unsigned long xorshift() {
  static unsigned long x=123456789, y=362436069, z=521288629, w=88675123;
  unsigned long t;
  
  t = (x ^ (x << 11));
  x = y;
  y = z;
  z = w;
  w = (w ^ (w >> 19)) ^ (t ^ (t >> 8));

  return w;
}