const track = document.getElementById('track');
const box   = document.getElementById('box');
const slides = document.querySelectorAll('.slide');
let idx = 0;

function goTo(n){
  idx = (n + slides.length) % slides.length;
  track.style.transform = `translateX(-${idx * 100}%)`;

  /* light blink */
  box.classList.add('swap');
  setTimeout(() => box.classList.remove('swap'), 250);
}

function move(d){ goTo(idx + d); }

/* auto-advance every 3 s */
setInterval(() => move(1), 3000);