force_type operator()(NODE n, double t) {
(void) t;
force_type force = force_type(0,0,0);
Node node2;
double distance;
double initial_spring_length;
double sprint_const_this_edge;
// iterate through each neighboring node and add spring forces
for(auto it = n.edge_begin(); it != n.edge_end(); ++it)
{
node2 = (*it).node2();
sprint_const_this_edge = (*it).value().spring_constant;
initial_spring_length = (*it).value().initial_length;
distance = norm(n.position()-node2.position()); // Euclidean distance
force += (-1)*sprint_const_this_edge*(n.position()-node2.position())*(distance-initial_spring_length)/distance;
}
return force;
}
