Classes

public class velocity{
  float delx, dely;

  //Constructor 1
  public velocity(){}

  //Constructor 2
  public velocity(float delx, float dely){
    this.delx = delx;
    this.dely = dely;
  }

  //Mutators for xvelocity and y velocity
  public float getdelx(){
    return this.delx;
  }

  public float getdely(){
    return this.dely;
  }

  //Accessors for xvelocity and y velocity
  public void setdelx(float delx){
    this.delx = delx;
  }

  public void setdely(float dely){
    this.dely = dely;
  }

}






public class particle{

  private float xpos , ypos, delx , dely,size, decay, mass;
  color colour;

  //constructor 1
  public particle(float x, float y){
  this.xpos = x;
  this.ypos = y;
  }

  //constructor 2
  public particle(float xpos, float ypos, float size, float delx, float dely, float mass){
    this.xpos = xpos;
    this.ypos = ypos;
    this.size = size;
    this.delx = delx;
    this.dely = dely;
    this.mass = mass;

  }

  //Mutators for size, x velocity y velocity and velocity vector
  public void setsize(float size){
    this.size = size;
  }

  public void setDX(float delx){
    this.delx = delx;
  }

  public void setDY(float dely){
    this.dely = dely;
  }




  //Accessors for size, x position, y position, x velocity and y velocity

  public float getsize(){
    return this.size;
  }

  public float getX(){
    return this.xpos;
  }

  public float getY(){
    return this.ypos;
  }

  public float getDX(){
    return this.delx;
  }

  public float getDY(){
    return this.dely;
  }

  public float getmass(){
    return this.mass;
  }

  public velocity getvel(){

    velocity v = new velocity(this.getDX(), this.getDY());
    return v;
  }



  //Functionality. Moving around, Bouncing off of walls, and basic display updates
  public void move(){
    this.xpos += this.delx;
    this.ypos += this.dely;
  }

  public void bounce(){
    if((this.xpos - this.size/2.0) < 0 || (this.xpos + this.size/2.0) > width){
      this.setDX(this.getDX()*-1);
    }
    if((this.ypos - this.size/2.0) < 0 || (this.ypos + this.size/2.0) > height){
       this.setDY(this.getDY()*-1);
    }
  }

  public void update(particle[] elec){

    for(int i =0; i< elec.length; i++){

      if(this == elec[i]) continue;
      if(dist(this.getX(),this.getY(),elec[i].getX(),elec[i].getY()) - this.getsize() <0){
        collision(this, elec[i]);
        //println(dist(this.getX(),this.getY(),elec[i].getX(),elec[i].getY()) - this.getsize()/2);
      }
    }
    display();
  }

  public void display(){
    stroke(0);
    fill(119,240,153);
    ellipse(this.xpos, this.ypos, this.size ,this.size);
  }
}



velocity rotate(velocity v, float angle){
  float x = v.getdelx()*cos(angle) - v.getdely()*sin(angle);
  float y = v.getdelx()*sin(angle) + v.getdely()*cos(angle);

  velocity vel = new velocity(x,y);
  return vel;
}



void collision(particle p1, particle p2){

  float xveldiff = p1.getDX()-p2.getDX();
  float yveldiff = p1.getDY()-p2.getDY();

  float xdist = p2.getX() - p1.getX();
  float ydist = p2.getY() - p1.getY();

  //Check for accidental overlaps of particles
  if(xveldiff*xdist + yveldiff*ydist > 0){

    float angle = -atan2(p2.getY() - p1.getY(), p2.getX() - p1.getY());

    float m1 = p1.getmass();
    float m2 = p2.getmass();

    velocity u1 = rotate(p1.getvel(),angle);
    velocity u2 = rotate(p2.getvel(),angle);

    velocity v1 = new velocity(u1.getdelx() * (m1 - m2) / (m1 + m2) + u2.getdelx() * 2 * m2 / (m1 + m2), u1.getdely());
    velocity v2 = new velocity(u2.getdelx() * (m1 - m2) / (m1 + m2) + u1.getdelx() * 2 * m2 / (m1 + m2), u2.getdely());

    velocity vf1 = rotate(v1, -angle);
    velocity vf2 = rotate(v2, -angle);

    p1.setDX(vf1.getdelx());
    p1.setDY(vf1.getdely());

    p2.setDX(vf2.getdelx());
    p2.setDY(vf2.getdely());
  }

}