float x,y,xD,yD,angle,angleD,speed,speedD;

void setup() {
  size(400,400);
  x=width/2;
  y=height/2;
  
  angle=random(360);
  speed=2;
  angleD=0.5;
  speedD=0.1;
  ellipseMode(CENTER_DIAMETER);
}

void loop() {
//  background(200);
  
  speedD=speedD+random(0.1)-0.05;
  if(speedD>0.2) speedD=0.2;
  else if(speedD<-0.2) speedD=0.2;

  speed=speed+speedD;
  if(speed>6) speed=6;
  else if(speed<0.1) speed=0.1;
  
  angleD=angleD+random(1)-0.5;
  if(angleD>5) angleD=5;
  else if(angleD<-5) angleD=-5;
  
  angle=angle+angleD;
  xD=cos(radians(angle));
  yD=sin(radians(angle));
  xD=xD*speed;
  yD=yD*speed;
  
  x=x+xD;
  y=y+yD;
  if(x>width) x=0;
  else if(x<0) x=width;
  if(y>height) y=0;
  else if(y<0) y=height;
  
  ellipse(x,y, 4*speed,4*speed);
}