class Particle{
  public float x, y;
  public float xprev, yprev;
  private float xt, yt;
  
  Particle(float xi, float yi, float xp, float yp){
    x=xt=xi;
    y=yt=yi;
    xprev=xp;
    yprev=yp;
    }
  
  void update (Force F, float friction){
    xt= (2.0f-friction)*x-(1-friction)*xprev+F.x;
    yt= (2.0f-friction)*y-(1-friction)*yprev+F.y;  
    xprev=x;
    yprev=y;
    x=xt;
    y=yt;
    
    
    }
    
    void constrain(float m, float M, float r){
      if(x<m){
        xt=xprev-x;
        x=2*m-x;
        xprev=x-r*xt;
        }
       if(x>M){
         xt=xprev-x;
         x=2*M-x;
         xprev=x-r*xt;
         }
      if(y<m){
        yt=yprev-y;
        y=2*m-y;
        yprev=y-r*yt;
        }
       if(y>M){
         yt=yprev-y;
         y=2*M-y;
         yprev=y-r*yt;
         }
      
      }
  
  }