boolean evo_step() {
  int vc;
  Circle old_c, new_c;
  float ol, x, y;
  
  // choose a random circle
  vc = int(random(n_circles));
  old_c = circles[vc];
  
  // make new circle with radius and color of old one
  x = old_c.r + random((width-2*old_c.r));
  y = old_c.r + random((height-2*old_c.r));
  new_c = new Circle(x, y, old_c.r, old_c.c);
  circles[vc] = new_c;
  
  // calculate new overlap values
  ol = overlap();

  if (ol >= last_overlap) {
    // switch back
    circles[vc] = old_c;
    return false;
  }
  // keep changes
  last_overlap = ol;
  println(ol);
  return true;
}


// calculate overlap-value (sum)
float overlap() {
  float o, sum_o = 0.0;
  Circle a, b;
  for (int i = 0; i < n_circles; i++) {
    a = circles[i];
    for (int j = i+1; j < n_circles; j++) {
      b = circles[j];
      o = additional_distance + a.r + b.r - dist(a.x, a.y, b.x, b.y);
      if (o > 0.0)
        sum_o += o;
    }
  }
  return sum_o;
}