// globals
Graviton gravioli;

void setup(){
  size(200,200);
  background(0);
  gravioli = new Graviton(); 
}

void draw(){
  background(0);
  gravioli.draw();
}

class Graviton{
  float x, y;
  float cnt, detail, radius, offset;
  float[] xlist = new float[200];
  float[] ylist = new float[200];
  // constructor
  Graviton(){
    radius = width * 0.4;       // orbit size
    offset = width / 2;         // center
    detail = 6.28 / 1000;      // speed
    cnt = random(-3.14, 3.14);  // starting location
  }
 
  // called every refresh from main draw loop
  void draw(){
    this.move();
    stroke(255);
    point(x,y);
  }
 
  void move(){
    x = sin(cnt) * radius + offset;
    y = cos(cnt) * radius + offset;
    cnt = cnt + detail;
    if (cnt > 3.14){
      cnt = -3.14;
    }
    this.trail();
  }
 
  void trail(){
    xlist = reverse(xlist);        // turn arrays around
    ylist = reverse(ylist);
    xlist = append(xlist, x);      // appends to back of array
    ylist = append(ylist, y);
    xlist = reverse(xlist);        // reflip so "back" is front
    ylist = reverse(ylist);
    xlist = contract(xlist, 200);  // resize to drop oldest element
    ylist = contract(ylist, 200);
    // draw trail
    for(int i=0; i<xlist.length; i++){
      float a = 100 - (0.5 * i);
      stroke(255,255,255,a);
      point(xlist[i], ylist[i]);
    }
  } 
}