/**
 * Perform the first step of Langevin integration.
 */

extern "C" __global__ void integrateDrudeLangevinPart1(mixed4* __restrict__ velm, const long long* __restrict__ force, mixed4* __restrict__ posDelta,
        const int* __restrict__ normalParticles, const int2* __restrict__ pairParticles, const mixed2* __restrict__ dt, mixed vscale, mixed fscale,
        mixed noisescale, mixed vscaleDrude, mixed fscaleDrude, mixed noisescaleDrude, const float4* __restrict__ random, unsigned int randomIndex) {
    mixed stepSize = dt[0].y;
    
    // Update normal particles.

    for (int i = blockIdx.x*blockDim.x+threadIdx.x; i < NUM_NORMAL_PARTICLES; i += blockDim.x*gridDim.x) {
        int index = normalParticles[i];
        mixed4 velocity = velm[index];
        if (velocity.w != 0) {
            mixed sqrtInvMass = SQRT(velocity.w);
            float4 rand = random[randomIndex+index];
            velocity.x = vscale*velocity.x + fscale*velocity.w*force[index] + noisescale*sqrtInvMass*rand.x;
            velocity.y = vscale*velocity.y + fscale*velocity.w*force[index+PADDED_NUM_ATOMS] + noisescale*sqrtInvMass*rand.y;
            velocity.z = vscale*velocity.z + fscale*velocity.w*force[index+PADDED_NUM_ATOMS*2] + noisescale*sqrtInvMass*rand.z;
            velm[index] = velocity;
            posDelta[index] = make_mixed4(stepSize*velocity.x, stepSize*velocity.y, stepSize*velocity.z, 0);
        }
    }
    
    // Update Drude particle pairs.
    
    randomIndex += NUM_NORMAL_PARTICLES;
    for (int i = blockIdx.x*blockDim.x+threadIdx.x; i < NUM_PAIRS; i += blockDim.x*gridDim.x) {
        int2 particles = pairParticles[i];
        mixed4 velocity1 = velm[particles.x];
        mixed4 velocity2 = velm[particles.y];
        mixed mass1 = RECIP(velocity1.w);
        mixed mass2 = RECIP(velocity2.w);
        mixed invTotalMass = RECIP(mass1+mass2);
        mixed invReducedMass = (mass1+mass2)*velocity1.w*velocity2.w;
        mixed mass1fract = invTotalMass*mass1;
        mixed mass2fract = invTotalMass*mass2;
        mixed sqrtInvTotalMass = SQRT(invTotalMass);
        mixed sqrtInvReducedMass = SQRT(invReducedMass);
        mixed4 cmVel = velocity1*mass1fract+velocity2*mass2fract;
        mixed4 relVel = velocity2-velocity1;
        mixed3 force1 = make_mixed3(force[particles.x], force[particles.x+PADDED_NUM_ATOMS], force[particles.x+PADDED_NUM_ATOMS*2]);
        mixed3 force2 = make_mixed3(force[particles.y], force[particles.y+PADDED_NUM_ATOMS], force[particles.y+PADDED_NUM_ATOMS*2]);
        mixed3 cmForce = force1+force2;
        mixed3 relForce = force2*mass1fract - force1*mass2fract;
        float4 rand1 = random[randomIndex+2*i];
        float4 rand2 = random[randomIndex+2*i+1];
        cmVel.x = vscale*cmVel.x + fscale*invTotalMass*cmForce.x + noisescale*sqrtInvTotalMass*rand1.x;
        cmVel.y = vscale*cmVel.y + fscale*invTotalMass*cmForce.y + noisescale*sqrtInvTotalMass*rand1.y;
        cmVel.z = vscale*cmVel.z + fscale*invTotalMass*cmForce.z + noisescale*sqrtInvTotalMass*rand1.z;
        relVel.x = vscaleDrude*relVel.x + fscaleDrude*invReducedMass*relForce.x + noisescaleDrude*sqrtInvReducedMass*rand2.x;
        relVel.y = vscaleDrude*relVel.y + fscaleDrude*invReducedMass*relForce.y + noisescaleDrude*sqrtInvReducedMass*rand2.y;
        relVel.z = vscaleDrude*relVel.z + fscaleDrude*invReducedMass*relForce.z + noisescaleDrude*sqrtInvReducedMass*rand2.z;
        velocity1.x = cmVel.x-relVel.x*mass2fract;
        velocity1.y = cmVel.y-relVel.y*mass2fract;
        velocity1.z = cmVel.z-relVel.z*mass2fract;
        velocity2.x = cmVel.x+relVel.x*mass1fract;
        velocity2.y = cmVel.y+relVel.y*mass1fract;
        velocity2.z = cmVel.z+relVel.z*mass1fract;
        velm[particles.x] = velocity1;
        velm[particles.y] = velocity2;
        posDelta[particles.x] = make_mixed4(stepSize*velocity1.x, stepSize*velocity1.y, stepSize*velocity1.z, 0);
        posDelta[particles.y] = make_mixed4(stepSize*velocity2.x, stepSize*velocity2.y, stepSize*velocity2.z, 0);
    }
}

/**
 * Perform the second step of Langevin integration.
 */

extern "C" __global__ void integrateDrudeLangevinPart2(real4* __restrict__ posq, real4* __restrict__ posqCorrection, const mixed4* __restrict__ posDelta, mixed4* __restrict__ velm, const mixed2* __restrict__ dt) {
    double invStepSize = 1.0/dt[0].y;
    int index = blockIdx.x*blockDim.x+threadIdx.x;
    while (index < NUM_ATOMS) {
        mixed4 vel = velm[index];
        if (vel.w != 0) {
#ifdef USE_MIXED_PRECISION
 
            real4 pos1 = posq[index];
            real4 pos2 = posqCorrection[index];
            mixed4 pos = make_mixed4(pos1.x+(mixed)pos2.x, pos1.y+(mixed)pos2.y, pos1.z+(mixed)pos2.z, pos1.w);
#else
            real4 pos = posq[index];
#endif
            mixed4 delta = posDelta[index];
            pos.x += delta.x;
            pos.y += delta.y;
            pos.z += delta.z;
            vel.x = (mixed) (invStepSize*delta.x);
            vel.y = (mixed) (invStepSize*delta.y);
            vel.z = (mixed) (invStepSize*delta.z);
#ifdef USE_MIXED_PRECISION
            posq[index] = make_real4((real) pos.x, (real) pos.y, (real) pos.z, (real) pos.w);
            posqCorrection[index] = make_real4(pos.x-(real) pos.x, pos.y-(real) pos.y, pos.z-(real) pos.z, 0);
#else
            posq[index] = pos;
#endif
            velm[index] = vel;
        }
        index += blockDim.x*gridDim.x;
    }
}