/******************************************************************************
 *  Compilation:  javac CollisionSystem.java
 *  Execution:    java CollisionSystem n               (n random particles)
 *                java CollisionSystem < input.txt     (from a file)
 *  Dependencies: StdDraw.java Particle.java MinPQ.java
 *  Data files:   https://algs4.cs.princeton.edu/61event/diffusion.txt
 *                https://algs4.cs.princeton.edu/61event/diffusion2.txt
 *                https://algs4.cs.princeton.edu/61event/diffusion3.txt
 *                https://algs4.cs.princeton.edu/61event/brownian.txt
 *                https://algs4.cs.princeton.edu/61event/brownian2.txt
 *                https://algs4.cs.princeton.edu/61event/billiards5.txt
 *                https://algs4.cs.princeton.edu/61event/pendulum.txt
 *
 *  Creates n random particles and simulates their motion according
 *  to the laws of elastic collisions.
 *
 ******************************************************************************/

package edu.princeton.cs.algs4;

import java.awt.Color;

/**
 *  The {@code CollisionSystem} class represents a collection of particles
 *  moving in the unit box, according to the laws of elastic collision.
 *  This event-based simulation relies on a priority queue.
 *  <p>
 *  For additional documentation,
 *  see <a href="https://algs4.cs.princeton.edu/61event">Section 6.1</a> of
 *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */
public class CollisionSystem {
    private static final double HZ = 0.5;    // number of redraw events per clock tick

    private MinPQ<Event> pq;          // the priority queue
    private double t  = 0.0;          // simulation clock time
    private Particle[] particles;     // the array of particles

    /**
     * Initializes a system with the specified collection of particles.
     * The individual particles will be mutated during the simulation.
     *
     * @param  particles the array of particles
     */
    public CollisionSystem(Particle[] particles) {
        this.particles = particles.clone();   // defensive copy
    }

    // updates priority queue with all new events for particle a
    private void predict(Particle a, double limit) {
        if (a == null) return;

        // particle-particle collisions
        for (int i = 0; i < particles.length; i++) {
            double dt = a.timeToHit(particles[i]);
            if (t + dt <= limit)
                pq.insert(new Event(t + dt, a, particles[i]));
        }

        // particle-wall collisions
        double dtX = a.timeToHitVerticalWall();
        double dtY = a.timeToHitHorizontalWall();
        if (t + dtX <= limit) pq.insert(new Event(t + dtX, a, null));
        if (t + dtY <= limit) pq.insert(new Event(t + dtY, null, a));
    }

    // redraw all particles
    private void redraw(double limit) {
        StdDraw.clear();
        for (int i = 0; i < particles.length; i++) {
            particles[i].draw();
        }
        StdDraw.show();
        StdDraw.pause(20);
        if (t < limit) {
            pq.insert(new Event(t + 1.0 / HZ, null, null));
        }
    }


    /**
     * Simulates the system of particles for the specified amount of time.
     *
     * @param  limit the amount of time
     */
    public void simulate(double limit) {

        // initialize PQ with collision events and redraw event
        pq = new MinPQ<Event>();
        for (int i = 0; i < particles.length; i++) {
            predict(particles[i], limit);
        }
        pq.insert(new Event(0, null, null));        // redraw event


        // the main event-driven simulation loop
        while (!pq.isEmpty()) {

            // get impending event, discard if invalidated
            Event e = pq.delMin();
            if (!e.isValid()) continue;
            Particle a = e.a;
            Particle b = e.b;

            // physical collision, so update positions, and then simulation clock
            for (int i = 0; i < particles.length; i++)
                particles[i].move(e.time - t);
            t = e.time;

            // process event
            if      (a != null && b != null) a.bounceOff(b);              // particle-particle collision
            else if (a != null && b == null) a.bounceOffVerticalWall();   // particle-wall collision
            else if (a == null && b != null) b.bounceOffHorizontalWall(); // particle-wall collision
            else if (a == null && b == null) redraw(limit);               // redraw event

            // update the priority queue with new collisions involving a or b
            predict(a, limit);
            predict(b, limit);
        }
    }


   /***************************************************************************
    *  An event during a particle collision simulation. Each event contains
    *  the time at which it will occur (assuming no supervening actions)
    *  and the particles a and b involved.
    *
    *    -  a and b both null:      redraw event
    *    -  a null, b not null:     collision with vertical wall
    *    -  a not null, b null:     collision with horizontal wall
    *    -  a and b both not null:  binary collision between a and b
    *
    ***************************************************************************/
    private static class Event implements Comparable<Event> {
        private final double time;         // time that event is scheduled to occur
        private final Particle a, b;       // particles involved in event, possibly null
        private final int countA, countB;  // collision counts at event creation


        // create a new event to occur at time t involving a and b
        public Event(double t, Particle a, Particle b) {
            this.time = t;
            this.a    = a;
            this.b    = b;
            if (a != null) countA = a.count();
            else           countA = -1;
            if (b != null) countB = b.count();
            else           countB = -1;
        }

        // compare times when two events will occur
        public int compareTo(Event that) {
            return Double.compare(this.time, that.time);
        }

        // has any collision occurred between when event was created and now?
        public boolean isValid() {
            if (a != null && a.count() != countA) return false;
            if (b != null && b.count() != countB) return false;
            return true;
        }

    }


    /**
     * Unit tests the {@code CollisionSystem} data type.
     * Reads in the particle collision system from a standard input
     * (or generates {@code N} random particles if a command-line integer
     * is specified); simulates the system.
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {

        StdDraw.setCanvasSize(600, 600);

        // enable double buffering
        StdDraw.enableDoubleBuffering();

        // the array of particles
        Particle[] particles;

        // create n random particles
        if (args.length == 1) {
            int n = Integer.parseInt(args[0]);
            particles = new Particle[n];
            for (int i = 0; i < n; i++)
                particles[i] = new Particle();
        }

        // or read from standard input
        else {
            int n = StdIn.readInt();
            particles = new Particle[n];
            for (int i = 0; i < n; i++) {
                double rx     = StdIn.readDouble();
                double ry     = StdIn.readDouble();
                double vx     = StdIn.readDouble();
                double vy     = StdIn.readDouble();
                double radius = StdIn.readDouble();
                double mass   = StdIn.readDouble();
                int r         = StdIn.readInt();
                int g         = StdIn.readInt();
                int b         = StdIn.readInt();
                Color color   = new Color(r, g, b);
                particles[i] = new Particle(rx, ry, vx, vy, radius, mass, color);
            }
        }

        // create collision system and simulate
        CollisionSystem system = new CollisionSystem(particles);
        system.simulate(10000);
    }

}

/******************************************************************************
 *  Copyright 2002-2022, Robert Sedgewick and Kevin Wayne.
 *
 *  This file is part of algs4.jar, which accompanies the textbook
 *
 *      Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne,
 *      Addison-Wesley Professional, 2011, ISBN 0-321-57351-X.
 *      http://algs4.cs.princeton.edu
 *
 *
 *  algs4.jar is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  algs4.jar is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with algs4.jar.  If not, see http://www.gnu.org/licenses.
 ******************************************************************************/