float radijus = 30;
float xc;    
float xbrzina = 2;
int xsmer = 1;

void setup() 
{
  size(640, 360);
  stroke(255);
  frameRate(30);
  strokeWeight(2);
  ellipseMode(RADIUS);
  noFill();
  xc = width / 3;
}

void draw() {
  background(0);
  xc = xc + xbrzina * xsmer;
 
  if (xc > width + radijus)
    xc = - radijus;

  ellipse(xc, height/2, radijus, radijus);
}