/* -------------------------------------------------------------------------- *
 *                                   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) 2009 Stanford University and the Authors.           *
 * Authors: Scott Le Grand, Peter Eastman                                     *
 * Contributors:                                                              *
 *                                                                            *
 * This program is free software: you can redistribute it and/or modify       *
 * it under the terms of the GNU Lesser General Public License as published   *
 * by the Free Software Foundation, either version 3 of the License, or       *
 * (at your option) any later version.                                        *
 *                                                                            *
 * This program is distributed in the hope that it will be useful,            *
 * but WITHOUT ANY WARRANTY; without even the implied warranty of             *
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the              *
 * GNU Lesser General Public License for more details.                        *
 *                                                                            *
 * You should have received a copy of the GNU Lesser General Public License   *
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.      *
 * -------------------------------------------------------------------------- */

#include <stdio.h>
#include <cuda.h>
#include <vector_functions.h>
#include <cstdlib>
#include <string>
#include <iostream>
//#include <fstream>
using namespace std;

#include "gputypes.h"

static __constant__ cudaGmxSimulation cSim;

void SetCalculateAndersenThermostatSim(gpuContext gpu)
{
    cudaError_t status;
    status = cudaMemcpyToSymbol(cSim, &gpu->sim, sizeof(cudaGmxSimulation));     
    RTERROR(status, "cudaMemcpyToSymbol: SetSim copy to cSim failed");
}

void GetCalculateAndersenThermostatSim(gpuContext gpu)
{
    cudaError_t status;
    status = cudaMemcpyFromSymbol(&gpu->sim, cSim, sizeof(cudaGmxSimulation));     
    RTERROR(status, "cudaMemcpyFromSymbol: SetSim copy from cSim failed");
}

__global__ void kCalculateAndersenThermostat_kernel()
{
    unsigned int pos            = threadIdx.x + blockIdx.x * blockDim.x;
    unsigned int rpos           = cSim.pRandomPosition[blockIdx.x];
    __syncthreads();

    float collisionProbability = 1.0f-exp(-cSim.collisionFrequency*cSim.pStepSize[0].y);
    float randomRange = erf(collisionProbability/sqrt(2.0f));
    while (pos < cSim.atoms)
    {
        float4 velocity         = cSim.pVelm4[pos];
        float4 random4a         = cSim.pRandom4a[rpos + pos];
        float scale = (random4a.w > -randomRange && random4a.w < randomRange ? 0.0f : 1.0f);
        float add = (1.0f-scale)*sqrt(cSim.kT*velocity.w);
        velocity.x = scale*velocity.x + add*random4a.x;
        velocity.y = scale*velocity.y + add*random4a.y;
        velocity.z = scale*velocity.z + add*random4a.z;
        cSim.pVelm4[pos]        = velocity;
         
        pos                    += blockDim.x * gridDim.x;    
    }

    // Update random position pointer
    if (threadIdx.x == 0)
    {
        rpos                   += cSim.paddedNumberOfAtoms;
        if (rpos > cSim.randoms)
            rpos               -= cSim.randoms;
        cSim.pRandomPosition[blockIdx.x] = rpos;
    }
}

extern void kGenerateRandoms(gpuContext gpu);
void kCalculateAndersenThermostat(gpuContext gpu)
{
//    printf("kCalculateAndersenThermostat\n");
    kCalculateAndersenThermostat_kernel<<<gpu->sim.blocks, gpu->sim.update_threads_per_block>>>();
    LAUNCHERROR("kCalculateAndersenThermostat");
    
    // Update randoms if necessary
    gpu->iterations++;
    if (gpu->iterations == gpu->sim.randomIterations)
    {
        kGenerateRandoms(gpu);
        gpu->iterations = 0;
    }
}

