VertexMesh.cpp

/*

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*/

#include "VertexMesh.hpp"
#include "RandomNumberGenerator.hpp"
#include "UblasCustomFunctions.hpp"

template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
VertexMesh<ELEMENT_DIM, SPACE_DIM>::VertexMesh(std::vector<Node<SPACE_DIM>*> nodes,
                                               std::vector<VertexElement<ELEMENT_DIM,SPACE_DIM>*> vertexElements)
    : mpDelaunayMesh(NULL)
{

    // Reset member variables and clear mNodes and mElements
    Clear();

    // Populate mNodes and mElements
    for (unsigned node_index=0; node_index<nodes.size(); node_index++)
    {
        Node<SPACE_DIM>* p_temp_node = nodes[node_index];
        this->mNodes.push_back(p_temp_node);
    }
    for (unsigned elem_index=0; elem_index<vertexElements.size(); elem_index++)
    {
        VertexElement<ELEMENT_DIM, SPACE_DIM>* p_temp_vertex_element = vertexElements[elem_index];
        mElements.push_back(p_temp_vertex_element);
    }

    // In 3D, populate mFaces
    if (SPACE_DIM == 3)
    {
        // Use a std::set to keep track of which faces have been added to mFaces
        std::set<unsigned> faces_counted;

        // Loop over mElements
        for (unsigned elem_index=0; elem_index<mElements.size(); elem_index++)
        {
            // Loop over faces of this element
            for (unsigned face_index=0; face_index<mElements[elem_index]->GetNumFaces(); face_index++)
            {
                VertexElement<ELEMENT_DIM-1, SPACE_DIM>* p_face = mElements[elem_index]->GetFace(face_index);
                unsigned global_index = p_face->GetIndex();

                // If this face is not already contained in mFaces, add it, and update faces_counted
                if (faces_counted.find(global_index) == faces_counted.end())
                {
                    mFaces.push_back(p_face);
                    faces_counted.insert(global_index);
                }
            }
        }
    }

    // Register elements with nodes
    for (unsigned index=0; index<mElements.size(); index++)
    {
        VertexElement<ELEMENT_DIM, SPACE_DIM>* p_element = mElements[index];

        unsigned element_index = p_element->GetIndex();
        unsigned num_nodes_in_element = p_element->GetNumNodes();

        for (unsigned node_index=0; node_index<num_nodes_in_element; node_index++)
        {
            p_element->GetNode(node_index)->AddElement(element_index);
        }
    }

    this->mMeshChangesDuringSimulation = false;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
VertexMesh<ELEMENT_DIM, SPACE_DIM>::VertexMesh(std::vector<Node<SPACE_DIM>*> nodes,
                           std::vector<VertexElement<ELEMENT_DIM-1, SPACE_DIM>*> faces,
                           std::vector<VertexElement<ELEMENT_DIM, SPACE_DIM>*> vertexElements)
    : mpDelaunayMesh(NULL)
{
    // Reset member variables and clear mNodes, mFaces and mElements
    Clear();

    // Populate mNodes mFaces and mElements
    for (unsigned node_index=0; node_index<nodes.size(); node_index++)
    {
        Node<SPACE_DIM>* p_temp_node = nodes[node_index];
        this->mNodes.push_back(p_temp_node);
    }

    for (unsigned face_index=0; face_index<faces.size(); face_index++)
    {
        VertexElement<ELEMENT_DIM-1, SPACE_DIM>* p_temp_face = faces[face_index];
        mFaces.push_back(p_temp_face);
    }

    for (unsigned elem_index=0; elem_index<vertexElements.size(); elem_index++)
    {
        VertexElement<ELEMENT_DIM, SPACE_DIM>* p_temp_vertex_element = vertexElements[elem_index];
        mElements.push_back(p_temp_vertex_element);
    }

    // Register elements with nodes
    for (unsigned index=0; index<mElements.size(); index++)
    {
        VertexElement<ELEMENT_DIM, SPACE_DIM>* p_temp_vertex_element = mElements[index];
        for (unsigned node_index=0; node_index<p_temp_vertex_element->GetNumNodes(); node_index++)
        {
            Node<SPACE_DIM>* p_temp_node = p_temp_vertex_element->GetNode(node_index);
            p_temp_node->AddElement(p_temp_vertex_element->GetIndex());
        }
    }

    this->mMeshChangesDuringSimulation = false;
}


template<>
VertexMesh<2,2>::VertexMesh(TetrahedralMesh<2,2>& rMesh, bool isPeriodic)
    : mpDelaunayMesh(&rMesh)
{
    //Note  !isPeriodic is not used except through polymorphic calls in rMesh

    // Reset member variables and clear mNodes, mFaces and mElements
    Clear();

    unsigned num_elements = mpDelaunayMesh->GetNumAllNodes();
    unsigned num_nodes = mpDelaunayMesh->GetNumAllElements();

    // Allocate memory for mNodes and mElements
    this->mNodes.reserve(num_nodes);

    // Create as many elements as there are nodes in the mesh
    mElements.reserve(num_elements);
    for (unsigned elem_index=0; elem_index<num_elements; elem_index++)
    {
        VertexElement<2,2>* p_element = new VertexElement<2,2>(elem_index);
        mElements.push_back(p_element);
    }

    // Populate mNodes
    GenerateVerticesFromElementCircumcentres(rMesh);

    // Loop over elements of the Delaunay mesh (which are nodes/vertices of this mesh)
    for (unsigned i=0; i<num_nodes; i++)
    {
        // Loop over nodes owned by this triangular element in the Delaunay mesh
        // Add this node/vertex to each of the 3 vertex elements
        for (unsigned local_index=0; local_index<3; local_index++)
        {
            unsigned elem_index = mpDelaunayMesh->GetElement(i)->GetNodeGlobalIndex(local_index);
            unsigned num_nodes_in_elem = mElements[elem_index]->GetNumNodes();
            unsigned end_index = num_nodes_in_elem>0 ? num_nodes_in_elem-1 : 0;

            mElements[elem_index]->AddNode(this->mNodes[i], end_index);
        }
    }

    // Reorder mNodes anticlockwise
    for (unsigned elem_index=0; elem_index<mElements.size(); elem_index++)
    {
        std::vector<std::pair<double, unsigned> > index_angle_list;
        for (unsigned local_index=0; local_index<mElements[elem_index]->GetNumNodes(); local_index++)
        {
            c_vector<double, 2> vectorA = mpDelaunayMesh->GetNode(elem_index)->rGetLocation();
            c_vector<double, 2> vectorB = mElements[elem_index]->GetNodeLocation(local_index);
            c_vector<double, 2> centre_to_vertex = mpDelaunayMesh->GetVectorFromAtoB(vectorA, vectorB);

            double angle = atan2(centre_to_vertex(1), centre_to_vertex(0));
            unsigned global_index = mElements[elem_index]->GetNodeGlobalIndex(local_index);

            std::pair<double, unsigned> pair(angle, global_index);
            index_angle_list.push_back(pair);
        }

        // Sort the list in order of increasing angle
        sort(index_angle_list.begin(), index_angle_list.end());

        // Create a new Voronoi element and pass in the appropriate Nodes, ordered anticlockwise
        VertexElement<2,2>* p_new_element = new VertexElement<2,2>(elem_index);
        for (unsigned count = 0; count < index_angle_list.size(); count++)
        {
            unsigned local_index = count>1 ? count-1 : 0;
            p_new_element->AddNode(mNodes[index_angle_list[count].second], local_index);

        }

        // Replace the relevant member of mElements with this Voronoi element
        delete mElements[elem_index];
        mElements[elem_index] = p_new_element;
    }

    this->mMeshChangesDuringSimulation = false;

}


template<>
VertexMesh<3,3>::VertexMesh(TetrahedralMesh<3,3>& rMesh)
    : mpDelaunayMesh(&rMesh)
{
    // Reset member variables and clear mNodes, mFaces and mElements
    Clear();

    unsigned num_nodes = mpDelaunayMesh->GetNumAllElements();

    // Allocate memory for mNodes
    this->mNodes.reserve(num_nodes);

    // Populate mNodes
    GenerateVerticesFromElementCircumcentres(rMesh);

    std::map<unsigned, VertexElement<3,3>*> index_element_map;
    unsigned face_index = 0;
    unsigned element_index = 0;

    // Loop over each edge of the Delaunay mesh and populate mFaces and mElements
    for (TetrahedralMesh<3,3>::EdgeIterator edge_iterator = mpDelaunayMesh->EdgesBegin();
         edge_iterator != mpDelaunayMesh->EdgesEnd();
         ++edge_iterator)
    {
        Node<3>* p_node_a = edge_iterator.GetNodeA();
        Node<3>* p_node_b = edge_iterator.GetNodeB();

        if ( !(p_node_a->IsBoundaryNode() && p_node_b->IsBoundaryNode()) )
        {
            std::set<unsigned>& node_a_element_indices = p_node_a->rGetContainingElementIndices();
            std::set<unsigned>& node_b_element_indices = p_node_b->rGetContainingElementIndices();
            std::set<unsigned> edge_element_indices;

            std::set_intersection(node_a_element_indices.begin(),
                                  node_a_element_indices.end(),
                                  node_b_element_indices.begin(),
                                  node_b_element_indices.end(),
                                  std::inserter(edge_element_indices, edge_element_indices.begin()));

            c_vector<double,3> edge_vector = p_node_b->rGetLocation() - p_node_a->rGetLocation();
            c_vector<double,3> mid_edge = edge_vector*0.5 + p_node_a->rGetLocation();

            unsigned element0_index = *(edge_element_indices.begin());

            c_vector<double,3> basis_vector1 = mNodes[element0_index]->rGetLocation() - mid_edge;

            c_vector<double,3> basis_vector2 = VectorProduct(edge_vector, basis_vector1);

            std::vector<std::pair<double, unsigned> > index_angle_list;

            // Loop over each element containing this edge (i.e. those containing both nodes of the edge)
            for (std::set<unsigned>::iterator index_iter = edge_element_indices.begin();
                 index_iter != edge_element_indices.end();
                 ++index_iter)
            {
                // Calculate angle
                c_vector<double, 3> vertex_vector =  mNodes[*index_iter]->rGetLocation() - mid_edge;

                double local_vertex_dot_basis_vector1 = inner_prod(vertex_vector, basis_vector1);
                double local_vertex_dot_basis_vector2 = inner_prod(vertex_vector, basis_vector2);

                double angle = atan2(local_vertex_dot_basis_vector2, local_vertex_dot_basis_vector1);

                std::pair<double, unsigned> pair(angle, *index_iter);
                index_angle_list.push_back(pair);
            }

            // Sort the list in order of increasing angle
            sort(index_angle_list.begin(), index_angle_list.end());

            // Create face
            VertexElement<2,3>* p_face = new VertexElement<2,3>(face_index);
            face_index++;
            for (unsigned count = 0; count < index_angle_list.size(); count++)
            {
                unsigned local_index = count>1 ? count-1 : 0;
                p_face->AddNode(mNodes[index_angle_list[count].second], local_index);
            }

            // Add face to list of faces
            mFaces.push_back(p_face);

            // Add face to appropriate elements
            if (!p_node_a->IsBoundaryNode())
            {
                unsigned node_a_index = p_node_a->GetIndex();
                if (index_element_map[node_a_index])
                {
                    // If there is already an element with this index, add the face to it...
                    index_element_map[node_a_index]->AddFace(p_face);
                }
                else
                {
                    // ...otherwise create an element, add the face to it, and add to the map
                    mVoronoiElementIndexMap[node_a_index] = element_index;
                    VertexElement<3,3>* p_element = new VertexElement<3,3>(element_index);
                    element_index++;
                    p_element->AddFace(p_face);
                    index_element_map[node_a_index] = p_element;
                }
            }
            if (!p_node_b->IsBoundaryNode())
            {
                unsigned node_b_index = p_node_b->GetIndex();
                if (index_element_map[node_b_index])
                {
                    // If there is already an element with this index, add the face to it...
                    index_element_map[node_b_index]->AddFace(p_face);
                }
                else
                {
                    // ...otherwise create an element, add the face to it, and add to the map
                    mVoronoiElementIndexMap[node_b_index] = element_index;
                    VertexElement<3,3>* p_element = new VertexElement<3,3>(element_index);
                    element_index++;
                    p_element->AddFace(p_face);
                    index_element_map[node_b_index] = p_element;
                }
            }
        }
    }

    // Populate mElements
    unsigned elem_count = 0;
    for (std::map<unsigned, VertexElement<3,3>*>::iterator element_iter = index_element_map.begin();
         element_iter != index_element_map.end();
         ++element_iter)
    {
        mElements.push_back(element_iter->second);
        mVoronoiElementIndexMap[element_iter->first] = elem_count;
        elem_count++;
    }

    this->mMeshChangesDuringSimulation = false;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void VertexMesh<ELEMENT_DIM, SPACE_DIM>::GenerateVerticesFromElementCircumcentres(TetrahedralMesh<ELEMENT_DIM, SPACE_DIM>& rMesh)
{
    c_matrix<double, SPACE_DIM, ELEMENT_DIM> jacobian;
    c_matrix<double, ELEMENT_DIM, SPACE_DIM> inverse_jacobian;
    double jacobian_det;

    // Loop over elements of the Delaunay mesh and populate mNodes
    for (unsigned i=0; i<rMesh.GetNumElements(); i++)
    {
        // Calculate the circumcentre of this element in the Delaunay mesh
        rMesh.GetInverseJacobianForElement(i, jacobian, jacobian_det, inverse_jacobian);
        c_vector<double, SPACE_DIM+1> circumsphere = rMesh.GetElement(i)->CalculateCircumsphere(jacobian, inverse_jacobian);

        c_vector<double, SPACE_DIM> circumcentre;
        for (unsigned j=0; j<SPACE_DIM; j++)
        {
            circumcentre(j) = circumsphere(j);
        }

        // Create a node in the Voronoi mesh at the location of this circumcentre
        this->mNodes.push_back(new Node<SPACE_DIM>(i, circumcentre));
    }
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetEdgeLength(unsigned elementIndex1, unsigned elementIndex2)
{
    assert(SPACE_DIM == 2);

    std::set<unsigned> node_indices_1;
    for (unsigned i=0; i<mElements[elementIndex1]->GetNumNodes(); i++)
    {
        node_indices_1.insert(mElements[elementIndex1]->GetNodeGlobalIndex(i));
    }
    std::set<unsigned> node_indices_2;
    for (unsigned i=0; i<mElements[elementIndex2]->GetNumNodes(); i++)
    {
        node_indices_2.insert(mElements[elementIndex2]->GetNodeGlobalIndex(i));
    }

    std::set<unsigned> shared_nodes;
    std::set_intersection(node_indices_1.begin(), node_indices_1.end(),
                          node_indices_2.begin(), node_indices_2.end(),
                          std::inserter(shared_nodes, shared_nodes.begin()));

    if (shared_nodes.size() == 1)
    {
        // It's possible that these two elements are actually infinite but are on the edge of the domain
        EXCEPTION("Elements "<< elementIndex1 <<" and "<< elementIndex2<< " share only one node.");
    }
    assert(shared_nodes.size() == 2);

    unsigned index1 = *(shared_nodes.begin());
    unsigned index2 = *(++(shared_nodes.begin()));

    double edge_length = this->GetDistanceBetweenNodes(index1, index2);
    return edge_length;
}

template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetElongationShapeFactorOfElement(unsigned index)
{
    assert(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;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
VertexMesh<ELEMENT_DIM, SPACE_DIM>::VertexMesh()
{
    mpDelaunayMesh = NULL;
    this->mMeshChangesDuringSimulation = false;
    Clear();
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
VertexMesh<ELEMENT_DIM, SPACE_DIM>::~VertexMesh()
{
    Clear();
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned VertexMesh<ELEMENT_DIM, SPACE_DIM>::SolveNodeMapping(unsigned index) const
{
    assert(index < this->mNodes.size());
    return index;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned VertexMesh<ELEMENT_DIM, SPACE_DIM>::SolveElementMapping(unsigned index) const
{
    assert(index < this->mElements.size());
    return index;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned VertexMesh<ELEMENT_DIM, SPACE_DIM>::SolveBoundaryElementMapping(unsigned index) const
{
//    assert(index < this->mBoundaryElements.size() );
    return index;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetDelaunayNodeIndexCorrespondingToVoronoiElementIndex(unsigned elementIndex)
{
    unsigned node_index = UNSIGNED_UNSET;

    if (mVoronoiElementIndexMap.empty())
    {
        node_index = elementIndex;
    }
    else
    {
        for (std::map<unsigned, unsigned>::iterator iter = mVoronoiElementIndexMap.begin();
             iter != mVoronoiElementIndexMap.end();
             ++iter)
        {
            if (iter->second == elementIndex)
            {
                node_index = iter->first;
                break;
            }
        }
    }
    assert(node_index != UNSIGNED_UNSET);
    return node_index;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetVoronoiElementIndexCorrespondingToDelaunayNodeIndex(unsigned nodeIndex)
{
    unsigned element_index = UNSIGNED_UNSET;

    if (mVoronoiElementIndexMap.empty())
    {
        element_index = nodeIndex;
    }
    else
    {
        std::map<unsigned, unsigned>::iterator iter = mVoronoiElementIndexMap.find(nodeIndex);

        if (iter == mVoronoiElementIndexMap.end())
        {
            EXCEPTION("This index does not correspond to a VertexElement");
        }
        else
        {
            element_index = iter->second;
        }
    }
    assert(element_index != UNSIGNED_UNSET);
    return element_index;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
void VertexMesh<ELEMENT_DIM, SPACE_DIM>::Clear()
{
    // Delete elements
    for (unsigned i=0; i<mElements.size(); i++)
    {
        delete mElements[i];
    }
    mElements.clear();

    // Delete faces
    for (unsigned i=0; i<mFaces.size(); i++)
    {
        delete mFaces[i];
    }
    mFaces.clear();

    // Delete nodes
    for (unsigned i=0; i<this->mNodes.size(); i++)
    {
        delete this->mNodes[i];
    }
    this->mNodes.clear();
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetNumNodes() const
{
    return this->mNodes.size();
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetNumElements() const
{
    return mElements.size();
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetNumAllElements() const
{
    return mElements.size();
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetNumFaces() const
{
    return mFaces.size();
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
VertexElement<ELEMENT_DIM, SPACE_DIM>* VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetElement(unsigned index) const
{
    assert(index < mElements.size());
    return mElements[index];
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
VertexElement<ELEMENT_DIM-1, SPACE_DIM>* VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetFace(unsigned index) const
{
    assert(index < mFaces.size());
    return mFaces[index];
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
c_vector<double, SPACE_DIM> VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetCentroidOfElement(unsigned index)
{
    VertexElement<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);

    switch (SPACE_DIM)
    {
        case 1:
        {
            centroid = 0.5*(p_element->GetNodeLocation(0) + p_element->GetNodeLocation(1));
        }
        break;
        case 2:
        {
            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);
        }
        break;
        case 3:
        {
            for (unsigned local_index=0; local_index<num_nodes; local_index++)
            {
                centroid += p_element->GetNodeLocation(local_index);
            }
            centroid /= ((double) num_nodes);
        }
        break;
        default:
            NEVER_REACHED;
    }
    return centroid;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
std::set<unsigned> VertexMesh<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>
std::set<unsigned> VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetNeighbouringNodeNotAlsoInElement(unsigned nodeIndex, unsigned elemIndex)
{
    // Get a pointer to this element
    VertexElement<ELEMENT_DIM, SPACE_DIM>* p_element = this->GetElement(elemIndex);

    // Get the indices of the nodes contained in this element
    std::set<unsigned> node_indices_in_this_element;
    for (unsigned local_index=0; local_index<p_element->GetNumNodes(); local_index++)
    {
        unsigned global_index = p_element->GetNodeGlobalIndex(local_index);
        node_indices_in_this_element.insert(global_index);
    }

    // Check that the node is in fact contained in the element
    if (node_indices_in_this_element.find(nodeIndex) == node_indices_in_this_element.end())
    {
        EXCEPTION("The given node is not contained in the given element.");
    }

    // Create a set of neighbouring node indices
    std::set<unsigned> neighbouring_node_indices_not_in_this_element;

    // Get the indices of this node's neighbours
    std::set<unsigned> node_neighbours = GetNeighbouringNodeIndices(nodeIndex);

    // Check if each neighbour is also in this element; if not, add it to the set
    for (std::set<unsigned>::iterator iter = node_neighbours.begin();
         iter != node_neighbours.end();
         ++iter)
    {
        if (node_indices_in_this_element.find(*iter) == node_indices_in_this_element.end())
        {
            neighbouring_node_indices_not_in_this_element.insert(*iter);
        }
    }

    return neighbouring_node_indices_not_in_this_element;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
std::set<unsigned> VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetNeighbouringElementIndices(unsigned elementIndex)
{
    // Get a pointer to this element
    VertexElement<ELEMENT_DIM, SPACE_DIM>* p_element = this->GetElement(elementIndex);

    // Create a set of neighbouring element indices
    std::set<unsigned> neighbouring_element_indices;

    // Loop over nodes owned by this element
    for (unsigned local_index=0; local_index<p_element->GetNumNodes(); local_index++)
    {
        // Get a pointer to this node
        Node<SPACE_DIM>* p_node = p_element->GetNode(local_index);

        // Find the indices of the elements owned by this node
        std::set<unsigned> containing_elem_indices = p_node->rGetContainingElementIndices();

        // Form the union of this set with the current set of neighbouring element indices
        std::set<unsigned> all_elements;
        std::set_union(neighbouring_element_indices.begin(), neighbouring_element_indices.end(),
                       containing_elem_indices.begin(), containing_elem_indices.end(),
                       std::inserter(all_elements, all_elements.begin()));

        // Update the set of neighbouring element indices
        neighbouring_element_indices = all_elements;
    }

    // Lastly remove this element's index from the set of neighbouring element indices
    neighbouring_element_indices.erase(elementIndex);

    return neighbouring_element_indices;
}


template<>
void VertexMesh<1,1>::ConstructFromMeshReader(AbstractMeshReader<1,1>& rMeshReader)
{
}

template<>
void VertexMesh<1,2>::ConstructFromMeshReader(AbstractMeshReader<1,2>& rMeshReader)
{
}

template<>
void VertexMesh<1,3>::ConstructFromMeshReader(AbstractMeshReader<1,3>& rMeshReader)
{
}

template<>
void VertexMesh<2,3>::ConstructFromMeshReader(AbstractMeshReader<2,3>& rMeshReader)
{
}

template<>
void VertexMesh<2,2>::ConstructFromMeshReader(AbstractMeshReader<2,2>& rMeshReader)
{
    assert(rMeshReader.HasNodePermutation() == false);
    // Store numbers of nodes and elements
    unsigned num_nodes = rMeshReader.GetNumNodes();
    unsigned num_elements = rMeshReader.GetNumElements();

    // Reserve memory for nodes
    this->mNodes.reserve(num_nodes);

    rMeshReader.Reset();

    // Add nodes
    std::vector<double> node_data;
    for (unsigned i=0; i<num_nodes; i++)
    {
        node_data = rMeshReader.GetNextNode();
        unsigned is_boundary_node = (bool) node_data[2];
        node_data.pop_back();
        this->mNodes.push_back(new Node<2>(i, node_data, is_boundary_node));
    }

    rMeshReader.Reset();

    // Reserve memory for nodes
    mElements.reserve(rMeshReader.GetNumElements());

    // Add elements
    for (unsigned elem_index=0; elem_index<num_elements; elem_index++)
    {
        // Get the data for this element
        ElementData element_data = rMeshReader.GetNextElementData();

        // Get the nodes owned by this element
        std::vector<Node<2>*> nodes;
        unsigned num_nodes_in_element = element_data.NodeIndices.size();
        for (unsigned j=0; j<num_nodes_in_element; j++)
        {
            assert(element_data.NodeIndices[j] < this->mNodes.size());
            nodes.push_back(this->mNodes[element_data.NodeIndices[j]]);
        }

        // Use nodes and index to construct this element
        VertexElement<2,2>* p_element = new VertexElement<2,2>(elem_index, nodes);
        mElements.push_back(p_element);

        if (rMeshReader.GetNumElementAttributes() > 0)
        {
            assert(rMeshReader.GetNumElementAttributes() == 1);
            unsigned attribute_value = (unsigned) element_data.AttributeValue;
            p_element->SetAttribute(attribute_value);
        }
    }
}

template<>
void VertexMesh<3,3>::ConstructFromMeshReader(AbstractMeshReader<3,3>& rMeshReader)
{
    assert(rMeshReader.HasNodePermutation() == false);

    // Store numbers of nodes and elements
    unsigned num_nodes = rMeshReader.GetNumNodes();
    unsigned num_elements = rMeshReader.GetNumElements();

    // Reserve memory for nodes
    this->mNodes.reserve(num_nodes);

    rMeshReader.Reset();

    // Add nodes
    std::vector<double> node_data;
    for (unsigned i=0; i<num_nodes; i++)
    {
        node_data = rMeshReader.GetNextNode();
        unsigned is_boundary_node = (bool) node_data[3];
        node_data.pop_back();
        this->mNodes.push_back(new Node<3>(i, node_data, is_boundary_node));
    }

    rMeshReader.Reset();

    // Reserve memory for nodes
    mElements.reserve(rMeshReader.GetNumElements());

    // Use a std::set to keep track of which faces have been added to mFaces
    std::set<unsigned> faces_counted;

    // Add elements
    for (unsigned elem_index=0; elem_index<num_elements; elem_index++)
    {
        typedef VertexMeshReader<3,3> VERTEX_MESH_READER;
        assert(dynamic_cast<VERTEX_MESH_READER*>(&rMeshReader) != NULL);

        // Get the data for this element
        VertexElementData element_data = static_cast<VERTEX_MESH_READER*>(&rMeshReader)->GetNextElementDataWithFaces();

        // Get the nodes owned by this element
        std::vector<Node<3>*> nodes;
        unsigned num_nodes_in_element = element_data.NodeIndices.size();
        for (unsigned j=0; j<num_nodes_in_element; j++)
        {
            assert(element_data.NodeIndices[j] < this->mNodes.size());
            nodes.push_back(this->mNodes[element_data.NodeIndices[j]]);
        }

        // Get the faces owned by this element
        std::vector<VertexElement<2,3>*> faces;
        unsigned num_faces_in_element = element_data.Faces.size();
        for (unsigned i=0; i<num_faces_in_element; i++)
        {
            // Get the data for this face
            ElementData face_data = element_data.Faces[i];

            // Get the face index
            unsigned face_index = (unsigned) face_data.AttributeValue;

            // Get the nodes owned by this face
            std::vector<Node<3>*> nodes_in_face;
            unsigned num_nodes_in_face = face_data.NodeIndices.size();
            for (unsigned j=0; j<num_nodes_in_face; j++)
            {
                assert(face_data.NodeIndices[j] < this->mNodes.size());
                nodes_in_face.push_back(this->mNodes[face_data.NodeIndices[j]]);
            }

            // If this face index is not already encountered, create a new face and update faces_counted...
            if (faces_counted.find(face_index) == faces_counted.end())
            {
                // Use nodes and index to construct this face
                VertexElement<2,3>* p_face = new VertexElement<2,3>(face_index, nodes_in_face);
                mFaces.push_back(p_face);
                faces_counted.insert(face_index);
                faces.push_back(p_face);
            }
            else
            {
                // ... otherwise use the member of mFaces with this index
                bool face_added = false;
                for (unsigned k=0; k<mFaces.size(); k++)
                {
                    if (mFaces[k]->GetIndex() == face_index)
                    {
                        faces.push_back(mFaces[k]);
                        face_added = true;
                        break;
                    }
                }
                UNUSED_OPT(face_added);
                assert(face_added == true);
            }
        }

        std::vector<bool> orientations = std::vector<bool>(num_faces_in_element, true);

        // Use faces and index to construct this element
        VertexElement<3,3>* p_element = new VertexElement<3,3>(elem_index, faces, orientations, nodes);
        mElements.push_back(p_element);

        if (rMeshReader.GetNumElementAttributes() > 0)
        {
            assert(rMeshReader.GetNumElementAttributes() == 1);
            unsigned attribute_value = element_data.AttributeValue;
            p_element->SetAttribute(attribute_value);
        }
    }
}


template <unsigned ELEMENT_DIM, unsigned SPACE_DIM>
c_vector<double, SPACE_DIM> VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetVectorFromAtoB(
    const c_vector<double, SPACE_DIM>& rLocationA, const c_vector<double, SPACE_DIM>& rLocationB)
{
    c_vector<double, SPACE_DIM> vector;
    if (mpDelaunayMesh)
    {
        vector = mpDelaunayMesh->GetVectorFromAtoB(rLocationA, rLocationB);
    }
    else
    {
        vector = AbstractMesh<ELEMENT_DIM, SPACE_DIM>::GetVectorFromAtoB(rLocationA, rLocationB);
    }
    return vector;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetVolumeOfElement(unsigned index)
{
    assert(SPACE_DIM == 2 || SPACE_DIM == 3);

    // Get pointer to this element
    VertexElement<ELEMENT_DIM, SPACE_DIM>* p_element = GetElement(index);

    double element_volume = 0.0;
    if (SPACE_DIM == 2)
    {
        // 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;
        }
    }
    else
    {
        // Loop over faces and add up pyramid volumes
        c_vector<double, SPACE_DIM> pyramid_apex = p_element->GetNodeLocation(0);
        for (unsigned face_index=0; face_index<p_element->GetNumFaces(); face_index++)
        {
            // Get pointer to face
            VertexElement<ELEMENT_DIM-1, SPACE_DIM>* p_face = p_element->GetFace(face_index);


            // Calculate the area of the face and get unit normal to this face
            c_vector<double, SPACE_DIM> unit_normal;
            double face_area = CalculateUnitNormalToFaceWithArea(p_face, unit_normal);

            // Calculate the perpendicular distance from the plane of the face to the chosen apex
            c_vector<double, SPACE_DIM> base_to_apex = GetVectorFromAtoB(p_face->GetNodeLocation(0), pyramid_apex);
            double perpendicular_distance = fabs(inner_prod(base_to_apex, unit_normal));


            // Use these to calculate the volume of the pyramid formed by the face and the point pyramid_apex
            element_volume += face_area * perpendicular_distance / 3;
        }
    }

    // We take the absolute value just in case the nodes were really oriented clockwise
    return fabs(element_volume);
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetSurfaceAreaOfElement(unsigned index)
{
    assert(SPACE_DIM == 2 || SPACE_DIM == 3);

    // Get pointer to this element
    VertexElement<ELEMENT_DIM, SPACE_DIM>* p_element = GetElement(index);

    double surface_area = 0.0;
    if (SPACE_DIM == 2)
    {
        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;
        }
    }
    else
    {
        // Loop over faces and add up areas
        for (unsigned face_index=0; face_index<p_element->GetNumFaces(); face_index++)
        {
            surface_area += CalculateAreaOfFace(p_element->GetFace(face_index));
        }
    }
    return surface_area;
}


//                        2D-specific methods                       //

template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
bool VertexMesh<ELEMENT_DIM, SPACE_DIM>::ElementIncludesPoint(const c_vector<double, SPACE_DIM>& rTestPoint, unsigned elementIndex)
{
    assert(SPACE_DIM == 2);
    assert(ELEMENT_DIM == SPACE_DIM);

    // Get the element
    VertexElement<ELEMENT_DIM, SPACE_DIM>* p_element = GetElement(elementIndex);
    unsigned num_nodes = p_element->GetNumNodes();

    // Initialise boolean
    bool element_includes_point = false;

//    // Initialise boolean
//    bool element_includes_point = true;
//
//    unsigned winding_number = 0;
//
//    c_vector<double, SPACE_DIM> first_node_location = p_element->GetNodeLocation(0);
//    c_vector<double, SPACE_DIM> test_point = this->GetVectorFromAtoB(first_node_location, rTestPoint);
//    c_vector<double, SPACE_DIM> this_node_location = zero_vector<double>(SPACE_DIM);
//
//    // Loop over edges of the element
//    for (unsigned local_index=0; local_index<num_nodes; local_index++)
//    {
//        c_vector<double, SPACE_DIM> untransformed_vector = p_element->GetNodeLocation((local_index+1)%num_nodes);
//        c_vector<double, SPACE_DIM> next_node_location = this->GetVectorFromAtoB(first_node_location, untransformed_vector);
//
//        // If this edge is crossing upward...
//        if (this_node_location[1] <= test_point[1])
//        {
//            if (next_node_location[1] > test_point[1])
//            {
//                double is_left =  (next_node_location[0] - this_node_location[0])*(test_point[1] - this_node_location[1])
//                                 - (test_point[0] - this_node_location[0])*(next_node_location[1] - this_node_location[1]);
//
//                // ...and the test point is to the left of the edge...
//                if (is_left > DBL_EPSILON)
//                {
//                    // ...then there is a valid upward edge-ray intersection to the right of the test point
//                    winding_number++;
//                }
//            }
//        }
//        else
//        {
//            // ...otherwise, if the edge is crossing downward
//            if (next_node_location[1] <= test_point[1])
//            {
//                double is_left =  (next_node_location[0] - this_node_location[0])*(test_point[1] - this_node_location[1])
//                                 - (test_point[0] - this_node_location[0])*(next_node_location[1] - this_node_location[1]);
//
//                // ...and the test point is to the right of the edge...
//                if (is_left < -DBL_EPSILON)
//                {
//                    // ...then there is a valid downward edge-ray intersection to the right of the test point
//                    winding_number--;
//                }
//            }
//        }
//        this_node_location = next_node_location;
//    }
//
//    if (winding_number == 0)
//    {
//        element_includes_point = false;
//    }

    // Remap the origin to the first vertex to allow alternative distance metrics to be used in subclasses
    c_vector<double, SPACE_DIM> first_vertex = p_element->GetNodeLocation(0);
    c_vector<double, SPACE_DIM> test_point = GetVectorFromAtoB(first_vertex, rTestPoint);

    // Loop over edges of the element
    c_vector<double, SPACE_DIM> vertexA = zero_vector<double>(SPACE_DIM);
    for (unsigned local_index=0; local_index<num_nodes; local_index++)
    {
        // Check if this edge crosses the ray running out horizontally (increasing x, fixed y) from the test point
        c_vector<double, SPACE_DIM> vector_a_to_point = GetVectorFromAtoB(vertexA, test_point);

        // Pathological case - test point coincides with vertexA
        // (we will check vertexB next time we go through the for loop)
        if (norm_2(vector_a_to_point) < DBL_EPSILON)
        {
            return false;
        }

        c_vector<double, SPACE_DIM> vertexB = GetVectorFromAtoB(first_vertex, p_element->GetNodeLocation((local_index+1)%num_nodes));
        c_vector<double, SPACE_DIM> vector_b_to_point = GetVectorFromAtoB(vertexB, test_point);
        c_vector<double, SPACE_DIM> vector_a_to_b = GetVectorFromAtoB(vertexA, vertexB);

        // Pathological case - ray coincides with horizontal edge
        if ( (fabs(vector_a_to_b[1]) < DBL_EPSILON) &&
             (fabs(vector_a_to_point[1]) < DBL_EPSILON) &&
             (fabs(vector_b_to_point[1]) < DBL_EPSILON) )
        {
            if ( (vector_a_to_point[0]>0) != (vector_b_to_point[0]>0) )
            {
                return false;
            }
        }

        // Non-pathological case
        // A and B on different sides of the line y = test_point[1]
        if ( (vertexA[1] > test_point[1]) != (vertexB[1] > test_point[1]) )
        {
            // Intersection of y=test_point[1] and vector_a_to_b is on the right of test_point
            if (test_point[0] < vertexA[0] + vector_a_to_b[0]*vector_a_to_point[1]/vector_a_to_b[1])
            {
                element_includes_point = !element_includes_point;
            }
        }

        vertexA = vertexB;
    }
    return element_includes_point;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
unsigned VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetLocalIndexForElementEdgeClosestToPoint(const c_vector<double, SPACE_DIM>& rTestPoint, unsigned elementIndex)
{
    // Make sure that we are in the correct dimension - this code will be eliminated at compile time
    assert(SPACE_DIM == 2);
    assert(ELEMENT_DIM == SPACE_DIM);

    // Get the element
    VertexElement<ELEMENT_DIM, SPACE_DIM>* p_element = GetElement(elementIndex);
    unsigned num_nodes = p_element->GetNumNodes();

    double min_squared_normal_distance = DBL_MAX;
    unsigned min_distance_edge_index = UINT_MAX;

    // Loop over edges of the element
    for (unsigned local_index=0; local_index<num_nodes; local_index++)
    {
        // Get the end points of this edge
        c_vector<double, SPACE_DIM> vertexA = p_element->GetNodeLocation(local_index);
        c_vector<double, SPACE_DIM> vertexB = p_element->GetNodeLocation((local_index+1)%num_nodes);

        c_vector<double, SPACE_DIM> vector_a_to_point = this->GetVectorFromAtoB(vertexA, rTestPoint);
        c_vector<double, SPACE_DIM> vector_a_to_b = this->GetVectorFromAtoB(vertexA, vertexB);
        double distance_a_to_b = norm_2(vector_a_to_b);

        c_vector<double, SPACE_DIM> edge_ab_unit_vector = vector_a_to_b/norm_2(vector_a_to_b);
        double distance_parallel_to_edge = inner_prod(vector_a_to_point, edge_ab_unit_vector);

        double squared_distance_normal_to_edge = SmallPow(norm_2(vector_a_to_point), 2) - SmallPow(distance_parallel_to_edge, 2);

        /*
         * If the point lies almost bang on the supporting line of the edge, then snap to the line.
         * This allows us to do floating point tie-breaks when line is exactly at a node.
         * We adopt a similar approach if the point is at the same position as a point in the
         * element.
         */
        if (squared_distance_normal_to_edge < DBL_EPSILON)
        {
            squared_distance_normal_to_edge = 0.0;
        }

        if (fabs(distance_parallel_to_edge) < DBL_EPSILON)
        {
            distance_parallel_to_edge = 0.0;
        }
        else if (fabs(distance_parallel_to_edge-distance_a_to_b) < DBL_EPSILON)
        {
            distance_parallel_to_edge = distance_a_to_b;
        }

        // Make sure node is within the confines of the edge and is the nearest edge to the node \this breaks for convex elements
        if (squared_distance_normal_to_edge < min_squared_normal_distance &&
                distance_parallel_to_edge >=0 &&
                distance_parallel_to_edge <= distance_a_to_b)
        {
            min_squared_normal_distance = squared_distance_normal_to_edge;
            min_distance_edge_index = local_index;
        }
    }

    assert(min_distance_edge_index < num_nodes);
    return min_distance_edge_index;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
c_vector<double, 3> VertexMesh<ELEMENT_DIM, SPACE_DIM>::CalculateMomentsOfElement(unsigned index)
{
    assert(SPACE_DIM == 2);

    // Define helper variables
    VertexElement<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;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
c_vector<double, SPACE_DIM> VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetShortAxisOfElement(unsigned index)
{
    assert(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);

    // 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) < 1e-10) 
    {
        // ...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 (moments(2) == 0.0)
        {
            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);

            double magnitude = norm_2(short_axis);
            short_axis = short_axis / magnitude;
        }
    }

    return short_axis;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
c_vector<double, SPACE_DIM> VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetAreaGradientOfElementAtNode(VertexElement<ELEMENT_DIM,SPACE_DIM>* pElement, unsigned localIndex)
{
    assert(SPACE_DIM==2);

    unsigned num_nodes_in_element = pElement->GetNumNodes();
    unsigned next_local_index = (localIndex+1)%num_nodes_in_element;

    // We add an extra num_nodes_in_element in the line below as otherwise this term can be negative, which breaks the % operator
    unsigned previous_local_index = (num_nodes_in_element+localIndex-1)%num_nodes_in_element;

    c_vector<double, SPACE_DIM> previous_node_location = pElement->GetNodeLocation(previous_local_index);
    c_vector<double, SPACE_DIM> next_node_location = pElement->GetNodeLocation(next_local_index);
    c_vector<double, SPACE_DIM> difference_vector = this->GetVectorFromAtoB(previous_node_location, next_node_location);

    c_vector<double, SPACE_DIM> area_gradient;

    area_gradient[0] = 0.5*difference_vector[1];
    area_gradient[1] = -0.5*difference_vector[0];

    return area_gradient;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
c_vector<double, SPACE_DIM> VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetPreviousEdgeGradientOfElementAtNode(VertexElement<ELEMENT_DIM,SPACE_DIM>* pElement, unsigned localIndex)
{
    assert(SPACE_DIM==2);

    unsigned num_nodes_in_element = pElement->GetNumNodes();

    // We add an extra num_nodes_in_element-1 in the line below as otherwise this term can be negative, which breaks the % operator
    unsigned previous_local_index = (num_nodes_in_element+localIndex-1)%num_nodes_in_element;

    unsigned this_global_index = pElement->GetNodeGlobalIndex(localIndex);
    unsigned previous_global_index = pElement->GetNodeGlobalIndex(previous_local_index);

    double previous_edge_length = this->GetDistanceBetweenNodes(this_global_index, previous_global_index);
    assert(previous_edge_length > DBL_EPSILON);

    c_vector<double, SPACE_DIM> previous_edge_gradient = this->GetVectorFromAtoB(pElement->GetNodeLocation(previous_local_index), pElement->GetNodeLocation(localIndex))/previous_edge_length;

    return previous_edge_gradient;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
c_vector<double, SPACE_DIM> VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetNextEdgeGradientOfElementAtNode(VertexElement<ELEMENT_DIM,SPACE_DIM>* pElement, unsigned localIndex)
{
    assert(SPACE_DIM==2);

    unsigned next_local_index = (localIndex+1)%(pElement->GetNumNodes());

    unsigned this_global_index = pElement->GetNodeGlobalIndex(localIndex);
    unsigned next_global_index = pElement->GetNodeGlobalIndex(next_local_index);

    double next_edge_length = this->GetDistanceBetweenNodes(this_global_index, next_global_index);
    assert(next_edge_length > DBL_EPSILON);

    c_vector<double, SPACE_DIM> next_edge_gradient = this->GetVectorFromAtoB(pElement->GetNodeLocation(next_local_index), pElement->GetNodeLocation(localIndex))/next_edge_length;

    return next_edge_gradient;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
c_vector<double, SPACE_DIM> VertexMesh<ELEMENT_DIM, SPACE_DIM>::GetPerimeterGradientOfElementAtNode(VertexElement<ELEMENT_DIM,SPACE_DIM>* pElement, unsigned localIndex)
{
    assert(SPACE_DIM==2);

    c_vector<double, SPACE_DIM> previous_edge_gradient = GetPreviousEdgeGradientOfElementAtNode(pElement, localIndex);
    c_vector<double, SPACE_DIM> next_edge_gradient = GetNextEdgeGradientOfElementAtNode(pElement, localIndex);

    return previous_edge_gradient + next_edge_gradient;
}


//                        3D-specific methods                       //


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double VertexMesh<ELEMENT_DIM, SPACE_DIM>::CalculateUnitNormalToFaceWithArea(VertexElement<ELEMENT_DIM-1, SPACE_DIM>* pFace, c_vector<double, SPACE_DIM>& rNormal)
{
    assert(SPACE_DIM == 3);
    // As we are in 3D, the face must have at least three vertices,
    assert( pFace->GetNumNodes() >= 3u );

    // Reset the answer
    rNormal = zero_vector<double>(SPACE_DIM);

    c_vector<double, SPACE_DIM> v_minus_v0 = this->GetVectorFromAtoB(pFace->GetNode(0)->rGetLocation(), pFace->GetNode(1)->rGetLocation());
    for (unsigned local_index=2; local_index<pFace->GetNumNodes(); local_index++)
    {

        c_vector<double, SPACE_DIM> vnext_minus_v0 = this->GetVectorFromAtoB(pFace->GetNode(0)->rGetLocation(), pFace->GetNode(local_index)->rGetLocation());
        rNormal += VectorProduct(v_minus_v0, vnext_minus_v0);
        v_minus_v0 = vnext_minus_v0;
    }
    double magnitude = norm_2(rNormal);
    if ( magnitude != 0.0 )
    {
        // Normalize the normal vector
        rNormal /= magnitude;
        // If all points are co-located, then magnitude==0.0 and there is potential for a floating point exception
        // here if we divide by zero, so we'll move on.
    }
    return magnitude/2.0;
}


template<unsigned ELEMENT_DIM, unsigned SPACE_DIM>
double VertexMesh<ELEMENT_DIM, SPACE_DIM>::CalculateAreaOfFace(VertexElement<ELEMENT_DIM-1, SPACE_DIM>* pFace)
{
    assert(SPACE_DIM == 3);

    // Get the unit normal to the plane of this face
    c_vector<double, SPACE_DIM> unit_normal;
    return CalculateUnitNormalToFaceWithArea(pFace, unit_normal);
}
#if defined(__xlC__)
template<>
double VertexMesh<1,1>::CalculateAreaOfFace(VertexElement<0,1>* pFace)
{
    NEVER_REACHED;
}
#endif


// Explicit instantiation

template class VertexMesh<1,1>;
template class VertexMesh<1,2>;
template class VertexMesh<1,3>;
template class VertexMesh<2,2>;
template class VertexMesh<2,3>;
template class VertexMesh<3,3>;


// Serialization for Boost >= 1.36
#include "SerializationExportWrapperForCpp.hpp"
EXPORT_TEMPLATE_CLASS_ALL_DIMS(VertexMesh)