void evolve(int count,double dt){
int step;
int i,j,k,l;
double dr, dx[3];
/*
* Loop over timesteps.
*/
for(step = 1;step<=count;step++){
printf("timestep %d\n",step);
printf("collisions %d\n",collisions);
/* set the viscosity term in the force calculation */
for(i=0;i<Nbody;i++){
visc_force(Ndim,particle[i].f, particle[i].visc, particle[i].vel);
/* calculate norm of seperation vector */
particle[i].r = 0.0;
add_norm(Ndim,&particle[i].r,particle[i].pos);
particle[i].r = sqrt(particle[i].r);
/* calculate central force */
for(l=0;l<Ndim;l++){
particle[i].f[l] = particle[i].f[l] -
force(G*particle[i].mass*M_central,particle[i].pos[l],particle[i].r);
}
}
/* calculate pairwise separation of particles */
for(i=0;i<Nbody;i++){
for(j=i+1;j<Nbody;j++){
for(l=0;l<Ndim;l++){
dx[l] = particle[i].pos[l] - particle[j].pos[l];
}
dr = 0.0;
add_norm(Ndim, &dr, dx);
dr = sqrt(dr);
for(l=0;l<Ndim;l++){
/* flip force if close in */
double force1 = force(G*particle[i].mass*particle[j].mass,dx[l],dr);
if( dr >= Size ){
particle[i].f[l] = particle[i].f[l] - force1;
particle[j].f[l] = particle[j].f[l] + force1;
}else{
particle[i].f[l] = particle[i].f[l] + force1;
particle[j].f[l] = particle[j].f[l] - force1;
collisions++;
}
}
}
}
/* update positions */
for(i=0;i<Nbody;i++){
for(j=0;j<Ndim;j++){
particle[i].pos[j] = particle[i].pos[j] + dt * particle[i].vel[j];
/* update velocities */
particle[i].vel[j] = particle[i].vel[j] + dt * (particle[i].f[j]/particle[i].mass);
}
}
}
}
void evolve(int count,double dt){
int step;
int i,j,k,l;
/*
* Loop over timesteps.
*/
for(step = 0;step<count;step++){
printf("timestep %d\n",step);
/* set the viscosity term in the force calculation */
for(j=0;j<Ndim;j++){
visc_force(Nbody,f[j],visc,vel[j]);
}
/* calculate pairwise separation of particles */
k = 0;
for(i=0;i<Nbody;i++){
for(j=i+1;j<Nbody;j++){
for(l=0;l<Ndim;l++){
delta_x[l][k] = pos[l][i] - pos[l][j];
}
k = k + 1;
}
}
/* calculate norm of seperation vector */
for(k=0;k<Npair;k++){
r[k] = 0.0;
}
for(i=0;i<Ndim;i++){
add_norm(Npair,r,delta_x[i]);
}
for(k=0;k<Npair;k++){
r[k] = sqrt(r[k]);
}
/*
* add pairwise forces.
*/
k = 0;
for(i=0;i<Nbody;i++){
for(j=i+1;j<Nbody;j++){
for(l=0;l<Ndim;l++){
/* flip force if close in */
if( r[k] >= Size ){
f[l][i] = f[l][i] -
force(G*mass[i]*mass[j],delta_x[l][k],r[k]);
f[l][j] = f[l][j] +
force(G*mass[i]*mass[j],delta_x[l][k],r[k]);
}else{
f[l][i] = f[l][i] +
force(G*mass[i]*mass[j],delta_x[l][k],r[k]);
f[l][j] = f[l][j] -
force(G*mass[i]*mass[j],delta_x[l][k],r[k]);
}
}
k = k + 1;
}
}
/* update positions */
for(i=0;i<Nbody;i++){
for(j=0;j<Ndim;j++){
pos[j][i] = pos[j][i] + dt * vel[j][i];
}
}
/* update velocities */
for(i=0;i<Nbody;i++){
for(j=0;j<Ndim;j++){
vel[j][i] = vel[j][i] + dt * (f[j][i]/mass[i]);
}
}
}
}
