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/*! \libinternal \file
 *
 * \brief Declares the class for GPU implementation of LINCS.
 *
 * \author Artem Zhmurov <zhmurov@gmail.com>
 *
 * \ingroup module_mdlib
 * \inlibraryapi
 */
#ifndef GMX_MDLIB_LINCS_GPU_CUH
#define GMX_MDLIB_LINCS_GPU_CUH

#include <memory>

#include "gromacs/gpu_utils/device_context.h"
#include "gromacs/gpu_utils/device_stream.h"
#include "gromacs/gpu_utils/devicebuffer_datatype.h"
#include "gromacs/gpu_utils/gputraits.h"
#include "gromacs/mdlib/constr.h"
#include "gromacs/pbcutil/pbc_aiuc.h"

class InteractionDefinitions;

namespace gmx
{

//! A pair of atoms indexes
struct AtomPair
{
    //! First atom
    int i;
    //! Second atom
    int j;
};

/* \brief LINCS parameters and GPU pointers
 *
 * This is used to accumulate all the parameters and pointers so they can be passed
 * to the GPU as a single structure.
 *
 */
struct LincsGpuKernelParameters
{
    //! Periodic boundary data
    PbcAiuc pbcAiuc;
    //! Order of expansion when inverting the matrix
    int expansionOrder;
    //! Number of iterations used to correct the projection
    int numIterations;
    //! 1/mass for all atoms (GPU)
    DeviceBuffer<float> d_inverseMasses;
    //! Scaled virial tensor (6 floats: [XX, XY, XZ, YY, YZ, ZZ], GPU)
    DeviceBuffer<float> d_virialScaled;
    /*! \brief Total number of threads.
     *
     *  This covers all constraints and gaps in the ends of the thread blocks
     *  that are necessary to avoid inter-block synchronizations.
     *  Should be a multiple of block size (the last block is filled with dummy to the end).
     */
    int numConstraintsThreads;
    //! List of constrained atoms (GPU memory)
    DeviceBuffer<AtomPair> d_constraints;
    //! Equilibrium distances for the constraints (GPU)
    DeviceBuffer<float> d_constraintsTargetLengths;
    /*! \brief Whether there are coupled constraints.
     *
     * In SYCL, the accessors can not be initialized with an empty buffer.
     * In case there are no coupled constraints, the respective buffers below are
     * empty. So we need to inform the kernel launcher that these are optional.
     */
    bool haveCoupledConstraints = false;
    //! Number of constraints, coupled with the current one (GPU)
    DeviceBuffer<int> d_coupledConstraintsCounts;
    //! List of coupled with the current one (GPU)
    DeviceBuffer<int> d_coupledConstraintsIndices;
    //! Elements of the coupling matrix.
    DeviceBuffer<float> d_matrixA;
    //! Mass factors (GPU)
    DeviceBuffer<float> d_massFactors;
};

/*! \internal \brief Class with interfaces and data for GPU version of LINCS. */
class LincsGpu
{

public:
    /*! \brief Constructor.
     *
     * \param[in] numIterations    Number of iteration for the correction of the projection.
     * \param[in] expansionOrder   Order of the matrix inversion algorithm.
     * \param[in] deviceContext    Device context (dummy in CUDA).
     * \param[in] deviceStream     Device command stream.
     */
    LincsGpu(int                  numIterations,
             int                  expansionOrder,
             const DeviceContext& deviceContext,
             const DeviceStream&  deviceStream);
    /*! \brief Destructor.*/
    ~LincsGpu();

    /*! \brief Apply LINCS.
     *
     * Applies LINCS to coordinates and velocities, stored on GPU.
     * The results are not automatically copied back to the CPU memory.
     * Method uses this class data structures which should be updated
     * when needed using set() method.
     *
     * \param[in]     d_x               Coordinates before timestep (in GPU memory)
     * \param[in,out] d_xp              Coordinates after timestep (in GPU memory). The
     *                                  resulting constrained coordinates will be saved here.
     * \param[in]     updateVelocities  If the velocities should be updated.
     * \param[in,out] d_v               Velocities to update (in GPU memory, can be nullptr
     *                                  if not updated)
     * \param[in]     invdt             Reciprocal timestep (to scale Lagrange
     *                                  multipliers when velocities are updated)
     * \param[in]     computeVirial     If virial should be updated.
     * \param[in,out] virialScaled      Scaled virial tensor to be updated.
     * \param[in]     pbcAiuc           PBC data.
     */
    void apply(const DeviceBuffer<Float3>& d_x,
               DeviceBuffer<Float3>        d_xp,
               bool                        updateVelocities,
               DeviceBuffer<Float3>        d_v,
               real                        invdt,
               bool                        computeVirial,
               tensor                      virialScaled,
               const PbcAiuc&              pbcAiuc);

    /*! \brief
     * Update data-structures (e.g. after NB search step).
     *
     * Updates the constraints data and copies it to the GPU. Should be
     * called if the particles were sorted, redistributed between domains, etc.
     * This version uses common data formats so it can be called from anywhere
     * in the code. Does not recycle the data preparation routines from the CPU
     * version. Works only with simple case when all the constraints in idef are
     * are handled by a single GPU. Triangles are not handled as special case.
     *
     * Information about constraints is taken from:
     *     idef.il[F_CONSTR].iatoms  --- type (T) of constraint and two atom indexes (i1, i2)
     *     idef.iparams[T].constr.dA --- target length for constraint of type T
     *
     * \param[in] idef      Local topology data to get information on constraints from.
     * \param[in] numAtoms  Number of atoms.
     * \param[in] invmass   Inverse masses of atoms.
     */
    void set(const InteractionDefinitions& idef, int numAtoms, ArrayRef<const real> invmass);

    /*! \brief
     * Returns whether the maximum number of coupled constraints is supported
     * by the GPU LINCS code.
     *
     * \param[in] mtop The molecular topology
     */
    static bool isNumCoupledConstraintsSupported(const gmx_mtop_t& mtop);

private:
    //! GPU context object
    const DeviceContext& deviceContext_;
    //! GPU stream
    const DeviceStream& deviceStream_;

    //! Parameters and pointers, passed to the GPU kernel
    LincsGpuKernelParameters kernelParams_;

    //! Scaled virial tensor (6 floats: [XX, XY, XZ, YY, YZ, ZZ])
    std::vector<float> h_virialScaled_;

    /*! \brief Maximum total number of constraints so far.
     *
     * If the new number of constraints is larger then previous maximum, the GPU data arrays are
     * reallocated.
     */
    int numConstraintsThreadsAlloc_;

    /*! \brief Maximum total number of atoms so far.
     *
     * If the new number of atoms is larger then previous maximum, the GPU array with masses is
     * reallocated.
     */
    int numAtomsAlloc_;

    //! Maximum number of coupled constraints
    int maxCoupledConstraints_ = 0;
};

} // namespace gmx

#endif // GMX_MDLIB_LINCS_GPU_CUH
