SimpleCollision.java

package org.openpixi.pixi.physics.collision.algorithms;

import java.util.ArrayList;

import org.openpixi.pixi.physics.collision.util.Pair;
import org.openpixi.pixi.physics.force.Force;
import org.openpixi.pixi.physics.particles.Particle;
import org.openpixi.pixi.physics.solver.Solver;

public class SimpleCollision extends CollisionAlgorithm{
	
	public SimpleCollision() {
		
		super();
	}
	
	private void doCollision(Particle p1, Particle p2) {
		
		//distance between the particles
		double distance = Math.sqrt((p1.getX() - p2.getX()) * (p1.getX() - p2.getX()) + (p1.getY() - p2.getY()) * (p1.getY() - p2.getY()));
		
		//finding the unit distance vector
		double dnX = (p1.getX() - p2.getX()) / distance;
		double dnY = (p1.getY() - p2.getY()) / distance;
		
		//finding the minimal distance if the ball are overlapping
		double minDistanceX = dnX * (p1.getRadius() + p2.getRadius() - distance);
		double minDistanceY = dnY * (p1.getRadius() + p2.getRadius() - distance);
		
		//moving the balls if they are overlapping (if not, the minimal distance is equal to zero)
		p1.setX(p1.getX() + minDistanceX * p2.getMass() / (p1.getMass() + p2.getMass()));
		p1.setY(p1.getY() + minDistanceY * p2.getMass() / (p1.getMass() + p2.getMass()));
		p2.setX(p2.getX() - minDistanceX * p1.getMass() / (p1.getMass() + p2.getMass()));
		p2.setY(p2.getY() - minDistanceY * p1.getMass() / (p1.getMass() + p2.getMass()));
		
		//defining variables for cleaner calculation
		double m21 = p2.getMass() / p1.getMass();
		double x21 = p2.getX() - p1.getX();
		double y21 = p2.getY() - p1.getY();
		double vx21 = p2.getVx() - p1.getVx();
		double vy21 = p2.getVy() - p1.getVy();
		
		//avoiding dividing with zero
		double helpCoef = 1.0e-5 * Math.abs(y21); 
		if(Math.abs(x21) < helpCoef) {
			if(x21 < 0) {
				helpCoef = - helpCoef;
			}
			x21 = helpCoef;
		}
		
		//doing the calculation
	    double angle = y21 / x21;
	    double dvx2 = -2 * (vx21 + angle * vy21) / ((1 + angle * angle) * (1 + m21)) ;
	    p2.setVx(p2.getVx() + dvx2);
	    p2.setVy(p2.getVy() + angle * dvx2);
	    p1.setVx(p1.getVx() - m21 * dvx2);
	    p1.setVy(p1.getVy() - angle * m21 * dvx2);
	}
	
	public void collide(ArrayList<Pair<Particle, Particle>> pairs, Force f, Solver s, double step) {
		
		for(int i = 0; i < pairs.size(); i++) {
			Particle p1 = (Particle) pairs.get(i).getFirst();
			Particle p2 = (Particle) pairs.get(i).getSecond();
		
			double distanceSquare = ((p1.getX() - p2.getX()) * (p1.getX() - p2.getX()) + (p1.getY() - p2.getY()) * (p1.getY() - p2.getY()));
			if(distanceSquare <= ((p1.getRadius() + p2.getRadius()) * (p1.getRadius() + p2.getRadius()))) {
				s.complete(p1, f, step);
				s.complete(p2, f, step);
				doCollision(p1, p2);
				s.prepare(p1, f, step);
				s.prepare(p2, f, step);
			}
		}
	}
}