import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Toolkit;

import javax.swing.JFrame;
import javax.swing.JPanel;

/**
 * Dynamics of a collision between 2 hard spheres
 * 
 * Based on equations from http://introcs.cs.princeton.edu/java/assignments/collisions.html
 * (I kept the same names for clarity)
 * 
 * @author Jan Lemeire
 *
 */

public class Collision {
	double rxi, ryi, vxi, vyi;
	double rxj, ryj, vxj, vyj;
	double ri, rj, rtot; // radius of both spheres
	double mi, mj; // masses 
	
	// exact time and place of collision
	double deltat;
	double rxiC, ryiC; // position sphere i
	double rxjC, ryjC; // position sphere j
	// velocities right after collision
	double vxiC, vyiC; // velocity sphere i
	double vxjC, vyjC; // velocity sphere j
	
	/**
	 * Calculates time and position of collision and velocities right after the collision.
	 * If spheres will not collide, an error is thrown.
	 * Properties sphere i: position (rxi, ryi), velocity (vxi, vyi), radius ri, mass mi
	 * Properties sphere j: position (rxj, ryj), velocity (vxj, vyj), radius rj, mass mj
	 */
	public Collision(double rxi, double ryi, double vxi, double vyi, double rxj, double ryj, double vxj, double vyj, double ri, double rj, double mi, double mj){
		this.rxi=rxi; this.ryi=ryi; this.vxi=vxi; this.vyi=vyi;
		this.rxj=rxj; this.ryj= ryj; this.vxj= vxj; this.vyj= vyj;
		this.ri=ri; this.rj=rj; rtot = ri+rj;
		this.mi=mi; this.mj= mj;
		
		//  (1) calculate exact time of collision
		double deltarx = rxj - rxi;
		double deltary = ryj - ryi;
		double deltavx = vxj - vxi;
		double deltavy = vyj - vyi;
		double deltav_dot_deltar = deltarx * deltavx + deltary * deltavy; 
		if (deltav_dot_deltar >= 0)
			throw new RuntimeException("Spheres will not collide.");
		double deltav_dot_deltav = deltavx*deltavx + deltavy*deltavy;
		double discriminant = Math.pow(deltav_dot_deltar, 2) - (deltav_dot_deltav) 
		* (deltarx * deltarx + deltary * deltary - rtot * rtot);
		if (discriminant<0)
			throw new RuntimeException("Spheres will not collide.");
		deltat = - (deltav_dot_deltar + Math.sqrt(discriminant)) / deltav_dot_deltav;
		
		// (2) calculate place of collision
		rxiC = rxi + deltat * vxi;
		ryiC = ryi + deltat * vyi;
		rxjC = rxj + deltat * vxj;
		ryjC = ryj + deltat * vyj;
		
		// (3) calculate velocities after collision
		double deltarxC = rxjC - rxiC;
		double deltaryC = ryjC - ryiC;
		double deltavxC = vxj - vxi;
		double deltavyC = vyj - vyi;
		double deltav_dot_deltarC = deltarxC * deltavxC + deltaryC * deltavyC; 
		// the equations are based on the preservation of impulse ("moment" in nederlands)
		// impulse = mass * velocity
		double J = 2 * mi * mj * deltav_dot_deltarC / (rtot * (mi + mj));
		double Jx = J * deltarxC / rtot;
		double Jy = J * deltaryC / rtot;
		vxiC = vxi + Jx/mi;
		vyiC = vyi + Jy/mi;
		vxjC = vxj - Jx/mj;
		vyjC = vyj - Jy/mj;
	}
	/**
	 * DEMO PROGRAM WITH EXAMPLE COLLISION
	 */
	public static void main(String[] args) {
		final int SCREEN_WIDTH = 800, SCREEN_HEIGHT = 800;
		JPanel panel = new JPanel(){
			public void paintComponent(Graphics g) {
				
				// * START CONDITION 
				double rxi = 100, ryi = 100, vxi = 20, vyi= 21;
				double rxj = 500, ryj=500, vxj=-20, vyj=-25;
				double ri=40, rj=40; // radius of both spheres
				double mi=1, mj=1; // masses 
				
				
				Graphics2D g2D = (Graphics2D)g; 
				// draw a grid on the panel where gap between the lines is 50
				StandardGraphics.drawGrid(g2D, 0, 0, getWidth(), getHeight(), 50);
				
				// SITUATION BEFORE COLLISION
				StandardGraphics.drawCircle(g, rxi, ryi, ri);
				StandardGraphics.DrawArrow(g2D, (int)rxi, (int)ryi, (int)(rxi+vxi), (int)(ryi+vyi), 15); 
				g.setColor(Color.RED);
				StandardGraphics.drawCircle(g, rxj, ryj, rj);		
				StandardGraphics.DrawArrow(g2D, (int)rxj, (int)ryj, (int)(rxj+vxj), (int)(ryj+vyj), 15); 
				
				try{
					// CALCULATION OF COLLISION
					Collision col = new Collision(rxi, ryi, vxi, vyi, rxj, ryj, vxj, vyj, ri, rj, mi, mj);
					System.out.println("Collision after "+col.deltat+" time units");
					g.setFont(new Font("Times", Font.BOLD, 24)); // change font
					g.drawString("Collision after "+col.deltat+" time units", 250, 100);
					
					// SITUATION JUST AFTER COLLISION
					g.setColor(Color.BLACK);
					StandardGraphics.drawCircle(g, col.rxiC, col.ryiC, ri);
					StandardGraphics.DrawArrow(g2D, (int)col.rxiC, (int)col.ryiC, (int)(col.rxiC+col.vxiC), (int)(col.ryiC+col.vyiC), 15); 
	
					g.setColor(Color.RED);
					StandardGraphics.drawCircle(g, col.rxjC, col.ryjC, rj);
					StandardGraphics.DrawArrow(g2D, (int)col.rxjC, (int)col.ryjC, (int)(col.rxjC+col.vxjC), (int)(col.ryjC+col.vyjC), 15);
				} catch (RuntimeException e){
					g.setFont(new Font("Times", Font.BOLD, 24)); // change font
					g.drawString("Spheres will not collide", 300, 100);
				}
			}
		};
		panel.setPreferredSize(new Dimension(SCREEN_WIDTH, SCREEN_HEIGHT));
		JFrame frame = new JFrame("Demo of StandardGraphics Functions");
		frame.add(panel);
		frame.pack(); // calculate frame size
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		 // show centered
		Dimension frameSize = frame.getSize();
        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        frame.setLocation(screenSize.width / 2 - frameSize.width / 2, screenSize.height / 2 - frameSize.height / 2);
	    frame.setVisible(true);
	}
}