var frame = 0;

function setup() {
  frameRate(3);
}

function draw() {
  createCanvas(400, 400);
  background(220);

  var small_square_length = 40;
  var diameter = 20;
  var y_step = diameter;
  var x_start = 10;
  var y_start = 10;
  var k = 18;

  for (var i = 0; i < 20; i++) {
    for (var j = 0; j < 20; j++) {
      if (j > k) {
        const c = color(255, 255, 255);
        fill(c);
        circle(x_start + y_step * j, y_start + y_step * i, 20);
      }
    }
    k -= 1;
  }

  const c = color(255, 0, 0);
  fill(c);
  circle(370 - y_step * frame, 10 + y_step * frame, 20);

  frame += 1;
  if (frame > 20) {
    frame = 0;
  }
}