void the_derive(double time, double y[], double dy[]) {
int i;
BallVector balls = decode_balls(y); // new BallVector();//Ball[num_balls];
BallVector dballs = new BallVector(); // new Ball[num_balls];
for (i = 0; i < num_balls; i++) {
Ball p = balls.get2(i);
Ball d = new Ball();
d.pos = p.speed;
d.speed = wall_power(p);
d.speed.y -= 1; // gravity
dballs.add(d);
}
for (i = 0; i < num_balls; i++)
for (int j = i + 1; j < num_balls; j++) {
Ball p1 = balls.get2(i);
Ball p2 = balls.get2(j);
if (far_away_fast_calc(p1.pos, p2.pos, radius * 2)) continue;
double dist = p1.pos.calc_dist(p2.pos);
// if (dist>radius*2)
// continue;
Vec collide_power = calc_collide_power(p1, p2, dist);
dballs.get2(i).speed.add_to(collide_power);
dballs.get2(j).speed.sub_to(collide_power);
}
for (i = 0; i < springs.size(); i++) {
Spring s = springs.get2(i);
Vec collide_power = calc_spring_power(balls.get2(s.start), balls.get2(s.end));
dballs.get2(s.start).speed.add_to(collide_power);
dballs.get2(s.end).speed.sub_to(collide_power);
}
encode_balls(dballs, dy);
};
void new_ball(Vec vec) {
Ball b = new Ball();
b.pos = vec;
balls.add(b);
}