int numBalls = 12;
float spring = 0.05;
float gravity = 0.03;
float friction = -0.9;
Ball[] balls = new Ball[numBalls];

void setup() 
{
 size(640, 200);
 noStroke();
 smooth();
 for (int i = 0; i < numBalls; i++) {
 balls[i] = new Ball(random(width), random(height), random(20, 40), i, balls);
 }
}

void draw() 
{
 background(0);
 for (int i = 0; i < numBalls; i++) {
 balls[i].collide();
 balls[i].move();
 balls[i].display(); 
 }
}

class Ball {
 float x, y;
 float diameter;
 float vx = 0;
 float vy = 0;
 int id;
 Ball[] others;
 
 Ball(float xin, float yin, float din, int idin, Ball[] oin) {
 x = xin;
 y = yin;
 diameter = din;
 id = idin;
 others = oin;
 } 
 
 void collide() {
 for (int i = id + 1; i < numBalls; i++) {
 float dx = others[i].x - x;
 float dy = others[i].y - y;
 float distance = sqrt(dx*dx + dy*dy);
 float minDist = others[i].diameter/2 + diameter/2;
 if (distance < minDist) { 
 float angle = atan2(dy, dx);
 float targetX = x + cos(angle) * minDist;
 float targetY = y + sin(angle) * minDist;
 float ax = (targetX - others[i].x) * spring;
 float ay = (targetY - others[i].y) * spring;
 vx -= ax;
 vy -= ay;
 others[i].vx += ax;
 others[i].vy += ay;
 }
 } 
 }
 
 void move() {
 vy += gravity;
 x += vx;
 y += vy;
 if (x + diameter/2 > width) {
 x = width - diameter/2;
 vx *= friction; 
 }
 else if (x - diameter/2 < 0) {
 x = diameter/2;
 vx *= friction;
 }
 if (y + diameter/2 > height) {
 y = height - diameter/2;
 vy *= friction; 
 } 
 else if (y - diameter/2 < 0) {
 y = diameter/2;
 vy *= friction;
 }
 }
 
 void display() {
 fill(255, 204);
 ellipse(x, y, diameter, diameter);
 }
}