ReferenceKernels.h

#ifndef OPENMM_REFERENCEKERNELS_H_
#define OPENMM_REFERENCEKERNELS_H_

/* -------------------------------------------------------------------------- *
 *                                   OpenMM                                   *
 * -------------------------------------------------------------------------- *
 * This is part of the OpenMM molecular simulation toolkit originating from   *
 * Simbios, the NIH National Center for Physics-Based Simulation of           *
 * Biological Structures at Stanford, funded under the NIH Roadmap for        *
 * Medical Research, grant U54 GM072970. See https://simtk.org.               *
 *                                                                            *
 * Portions copyright (c) 2008-2009 Stanford University and the Authors.      *
 * Authors: Peter Eastman                                                     *
 * Contributors:                                                              *
 *                                                                            *
 * Permission is hereby granted, free of charge, to any person obtaining a    *
 * copy of this software and associated documentation files (the "Software"), *
 * to deal in the Software without restriction, including without limitation  *
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,   *
 * and/or sell copies of the Software, and to permit persons to whom the      *
 * Software is furnished to do so, subject to the following conditions:       *
 *                                                                            *
 * The above copyright notice and this permission notice shall be included in *
 * all copies or substantial portions of the Software.                        *
 *                                                                            *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR *
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,   *
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL    *
 * THE AUTHORS, CONTRIBUTORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,    *
 * DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR      *
 * OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE  *
 * USE OR OTHER DEALINGS IN THE SOFTWARE.                                     *
 * -------------------------------------------------------------------------- */

#include "ReferencePlatform.h"
#include "openmm/kernels.h"
#include "SimTKUtilities/SimTKOpenMMRealType.h"
#include "SimTKReference/ReferenceNeighborList.h"
#include "lepton/ExpressionProgram.h"

class CpuObc;
class CpuGBVI;
class ReferenceAndersenThermostat;
class ReferenceBrownianDynamics;
class ReferenceStochasticDynamics;
class ReferenceConstraintAlgorithm;
class ReferenceVariableStochasticDynamics;
class ReferenceVariableVerletDynamics;
class ReferenceVerletDynamics;

namespace OpenMM {

/**
 * This kernel is invoked at the beginning and end of force and energy computations.  It gives the
 * Platform a chance to clear buffers and do other initialization at the beginning, and to do any
 * necessary work at the end to determine the final results.
 */
class ReferenceCalcForcesAndEnergyKernel : public CalcForcesAndEnergyKernel {
public:
    ReferenceCalcForcesAndEnergyKernel(std::string name, const Platform& platform) : CalcForcesAndEnergyKernel(name, platform) {
    }
    /**
     * Initialize the kernel.
     * 
     * @param system     the System this kernel will be applied to
     */
    void initialize(const System& system);
    /**
     * This is called at the beginning of each force computation, before calcForces() has been called on
     * any ForceImpl.
     *
     * @param context    the context in which to execute this kernel
     */
    void beginForceComputation(ContextImpl& context);
    /**
     * This is called at the end of each force computation, after calcForces() has been called on
     * every ForceImpl.
     *
     * @param context    the context in which to execute this kernel
     */
    void finishForceComputation(ContextImpl& context);
    /**
     * This is called at the beginning of each energy computation, before calcEnergy() has been called on
     * any ForceImpl.
     *
     * @param context    the context in which to execute this kernel
     */
    void beginEnergyComputation(ContextImpl& context);
    /**
     * This is called at the end of each energy computation, after calcEnergy() has been called on
     * every ForceImpl.
     *
     * @param context    the context in which to execute this kernel
     * @return the potential energy of the system.  This value is added to all values returned by ForceImpls'
     * calcEnergy() methods.  That is, each force kernel may <i>either</i> return its contribution to the
     * energy directly, <i>or</i> add it to an internal buffer so that it will be included here.
     */
    double finishEnergyComputation(ContextImpl& context);
};

/**
 * This kernel provides methods for setting and retrieving various state data: time, positions,
 * velocities, and forces.
 */
class ReferenceUpdateStateDataKernel : public UpdateStateDataKernel {
public:
    ReferenceUpdateStateDataKernel(std::string name, const Platform& platform, ReferencePlatform::PlatformData& data) : UpdateStateDataKernel(name, platform), data(data) {
    }
    /**
     * Initialize the kernel.
     *
     * @param system     the System this kernel will be applied to
     */
    void initialize(const System& system);
    /**
     * Get the current time (in picoseconds).
     *
     * @param context    the context in which to execute this kernel
     */
    double getTime(const ContextImpl& context) const;
    /**
     * Set the current time (in picoseconds).
     *
     * @param context    the context in which to execute this kernel
     */
    void setTime(ContextImpl& context, double time);
    /**
     * Get the positions of all particles.
     *
     * @param positions  on exit, this contains the particle positions
     */
    void getPositions(ContextImpl& context, std::vector<Vec3>& positions);
    /**
     * Set the positions of all particles.
     *
     * @param positions  a vector containg the particle positions
     */
    void setPositions(ContextImpl& context, const std::vector<Vec3>& positions);
    /**
     * Get the velocities of all particles.
     *
     * @param velocities  on exit, this contains the particle velocities
     */
    void getVelocities(ContextImpl& context, std::vector<Vec3>& velocities);
    /**
     * Set the velocities of all particles.
     *
     * @param velocities  a vector containg the particle velocities
     */
    void setVelocities(ContextImpl& context, const std::vector<Vec3>& velocities);
    /**
     * Get the current forces on all particles.
     *
     * @param forces  on exit, this contains the forces
     */
    void getForces(ContextImpl& context, std::vector<Vec3>& forces);
private:
    ReferencePlatform::PlatformData& data;
};

/**
 * This kernel is invoked by HarmonicBondForce to calculate the forces acting on the system and the energy of the system.
 */
class ReferenceCalcHarmonicBondForceKernel : public CalcHarmonicBondForceKernel {
public:
    ReferenceCalcHarmonicBondForceKernel(std::string name, const Platform& platform) : CalcHarmonicBondForceKernel(name, platform) {
    }
    ~ReferenceCalcHarmonicBondForceKernel();
    /**
     * Initialize the kernel.
     * 
     * @param system     the System this kernel will be applied to
     * @param force      the HarmonicBondForce this kernel will be used for
     */
    void initialize(const System& system, const HarmonicBondForce& force);
    /**
     * Execute the kernel to calculate the forces.
     * 
     * @param context    the context in which to execute this kernel
     */
    void executeForces(ContextImpl& context);
    /**
     * Execute the kernel to calculate the energy.
     * 
     * @param context    the context in which to execute this kernel
     * @return the potential energy due to the HarmonicBondForce
     */
    double executeEnergy(ContextImpl& context);
private:
    int numBonds;
    int **bondIndexArray;
    RealOpenMM **bondParamArray;
};

/**
 * This kernel is invoked by CustomBondForce to calculate the forces acting on the system and the energy of the system.
 */
class ReferenceCalcCustomBondForceKernel : public CalcCustomBondForceKernel {
public:
    ReferenceCalcCustomBondForceKernel(std::string name, const Platform& platform) : CalcCustomBondForceKernel(name, platform) {
    }
    ~ReferenceCalcCustomBondForceKernel();
    /**
     * Initialize the kernel.
     *
     * @param system     the System this kernel will be applied to
     * @param force      the CustomBondForce this kernel will be used for
     */
    void initialize(const System& system, const CustomBondForce& force);
    /**
     * Execute the kernel to calculate the forces.
     *
     * @param context    the context in which to execute this kernel
     */
    void executeForces(ContextImpl& context);
    /**
     * Execute the kernel to calculate the energy.
     *
     * @param context    the context in which to execute this kernel
     * @return the potential energy due to the CustomBondForce
     */
    double executeEnergy(ContextImpl& context);
private:
    int numBonds;
    int **bondIndexArray;
    RealOpenMM **bondParamArray;
    Lepton::ExpressionProgram energyExpression, forceExpression;
    std::vector<std::string> parameterNames, globalParameterNames;
};

/**
 * This kernel is invoked by HarmonicAngleForce to calculate the forces acting on the system and the energy of the system.
 */
class ReferenceCalcHarmonicAngleForceKernel : public CalcHarmonicAngleForceKernel {
public:
    ReferenceCalcHarmonicAngleForceKernel(std::string name, const Platform& platform) : CalcHarmonicAngleForceKernel(name, platform) {
    }
    ~ReferenceCalcHarmonicAngleForceKernel();
    /**
     * Initialize the kernel.
     * 
     * @param system     the System this kernel will be applied to
     * @param force      the HarmonicAngleForce this kernel will be used for
     */
    void initialize(const System& system, const HarmonicAngleForce& force);
    /**
     * Execute the kernel to calculate the forces.
     * 
     * @param context    the context in which to execute this kernel
     */
    void executeForces(ContextImpl& context);
    /**
     * Execute the kernel to calculate the energy.
     * 
     * @param context    the context in which to execute this kernel
     * @return the potential energy due to the HarmonicAngleForce
     */
    double executeEnergy(ContextImpl& context);
private:
    int numAngles;
    int **angleIndexArray;
    RealOpenMM **angleParamArray;
};

/**
 * This kernel is invoked by PeriodicTorsionForce to calculate the forces acting on the system and the energy of the system.
 */
class ReferenceCalcPeriodicTorsionForceKernel : public CalcPeriodicTorsionForceKernel {
public:
    ReferenceCalcPeriodicTorsionForceKernel(std::string name, const Platform& platform) : CalcPeriodicTorsionForceKernel(name, platform) {
    }
    ~ReferenceCalcPeriodicTorsionForceKernel();
    /**
     * Initialize the kernel.
     * 
     * @param system     the System this kernel will be applied to
     * @param force      the PeriodicTorsionForce this kernel will be used for
     */
    void initialize(const System& system, const PeriodicTorsionForce& force);
    /**
     * Execute the kernel to calculate the forces.
     * 
     * @param context    the context in which to execute this kernel
     */
    void executeForces(ContextImpl& context);
    /**
     * Execute the kernel to calculate the energy.
     * 
     * @param context    the context in which to execute this kernel
     * @return the potential energy due to the PeriodicTorsionForce
     */
    double executeEnergy(ContextImpl& context);
private:
    int numTorsions;
    int **torsionIndexArray;
    RealOpenMM **torsionParamArray;
};

/**
 * This kernel is invoked by RBTorsionForce to calculate the forces acting on the system and the energy of the system.
 */
class ReferenceCalcRBTorsionForceKernel : public CalcRBTorsionForceKernel {
public:
    ReferenceCalcRBTorsionForceKernel(std::string name, const Platform& platform) : CalcRBTorsionForceKernel(name, platform) {
    }
    ~ReferenceCalcRBTorsionForceKernel();
    /**
     * Initialize the kernel.
     * 
     * @param system     the System this kernel will be applied to
     * @param force      the RBTorsionForce this kernel will be used for
     */
    void initialize(const System& system, const RBTorsionForce& force);
    /**
     * Execute the kernel to calculate the forces.
     * 
     * @param context    the context in which to execute this kernel
     */
    void executeForces(ContextImpl& context);
    /**
     * Execute the kernel to calculate the energy.
     * 
     * @param context    the context in which to execute this kernel
     * @return the potential energy due to the RBTorsionForce
     */
    double executeEnergy(ContextImpl& context);
private:
    int numTorsions;
    int **torsionIndexArray;
    RealOpenMM **torsionParamArray;
};

/**
 * This kernel is invoked by NonbondedForce to calculate the forces acting on the system.
 */
class ReferenceCalcNonbondedForceKernel : public CalcNonbondedForceKernel {
public:
    ReferenceCalcNonbondedForceKernel(std::string name, const Platform& platform) : CalcNonbondedForceKernel(name, platform) {
    }
    ~ReferenceCalcNonbondedForceKernel();
    /**
     * Initialize the kernel.
     * 
     * @param system     the System this kernel will be applied to
     * @param force      the NonbondedForce this kernel will be used for
     */
    void initialize(const System& system, const NonbondedForce& force);
    /**
     * Execute the kernel to calculate the forces.
     * 
     * @param context    the context in which to execute this kernel
     */
    void executeForces(ContextImpl& context);
    /**
     * Execute the kernel to calculate the energy.
     * 
     * @param context    the context in which to execute this kernel
     * @return the potential energy due to the NonbondedForce
     */
    double executeEnergy(ContextImpl& context);
private:
    int numParticles, num14;
    int **exclusionArray, **bonded14IndexArray;
    RealOpenMM **particleParamArray, **bonded14ParamArray;
    RealOpenMM nonbondedCutoff, periodicBoxSize[3], rfDielectric, ewaldAlpha;
    int kmax[3], gridSize[3];
    std::vector<std::set<int> > exclusions;
    NonbondedMethod nonbondedMethod;
    NeighborList* neighborList;
};

/**
 * This kernel is invoked by CustomNonbondedForce to calculate the forces acting on the system.
 */
class ReferenceCalcCustomNonbondedForceKernel : public CalcCustomNonbondedForceKernel {
public:
    ReferenceCalcCustomNonbondedForceKernel(std::string name, const Platform& platform) : CalcCustomNonbondedForceKernel(name, platform) {
    }
    ~ReferenceCalcCustomNonbondedForceKernel();
    /**
     * Initialize the kernel.
     *
     * @param system     the System this kernel will be applied to
     * @param force      the CustomNonbondedForce this kernel will be used for
     */
    void initialize(const System& system, const CustomNonbondedForce& force);
    /**
     * Execute the kernel to calculate the forces.
     *
     * @param context    the context in which to execute this kernel
     */
    void executeForces(ContextImpl& context);
    /**
     * Execute the kernel to calculate the energy.
     *
     * @param context    the context in which to execute this kernel
     * @return the potential energy due to the CustomNonbondedForce
     */
    double executeEnergy(ContextImpl& context);
private:
    int numParticles, numExceptions;
    int **exclusionArray, **exceptionIndexArray;
    RealOpenMM **particleParamArray, **exceptionParamArray;
    RealOpenMM nonbondedCutoff, periodicBoxSize[3];
    std::vector<std::set<int> > exclusions;
    Lepton::ExpressionProgram energyExpression, forceExpression;
    std::vector<std::string> parameterNames, globalParameterNames;
    std::vector<Lepton::ExpressionProgram> combiningRules;
    NonbondedMethod nonbondedMethod;
    NeighborList* neighborList;
    class TabulatedFunction;
};

/**
 * This kernel is invoked by GBSAOBCForce to calculate the forces acting on the system.
 */
class ReferenceCalcGBSAOBCForceKernel : public CalcGBSAOBCForceKernel {
public:
    ReferenceCalcGBSAOBCForceKernel(std::string name, const Platform& platform) : CalcGBSAOBCForceKernel(name, platform) {
    }
    ~ReferenceCalcGBSAOBCForceKernel();
    /**
     * Initialize the kernel.
     * 
     * @param system     the System this kernel will be applied to
     * @param force      the GBSAOBCForce this kernel will be used for
     */
    void initialize(const System& system, const GBSAOBCForce& force);
    /**
     * Execute the kernel to calculate the forces.
     * 
     * @param context    the context in which to execute this kernel
     */
    void executeForces(ContextImpl& context);
    /**
     * Execute the kernel to calculate the energy.
     * 
     * @param context    the context in which to execute this kernel
     * @return the potential energy due to the GBSAOBCForce
     */
    double executeEnergy(ContextImpl& context);
private:
    CpuObc* obc;
    std::vector<RealOpenMM> charges;
};

/**
 * This kernel is invoked by GBVIForce to calculate the forces acting on the system.
 */
class ReferenceCalcGBVIForceKernel : public CalcGBVIForceKernel {
public:
    ReferenceCalcGBVIForceKernel(std::string name, const Platform& platform) : CalcGBVIForceKernel(name, platform) {
    }
    ~ReferenceCalcGBVIForceKernel();
    /**
     * Initialize the kernel.
     * 
     * @param system       the System this kernel will be applied to
     * @param force        the GBVIForce this kernel will be used for
     * @param scaled radii the scaled radii (Eq. 5 of Labute paper)
     */
    void initialize(const System& system, const GBVIForce& force, const std::vector<double> & scaledRadii);
    /**
     * Execute the kernel to calculate the forces.
     * 
     * @param context    the context in which to execute this kernel
     */
    void executeForces(ContextImpl& context);
    /**
     * Execute the kernel to calculate the energy.
     * 
     * @param context    the context in which to execute this kernel
     * @return the potential energy due to the GBVIForce
     */
    double executeEnergy(ContextImpl& context);
private:
    CpuGBVI * gbvi;
    std::vector<RealOpenMM> charges;
};

/**
 * This kernel is invoked by CustomExternalForce to calculate the forces acting on the system and the energy of the system.
 */
class ReferenceCalcCustomExternalForceKernel : public CalcCustomExternalForceKernel {
public:
    ReferenceCalcCustomExternalForceKernel(std::string name, const Platform& platform) : CalcCustomExternalForceKernel(name, platform) {
    }
    ~ReferenceCalcCustomExternalForceKernel();
    /**
     * Initialize the kernel.
     *
     * @param system     the System this kernel will be applied to
     * @param force      the CustomExternalForce this kernel will be used for
     */
    void initialize(const System& system, const CustomExternalForce& force);
    /**
     * Execute the kernel to calculate the forces.
     *
     * @param context    the context in which to execute this kernel
     */
    void executeForces(ContextImpl& context);
    /**
     * Execute the kernel to calculate the energy.
     *
     * @param context    the context in which to execute this kernel
     * @return the potential energy due to the CustomExternalForce
     */
    double executeEnergy(ContextImpl& context);
private:
    int numParticles;
    std::vector<int> particles;
    RealOpenMM **particleParamArray;
    Lepton::ExpressionProgram energyExpression, forceExpressionX, forceExpressionY, forceExpressionZ;
    std::vector<std::string> parameterNames, globalParameterNames;
};

/**
 * This kernel is invoked by VerletIntegrator to take one time step.
 */
class ReferenceIntegrateVerletStepKernel : public IntegrateVerletStepKernel {
public:
    ReferenceIntegrateVerletStepKernel(std::string name, const Platform& platform, ReferencePlatform::PlatformData& data) : IntegrateVerletStepKernel(name, platform),
        data(data), dynamics(0), constraints(0), masses(0), constraintDistances(0), constraintIndices(0) {
    }
    ~ReferenceIntegrateVerletStepKernel();
    /**
     * Initialize the kernel.
     * 
     * @param system     the System this kernel will be applied to
     * @param integrator the VerletIntegrator this kernel will be used for
     */
    void initialize(const System& system, const VerletIntegrator& integrator);
    /**
     * Execute the kernel.
     * 
     * @param context    the context in which to execute this kernel
     * @param integrator the VerletIntegrator this kernel is being used for
     */
    void execute(ContextImpl& context, const VerletIntegrator& integrator);
private:
    ReferencePlatform::PlatformData& data;
    ReferenceVerletDynamics* dynamics;
    ReferenceConstraintAlgorithm* constraints;
    RealOpenMM* masses;
    RealOpenMM* constraintDistances;
    int** constraintIndices;
    int numConstraints;
    double prevStepSize;
};

/**
 * This kernel is invoked by LangevinIntegrator to take one time step.
 */
class ReferenceIntegrateLangevinStepKernel : public IntegrateLangevinStepKernel {
public:
    ReferenceIntegrateLangevinStepKernel(std::string name, const Platform& platform, ReferencePlatform::PlatformData& data) : IntegrateLangevinStepKernel(name, platform),
        data(data), dynamics(0), constraints(0), masses(0), constraintDistances(0), constraintIndices(0) {
    }
    ~ReferenceIntegrateLangevinStepKernel();
    /**
     * Initialize the kernel, setting up the particle masses.
     * 
     * @param system     the System this kernel will be applied to
     * @param integrator the LangevinIntegrator this kernel will be used for
     */
    void initialize(const System& system, const LangevinIntegrator& integrator);
    /**
     * Execute the kernel.
     * 
     * @param context    the context in which to execute this kernel
     * @param integrator the LangevinIntegrator this kernel is being used for
     */
    void execute(ContextImpl& context, const LangevinIntegrator& integrator);
private:
    ReferencePlatform::PlatformData& data;
    ReferenceStochasticDynamics* dynamics;
    ReferenceConstraintAlgorithm* constraints;
    RealOpenMM* masses;
    RealOpenMM* constraintDistances;
    int** constraintIndices;
    int numConstraints;
    double prevTemp, prevFriction, prevStepSize;
};

/**
 * This kernel is invoked by BrownianIntegrator to take one time step.
 */
class ReferenceIntegrateBrownianStepKernel : public IntegrateBrownianStepKernel {
public:
    ReferenceIntegrateBrownianStepKernel(std::string name, const Platform& platform, ReferencePlatform::PlatformData& data) : IntegrateBrownianStepKernel(name, platform),
        data(data), dynamics(0), constraints(0), masses(0), constraintDistances(0), constraintIndices(0) {
    }
    ~ReferenceIntegrateBrownianStepKernel();
    /**
     * Initialize the kernel.
     * 
     * @param system     the System this kernel will be applied to
     * @param integrator the BrownianIntegrator this kernel will be used for
     */
    void initialize(const System& system, const BrownianIntegrator& integrator);
    /**
     * Execute the kernel.
     * 
     * @param context    the context in which to execute this kernel
     * @param integrator the BrownianIntegrator this kernel is being used for
     */
    void execute(ContextImpl& context, const BrownianIntegrator& integrator);
private:
    ReferencePlatform::PlatformData& data;
    ReferenceBrownianDynamics* dynamics;
    ReferenceConstraintAlgorithm* constraints;
    RealOpenMM* masses;
    RealOpenMM* constraintDistances;
    int** constraintIndices;
    int numConstraints;
    double prevTemp, prevFriction, prevStepSize;
};

/**
 * This kernel is invoked by VariableLangevinIntegrator to take one time step.
 */
class ReferenceIntegrateVariableLangevinStepKernel : public IntegrateVariableLangevinStepKernel {
public:
    ReferenceIntegrateVariableLangevinStepKernel(std::string name, const Platform& platform, ReferencePlatform::PlatformData& data) : IntegrateVariableLangevinStepKernel(name, platform),
        data(data), dynamics(0), constraints(0), masses(0), constraintDistances(0), constraintIndices(0) {
    }
    ~ReferenceIntegrateVariableLangevinStepKernel();
    /**
     * Initialize the kernel.
     *
     * @param system     the System this kernel will be applied to
     * @param integrator the LangevinIntegrator this kernel will be used for
     */
    void initialize(const System& system, const VariableLangevinIntegrator& integrator);
    /**
     * Execute the kernel.
     *
     * @param context    the context in which to execute this kernel
     * @param integrator the LangevinIntegrator this kernel is being used for
     * @param maxTime    the maximum time beyond which the simulation should not be advanced
     */
    void execute(ContextImpl& context, const VariableLangevinIntegrator& integrator, double maxTime);
private:
    ReferencePlatform::PlatformData& data;
    ReferenceVariableStochasticDynamics* dynamics;
    ReferenceConstraintAlgorithm* constraints;
    RealOpenMM* masses;
    RealOpenMM* constraintDistances;
    int** constraintIndices;
    int numConstraints;
    double prevTemp, prevFriction, prevErrorTol;
};

/**
 * This kernel is invoked by VariableVerletIntegrator to take one time step.
 */
class ReferenceIntegrateVariableVerletStepKernel : public IntegrateVariableVerletStepKernel {
public:
    ReferenceIntegrateVariableVerletStepKernel(std::string name, const Platform& platform, ReferencePlatform::PlatformData& data) : IntegrateVariableVerletStepKernel(name, platform),
        data(data), dynamics(0), constraints(0), masses(0), constraintDistances(0), constraintIndices(0) {
    }
    ~ReferenceIntegrateVariableVerletStepKernel();
    /**
     * Initialize the kernel.
     *
     * @param system     the System this kernel will be applied to
     * @param integrator the VerletIntegrator this kernel will be used for
     */
    void initialize(const System& system, const VariableVerletIntegrator& integrator);
    /**
     * Execute the kernel.
     *
     * @param context    the context in which to execute this kernel
     * @param integrator the VerletIntegrator this kernel is being used for
     * @param maxTime    the maximum time beyond which the simulation should not be advanced
     */
    void execute(ContextImpl& context, const VariableVerletIntegrator& integrator, double maxTime);
private:
    ReferencePlatform::PlatformData& data;
    ReferenceVariableVerletDynamics* dynamics;
    ReferenceConstraintAlgorithm* constraints;
    RealOpenMM* masses;
    RealOpenMM* constraintDistances;
    int** constraintIndices;
    int numConstraints;
    double prevErrorTol;
};

/**
 * This kernel is invoked by AndersenThermostat at the start of each time step to adjust the particle velocities.
 */
class ReferenceApplyAndersenThermostatKernel : public ApplyAndersenThermostatKernel {
public:
    ReferenceApplyAndersenThermostatKernel(std::string name, const Platform& platform) : ApplyAndersenThermostatKernel(name, platform), thermostat(0) {
    }
    ~ReferenceApplyAndersenThermostatKernel();
    /**
     * Initialize the kernel.
     * 
     * @param system     the System this kernel will be applied to
     * @param thermostat the AndersenThermostat this kernel will be used for
     */
    void initialize(const System& system, const AndersenThermostat& thermostat);
    /**
     * Execute the kernel.
     * 
     * @param context    the context in which to execute this kernel
     */
    void execute(ContextImpl& context);
private:
    ReferenceAndersenThermostat* thermostat;
    RealOpenMM* masses;
};

/**
 * This kernel is invoked to calculate the kinetic energy of the system.
 */
class ReferenceCalcKineticEnergyKernel : public CalcKineticEnergyKernel {
public:
    ReferenceCalcKineticEnergyKernel(std::string name, const Platform& platform) : CalcKineticEnergyKernel(name, platform) {
    }
    /**
     * Initialize the kernel.
     * 
     * @param system     the System this kernel will be applied to
     */
    void initialize(const System& system);
    /**
     * Execute the kernel.
     * 
     * @param context    the context in which to execute this kernel
     */
    double execute(ContextImpl& context);
private:
    std::vector<double> masses;
};

/**
 * This kernel is invoked to remove center of mass motion from the system.
 */
class ReferenceRemoveCMMotionKernel : public RemoveCMMotionKernel {
public:
    ReferenceRemoveCMMotionKernel(std::string name, const Platform& platform, ReferencePlatform::PlatformData& data) : RemoveCMMotionKernel(name, platform), data(data) {
    }
    /**
     * Initialize the kernel, setting up the particle masses.
     * 
     * @param system     the System this kernel will be applied to
     * @param force      the CMMotionRemover this kernel will be used for
     */
    void initialize(const System& system, const CMMotionRemover& force);
    /**
     * Execute the kernel.
     * 
     * @param context    the context in which to execute this kernel
     */
    void execute(ContextImpl& context);
private:
    ReferencePlatform::PlatformData& data;
    std::vector<double> masses;
    int frequency;
};

} // namespace OpenMM

#endif /*OPENMM_REFERENCEKERNELS_H_*/