// A class to describe a group of Particles
  // An ArrayList is used to manage the list of Particles 
  
  class ParticleSystem {
  
    ArrayList particles;    // An arraylist for all the particles
    PVector origin;        // An origin point for where particles are birthed
    PImage img;
    
    ParticleSystem(int num, PVector v, PImage img_) {
      particles = new ArrayList();              // Initialize the arraylist
      origin = v.get();                        // Store the origin point
      img = img_;
      for (int i = 0; i < num; i++) {
        particles.add(new Particle(origin, img));    // Add "num" amount of particles to the arraylist
      }
    }
  
    void run() {
      // Cycle through the ArrayList backwards b/c we are deleting
      for (int i = particles.size()-1; i >= 0; i--) {
        Particle p = (Particle) particles.get(i);
        p.run();
        if (p.dead()) {
          particles.remove(i);
        }
      }
    }
    
    // Method to add a force vector to all particles currently in the system
    void add_force(PVector dir) {
      for (int i = particles.size()-1; i >= 0; i--) {
        Particle p = (Particle) particles.get(i);
        p.add_force(dir);
      }
    
    }  
  
    void addParticle() {
      particles.add(new Particle(origin,img));
    }
  
    void addParticle(Particle p) {
      particles.add(p);
    }
  
    // A method to test if the particle system still has particles
    boolean dead() {
      if (particles.isEmpty()) {
        return true;
      } else {
        return false;
      }
    }
  
  }