/*=========================================================================

Library: iMSTK

Copyright (c) Kitware, Inc. & Center for Modeling, Simulation,
& Imaging in Medicine, Rensselaer Polytechnic Institute.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0.txt

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONinitiDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

=========================================================================*/

#include "imstkSPHModel.h"
#include "imstkParallelUtils.h"
#include "imstkPointSet.h"
#include "imstkTaskGraph.h"
#include "imstkVTKMeshIO.h"

namespace imstk
{
SPHModelConfig::SPHModelConfig(const Real particleRadius)
{
    // \todo Warning in all paths?
    if (std::abs(particleRadius) > Real(1.e-6))
    {
        LOG_IF(WARNING, (particleRadius < 0)) << "Particle radius supplied is negative! Using absolute value of the supplied radius.";
        m_particleRadius = std::abs(particleRadius);
    }
    else
    {
        LOG(WARNING) << "Particle radius too small! Setting to 1.e-6";
        m_particleRadius = 1.e-6;
    }
    initialize();
}

SPHModelConfig::SPHModelConfig(const Real particleRadius, const Real speedOfSound, const Real restDensity)
{
    if (std::abs(particleRadius) > Real(1.e-6))
    {
        LOG_IF(WARNING, (particleRadius < 0)) << "Particle radius supplied is negative! Using absolute value of the supplied radius.";
        m_particleRadius = std::abs(particleRadius);
    }
    else
    {
        LOG(WARNING) << "Particle radius too small! Setting to 1.e-6";
        m_particleRadius = 1.e-6;
    }

    if (speedOfSound < 0)
    {
        LOG(WARNING) << "Speed of sound is negative! Setting speed of sound to default value.";
    }
    else
    {
        m_speedOfSound = speedOfSound;
    }

    if (restDensity < 0)
    {
        LOG(WARNING) << "Rest density is negative! Setting rest density to default value.";
    }
    else
    {
        m_restDensity = restDensity;
    }
    initialize();
}

void
SPHModelConfig::initialize()
{
    // Compute the derived quantities
    m_particleRadiusSqr = m_particleRadius * m_particleRadius;

    m_particleMass   = Real(std::pow(Real(2.0) * m_particleRadius, 3)) * m_restDensity * m_particleMassScale;
    m_restDensitySqr = m_restDensity * m_restDensity;
    m_restDensityInv = Real(1.0) / m_restDensity;

    m_kernelRadius    = m_particleRadius * m_kernelOverParticleRadiusRatio;
    m_kernelRadiusSqr = m_kernelRadius * m_kernelRadius;

    m_pressureStiffness = m_restDensity * m_speedOfSound * m_speedOfSound / 7;
}

SPHModel::SPHModel() : DynamicalModel<SPHState>(DynamicalModelType::SmoothedParticleHydrodynamics)
{
    m_validGeometryTypes = { "PointSet" };

    m_findParticleNeighborsNode = m_taskGraph->addFunction("SPHModel_Partition", std::bind(&SPHModel::findParticleNeighbors, this));
    m_computeDensityNode = m_taskGraph->addFunction("SPHModel_ComputeDensity", [&]()
        {
            computeNeighborRelativePositions();
            computeDensity();
        });

    m_normalizeDensityNode = m_taskGraph->addFunction("SPHModel_NormalizeDensity", std::bind(&SPHModel::normalizeDensity, this));

    m_collectNeighborDensityNode = m_taskGraph->addFunction("SPHModel_CollectNeighborDensity", std::bind(&SPHModel::collectNeighborDensity, this));

    m_computeTimeStepSizeNode =
        m_taskGraph->addFunction("SPHModel_ComputeTimestep", std::bind(&SPHModel::computeTimeStepSize, this));

    m_computePressureAccelNode =
        m_taskGraph->addFunction("SPHModel_ComputePressureAccel", std::bind(&SPHModel::computePressureAcceleration, this));

    m_computeSurfaceTensionNode =
        m_taskGraph->addFunction("SPHModel_ComputeSurfaceTensionAccel", std::bind(&SPHModel::computeSurfaceTension, this));

    m_computeViscosityNode =
        m_taskGraph->addFunction("SPHModel_ComputeViscosity", std::bind(&SPHModel::computeViscosity, this));

    m_integrateNode =
        m_taskGraph->addFunction("SPHModel_Integrate", std::bind(&SPHModel::sumAccels, this));

    m_updateVelocityNode =
        m_taskGraph->addFunction("SPHModel_UpdateVelocity", [&]()
        {
            updateVelocity(getTimeStep());
        });

    m_moveParticlesNode =
        m_taskGraph->addFunction("SPHModel_MoveParticles", [&]()
        {
            moveParticles(getTimeStep());
        });

    //m_computePositionNode =
    //    m_taskGraph->addFunction("SPHModel_ComputePositions", [&]()
    //    {
    //        moveParticles(getTimeStep());
    //        });
}

bool
SPHModel::initialize()
{
    LOG_IF(FATAL, (!this->getModelGeometry())) << "Model geometry is not yet set! Cannot initialize without model geometry.";
    m_pointSetGeometry = std::dynamic_pointer_cast<PointSet>(m_geometry);
    const int numParticles = m_pointSetGeometry->getNumVertices();

    // Allocate init and current state
    m_initialState = std::make_shared<SPHState>(numParticles);
    m_currentState = std::make_shared<SPHState>(numParticles);

    // If there were initial velocities (set them)
    if (m_initialVelocities != nullptr)
    {
        m_currentState->setVelocities(m_initialVelocities);
    }

    // Copy current to initial
    m_initialState->setState(m_currentState);

    // Share geometry and state position arrays
    m_currentState->setPositions(m_pointSetGeometry->getVertexPositions());
    m_initialState->setPositions(m_pointSetGeometry->getInitialVertexPositions());

    // Initialize simulation dependent parameters and kernel data
    m_kernels.initialize(m_modelParameters->m_kernelRadius);

    // Initialize neighbor searcher
    m_neighborSearcher = std::make_shared<NeighborSearch>(m_modelParameters->m_NeighborSearchMethod,
      m_modelParameters->m_kernelRadius);

    m_pressureAccels = std::make_shared<VecDataArray<double, 3>>(numParticles);
    std::fill_n(m_pressureAccels->getPointer(), m_pressureAccels->size(), Vec3d(0, 0, 0));

    // initialize surface tension to 0 in case you remove the surface tension node
    m_surfaceTensionAccels = std::make_shared<VecDataArray<double, 3>>(numParticles);
    std::fill_n(m_surfaceTensionAccels->getPointer(), m_surfaceTensionAccels->size(), Vec3d(0, 0, 0));

    m_viscousAccels = std::make_shared<VecDataArray<double, 3>>(numParticles);
    std::fill_n(m_viscousAccels->getPointer(), m_viscousAccels->size(), Vec3d(0, 0, 0));

    m_neighborVelContr = std::make_shared<VecDataArray<double, 3>>(numParticles);
    std::fill_n(m_neighborVelContr->getPointer(), m_neighborVelContr->size(), Vec3d(0, 0, 0));

    m_particleShift = std::make_shared<VecDataArray<double, 3>>(numParticles);
    std::fill_n(m_particleShift->getPointer(), m_particleShift->size(), Vec3d(0, 0, 0));

    // Add all the attributes to the geometry
    m_pointSetGeometry->setVertexAttribute("Pressure Accels", m_pressureAccels);
    m_pointSetGeometry->setVertexAttribute("Surface Tension Accels", m_surfaceTensionAccels);
    m_pointSetGeometry->setVertexAttribute("Viscous Accels", m_viscousAccels);
    m_pointSetGeometry->setVertexAttribute("Densities", m_currentState->getDensities());
    m_pointSetGeometry->setVertexAttribute("Velocities", m_currentState->getVelocities());
    m_pointSetGeometry->setVertexAttribute("Diffuse Velocities", m_currentState->getDiffuseVelocities());
    m_pointSetGeometry->setVertexAttribute("Normals", m_currentState->getNormals());
    m_pointSetGeometry->setVertexAttribute("Accels", m_currentState->getAccelerations());

    return true;
}

void
SPHModel::initGraphEdges(std::shared_ptr<TaskNode> source, std::shared_ptr<TaskNode> sink)
{
    // Setup graph connectivity
    m_taskGraph->addEdge(source, m_findParticleNeighborsNode);
    m_taskGraph->addEdge(m_findParticleNeighborsNode, m_computeDensityNode);
    m_taskGraph->addEdge(m_computeDensityNode, m_normalizeDensityNode);
    m_taskGraph->addEdge(m_normalizeDensityNode, m_collectNeighborDensityNode);

    // Pressure, Surface Tension, and time step size can be done in parallel
    m_taskGraph->addEdge(m_collectNeighborDensityNode, m_computePressureAccelNode);
    m_taskGraph->addEdge(m_collectNeighborDensityNode, m_computeSurfaceTensionNode);
    m_taskGraph->addEdge(m_collectNeighborDensityNode, m_computeViscosityNode);
    m_taskGraph->addEdge(m_collectNeighborDensityNode, m_computeTimeStepSizeNode);

    m_taskGraph->addEdge(m_computePressureAccelNode, m_integrateNode);
    m_taskGraph->addEdge(m_computeSurfaceTensionNode, m_integrateNode);
    m_taskGraph->addEdge(m_computeViscosityNode, m_integrateNode);
    m_taskGraph->addEdge(m_computeTimeStepSizeNode, m_integrateNode);

    m_taskGraph->addEdge(m_integrateNode, m_updateVelocityNode);
    m_taskGraph->addEdge(m_updateVelocityNode, m_moveParticlesNode);
    m_taskGraph->addEdge(m_moveParticlesNode, sink);
}

void
SPHModel::computeTimeStepSize()
{
    m_dt = (this->m_timeStepSizeType == TimeSteppingType::Fixed) ? m_defaultDt : computeCFLTimeStepSize();
}

Real
SPHModel::computeCFLTimeStepSize()
{
    auto maxVel = ParallelUtils::findMaxL2Norm(*getCurrentState()->getFullStepVelocities());

    // dt = CFL * 2r / (speed of sound + max{|| v ||})
    Real timestep = maxVel > Real(1e-6) ?
                    m_modelParameters->m_CFLFactor * (Real(2.0) * m_modelParameters->m_particleRadius / (m_modelParameters->m_speedOfSound + maxVel)) :
                    m_modelParameters->m_maxTimestep;

    // clamp the time step size to be within a given range
    if (timestep > m_modelParameters->m_maxTimestep)
    {
        timestep = m_modelParameters->m_maxTimestep;
    }
    else if (timestep < m_modelParameters->m_minTimestep)
    {
        timestep = m_modelParameters->m_minTimestep;
    }
    return timestep;
}

void
SPHModel::findParticleNeighbors()
{
    m_neighborSearcher->getNeighbors(getCurrentState()->getFluidNeighborLists(), *getCurrentState()->getPositions());

    if (m_modelParameters->m_bDensityWithBoundary)   // if considering boundary particles for computing fluid density
    {
        m_neighborSearcher->getNeighbors(getCurrentState()->getBoundaryNeighborLists(),
            *getCurrentState()->getPositions(),
            *getCurrentState()->getBoundaryParticlePositions());
    }
}

void
SPHModel::computeNeighborRelativePositions()
{
    auto computeRelativePositions = [&](const Vec3r& ppos, const std::vector<size_t>& neighborList,
                                        const VecDataArray<double, 3>& allPositions, std::vector<NeighborInfo>& neighborInfo)
                                    {
                                        for (const size_t q : neighborList)
                                        {
                                            const Vec3d& qpos = allPositions[q];
                                            const Vec3d  r    = ppos - qpos;
                                            neighborInfo.push_back({ r, m_modelParameters->m_restDensity });
                                        }
                                    };

    std::shared_ptr<VecDataArray<double, 3>> positionsPtr = getCurrentState()->getPositions();
    const VecDataArray<double, 3>&           positions = *positionsPtr;

    std::vector<std::vector<NeighborInfo>>& neighborInfos = getCurrentState()->getNeighborInfo();

    ParallelUtils::parallelFor(getCurrentState()->getNumParticles(),
        [&](const size_t p)
        {
            if (m_sphBoundaryConditions
                && m_sphBoundaryConditions->getParticleTypes()[p] == SPHBoundaryConditions::ParticleType::Buffer)
            {
                return;
            }

            const Vec3d& ppos = positions[p];
            std::vector<NeighborInfo>& neighborInfo = neighborInfos[p];
            neighborInfo.resize(0);
            neighborInfo.reserve(48);

            computeRelativePositions(ppos, getCurrentState()->getFluidNeighborLists()[p], *getCurrentState()->getPositions(), neighborInfo);
            // if considering boundary particles then also cache relative positions with them
            if (m_modelParameters->m_bDensityWithBoundary)
            {
                computeRelativePositions(ppos, getCurrentState()->getBoundaryNeighborLists()[p], *getCurrentState()->getBoundaryParticlePositions(), neighborInfo);
            }
      });
}

void
SPHModel::collectNeighborDensity()
{
    // After computing particle densities, cache them into neighborInfo variable, next to relative positions
    // this is useful because relative positions and densities are accessed together multiple times
    // caching relative positions and densities therefore can reduce computation time significantly (tested)
    std::shared_ptr<DataArray<double>> densitiesPtr = getCurrentState()->getDensities();
    DataArray<double>&                 densities    = *densitiesPtr;

    const std::vector<std::vector<size_t>>&                 neighborLists = getCurrentState()->getFluidNeighborLists();
    const std::vector<SPHBoundaryConditions::ParticleType>& particleTypes = m_sphBoundaryConditions->getParticleTypes();

    ParallelUtils::parallelFor(getCurrentState()->getNumParticles(),
        [&](const size_t p)
        {
            if (m_sphBoundaryConditions && particleTypes[p] == SPHBoundaryConditions::ParticleType::Buffer)
            {
                return;
            }

            auto& neighborInfo = getCurrentState()->getNeighborInfo()[p];
            if (neighborInfo.size() <= 1)
            {
                return; // the particle has no neighbor
            }

            const auto& fluidNeighborList = neighborLists[p];
            for (size_t i = 0; i < fluidNeighborList.size(); ++i)
            {
                auto q = fluidNeighborList[i];
                neighborInfo[i].density = densities[q];
            }
      });
}

void
SPHModel::computeDensity()
{
    std::shared_ptr<DataArray<double>> densitiesPtr = getCurrentState()->getDensities();
    DataArray<double>&                 densities    = *densitiesPtr;

    const std::vector<std::vector<NeighborInfo>>& neighborInfos = getCurrentState()->getNeighborInfo();

    ParallelUtils::parallelFor(getCurrentState()->getNumParticles(),
        [&](const size_t p)
        {
            if (m_sphBoundaryConditions && m_sphBoundaryConditions->getParticleTypes()[p] == SPHBoundaryConditions::ParticleType::Buffer)
            {
                return;
            }

            const std::vector<NeighborInfo>& neighborInfo = neighborInfos[p];
            if (neighborInfo.size() <= 1)
            {
                return; // the particle has no neighbor
            }

            Real pdensity = 0.0;
            for (const auto& qInfo : neighborInfo)
            {
                pdensity += m_kernels.W(qInfo.xpq);
            }
            pdensity    *= m_modelParameters->m_particleMass;
            densities[p] = pdensity;
      });
}

//void
//SPHModel::computePressureOutlet()
//{
//  ParallelUtils::parallelFor(getState().getNumParticles(),
//    [&](const size_t p) {
//      if (getState().getPositions()[p].x() > m_outletRegionXCoord && getState().getPositions()[p].x() < m_maxXCoord)
//      {
//        getState().getDensities()[p] = m_outletDensity;
//      }
//    });
//}

void
SPHModel::normalizeDensity()
{
    if (!m_modelParameters->m_bNormalizeDensity)
    {
        return;
    }

    std::shared_ptr<DataArray<double>> densitiesPtr = getCurrentState()->getDensities();
    DataArray<double>&                 densities    = *densitiesPtr;

    const std::vector<std::vector<size_t>>&                 neighborLists = getCurrentState()->getFluidNeighborLists();
    const std::vector<std::vector<NeighborInfo>>&           neighborInfos = getCurrentState()->getNeighborInfo();
    const std::vector<SPHBoundaryConditions::ParticleType>& particleTypes = m_sphBoundaryConditions->getParticleTypes();

    ParallelUtils::parallelFor(getCurrentState()->getNumParticles(),
        [&](const size_t p)
        {
            if (m_sphBoundaryConditions && particleTypes[p] == SPHBoundaryConditions::ParticleType::Buffer)
            {
                return;
            }

            const std::vector<NeighborInfo>& neighborInfo = neighborInfos[p];
            if (neighborInfo.size() <= 1)
            {
                return; // the particle has no neighbor
            }

            const auto& fluidNeighborList = neighborLists[p];
            Real tmp = 0;

            for (size_t i = 0; i < fluidNeighborList.size(); ++i)
            {
                const auto& qInfo = neighborInfo[i];

                // because we're not done with density computation, qInfo does not contain desity of particle q yet
                const auto q = fluidNeighborList[i];
                const auto qdensity = densities[q];
                tmp += m_kernels.W(qInfo.xpq) / qdensity;
            }

            densities[p] /= (tmp * m_modelParameters->m_particleMass);
      });
}

void
SPHModel::computePressureAcceleration()
{
    std::shared_ptr<DataArray<double>> densitiesPtr   = getCurrentState()->getDensities();
    const DataArray<double>&           densities      = *densitiesPtr;
    VecDataArray<double, 3>&           pressureAccels = *m_pressureAccels;

    const std::vector<std::vector<NeighborInfo>>&           neighborInfos = getCurrentState()->getNeighborInfo();
    const std::vector<SPHBoundaryConditions::ParticleType>& particleTypes = m_sphBoundaryConditions->getParticleTypes();

    ParallelUtils::parallelFor(getCurrentState()->getNumParticles(),
        [&](const size_t p)
        {
            if (m_sphBoundaryConditions && particleTypes[p] == SPHBoundaryConditions::ParticleType::Buffer)
            {
                return;
            }

            Vec3r accel(0, 0, 0);
            const std::vector<NeighborInfo>& neighborInfo = neighborInfos[p];
            if (neighborInfo.size() <= 1)
            {
                pressureAccels[p] = accel;
                return;
            }

            const auto pdensity  = densities[p];
            const auto ppressure = particlePressure(pdensity);

            for (size_t idx = 0; idx < neighborInfo.size(); ++idx)
            {
                const auto& qInfo    = neighborInfo[idx];
                const auto r         = qInfo.xpq;
                const auto qdensity  = qInfo.density;
                const auto qpressure = particlePressure(qdensity);
                // pressure forces
                accel += -(ppressure / (pdensity * pdensity) + qpressure / (qdensity * qdensity)) * m_kernels.gradW(r);
            }

            accel *= m_modelParameters->m_particleMass;

            //getState().getAccelerations()[p] = accel;
            pressureAccels[p] = accel;
      });
}

void
SPHModel::computeViscosity()
{
    VecDataArray<double, 3>&       viscousAccels      = *m_viscousAccels;
    VecDataArray<double, 3>&       neighborVelContr   = *m_neighborVelContr;
    VecDataArray<double, 3>&       particleShift      = *m_particleShift;
    const VecDataArray<double, 3>& halfStepVelocities = *getCurrentState()->getHalfStepVelocities();

    const std::vector<std::vector<NeighborInfo>>& neighborInfos = getCurrentState()->getNeighborInfo();
    const std::vector<std::vector<size_t>>&       neighborLists = getCurrentState()->getFluidNeighborLists();

    ParallelUtils::parallelFor(getCurrentState()->getNumParticles(),
        [&](const size_t p)
        {
            if (m_sphBoundaryConditions
                && (m_sphBoundaryConditions->getParticleTypes()[p] == SPHBoundaryConditions::ParticleType::Buffer
                    || m_sphBoundaryConditions->getParticleTypes()[p] == SPHBoundaryConditions::ParticleType::Wall))
            {
                return;
            }

            const std::vector<NeighborInfo>& neighborInfo = neighborInfos[p];
            if (neighborInfo.size() <= 1)
            {
                neighborVelContr[p] = Vec3r(0, 0, 0);
                viscousAccels[p]    = Vec3r(0, 0, 0);
                return;
            }

            Vec3r neighborVelContributions(0, 0, 0);
            Vec3r neighborVelContributionsNumerator(0, 0, 0);
            Real neighborVelContributionsDenominator = 0;
            Vec3r particleShifts(0, 0, 0);

            const Vec3d& pvel = halfStepVelocities[p];
            const std::vector<size_t>& fluidNeighborList = neighborLists[p];

            Vec3r diffuseFluid(0, 0, 0);
            for (size_t i = 0; i < fluidNeighborList.size(); ++i)
            {
                const auto q        = fluidNeighborList[i];
                const auto& qvel    = halfStepVelocities[q];
                const auto& qInfo   = neighborInfo[i];
                const auto r        = qInfo.xpq;
                const auto qdensity = qInfo.density;
                diffuseFluid       += (Real(1.0) / qdensity) * m_kernels.laplace(r) * (qvel - pvel);

                neighborVelContributionsNumerator   += (qvel - pvel) * m_kernels.W(r);
                neighborVelContributionsDenominator += m_kernels.W(r);
                particleShifts += m_kernels.gradW(r);
                //diffuseFluid       += (Real(1.0) / qdensity) * m_kernels.W(r) * (qvel - pvel);
            }
            //diffuseFluid *= m_modelParameters->m_dynamicViscosityCoeff / getState().getDensities()[p];
            const double particleRadius = m_modelParameters->m_particleRadius;
            particleShifts     *= 4 / 3 * PI * particleRadius * particleRadius * particleRadius * 0.5 * m_modelParameters->m_kernelRadius * halfStepVelocities[p].norm();
            diffuseFluid       *= m_modelParameters->m_dynamicViscosityCoeff * m_modelParameters->m_particleMass;
            neighborVelContr[p] = neighborVelContributionsNumerator * m_modelParameters->m_eta / neighborVelContributionsDenominator;
            particleShift[p]    = -particleShifts;

            viscousAccels[p] = diffuseFluid;
      });
}

void
SPHModel::computeSurfaceTension()
{
    VecDataArray<double, 3>& surfaceNormals = *getCurrentState()->getNormals();

    const std::vector<std::vector<NeighborInfo>>& neighborInfos = getCurrentState()->getNeighborInfo();

    // First, compute surface normal for all particles
    ParallelUtils::parallelFor(getCurrentState()->getNumParticles(),
        [&](const size_t p)
        {
            if (m_sphBoundaryConditions && m_sphBoundaryConditions->getParticleTypes()[p] == SPHBoundaryConditions::ParticleType::Buffer)
            {
                return;
            }

            Vec3d n(0.0, 0.0, 0.0);
            const std::vector<NeighborInfo>& neighborInfo = neighborInfos[p];
            if (neighborInfo.size() <= 1)
            {
                surfaceNormals[p] = n;
                return;
            }

            for (size_t i = 0; i < neighborInfo.size(); ++i)
            {
                const auto& qInfo   = neighborInfo[i];
                const auto r        = qInfo.xpq;
                const auto qdensity = qInfo.density;
                n += (Real(1.0) / qdensity) * m_kernels.gradW(r);
            }

            n *= m_modelParameters->m_kernelRadius * m_modelParameters->m_particleMass;
            surfaceNormals[p] = n;
        });

    VecDataArray<double, 3>& surfaceTensionAccels = *m_surfaceTensionAccels;
    const DataArray<double>& densities = *getCurrentState()->getDensities();

    const std::vector<std::vector<size_t>>& neighborLists = getCurrentState()->getFluidNeighborLists();

    // Second, compute surface tension acceleration
    ParallelUtils::parallelFor(getCurrentState()->getNumParticles(),
        [&](const size_t p)
        {
            if (m_sphBoundaryConditions
                && (m_sphBoundaryConditions->getParticleTypes()[p] == SPHBoundaryConditions::ParticleType::Buffer
                    || m_sphBoundaryConditions->getParticleTypes()[p] == SPHBoundaryConditions::ParticleType::Wall))
            {
                return;
            }

            const auto& fluidNeighborList = neighborLists[p];
            if (fluidNeighborList.size() <= 1)
            {
                return; // the particle has no neighbor
            }

            const auto ni            = surfaceNormals[p];
            const auto pdensity      = densities[p];
            const auto& neighborInfo = neighborInfos[p];

            Vec3r accel(0, 0, 0);
            for (size_t i = 0; i < fluidNeighborList.size(); ++i)
            {
                const auto q = fluidNeighborList[i];
                if (p == q)
                {
                    continue;
                }
                const auto& qInfo   = neighborInfo[i];
                const auto qdensity = qInfo.density;

                // Correction factor
                const auto K_ij = Real(2) * m_modelParameters->m_restDensity / (pdensity + qdensity);

                // Cohesion acc
                const auto r  = qInfo.xpq;
                const auto d2 = r.squaredNorm();
                if (d2 > Real(1e-20))
                {
                    accel -= K_ij * m_modelParameters->m_particleMass * (r / std::sqrt(d2)) * m_kernels.cohesionW(r);
                }

                // Curvature acc
                const auto nj = surfaceNormals[q];
                accel -= K_ij * (ni - nj);
            }

            accel *= m_modelParameters->m_surfaceTensionStiffness;
            //getState().getAccelerations()[p] += accel;
            surfaceTensionAccels[p] = accel;
        });
}

void
SPHModel::sumAccels()
{
    const VecDataArray<double, 3>& pressureAccels       = *m_pressureAccels;
    const VecDataArray<double, 3>& surfaceTensionAccels = *m_surfaceTensionAccels;
    const VecDataArray<double, 3>& viscousAccels = *m_viscousAccels;
    VecDataArray<double, 3>&       accels = *getCurrentState()->getAccelerations();

    ParallelUtils::parallelFor(getCurrentState()->getNumParticles(),
        [&](const size_t p)
        {
            if (m_sphBoundaryConditions
                && (m_sphBoundaryConditions->getParticleTypes()[p] == SPHBoundaryConditions::ParticleType::Buffer
                    || m_sphBoundaryConditions->getParticleTypes()[p] == SPHBoundaryConditions::ParticleType::Wall))
            {
                return;
            }

            accels[p] = pressureAccels[p] + surfaceTensionAccels[p] + viscousAccels[p];
      });
}

void
SPHModel::updateVelocity(const Real timestep)
{
    VecDataArray<double, 3>&       halfStepVelocities = *getCurrentState()->getHalfStepVelocities();
    VecDataArray<double, 3>&       fullStepVelocities = *getCurrentState()->getFullStepVelocities();
    const VecDataArray<double, 3>& positions = *getCurrentState()->getPositions();
    const VecDataArray<double, 3>& accels    = *getCurrentState()->getAccelerations();

    ParallelUtils::parallelFor(getCurrentState()->getNumParticles(),
        [&](const size_t p)
        {
            if (m_sphBoundaryConditions
                && (m_sphBoundaryConditions->getParticleTypes()[p] == SPHBoundaryConditions::ParticleType::Buffer
                    || m_sphBoundaryConditions->getParticleTypes()[p] == SPHBoundaryConditions::ParticleType::Wall))
            {
                return;
            }

            // todo - simply run SPH for half a time step to start to we don't need to perform this check at every time step
            if (m_timeStepCount == 0)
            {
                halfStepVelocities[p]  = fullStepVelocities[p] + (m_modelParameters->m_gravity + accels[p]) * timestep * 0.5;
                fullStepVelocities[p] += (m_modelParameters->m_gravity + accels[p]) * timestep;
            }
            else
            {
                halfStepVelocities[p] += (m_modelParameters->m_gravity + accels[p]) * timestep;
                fullStepVelocities[p]  = halfStepVelocities[p] + (m_modelParameters->m_gravity + accels[p]) * timestep * 0.5;
            }
            if (m_sphBoundaryConditions && m_sphBoundaryConditions->getParticleTypes()[p] == SPHBoundaryConditions::ParticleType::Inlet)
            {
                halfStepVelocities[p] = m_sphBoundaryConditions->computeParabolicInletVelocity(positions[p]);
                fullStepVelocities[p] = m_sphBoundaryConditions->computeParabolicInletVelocity(positions[p]);
            }
      });
}

void
SPHModel::moveParticles(const Real timestep)
{
    //ParallelUtils::parallelFor(getState().getNumParticles(),
    //  [&](const size_t p) {

    VecDataArray<double, 3>& neighborVelContr = *m_neighborVelContr;
    VecDataArray<double, 3>& particleShift    = *m_particleShift;
    VecDataArray<double, 3>& positions = *getCurrentState()->getPositions();
    VecDataArray<double, 3>& halfStepVelocities = *getCurrentState()->getHalfStepVelocities();
    VecDataArray<double, 3>& fullStepVelocities = *getCurrentState()->getFullStepVelocities();

    for (int p = 0; p < static_cast<int>(getCurrentState()->getNumParticles()); p++)
    {
        if (m_sphBoundaryConditions
            && (m_sphBoundaryConditions->getParticleTypes()[p] == SPHBoundaryConditions::ParticleType::Buffer
                || m_sphBoundaryConditions->getParticleTypes()[p] == SPHBoundaryConditions::ParticleType::Wall))
        {
            continue;
        }

        Vec3r oldPosition = positions[p];
        Vec3r newPosition = oldPosition + particleShift[p] * timestep + (halfStepVelocities[p] + neighborVelContr[p]) * timestep;

        positions[p] = newPosition;

        if (m_sphBoundaryConditions)
        {
            std::vector<SPHBoundaryConditions::ParticleType>& particleTypes = m_sphBoundaryConditions->getParticleTypes();
            if (particleTypes[p] == SPHBoundaryConditions::ParticleType::Inlet
                && !m_sphBoundaryConditions->isInInletDomain(newPosition))
            {
                // change particle type to fluid
                particleTypes[p] = SPHBoundaryConditions::ParticleType::Fluid;
                // insert particle into inlet domain from buffer domain
                // todo: come up with a better way to find buffer indices
                // right now, the buffer index is limiting the parallel ability of this function
                const size_t bufferParticleIndex = m_sphBoundaryConditions->getBufferIndices().back();
                m_sphBoundaryConditions->getBufferIndices().pop_back();
                particleTypes[bufferParticleIndex] = SPHBoundaryConditions::ParticleType::Inlet;

                positions[bufferParticleIndex] = m_sphBoundaryConditions->placeParticleAtInlet(oldPosition);
                halfStepVelocities[bufferParticleIndex] = m_sphBoundaryConditions->computeParabolicInletVelocity(positions[bufferParticleIndex]);
                fullStepVelocities[bufferParticleIndex] = m_sphBoundaryConditions->computeParabolicInletVelocity(positions[bufferParticleIndex]);
            }
            else if (particleTypes[p] == SPHBoundaryConditions::ParticleType::Outlet
                     && !m_sphBoundaryConditions->isInOutletDomain(newPosition))
            {
                particleTypes[p] = SPHBoundaryConditions::ParticleType::Buffer;
                // insert particle into buffer domain after it leaves outlet domain
                positions[p] = m_sphBoundaryConditions->getBufferCoord();
                m_sphBoundaryConditions->getBufferIndices().push_back(p);
            }
            else if (particleTypes[p] == SPHBoundaryConditions::ParticleType::Fluid
                     && m_sphBoundaryConditions->isInOutletDomain(newPosition))
            {
                particleTypes[p] = SPHBoundaryConditions::ParticleType::Outlet;
            }
            else if (particleTypes[p] == SPHBoundaryConditions::ParticleType::Fluid
                     && !m_sphBoundaryConditions->isInFluidDomain(newPosition))
            {
                particleTypes[p] = SPHBoundaryConditions::ParticleType::Buffer;
                positions[p]     = m_sphBoundaryConditions->getBufferCoord();
                m_sphBoundaryConditions->getBufferIndices().push_back(p);
            }
        }
    }
    m_timeStepCount++;
}

Real
SPHModel::particlePressure(const double density)
{
    const double d     = density / m_modelParameters->m_restDensity;
    const double d2    = d * d;
    const double d4    = d2 * d2;
    const Real   error = m_modelParameters->m_pressureStiffness * (d4 * d2 * d - Real(1.0));
    // clamp pressure error to zero to maintain stability
    return error > Real(0) ? error : Real(0);
}

void
SPHModel::setInitialVelocities(const size_t numParticles, const Vec3d& initialVelocity)
{
    m_initialVelocities->clear();
    m_initialVelocities->reserve(static_cast<int>(numParticles));
    for (size_t p = 0; p < numParticles; p++)
    {
        if (m_sphBoundaryConditions
            && (m_sphBoundaryConditions->getParticleTypes()[p] == SPHBoundaryConditions::ParticleType::Buffer
                || m_sphBoundaryConditions->getParticleTypes()[p] == SPHBoundaryConditions::ParticleType::Wall))
        {
            m_initialVelocities->push_back(Vec3d(0.0, 0.0, 0.0));
        }
        else
        {
            m_initialVelocities->push_back(initialVelocity);
        }
    }
}

void
SPHModel::findNearestParticleToVertex(const VecDataArray<double, 3>& points, const std::vector<std::vector<size_t>>& indices)
{
    const VecDataArray<double, 3>& positions = *getCurrentState()->getPositions();
    for (size_t i = 0; i < static_cast<size_t>(points.size()); i++)
    {
        double minDistance = 1e10;
        size_t minIndex    = 0;
        for (const size_t j : indices[i])
        {
            const Vec3d  p1       = positions[j];
            const double distance = (points[i] - p1).norm();
            if (distance < minDistance)
            {
                minDistance = distance;
                minIndex    = j;
            }
        }
        m_minIndices[i] = minIndex;
    }
}
} // end namespace imstk
