Code:

package physic;public class Manifold {public static final class Pair {public final Circle circleA;public final Circle circleB;public Pair(Circle circleA, Circle circleB) {this.circleA = circleA;this.circleB = circleB;}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + circleA.id;result = prime * result + circleB.id;return result;}@Overridepublic boolean equals(Object obj) {if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;Pair other = (Pair) obj;if ((circleA.id == other.circleA.id) && (circleB.id == other.circleB.id))return true;return false;}}public Circle circleA;public Circle circleB;public Contact contact = new Contact();public Manifold(Pair pair) {this.circleA = pair.circleA;this.circleB = pair.circleB;}public boolean collide() {Circle a = circleA;Circle b = circleB;Vec2 relPos = b.position.sub(a.position);double distSqr = relPos.lenSqr();double radiusSum = a.radius + b.radius;if (distSqr > (radiusSum * radiusSum))return false;double distance = Math.sqrt(distSqr);if (distance == 0d) {contact.normal.set(1d, 0d);contact.position.set(a.position);} else {contact.normal.set(relPos).sclLocal(1d / distance);contact.position.set(contact.normal).sclLocal(a.radius).addLocal(a.position);}return true;}public void updateContact(Contact newContact) {if (Settings.warmstarting) {Vec2 relPos = newContact.position.sub(contact.position);if (relPos.len() <= Settings.contactMatchingDistance) {newContact.accImpulses = contact.accImpulses;}}contact = newContact;}private double getRelativeVelocityAlongNormal() {return circleB.velocity.sub(circleA.velocity).dot(contact.normal);}public void preStep() {contact.normalMass = circleA.invMass + circleB.invMass;contact.normalMass = contact.normalMass > 0d ? 1d / contact.normalMass : 0d;double resting = (Settings.frequency * Settings.gravity) + Settings.epsilon;double rvn = getRelativeVelocityAlongNormal();contact.velocityBias = rvn < -resting ? -Settings.restitution * rvn : 0d;warmStart();}private void warmStart() {Vec2 J = new Vec2();J.x = contact.normal.x * contact.accImpulses;J.y = contact.normal.y * contact.accImpulses;circleA.velocity.x -= J.x * circleA.invMass;circleA.velocity.y -= J.y * circleA.invMass;circleB.velocity.x += J.x * circleB.invMass;circleB.velocity.y += J.y * circleB.invMass;}public void applyImpulses() {double j = contact.normalMass * (-getRelativeVelocityAlongNormal() + contact.velocityBias);double pn0 = contact.accImpulses;contact.accImpulses = Math.max(pn0 + j, 0d);j = contact.accImpulses - pn0;Vec2 jn = contact.normal.scl(j);circleA.velocity.x -= jn.x * circleA.invMass;circleA.velocity.y -= jn.y * circleA.invMass;circleB.velocity.x += jn.x * circleB.invMass;circleB.velocity.y += jn.y * circleB.invMass;}}

Code:

package physic;import java.util.ArrayList;import java.util.HashMap;import java.util.List;import java.util.Map;import physic.Manifold.Pair;public class World {public final List<Circle> circles = new ArrayList<>();public final Map<Pair, Manifold> contacts = new HashMap<>();public Circle addCircle(double radius, double x, double y, boolean isStatic) {Circle c = new Circle(radius, x, y, isStatic);circles.add(c);return c;}public void update() {broadphase();integrateGravity();preStep();for (int i = 0; i < Settings.solverIterations; i++)applyImpulses();integrateVelocities();}private void broadphase() {for (int i = 0; i < circles.size(); i++) {for (int j = i + 1; j < circles.size(); j++) {Circle circleA = circles.get(i);Circle circleB = circles.get(j);if (circleA.invMass == 0f && circleB.invMass == 0f)continue;Pair pair = new Pair(circleA, circleB);Manifold newManifold = new Manifold(pair);if (newManifold.collide()) {if (contacts.containsKey(pair)) {contacts.get(pair).updateContact(newManifold.contact);} else {contacts.put(pair, newManifold);}} else {contacts.remove(pair);}}}}private void integrateGravity() {for (Circle c : circles) {if (c.invMass == 0f)continue;c.velocity.y += Settings.frequency * Settings.gravity;}}private void preStep() {for (Manifold m : contacts.values())m.preStep();}private void applyImpulses() {for (Manifold m : contacts.values())m.applyImpulses();}private void integrateVelocities() {for (Circle c : circles) {if (c.invMass == 0f)continue;c.position.x += Settings.frequency * c.velocity.x;c.position.y += Settings.frequency * c.velocity.y;}}}