class ParticleSystem{
  ArrayList parts;
  PVector origin;
  
  //Constructor
  ParticleSystem(int num, PVector v, float r, float mass, float grav, float timer){
    parts = new ArrayList();
    origin = v.get();
    for(int i=0; i<num; i++){
      parts.add(new Particle(origin, r, mass, grav, timer));
    }
  }

  void run(){
    for(int i=parts.size()-1; i>=0; i--){ 
      Particle p = (Particle) parts.get(i);
      p.run(parts); //run all the main actions for every particle in the ArrayList
      if(p.dead()){ //if particle p timer is = 0, remove it from the ArrayList
        parts.remove(i);
      }
    }
    
    //display how many particles are alive
    fill(0);
    rect(width-120, height-40, width, height);
    fill(255);
    text("Particles: "+parts.size(), width-100, height-16);
  }

  //Add a new particle
  void addParticle(float x, float y, float r, float mass, float grav, float timer){
    parts.add(new Particle(new PVector(x, y), r, mass, grav, timer));
  }
}

class Particle{
  PVector loc;   //2d vector storing x and y locations
  PVector acc;   //rate of acceleration
  PVector vel;   //controlling direction
  float r;       //radius of particle
  float timer;   //particle lifetime
  float mass;    //"weight" of particle, need for attraction algorithm
  float velLim;  //limit the speed at which the particles can travel
  float g;       //gravity constant
  ArrayList parts;

  Particle(PVector l, float _r, float _mass, float _grav, float _timer){
    loc = l.get(); //x and y values
    acc = new PVector(random(0, 2), random(0, 2), 0);  //set initial acceleration
    vel = new PVector(random(-2, 2), random(-1, 1), 0); //set initial velocity
    r = _r;
    mass = _mass;
    g = _grav;
    timer = _timer;
    velLim = 5.0;
  }

  void applyAttraction(ArrayList parts){
    //compare location values between different particles in the arraylist
    for(int i=0; i<parts.size();i++){
      Particle p1 = (Particle)parts.get(i);
      for(int j=i+1;j<parts.size();j++){
        Particle p2 = (Particle)parts.get(j);

        PVector dir = PVector.sub(p1.loc, p2.loc);
        float d = dir.mag();
        dir.normalize();
        
        //calculate strength of attraction
        float force = (g * p1.mass * p2.mass) / (d*d);
        dir.mult(force);
      
        p2.applyForce(dir);
        dir.mult(-1.0);
        p1.applyForce(dir);
      }
    }
  }
  
  //used to apply the attraction force
  void applyForce(PVector force){
    force.div(mass);
    acc.add(force);
  }
  
  //main
  void run(ArrayList parts){
    bounds();
    applyAttraction(parts);
    update();
    render();
  }
  
  //update vector values
  void update(){
    vel.add(acc);
    vel.limit(velLim);
    loc.add(vel);
    acc.mult(0);
    timer -= 1.0;
  }

  //bounces particles off canvas edge
  void bounds(){
    if(loc.y > height || loc.y < 0){
      vel.y *= -1;
    }
    if(loc.x > width || loc.x < 0){
      vel.x *= -1;
    }
  }
  
  //set the visual characteristics of the individual particles here
  void render(){
    ellipseMode(CENTER);
    noStroke();
    fill(255,  timer);
    ellipse(loc.x, loc.y, r, r);
  }
  
  //check if the particle's timer is 0
  boolean dead(){
    if(timer <= 0.0){
      return true;
    }
    else{
      return false;
    }
  }
}

ParticleSystem ps;
PFont font;

void setup(){
  size(800, 500, JAVA2D);
  background(0);
  ps = new ParticleSystem(1, new PVector(width/2, height/2, 0), 2, 5, 0.1, 255); //create new particle system ArrayList
  smooth();
  
  font = loadFont("Monaco-12.vlw");
  textFont(font, 12);
}

void draw(){
  fill(0, 3);
  rect(0, 0, width, height); //show particle trails
 
  ps.run();
  
  if(mousePressed){
    ps.addParticle(mouseX, mouseY, 2, 5, 0.1, 255);
  }
}