class particle{
  vector3D pos, vel, force;
  float mass;
  float charge;
  vector3D[] pbuffer;
  vector3D[] vbuffer;
  int bl;
  int cb;

  particle(){
  }

   particle(float x, float y, float z){
    pos = new vector3D(x,y,z);
    vel = new vector3D(random(-5.0,5.0),random(-5.0,5.0),random(-5.0,5.0));
    force = new vector3D(0.0,0.0,0.0);
    initBuffer();
  }

  void initBuffer(){
    bl=128;
    cb=0;
    pbuffer= new vector3D[bl];
    vbuffer= new vector3D[bl];
    for(int i=0;i<bl;i++){
      pbuffer[i]=new vector3D(pos);
      vbuffer[i]=new vector3D(vel);     
    }
  }

  void updateBuffer(){
    cb=(cb+1)%bl;
    pbuffer[cb].assign(pos);
    vbuffer[cb].assign(vel);
  }

  void update(float dt){
    vel.add(vector3D.mul(dt, force));
    pos.add(vector3D.mul(dt, vel));   
    updateBuffer();
  }
}