    }
 
    // Add laminas
    rIBMeshReader.Reset();
    mLaminas.reserve(num_laminas);
    for (unsigned lam_idx = 0; lam_idx < num_laminas; lam_idx++)
    {
        // Get the data for this element
        ImmersedBoundaryElementData lamina_data = rIBMeshReader.GetNextImmersedBoundaryLaminaData();
 
        // Get the nodes owned by this element
        std::vector<Node<2>*> nodes;
        unsigned num_nodes_in_lamina = lamina_data.NodeIndices.size();
        for (unsigned node_idx = 0; node_idx < num_nodes_in_lamina; node_idx++)
        {
            assert(lamina_data.NodeIndices[node_idx] < this->mNodes.size());
            nodes.push_back(this->mNodes[lamina_data.NodeIndices[node_idx]]);
        }
 
        // Use nodes and index to construct this element
        ImmersedBoundaryElement<1, 2>* p_lamina = new ImmersedBoundaryElement<1, 2>(lam_idx, nodes);
        mLaminas.push_back(p_lamina);
 
        if (rIBMeshReader.GetNumLaminaAttributes() > 0)
        {
            assert(rIBMeshReader.GetNumLaminaAttributes() == 1);
            unsigned attribute_value = lamina_data.AttributeValue;
            p_lamina->SetAttribute(attribute_value);
        }
    }
 
    // Add elements
    rIBMeshReader.Reset();
    mElements.reserve(num_elements);
    for (unsigned elem_idx = 0; elem_idx < num_elements; elem_idx++)
    {
        // Get the data for this element
        ImmersedBoundaryElementData element_data = rIBMeshReader.GetNextImmersedBoundaryElementData();
 
        // Get the nodes owned by this element
        std::vector<Node<2>*> nodes;
        unsigned num_nodes_in_element = element_data.NodeIndices.size();
        for (unsigned node_idx = 0; node_idx < num_nodes_in_element; node_idx++)
        {
            assert(element_data.NodeIndices[node_idx] < this->mNodes.size());
            nodes.push_back(this->mNodes[element_data.NodeIndices[node_idx]]);
        }
 
        // Use nodes and index to construct this element
        ImmersedBoundaryElement<2, 2>* p_element = new ImmersedBoundaryElement<2, 2>(elem_idx, nodes);
        mElements.push_back(p_element);
 
        if (rIBMeshReader.GetNumElementAttributes() > 0)
        {
            assert(rIBMeshReader.GetNumElementAttributes() == 1);
            unsigned attribute_value = element_data.AttributeValue;
            p_element->SetAttribute(attribute_value);
        }
    }
 
    // Get grid dimensions from grid file and set up grids accordingly
    this->mNumGridPtsX = rIBMeshReader.GetNumGridPtsX();
    this->mNumGridPtsY = rIBMeshReader.GetNumGridPtsY();
    m2dVelocityGrids.resize(extents[2][mNumGridPtsX][mNumGridPtsY]);
 
    // Construct the velocity grids from mesh reader
    for (unsigned dim = 0; dim < 2; dim++)
    {
        for (unsigned grid_row = 0; grid_row < mNumGridPtsY; grid_row++)
        {
            std::vector<double> next_row = rIBMeshReader.GetNextGridRow();
            assert(next_row.size() == mNumGridPtsX);
 
            for (unsigned i = 0; i < mNumGridPtsX; i++)
            {
                m2dVelocityGrids[dim][i][grid_row] = next_row[i];
            }
        }
    }
}
 
template <>
void ImmersedBoundaryMesh<3, 3>::ConstructFromMeshReader(AbstractMeshReader<3, 3>& rMeshReader)
{
    EXCEPTION("ImmersedBoundaryMesh not yet supported for the specified dimensions");
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetVolumeOfElement(unsigned index)
{
    if constexpr (SPACE_DIM == 2)
    {
        // Get pointer to this element
        ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>* p_element = GetElement(index);
 
        double element_volume = 0.0;
 
        // We map the first vertex to the origin and employ GetVectorFromAtoB() to allow for periodicity
        c_vector<double, SPACE_DIM> first_node_location = p_element->GetNodeLocation(0);
        c_vector<double, SPACE_DIM> pos_1 = zero_vector<double>(SPACE_DIM);
 
        unsigned num_nodes = p_element->GetNumNodes();
        for (unsigned local_index = 0; local_index < num_nodes; local_index++)
        {
            c_vector<double, SPACE_DIM> next_node_location = p_element->GetNodeLocation((local_index + 1) % num_nodes);
            c_vector<double, SPACE_DIM> pos_2 = GetVectorFromAtoB(first_node_location, next_node_location);
 
            double this_x = pos_1[0];
            double this_y = pos_1[1];
            double next_x = pos_2[0];
            double next_y = pos_2[1];
 
            element_volume += 0.5 * (this_x * next_y - next_x * this_y);
 
            pos_1 = pos_2;
        }
 
        // We take the absolute value just in case the nodes were really oriented clockwise
        return fabs(element_volume);
    }
    else
    {
        NEVER_REACHED;
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetSurfaceAreaOfElement(unsigned index)
{
    if constexpr (SPACE_DIM == 2)
    {
        // Get pointer to this element
        ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>* p_element = GetElement(index);
 
        double surface_area = 0.0;
        unsigned num_nodes = p_element->GetNumNodes();
        unsigned this_node_index = p_element->GetNodeGlobalIndex(0);
        for (unsigned local_index = 0; local_index < num_nodes; local_index++)
        {
            unsigned next_node_index = p_element->GetNodeGlobalIndex((local_index + 1) % num_nodes);
            surface_area += this->GetDistanceBetweenNodes(this_node_index, next_node_index);
            this_node_index = next_node_index;
        }
 
        return surface_area;
    }
    else
    {
        NEVER_REACHED;
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetVoronoiSurfaceAreaOfElement(const unsigned elemIdx)
{
    if constexpr (SPACE_DIM == 2)
    {
        UpdateNodeLocationsVoronoiDiagramIfOutOfDate();
        ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>* p_element = GetElement(elemIdx);
        double surface_area = 0.0;
 
        // Sum contributions from each node in the element
        for (unsigned local_idx = 0; local_idx < p_element->GetNumNodes(); local_idx++)
        {
            const unsigned global_node_idx = p_element->GetNodeGlobalIndex(local_idx);
 
            // Get the voronoi cell that corresponds to this node
            const unsigned voronoi_cell_id = mVoronoiCellIdsIndexedByNodeIndex[global_node_idx];
            const auto& voronoi_cell = mNodeLocationsVoronoiDiagram.cells()[voronoi_cell_id];
 
            // Iterate over the edges of this voronoi cell
            auto p_edge = voronoi_cell.incident_edge();
            do
            {
                // The global node index corresponding to a voronoi cell is encoded in its 'color' variable
                const unsigned twin_node_idx = p_edge->twin()->cell()->color();
                const unsigned twin_elem_idx = *this->GetNode(twin_node_idx)->ContainingElementsBegin();
 
                // Check if the nodes are in different elements
                if (elemIdx != twin_elem_idx)
                {
                    surface_area += this->CalculateLengthOfVoronoiEdge(*p_edge);
                }
 
                p_edge = p_edge->next();
            } while (p_edge != voronoi_cell.incident_edge());
        }
 
        return surface_area;
    }
    else
    {
        NEVER_REACHED;
    }
}
 
// Excluded from coverage as it is impossible to construct a 1 dimensional element currently
//LCOV_EXCL_START
template <>
double ImmersedBoundaryMesh<1, 1>::GetVoronoiSurfaceAreaOfElement(const unsigned elemIdx)
{
    return 0.0;
}
//LCOV_EXCL_STOP
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetAverageNodeSpacingOfElement(unsigned index, bool recalculate)
{
    if (recalculate || (this->GetElement(index)->GetAverageNodeSpacing() == DOUBLE_UNSET))
    {
        double average_node_spacing = this->GetSurfaceAreaOfElement(index) / this->GetElement(index)->GetNumNodes();
        this->GetElement(index)->SetAverageNodeSpacing(average_node_spacing);
 
        return average_node_spacing;
    }
    else
    {
        return this->GetElement(index)->GetAverageNodeSpacing();
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetAverageNodeSpacingOfLamina(unsigned index, bool recalculate)
{
    if (recalculate || (this->GetLamina(index)->GetAverageNodeSpacing() == DOUBLE_UNSET))
    {
        ImmersedBoundaryElement<ELEMENT_DIM - 1, SPACE_DIM>* p_lam = this->GetLamina(index);
 
        // Explicitly calculate the average node spacing
        double average_node_spacing = 0.0;
        for (unsigned node_it = 1; node_it < p_lam->GetNumNodes(); node_it++)
        {
            average_node_spacing += this->GetDistanceBetweenNodes(p_lam->GetNodeGlobalIndex(node_it),
                                                                  p_lam->GetNodeGlobalIndex(node_it - 1));
        }
 
        average_node_spacing /= (p_lam->GetNumNodes() - 1);
 
        // Set it for quick retrieval next time
        p_lam->SetAverageNodeSpacing(average_node_spacing);
        return average_node_spacing;
    }
    else
    {
        return this->GetLamina(index)->GetAverageNodeSpacing();
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetElementDivisionSpacing()
{
    return mElementDivisionSpacing;
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::SetElementDivisionSpacing(double elementDivisionSpacing)
{
    mElementDivisionSpacing = elementDivisionSpacing;
}
 
//                        2D-specific methods                       //
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
c_vector<double, 3> ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::CalculateMomentsOfElement(unsigned index)
{
    if constexpr (SPACE_DIM == 2)
    {
        // Define helper variables
        ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>* p_element = GetElement(index);
        unsigned num_nodes = p_element->GetNumNodes();
        c_vector<double, 3> moments = zero_vector<double>(3);
 
        // Since we compute I_xx, I_yy and I_xy about the centroid, we must shift each vertex accordingly
        c_vector<double, SPACE_DIM> centroid = GetCentroidOfElement(index);
 
        c_vector<double, SPACE_DIM> this_node_location = p_element->GetNodeLocation(0);
        c_vector<double, SPACE_DIM> pos_1 = this->GetVectorFromAtoB(centroid, this_node_location);
 
        for (unsigned local_index = 0; local_index < num_nodes; local_index++)
        {
            unsigned next_index = (local_index + 1) % num_nodes;
            c_vector<double, SPACE_DIM> next_node_location = p_element->GetNodeLocation(next_index);
            c_vector<double, SPACE_DIM> pos_2 = this->GetVectorFromAtoB(centroid, next_node_location);
 
            double signed_area_term = pos_1(0) * pos_2(1) - pos_2(0) * pos_1(1);
            // Ixx
            moments(0) += (pos_1(1) * pos_1(1) + pos_1(1) * pos_2(1) + pos_2(1) * pos_2(1)) * signed_area_term;
 
            // Iyy
            moments(1) += (pos_1(0) * pos_1(0) + pos_1(0) * pos_2(0) + pos_2(0) * pos_2(0)) * signed_area_term;
 
            // Ixy
            moments(2) += (pos_1(0) * pos_2(1) + 2 * pos_1(0) * pos_1(1) + 2 * pos_2(0) * pos_2(1) + pos_2(0) * pos_1(1)) * signed_area_term;
 
            pos_1 = pos_2;
        }
 
        moments(0) /= 12;
        moments(1) /= 12;
        moments(2) /= 24;
 
        /*
        * If the nodes owned by the element were supplied in a clockwise rather
        * than anticlockwise manner, or if this arose as a result of enforcing
        * periodicity, then our computed quantities will be the wrong sign, so
        * we need to fix this.
        */
        if (moments(0) < 0.0)
        {
            moments(0) = -moments(0);
            moments(1) = -moments(1);
            moments(2) = -moments(2);
        }
        return moments;
    }
    else
    {
        NEVER_REACHED;
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
c_vector<double, SPACE_DIM> ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetShortAxisOfElement(unsigned index)
{
    if constexpr (SPACE_DIM == 2)
    {
        c_vector<double, SPACE_DIM> short_axis = zero_vector<double>(SPACE_DIM);
 
        // Calculate the moments of the element about its centroid (recall that I_xx and I_yy must be non-negative)
        c_vector<double, 3> moments = CalculateMomentsOfElement(index);
 
        // Normalise the moments vector to remove problem of a very small discriminant (see #2874)
        moments /= norm_2(moments);
 
        // If the principal moments are equal...
        double discriminant = (moments(0) - moments(1)) * (moments(0) - moments(1)) + 4.0 * moments(2) * moments(2);
        if (fabs(discriminant) < DBL_EPSILON)
        {
            // ...then every axis through the centroid is a principal axis, so return a random unit vector
            short_axis(0) = RandomNumberGenerator::Instance()->ranf();
            short_axis(1) = sqrt(1.0 - short_axis(0) * short_axis(0));
        }
        else
        {
            // If the product of inertia is zero, then the coordinate axes are the principal axes
            if (fabs(moments(2)) < DBL_EPSILON)
            {
                if (moments(0) < moments(1))
                {
                    short_axis(0) = 0.0;
                    short_axis(1) = 1.0;
                }
                else
                {
                    short_axis(0) = 1.0;
                    short_axis(1) = 0.0;
                }
            }
            else
            {
                // Otherwise we find the eigenvector of the inertia matrix corresponding to the largest eigenvalue
                double lambda = 0.5 * (moments(0) + moments(1) + sqrt(discriminant));
 
                short_axis(0) = 1.0;
                short_axis(1) = (moments(0) - lambda) / moments(2);
 
                // Normalise the short axis before returning it
                short_axis /= norm_2(short_axis);
            }
        }
 
        return short_axis;
    }
    else
    {
        NEVER_REACHED;
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::DivideElementAlongGivenAxis(ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>* pElement,
                                                                                   c_vector<double, SPACE_DIM> axisOfDivision,
                                                                                   bool placeOriginalElementBelow)
{
    if constexpr (SPACE_DIM == 2 && ELEMENT_DIM == 2)
    {
        // Get the centroid of the element
        c_vector<double, SPACE_DIM> centroid = this->GetCentroidOfElement(pElement->GetIndex());
 
        // Create a vector perpendicular to the axis of division
        c_vector<double, SPACE_DIM> perp_axis;
        perp_axis(0) = -axisOfDivision(1);
        perp_axis(1) = axisOfDivision(0);
 
        /*
        * Find which edges the axis of division crosses by finding any node
        * that lies on the opposite side of the axis of division to its next
        * neighbour.
        */
 
        unsigned num_nodes = pElement->GetNumNodes();
        std::vector<unsigned> intersecting_nodes;
        bool is_current_node_on_left = (inner_prod(this->GetVectorFromAtoB(pElement->GetNodeLocation(0), centroid), perp_axis) >= 0);
        for (unsigned i = 0; i < num_nodes; i++)
        {
            bool is_next_node_on_left = (inner_prod(this->GetVectorFromAtoB(pElement->GetNodeLocation((i + 1) % num_nodes), centroid), perp_axis) >= 0);
            if (is_current_node_on_left != is_next_node_on_left)
            {
                intersecting_nodes.push_back(i);
            }
            is_current_node_on_left = is_next_node_on_left;
        }
 
        // If the axis of division does not cross two edges then we cannot proceed
        if (intersecting_nodes.size() != 2)
        {
            EXCEPTION("Cannot proceed with element division: the given axis of division does not cross two edges of the element"); // LCOV_EXCL_LINE
        }
 
        // Now call DivideElement() to divide the element using the nodes found above
        //unsigned new_element_index = 0;
        unsigned new_element_index = DivideElement(pElement,
                                                intersecting_nodes[0],
                                                intersecting_nodes[1],
                                                centroid,
                                                axisOfDivision);
 
        return new_element_index;
    }
    else
    {
        NEVER_REACHED;
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::DivideElementAlongShortAxis(ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>* pElement,
                                                                                   bool placeOriginalElementBelow)
{
    if constexpr (SPACE_DIM == 2 && ELEMENT_DIM == 2)
    {
        c_vector<double, SPACE_DIM> short_axis = this->GetShortAxisOfElement(pElement->GetIndex());
        unsigned new_element_index = DivideElementAlongGivenAxis(pElement, short_axis, placeOriginalElementBelow);
        return new_element_index;
    }
    else
    {
        NEVER_REACHED;
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::DivideElement(ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>* pElement,
                                                                     unsigned nodeAIndex,
                                                                     unsigned nodeBIndex,
                                                                     c_vector<double, SPACE_DIM> centroid,
                                                                     c_vector<double, SPACE_DIM> axisOfDivision)
{
    if constexpr (SPACE_DIM == 2 && ELEMENT_DIM == 2)
    {
        if (mElementDivisionSpacing == DOUBLE_UNSET)
        {
            EXCEPTION("The value of mElementDivisionSpacing has not been set.");
        }
 
        /*
        * Method outline:
        *
        *   Each element needs to end up with the same number of nodes as the original element, and those nodes will be
        *   equally spaced around the outline of each of the two daughter elements.
        *
        *   The two elements need to be divided by a distance of mElementDivisionSpacing, where the distance is measured
        *   perpendicular to the axis of division.
        *
        *   To achieve this, we find four 'corner' locations, each of which has a perpendicular distance from the axis of
        *   half the required spacing, and are found by using the locations from the existing element as a stencil.
        */
 
        double half_spacing = 0.5 * mElementDivisionSpacing;
 
        // Get unit vectors in the direction of the division axis, and the perpendicular
        c_vector<double, SPACE_DIM> unit_axis = axisOfDivision / norm_2(axisOfDivision);
        c_vector<double, SPACE_DIM> unit_perp;
        unit_perp[0] = -unit_axis[1];
        unit_perp[1] = unit_axis[0];
 
        unsigned num_nodes = pElement->GetNumNodes();
 
        /*
        * We first identify the start and end indices of the nodes which will form the location stencil for each daughter
        * cell.  Our starting point is the node indices already identified.
        *
        * In order to ensure the resulting gap between the elements is the correct size, we remove as many nodes as
        * necessary until the perpendicular distance between the centroid and the node is at least half the required
        * spacing.
        *
        * Finally, we move the relevant node to be exactly half the required spacing.
        */
        unsigned start_a = (nodeAIndex + 1) % num_nodes;
        unsigned end_a = nodeBIndex;
 
        unsigned start_b = (nodeBIndex + 1) % num_nodes;
        unsigned end_b = nodeAIndex;
 
        // Find correct start_a
        bool no_node_satisfied_condition_1 = true;
        for (unsigned i = start_a; i != end_a;)
        {
            c_vector<double, SPACE_DIM> centroid_to_i = this->GetVectorFromAtoB(centroid, pElement->GetNode(i)->rGetLocation());
            double perpendicular_dist = inner_prod(centroid_to_i, unit_perp);
 
            if (fabs(perpendicular_dist) >= half_spacing)
            {
                no_node_satisfied_condition_1 = false;
                start_a = i;
 
                // Calculate position so it's exactly 0.5 * elem_spacing perpendicular distance from the centroid
                c_vector<double, SPACE_DIM> new_location = pElement->GetNode(i)->rGetLocation();
                new_location -= unit_perp * copysign(fabs(perpendicular_dist) - half_spacing, perpendicular_dist);
 
                pElement->GetNode(i)->SetPoint(ChastePoint<SPACE_DIM>(new_location));
                break;
            }
 
            // Go to the next node
            i = (i + 1) % num_nodes;
        }
 
        // Find correct end_a
        bool no_node_satisfied_condition_2 = true;
        for (unsigned i = end_a; i != start_a;)
        {
            c_vector<double, SPACE_DIM> centroid_to_i = this->GetVectorFromAtoB(centroid, pElement->GetNode(i)->rGetLocation());
            double perpendicular_dist = inner_prod(centroid_to_i, unit_perp);
 
            if (fabs(perpendicular_dist) >= half_spacing)
            {
                no_node_satisfied_condition_2 = false;
                end_a = i;
 
                // Calculate position so it's exactly 0.5 * elem_spacing perpendicular distance from the centroid
                c_vector<double, SPACE_DIM> new_location = pElement->GetNode(i)->rGetLocation();
                new_location -= unit_perp * copysign(fabs(perpendicular_dist) - half_spacing, perpendicular_dist);
 
                pElement->GetNode(i)->SetPoint(ChastePoint<SPACE_DIM>(new_location));
                break;
            }
 
            // Go to the previous node
            i = (i + num_nodes - 1) % num_nodes; // LCOV_EXCL_LINE
        }
 
        // Find correct start_b
        bool no_node_satisfied_condition_3 = true;
        for (unsigned i = start_b; i != end_b;)
        {
            c_vector<double, SPACE_DIM> centroid_to_i = this->GetVectorFromAtoB(centroid, pElement->GetNode(i)->rGetLocation());
            double perpendicular_dist = inner_prod(centroid_to_i, unit_perp);
 
            if (fabs(perpendicular_dist) >= half_spacing)
            {
                no_node_satisfied_condition_3 = false;
                start_b = i;
 
                // Calculate position so it's exactly 0.5 * elem_spacing perpendicular distance from the centroid
                c_vector<double, SPACE_DIM> new_location = pElement->GetNode(i)->rGetLocation();
                new_location -= unit_perp * copysign(fabs(perpendicular_dist) - half_spacing, perpendicular_dist);
 
                pElement->GetNode(i)->SetPoint(ChastePoint<SPACE_DIM>(new_location));
                break;
            }
 
            // Go to the next node
            i = (i + 1) % num_nodes;
        }
 
        // Find correct end_b
        bool no_node_satisfied_condition_4 = true;
        for (unsigned i = end_b; i != start_b;)
        {
            c_vector<double, SPACE_DIM> centroid_to_i = this->GetVectorFromAtoB(centroid, pElement->GetNode(i)->rGetLocation());
            double perpendicular_dist = inner_prod(centroid_to_i, unit_perp);
 
            if (fabs(perpendicular_dist) >= half_spacing)
            {
                no_node_satisfied_condition_4 = false;
                end_b = i;
 
                // Calculate position so it's exactly 0.5 * elem_spacing perpendicular distance from the centroid
                c_vector<double, SPACE_DIM> new_location = pElement->GetNode(i)->rGetLocation();
                new_location -= unit_perp * copysign(fabs(perpendicular_dist) - half_spacing, perpendicular_dist);
 
                pElement->GetNode(i)->SetPoint(ChastePoint<SPACE_DIM>(new_location));
                break;
            }
 
            // Go to the previous node
            i = (i + num_nodes - 1) % num_nodes;
        }
 
        if (no_node_satisfied_condition_1 || no_node_satisfied_condition_2 || no_node_satisfied_condition_3 || no_node_satisfied_condition_4)
        {
            EXCEPTION("Could not space elements far enough apart during cell division.  Cannot currently handle this case");
        }
 
        /*
        * Create location stencils for each of the daughter cells
        */
        std::vector<c_vector<double, SPACE_DIM> > daughter_a_location_stencil;
        for (unsigned node_idx = start_a; node_idx != (end_a + 1) % num_nodes;)
        {
            daughter_a_location_stencil.push_back(c_vector<double, SPACE_DIM>(pElement->GetNode(node_idx)->rGetLocation()));
 
            // Go to next node
            node_idx = (node_idx + 1) % num_nodes;
        }
 
        std::vector<c_vector<double, SPACE_DIM> > daughter_b_location_stencil;
        for (unsigned node_idx = start_b; node_idx != (end_b + 1) % num_nodes;)
        {
            daughter_b_location_stencil.push_back(c_vector<double, SPACE_DIM>(pElement->GetNode(node_idx)->rGetLocation()));
 
            // Go to next node
            node_idx = (node_idx + 1) % num_nodes;
        }
 
        assert(daughter_a_location_stencil.size() > 1);
        assert(daughter_b_location_stencil.size() > 1);
 
        // To help calculating cumulative distances, add the first location on to the end
        daughter_a_location_stencil.push_back(daughter_a_location_stencil[0]);
        daughter_b_location_stencil.push_back(daughter_b_location_stencil[0]);
 
        // Calculate the cumulative distances around the stencils
        std::vector<double> cumulative_distances_a;
        std::vector<double> cumulative_distances_b;
        cumulative_distances_a.push_back(0.0);
        cumulative_distances_b.push_back(0.0);
        for (unsigned loc_idx = 1; loc_idx < daughter_a_location_stencil.size(); loc_idx++)
        {
            cumulative_distances_a.push_back(cumulative_distances_a.back() + norm_2(this->GetVectorFromAtoB(daughter_a_location_stencil[loc_idx - 1], daughter_a_location_stencil[loc_idx])));
        }
        for (unsigned loc_idx = 1; loc_idx < daughter_b_location_stencil.size(); loc_idx++)
        {
            cumulative_distances_b.push_back(cumulative_distances_b.back() + norm_2(this->GetVectorFromAtoB(daughter_b_location_stencil[loc_idx - 1], daughter_b_location_stencil[loc_idx])));
        }
 
        // Find the target node spacing for each of the daughter elements
        double target_spacing_a = cumulative_distances_a.back() / (double)num_nodes;
        double target_spacing_b = cumulative_distances_b.back() / (double)num_nodes;
 
        // Move the existing nodes into position to become daughter-A nodes
        unsigned last_idx_used = 0;
        for (unsigned node_idx = 0; node_idx < num_nodes; node_idx++)
        {
            double location_along_arc = (double)node_idx * target_spacing_a;
 
            while (location_along_arc > cumulative_distances_a[last_idx_used + 1])
            {
                last_idx_used++;
            }
 
            // Interpolant is the extra distance past the last index used divided by the length of the next line segment
            double interpolant = (location_along_arc - cumulative_distances_a[last_idx_used]) / (cumulative_distances_a[last_idx_used + 1] - cumulative_distances_a[last_idx_used]);
 
            c_vector<double, SPACE_DIM> this_to_next = this->GetVectorFromAtoB(daughter_a_location_stencil[last_idx_used],
                                                                            daughter_a_location_stencil[last_idx_used + 1]);
 
            c_vector<double, SPACE_DIM> new_location_a = daughter_a_location_stencil[last_idx_used] + interpolant * this_to_next;
 
            pElement->GetNode(node_idx)->SetPoint(ChastePoint<SPACE_DIM>(new_location_a));
        }
 
        // Create new nodes at positions around the daughter-B stencil
        last_idx_used = 0;
        std::vector<Node<SPACE_DIM>*> new_nodes_vec;
        for (unsigned node_idx = 0; node_idx < num_nodes; node_idx++)
        {
            double location_along_arc = (double)node_idx * target_spacing_b;
 
            while (location_along_arc > cumulative_distances_b[last_idx_used + 1])
            {
                last_idx_used++;
            }
 
            // Interpolant is the extra distance past the last index used divided by the length of the next line segment
            double interpolant = (location_along_arc - cumulative_distances_b[last_idx_used]) / (cumulative_distances_b[last_idx_used + 1] - cumulative_distances_b[last_idx_used]);
 
            c_vector<double, SPACE_DIM> this_to_next = this->GetVectorFromAtoB(daughter_b_location_stencil[last_idx_used],
                                                                            daughter_b_location_stencil[last_idx_used + 1]);
 
            c_vector<double, SPACE_DIM> new_location_b = daughter_b_location_stencil[last_idx_used] + interpolant * this_to_next;
 
            unsigned new_node_idx = this->mNodes.size();
            this->mNodes.push_back(new Node<SPACE_DIM>(new_node_idx, new_location_b, true));
            new_nodes_vec.push_back(this->mNodes.back());
        }
 
        // Copy node attributes
        for (unsigned node_idx = 0; node_idx < num_nodes; node_idx++)
        {
            new_nodes_vec[node_idx]->SetRegion(pElement->GetNode(node_idx)->GetRegion());
 
            for (unsigned node_attribute = 0; node_attribute < pElement->GetNode(node_idx)->GetNumNodeAttributes(); node_attribute++)
            {
                new_nodes_vec[node_idx]->AddNodeAttribute(pElement->GetNode(node_idx)->rGetNodeAttributes()[node_attribute]);
            }
        }
 
        // Create the new element
        unsigned new_elem_idx = this->mElements.size();
        this->mElements.push_back(new ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>(new_elem_idx, new_nodes_vec));
        this->mElements.back()->RegisterWithNodes();
 
        // Copy any element attributes
        for (unsigned elem_attribute = 0; elem_attribute < pElement->GetNumElementAttributes(); elem_attribute++)
        {
            this->mElements.back()->AddElementAttribute(pElement->rGetElementAttributes()[elem_attribute]);
        }
 
        // Add the necessary corners to keep consistency with the other daughter element
        for (unsigned corner = 0; corner < pElement->rGetCornerNodes().size(); corner++)
        {
            this->mElements.back()->rGetCornerNodes().push_back(pElement->rGetCornerNodes()[corner]);
        }
 
        // Update fluid source location for the existing element
        pElement->GetFluidSource()->rGetModifiableLocation() = this->GetCentroidOfElement(pElement->GetIndex());
 
        // Add a fluid source for the new element
        c_vector<double, SPACE_DIM> new_centroid = this->GetCentroidOfElement(new_elem_idx);
        mElementFluidSources.push_back(std::make_shared<FluidSource<SPACE_DIM>>(new_elem_idx, new_centroid));
 
        // Set source parameters
        mElementFluidSources.back()->SetAssociatedElementIndex(new_elem_idx);
        mElementFluidSources.back()->SetStrength(0.0);
 
        // Associate source with element
        mElements[new_elem_idx]->SetFluidSource(mElementFluidSources.back());
 
        return new_elem_idx;
    }
    else
    {
        NEVER_REACHED;
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::ReMesh(bool randomOrder)
{
    // Iterate over elements and remesh each one
    for (auto p_elem : mElements)
    {
        ReMeshElement(p_elem, randomOrder);
    }
 
    // Iterate over laminas and remesh each one
    for (auto p_lam : mLaminas)
    {
        ReMeshLamina(p_lam, randomOrder);
    }
 
    // Reposition fluid sources to the centroid of cells
    for (auto& p_source : mElementFluidSources)
    {
        p_source->rGetModifiableLocation() = this->GetCentroidOfElement(p_source->GetAssociatedElementIndex());
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::ReMeshElement(ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>* pElement,
                                                                 bool randomOrder)
{
    if constexpr (SPACE_DIM == 2)
    {
        const unsigned num_nodes = pElement->GetNumNodes();
 
        // Start at a random location in the vector of element nodes if requested
        const unsigned start_idx = randomOrder ? RandomNumberGenerator::Instance()->randMod(num_nodes) : 0u;
 
        // Straighten out locations to a contiguous polygon rather than wrapped around the domain due to periodicity
        std::vector<c_vector<double, SPACE_DIM>> locations_straightened;
        locations_straightened.reserve(num_nodes);
 
        locations_straightened.emplace_back(pElement->GetNodeLocation(start_idx));
 
        for (unsigned node_idx = 1; node_idx < num_nodes; ++node_idx)
        {
            const unsigned prev_idx = AdvanceMod(start_idx, node_idx - 1, num_nodes);
            const unsigned this_idx = AdvanceMod(start_idx, node_idx, num_nodes);
 
            const c_vector<double, SPACE_DIM>& r_last_location_added = locations_straightened.back();
 
            const c_vector<double, SPACE_DIM>& r_prev_location = pElement->GetNodeLocation(prev_idx);
            const c_vector<double, SPACE_DIM>& r_this_location = pElement->GetNodeLocation(this_idx);
 
            locations_straightened.emplace_back(r_last_location_added +
                                                this->GetVectorFromAtoB(r_prev_location, r_this_location));
        }
 
        assert(locations_straightened.size() == num_nodes);
 
        const bool closed_path = true;
        const bool permute_order = false;
        const std::size_t num_pts_to_place = num_nodes;
 
        std::vector<c_vector<double, SPACE_DIM>> evenly_spaced_locations = EvenlySpaceAlongPath(
                locations_straightened,
                closed_path,
                permute_order,
                num_pts_to_place
        );
 
        assert(evenly_spaced_locations.size() == num_nodes);
 
        // Conform all locations to geometry
        for (c_vector<double, SPACE_DIM>& r_loc : evenly_spaced_locations)
        {
            ConformToGeometry(r_loc);
        }
 
        // Update the node locations
        for (unsigned node_idx = 0; node_idx < num_nodes; ++node_idx)
        {
            const unsigned this_idx = AdvanceMod(node_idx, start_idx, num_nodes);
            pElement->GetNode(this_idx)->rGetModifiableLocation() = evenly_spaced_locations[node_idx];
        }
    }
    else
    {
        NEVER_REACHED;
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::ReMeshLamina(ImmersedBoundaryElement<ELEMENT_DIM - 1, SPACE_DIM>* pLamina,
                                                                bool randomOrder)
{
    if constexpr (SPACE_DIM == 2)
    {
        const unsigned num_nodes = pLamina->GetNumNodes();
 
        // Start at a random location in the vector of element nodes
        const unsigned start_idx = randomOrder ? RandomNumberGenerator::Instance()->randMod(num_nodes) : 0u;
 
        // Straighten out locations to a contiguous line rather than wrapped around the domain due to periodicity
        std::vector<c_vector<double, SPACE_DIM>> locations_straightened;
        locations_straightened.reserve(1 + num_nodes);
 
        locations_straightened.emplace_back(pLamina->GetNodeLocation(start_idx));
 
        // We go to 1 + num_nodes so that we add on a node in a location congruent to the first location added
        for (unsigned node_idx = 1; node_idx < 1 + num_nodes; ++node_idx)
        {
            const unsigned prev_idx = AdvanceMod(start_idx, node_idx - 1, num_nodes);
            const unsigned this_idx = AdvanceMod(start_idx, node_idx, num_nodes);
 
            const c_vector<double, SPACE_DIM>& r_last_location_added = locations_straightened.back();
 
            const c_vector<double, SPACE_DIM>& r_prev_location = pLamina->GetNodeLocation(prev_idx);
            const c_vector<double, SPACE_DIM>& r_this_location = pLamina->GetNodeLocation(this_idx);
 
            locations_straightened.emplace_back(r_last_location_added +
                                                this->GetVectorFromAtoB(r_prev_location, r_this_location));
        }
 
        assert(locations_straightened.size() == 1 + num_nodes);
 
        const bool closed_path = false;
        const bool permute_order = false;
        const std::size_t num_pts_to_place = 1 + num_nodes;
 
        std::vector<c_vector<double, SPACE_DIM>> evenly_spaced_locations = EvenlySpaceAlongPath(
                locations_straightened,
                closed_path,
                permute_order,
                num_pts_to_place
        );
 
        assert(evenly_spaced_locations.size() == 1 + num_nodes);
 
        // Conform all locations to geometry
        for (c_vector<double, SPACE_DIM>& r_loc : evenly_spaced_locations)
        {
            ConformToGeometry(r_loc);
        }
 
        // Update the node locations, ignoring the very last location that was added to make the path spacing even
        for (unsigned node_idx = 0; node_idx < num_nodes; ++node_idx)
        {
            const unsigned this_idx = AdvanceMod(start_idx, node_idx, num_nodes);
            pLamina->GetNode(this_idx)->rGetModifiableLocation() = evenly_spaced_locations[node_idx];
        }
    }
    else
    {
        NEVER_REACHED;
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::ConformToGeometry(c_vector<double, SPACE_DIM>& rLocation)
{
    for (unsigned dim = 0; dim < SPACE_DIM; ++dim)
    {
        assert(rLocation[dim] >= -2.0);  // It's expected that the location is near the domain.  This method is
        assert(rLocation[dim] < 3.0);    // inefficient if the location is far away.
 
        while (rLocation[dim] < 0.0)
        {
            rLocation[dim] += 1.0;
        }
 
        while (rLocation[dim] >= 1.0)
        {
            rLocation[dim] -= 1.0;
        }
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
bool ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::NodesInDifferentElementOrLamina(Node<SPACE_DIM>* pNodeA,
                                                                                   Node<SPACE_DIM>* pNodeB)
{
    // If neither are lamina nodes, we can just check equality of first containing element indices
    if (pNodeA->GetRegion() != LAMINA_REGION && pNodeB->GetRegion() != LAMINA_REGION)
    {
        return *(pNodeA->ContainingElementsBegin()) != *(pNodeB->ContainingElementsBegin());
    }
    else // either one or both nodes is in lamina; assume that two laminas never interact
    {
        return true;
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
std::set<unsigned> ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetNeighbouringElementIndices(unsigned elemIdx)
{
    if (!mLaminas.empty())
    {
        EXCEPTION("This method does not yet work in the presence of laminas");
    }
 
    /*
     * This method uses geometric information from a voronoi diagram of every node.  The voronoi cell of an element
     * (the union of voronoi cells of nodes in the element) provides a precise boundary between elements on which the
     * concept of 'neighbourhood' is well-defined.
     *
     * We first update the voronoi diagram, if it is out of date.
     */
    UpdateNodeLocationsVoronoiDiagramIfOutOfDate();
 
    std::set<unsigned> neighbouring_element_indices;
 
    for (unsigned node_local_idx = 0; node_local_idx < this->GetElement(elemIdx)->GetNumNodes(); ++node_local_idx)
    {
        const unsigned node_global_idx = this->GetElement(elemIdx)->GetNodeGlobalIndex(node_local_idx);
        const unsigned voronoi_cell_id = mVoronoiCellIdsIndexedByNodeIndex[node_global_idx];
 
        /*
         * Iterate over the edges of the voronoi cell corresponding to the current node.  Each primary edge has a twin
         * in a voronoi cell corresponding to a different node.  The element containing that node (which may be the
         * current element under consideration) is added to the set of neighbours precisely if the distance between
         * nodes is less than mNeighbourDist.
         */
        const auto voronoi_cell = mNodeLocationsVoronoiDiagram.cells()[voronoi_cell_id];
        auto p_edge = voronoi_cell.incident_edge();
 
        do
        {
            // The global node index corresponding to a voronoi cell cell is encoded in its 'color' variable
            const unsigned twin_node_idx = p_edge->twin()->cell()->color();
            const unsigned twin_elem_idx = *this->GetNode(twin_node_idx)->ContainingElementsBegin();
 
            // Only bother to check the node distances if the nodes are in different elements
            if (twin_elem_idx != elemIdx)
            {
                if (this->GetDistanceBetweenNodes(node_global_idx, twin_node_idx) < mNeighbourDist)
                {
                    neighbouring_element_indices.insert(twin_elem_idx);
                }
            }
            p_edge = p_edge->next();
        } while (p_edge != voronoi_cell.incident_edge());
    }
 
    return neighbouring_element_indices;
} // LCOV_EXCL_LINE
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
std::array<unsigned, 13> ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetPolygonDistribution()
{
    if constexpr (SPACE_DIM == 2)
    {
        std::array<unsigned, 13> polygon_dist = {{0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u}};
 
        for (auto elem_it = this->GetElementIteratorBegin(); elem_it != this->GetElementIteratorEnd(); ++elem_it)
        {
            if (!elem_it->IsElementOnBoundary())
            {
                // Accumulate all 12+ sided shapes
                unsigned num_neighbours = std::min<unsigned>(12u, GetNeighbouringElementIndices(elem_it->GetIndex()).size());
                polygon_dist[num_neighbours]++;
            }
        }
 
        return polygon_dist;
    }
    else
    {
        NEVER_REACHED;
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::CalculateLengthOfVoronoiEdge(const boost::polygon::voronoi_diagram<double>::edge_type& rEdge)
{
    assert(rEdge.is_finite());
 
    const double d_x = rEdge.vertex1()->x() - rEdge.vertex0()->x();
    const double d_y = rEdge.vertex1()->y() - rEdge.vertex0()->y();
 
    return ScaleDistanceDownFromVoronoi(std::sqrt(d_x * d_x + d_y * d_y));
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetMaxNodeIndex() const
{
    if (this->mNodes.begin() == this->mNodes.end())
    {
        return UINT_MAX;
    }
    else
    {
        const auto max_idx_it = std::max_element(this->mNodes.begin(), this->mNodes.end(),
                                                 [](Node<SPACE_DIM>* const a, Node<SPACE_DIM>* const b)
                                                 {
                                                     return a->GetIndex() < b->GetIndex();
                                                 });
 
        return (*max_idx_it)->GetIndex();
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetMaxElementIndex() const
{
    if (this->mElements.begin() == this->mElements.end())
    {
        return UINT_MAX;
    }
    else
    {
        using IbElem = ImmersedBoundaryElement<ELEMENT_DIM, SPACE_DIM>;
 
        const auto max_idx_it = std::max_element(this->mElements.begin(), this->mElements.end(),
                                                 [](IbElem* const a, IbElem* const b)
                                                 {
                                                     return a->GetIndex() < b->GetIndex();
                                                 });
 
        return (*max_idx_it)->GetIndex();
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetMaxLaminaIndex() const
{
    if (this->mLaminas.begin() == this->mLaminas.end())
    {
        return UINT_MAX;
    }
    else
    {
        using IbLam = ImmersedBoundaryElement<ELEMENT_DIM - 1, SPACE_DIM>;
 
        const auto max_idx_it = std::max_element(this->mLaminas.begin(), this->mLaminas.end(),
                                                 [](IbLam* const a, IbLam* const b)
                                                 {
                                                     return a->GetIndex() < b->GetIndex();
                                                 });
 
        return (*max_idx_it)->GetIndex();
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::TagBoundaryElements()
{
    if (!mLaminas.empty())
    {
        EXCEPTION("This method does not yet work in the presence of laminas");
    }
 
    // Need a large double, but since we also perform calculations with it, we can't really use DBL_MAX
    const double LARGE_DOUBLE = 1e6;
 
    /*
     * This method uses geometric information from a voronoi diagram of every node.
     *
     * Nodes with infinite voronoi cells must be in a boundary element.
     * In addition, some nodes in boundary elements will have unusually long shared-edges, and the voronoi perimeter
     * of boundary elements will exceed the true perimeter by an unusually large amount.
     * We use a threshold on the median values of these identifiers to robustly determine boundary elements.
     */
    UpdateNodeLocationsVoronoiDiagramIfOutOfDate();
 
    // Get the maximum element index in the mesh
    const unsigned max_elem_idx = GetMaxElementIndex();
 
    // Keep track of all finite shared edge lengths (between two elements), as well as (for each element) the maximum
    // shared edge length.
    std::vector<double> max_shared_lengths(1 + max_elem_idx, 0.0);
    std::vector<double> voronoi_perimeter(1 + max_elem_idx, 0.0);
 
    // Loop over nodes and calculate values to fill the containers defined above
    for (const auto& p_node : this->mNodes)
    {
        const unsigned this_node_idx = p_node->GetIndex();
        const unsigned this_elem_idx = *p_node->ContainingElementsBegin();
 
        // Get the voronoi cell that corresponds to this node
        const unsigned voronoi_cell_id = mVoronoiCellIdsIndexedByNodeIndex[this_node_idx];
        const auto& voronoi_cell = mNodeLocationsVoronoiDiagram.cells()[voronoi_cell_id];
 
        // Iterate over the edges of this cell.  Note that due to the halo region none of these edges should be infinite.
        auto p_edge = voronoi_cell.incident_edge();
 
        // Loop over the voronoi edges associated with this node's voronoi cell
        do
        {
            // If the edge is infinite, the element containing it must be on the boundary
            if (p_edge->is_infinite())
            {
                max_shared_lengths[this_elem_idx] = LARGE_DOUBLE;
                voronoi_perimeter[this_elem_idx] += LARGE_DOUBLE;
            }
            else
            {
                // The global node index corresponding to a voronoi cell cell is encoded in its 'color' variable
                const unsigned twin_node_idx = p_edge->twin()->cell()->color();
                const unsigned twin_elem_idx = *this->GetNode(twin_node_idx)->ContainingElementsBegin();
 
                // If the edge is between two elements, it counts
                if (this_elem_idx != twin_elem_idx)
                {
                    const double edge_length = CalculateLengthOfVoronoiEdge(*p_edge);
                    max_shared_lengths[this_elem_idx] = std::max(max_shared_lengths[this_elem_idx], edge_length);
                    voronoi_perimeter[this_elem_idx] += edge_length;
                }
            }
            p_edge = p_edge->next();
        } while (p_edge != voronoi_cell.incident_edge());
    }
 
    // Calculate what proportion bigger the voronoi perimeter is than the actual one
    std::vector<double> perimeter_multiples(voronoi_perimeter.size());
    for (const auto& p_elem : this->mElements)
    {
        const unsigned idx = p_elem->GetIndex();
        perimeter_multiples[idx] = voronoi_perimeter[idx] / this->GetSurfaceAreaOfElement(idx);
    }
 
    // Calculate the medians of each vector.  Copy the vector accounting for possible non-contiguous element indices.
    std::vector<double> copy_max_shared_lengths;
    std::vector<double> copy_perimeter_multiples;
 
    for (const auto& p_elem : this->mElements)
    {
        const unsigned idx = p_elem->GetIndex();
        copy_max_shared_lengths.emplace_back(max_shared_lengths[idx]);
        copy_perimeter_multiples.emplace_back(perimeter_multiples[idx]);
    }
 
    const std::size_t half_way = copy_max_shared_lengths.size() / 2;
    assert(half_way == copy_perimeter_multiples.size() / 2);
 
    std::nth_element(copy_max_shared_lengths.begin(), copy_max_shared_lengths.begin() + half_way, copy_max_shared_lengths.end());
    std::nth_element(copy_perimeter_multiples.begin(), copy_perimeter_multiples.begin() + half_way, copy_perimeter_multiples.end());
 
    double median_max_edge_length = copy_max_shared_lengths[half_way];
    double median_perimeter_multiple = copy_perimeter_multiples[half_way];
 
    // In the event that most elements have infinite edges, for instance if there are a small number of elements,
    // a reduced median can be used
    if (median_max_edge_length == LARGE_DOUBLE || median_perimeter_multiple >= LARGE_DOUBLE)
    {
        median_max_edge_length = 0.5 * LARGE_DOUBLE;
        median_perimeter_multiple = 0.5 * LARGE_DOUBLE;
    }
 
    // Finally, tag the boundary elements
    for (const auto& p_elem : this->mElements)
    {
        const unsigned idx = p_elem->GetIndex();
 
        const bool large_shared_edge = max_shared_lengths[idx] > 1.1 * median_max_edge_length;
        const bool large_perimeter = perimeter_multiples[idx] > 1.1 * median_perimeter_multiple;
 
        p_elem->SetIsBoundaryElement(large_shared_edge && large_perimeter);
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::UpdateNodeLocationsVoronoiDiagramIfOutOfDate()
{
    if constexpr (SPACE_DIM == 2)
    {
        using boost_point = boost::polygon::point_data<int>;
 
        /*
        * The voronoi diagram needs updating only if the node locations have changed (i.e. not more than once per
        * timestep).  We test a chosen summary of node locations against the cached mSummaryOfNodeLocations to determine
        * this.
        */
        double new_location_summary = this->mNodes.front()->rGetLocation()[0] +
                                    this->mNodes.front()->rGetLocation()[1] +
                                    this->mNodes.back()->rGetLocation()[0] +
                                    this->mNodes.back()->rGetLocation()[1];
 
        bool voronoi_needs_updating = std::fabs(mSummaryOfNodeLocations - new_location_summary) > DBL_EPSILON;
 
        /*
        * Method outline:
        * - Add all node locations as boost points, correctly scaled
        * - Add extra locations for any nodes within distance mVoronoiHalo of the domain edge for periodicity
        * - Calculate the voronoi diagram
        * - Populate mVoronoiCellIdsIndexedByNodeIndex for efficient mapping between node index and corresponding voronoi
        *   cell
        * - Store the corresponding node index in each voronoi cell's "color" variable for efficient reverse-lookup
        * - Tag boundary elements
        */
        if (voronoi_needs_updating)
        {
            mSummaryOfNodeLocations = new_location_summary;
 
            // Helper c_vectors for halo region
            const c_vector<double, SPACE_DIM> halo_up = Create_c_vector(0.0, 1.0);
            const c_vector<double, SPACE_DIM> halo_down = Create_c_vector(0.0, -1.0);
            const c_vector<double, SPACE_DIM> halo_left = Create_c_vector(-1.0, 0.0);
            const c_vector<double, SPACE_DIM> halo_right = Create_c_vector(1.0, 0.0);
 
            std::vector<std::pair<unsigned, c_vector<double, SPACE_DIM>>> halo_ids_and_locations;
            std::vector<unsigned> node_ids_in_source_idx_order;
 
            // We need to translate node locations into boost points, which are scaled to an integer grid
            std::vector<boost_point> points;
 
            // First add a point for each node, and calculate any additional locations needed within the halo
            for (const auto& p_node : this->mNodes)
            {
                const double x_pos = p_node->rGetLocation()[0];
                const double y_pos = p_node->rGetLocation()[1];
 
                // Put the integer voronoi coordinate in for this node's location
                points.emplace_back(boost_point(ScaleUpToVoronoiCoordinate(x_pos), ScaleUpToVoronoiCoordinate(y_pos)));
                node_ids_in_source_idx_order.emplace_back(p_node->GetIndex());
 
                // Now, for this location, decide whether any copies of this location are needed in the halo region
                const bool needed_up = y_pos < mVoronoiHalo;
                const bool needed_down = y_pos > 1.0 - mVoronoiHalo;
                const bool needed_left = x_pos > 1.0 - mVoronoiHalo;
                const bool needed_right = x_pos < mVoronoiHalo;
 
                if (needed_up)
                {
                    halo_ids_and_locations.emplace_back(std::make_pair(p_node->GetIndex(),
                                                                    p_node->rGetLocation() + halo_up));
                }
                if (needed_down)
                {
                    halo_ids_and_locations.emplace_back(std::make_pair(p_node->GetIndex(),
                                                                    p_node->rGetLocation() + halo_down));
                }
                if (needed_left)
                {
                    halo_ids_and_locations.emplace_back(std::make_pair(p_node->GetIndex(),
                                                                    p_node->rGetLocation() + halo_left));
                }
                if (needed_right)
                {
                    halo_ids_and_locations.emplace_back(std::make_pair(p_node->GetIndex(),
                                                                    p_node->rGetLocation() + halo_right));
                }
                if (needed_up && needed_left)
                {
                    halo_ids_and_locations.emplace_back(std::make_pair(p_node->GetIndex(),
                                                                    p_node->rGetLocation() + halo_up + halo_left));
                }
                if (needed_up && needed_right)
                {
                    halo_ids_and_locations.emplace_back(std::make_pair(p_node->GetIndex(),
                                                                    p_node->rGetLocation() + halo_up + halo_right));
                }
                if (needed_down && needed_left)
                {
                    halo_ids_and_locations.emplace_back(std::make_pair(p_node->GetIndex(),
                                                                    p_node->rGetLocation() + halo_down + halo_left));
                }
                if (needed_down && needed_right)
                {
                    halo_ids_and_locations.emplace_back(std::make_pair(p_node->GetIndex(),
                                                                    p_node->rGetLocation() + halo_down + halo_right));
                }
            }
 
            // Next add the additional points
            for (const auto& pair : halo_ids_and_locations)
            {
                const unsigned node_idx = pair.first;
                c_vector<double, SPACE_DIM> location = pair.second;
 
                const int x_coord = ScaleUpToVoronoiCoordinate(location[0]);
                const int y_coord = ScaleUpToVoronoiCoordinate(location[1]);
 
                points.emplace_back(boost_point(x_coord, y_coord));
                node_ids_in_source_idx_order.emplace_back(node_idx);
            }
 
            // Construct the voronoi diagram.  This is the costly part of this method.
            mNodeLocationsVoronoiDiagram.clear();
            construct_voronoi(std::begin(points), std::end(points), &mNodeLocationsVoronoiDiagram);
 
            // We need an efficient map from node global index to the voronoi cell representing it, and we can't assume
            // that the nodes are ordered sequentially.  We first identify the largest node index.
            const unsigned max_node_idx = GetMaxNodeIndex();
 
            mVoronoiCellIdsIndexedByNodeIndex.resize(1 + max_node_idx);
 
            for (unsigned vor_cell_id = 0; vor_cell_id < mNodeLocationsVoronoiDiagram.cells().size(); ++vor_cell_id)
            {
                // Source index is incrementally given to each input point, which is in order of nodes in this->mNodes.
                // We need to be able to identify each vor_cell_id by the global node index
 
                auto& r_this_cell = mNodeLocationsVoronoiDiagram.cells()[vor_cell_id];
                const auto source_idx = r_this_cell.source_index();
                const unsigned node_idx = node_ids_in_source_idx_order[source_idx];
 
                r_this_cell.color(node_idx);
 
                if (source_idx < this->mNodes.size())
                {
                    mVoronoiCellIdsIndexedByNodeIndex[node_idx] = vor_cell_id;
                }
            }
 
            // Finally, if the diagram was out-of-date, we will need to re-tag boundary elements.
            this->TagBoundaryElements();
        }
    }
    else
    {
        NEVER_REACHED;
    }
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
int ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::ScaleUpToVoronoiCoordinate(const double location) const
{
    assert(location >= -mVoronoiHalo);
    assert(location <= 1.0 + mVoronoiHalo);
 
    constexpr auto DBL_INT_MIN = static_cast<double>(INT_MIN);
    constexpr auto DBL_INT_RANGE = static_cast<double>(INT_MAX) - static_cast<double>(INT_MIN);
 
    // To handle periodicity, we add (again) any nodes within a halo region of the unit square
    constexpr double scale_factor = DBL_INT_RANGE / (1.0 + 2.0 * mVoronoiHalo);
 
    // By construction there is no narrowing, so this static_cast might not be necessary
    return static_cast<int>(std::lround(DBL_INT_MIN + (mVoronoiHalo + location) * scale_factor));
}
 
template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::ScaleDistanceDownFromVoronoi(const double distance) const
{
    constexpr auto DBL_INT_RANGE = static_cast<double>(INT_MAX) - static_cast<double>(INT_MIN);
    constexpr double scale_factor = (1.0 + 2.0 * mVoronoiHalo) / DBL_INT_RANGE;
 
    return scale_factor * distance;
}
 
template<unsigned int ELEMENT_DIM, unsigned int SPACE_DIM>
const boost::polygon::voronoi_diagram<double>& ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::rGetNodeLocationsVoronoiDiagram(bool update)
{
    if (update)
    {
        this->UpdateNodeLocationsVoronoiDiagramIfOutOfDate();
    }
 
    return mNodeLocationsVoronoiDiagram;
}
 
template<unsigned int ELEMENT_DIM, unsigned int SPACE_DIM>
const std::vector<unsigned int>& ImmersedBoundaryMesh<ELEMENT_DIM, SPACE_DIM>::GetVoronoiCellIdsIndexedByNodeIndex() const
{
    return mVoronoiCellIdsIndexedByNodeIndex;
}
 
// Explicit instantiation
template class ImmersedBoundaryMesh<1, 1>;
template class ImmersedBoundaryMesh<1, 2>;
template class ImmersedBoundaryMesh<1, 3>;
template class ImmersedBoundaryMesh<2, 2>;
template class ImmersedBoundaryMesh<2, 3>;
template class ImmersedBoundaryMesh<3, 3>;
 
// Serialization for Boost >= 1.36
#include "SerializationExportWrapperForCpp.hpp"
EXPORT_TEMPLATE_CLASS_ALL_DIMS(ImmersedBoundaryMesh)