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

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 CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

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

#pragma once

#include "imstkPbdConstraint.h"
#include "imstkNeighborSearch.h"

namespace imstk
{
///
/// \class PbdConstantDensityConstraint
///
/// \brief Implements the constant density constraint to simulate fluids
///
class PbdConstantDensityConstraint : public PbdConstraint
{
public:
    ///
    /// \brief constructor
    ///
    PbdConstantDensityConstraint() : PbdConstraint()
    {
        // constraint parameters
        // Refer: Miller, et al 2003 "Particle-Based Fluid Simulation for Interactive Applications."
        /// \todo Check if these numbers can be variable
        m_wPoly6Coeff = 315.0 / (64.0 * PI * pow(m_maxDist, 9));
        m_wSpikyCoeff = 15.0 / (PI * pow(m_maxDist, 6));
    }

    ///
    /// \Constant Density Constraint Initialization
    ///
    void initConstraint(const StdVectorOfVec3d& initVertexPositions, const double k);

    ///
    /// \brief Returns PBD constraint of type Type::ConstantDensity
    ///
    Type getType() const override { return Type::ConstantDensity; }

    ///
    /// \brief Solves the constant density constraint
    ///
    void projectConstraint(const StdVectorOfReal&           currInvMasses,
                           const double                     dt,
                           const PbdConstraint::SolverType& type,
                           StdVectorOfVec3d&                currVertexPositions) override;

    bool computeValueAndGradient(const StdVectorOfVec3d& currVertexPositions,
                                 double&                 c,
                                 StdVectorOfVec3d&       dcdx) const
    {
        return true;
    }

private:
    ///
    /// \brief Smoothing kernel WPoly6 for density estimation
    ///
    double wPoly6(const Vec3d& pi, const Vec3d& pj);

    ///
    /// \brief Smoothing kernel Spiky for gradient calculation
    ///
    double wSpiky(const Vec3d& pi, const Vec3d& pj);

    ///
    /// \brief
    ///
    Vec3d gradSpiky(const Vec3d& pi, const Vec3d& pj);

    ///
    /// \brief
    ///
    void computeDensity(const Vec3d& pi, const size_t index, const StdVectorOfVec3d& positions);

    ///
    /// \brief
    ///
    void computeLambdaScalingFactor(const Vec3d& pi, const size_t index, const StdVectorOfVec3d& positions);

    ///
    /// \brief
    ///
    void updatePositions(const Vec3d& pi, const size_t index, StdVectorOfVec3d& positions);

    ///
    /// \brief Set/Get rest density
    ///
    void setDensity(const double density) { m_restDensity = density; }
    double getDensity() { return m_restDensity; }

    ///
    /// \brief Set/Get max. neighbor distance
    ///
    void setMaxNeighborDistance(const double dist);
    double getMaxNeighborDistance() { return m_maxDist; }

    ///
    /// \brief Set/Get neighbor search method
    ///
    void setNeighborSearchMethod(NeighborSearch::Method method) { m_NeighborSearchMethod = method; }
    NeighborSearch::Method getNeighborSearchMethod() { return m_NeighborSearchMethod; }

private:

    double m_wPoly6Coeff;
    double m_wSpikyCoeff;

    double m_maxDist    = 0.2;                       ///> Max. neighbor distance
    double m_maxDistSqr = 0.04;                      ///> Max. neighbor squared distance
    double m_relaxationParameter = 600.0;            ///> Relaxation parameter
    double m_restDensity = 6378.0;                   ///> Fluid density

    std::vector<double> m_lambdas;                   ///> lambdas
    std::vector<double> m_densities;                 ///> densities
    std::vector<Vec3d>  m_deltaPositions;            ///> delta positions
    std::vector<std::vector<size_t>> m_neighborList; ///> indices of neighbor particles

    NeighborSearch::Method m_NeighborSearchMethod = NeighborSearch::Method::UniformGridBasedSearch;
    std::shared_ptr<NeighborSearch> m_NeighborSearcher;  ///> neighbor searcher, must be initialized during model initialization
};
}
