void calculate_force(double forces[][3], double ****neighbor, double particles[][3], double region, int amount){
// Updates the force for each particle based on the neighbor list
int i,j,k;
double r = region/2.;
for (i=0; i<amount; i++){
for (j=0; j<3; j++){
forces[i][j] = 0;
}
}
omp_set_num_threads(NUM_THREADS);
#pragma omp parallel for private(j,k)
for (i=0; i<amount; i++){
int neighbors = *neighbor[i][0][0];
for (j=1; j<=neighbors; j++){
double d = distance_periodic(particles[i],*neighbor[i][j],region);
double di = 1./d;
double d3 = di * di *di;
for (k=0; k<3; k++){
double dl = particles[i][k] - *neighbor[i][j][k];
if (dl > r){
dl -= region;
}
else if (dl < -r){
dl += region;
}
forces[i][k] += 48.*d3*(d3-0.5)*di*dl;
}
}
}
#pragma omp barrier
}
void update_neighborlist(double ****neighbor, double *amount_neighbor, double particles[][3], double radius, int amount, double region){
// Updates the neighbor list for each particle within a given radius
// The first element of each row contains a pointer to the amount of neighbors
int i,j,k;
omp_set_num_threads(NUM_THREADS);
#pragma omp parallel for private(j,k)
for (i=0; i<amount; i++){
int l = 0;
for (j=0; j<amount; j++){
if (i!=j){
double distance_ij = distance_periodic(particles[i],particles[j],region);
if (distance_ij<radius*radius){
for (k=0; k<3; k++){
neighbor[i][l+1][k] = &particles[j][k];
}
l++;
}
}
}
amount_neighbor[i] = (double)l;
neighbor[i][0][0] = &amount_neighbor[i];
}
#pragma omp barrier
}
void test_distance_periodic(void)
{
//real distance_periodic(Vector p1, Vector p2)
real L = 32;
Vector a, b;
a.x = 1; a.y = 1;
b.x = 31; b.y = 1;
assert_almost_equal(distance_periodic(a, b, L), real(2));
a.x = 1; a.y = 1;
b.x = 1; b.y = 31;
assert_almost_equal(distance_periodic(a, b, L), real(2));
a.x = 1; a.y = 1;
b.x = 31; b.y = 31;
assert_almost_equal(distance_periodic(a, b, L), real(2)*std::sqrt(real(2)));
a.x = 1; a.y = 1;
b.x = 3; b.y = 1;
assert_almost_equal(distance_periodic(a, b, L), real(2));
a.x = 1; a.y = 1;
b.x = 1; b.y = 3;
assert_almost_equal(distance_periodic(a, b, L), real(2));
a.x = 1; a.y = 1;
b.x = 3; b.y = 3;
assert_almost_equal(distance_periodic(a, b, L), real(2)*std::sqrt(real(2)));
}
