function sm2(prev, quality) {
  let { ease_factor: e, interval_days: i, repetitions: r } = prev;
  if (quality < 3) {
    r = 0;
    i = 1;
  } else {
    if (r === 0) i = 1;
    else if (r === 1) i = 6;
    else i = Math.round(i * e);
    r += 1;
  }
  e = Math.max(1.3, e + (0.1 - (5 - quality) * (0.08 + (5 - quality) * 0.02)));
  // next_review uses local timezone — acceptable for a personal study app
  const next = new Date();
  next.setDate(next.getDate() + i);
  return {
    ease_factor: +e.toFixed(2),
    interval_days: i,
    repetitions: r,
    next_review: next.toISOString().slice(0, 10),
  };
}

module.exports = { sm2 };