/* SM-2 lite. Four grades, day-granularity intervals. */
export const AGAIN = 0, HARD = 1, GOOD = 2, EASY = 3;
export const NEW = 0, LEARNING = 1, REVIEW = 2;
export const SECURE_INTERVAL = 21;          // days at which a card counts as known

export const newCard = () => ({ state: NEW, interval: 0, ease: 2.5, due: 0, reps: 0, lapses: 0 });

export function grade(card, g, today) {
  const c = { ...card };
  if (c.state === NEW || c.state === LEARNING) {
    if (g <= HARD)      { c.state = LEARNING; c.interval = 0; c.due = today; }
    else if (g === GOOD){ c.state = REVIEW;   c.interval = 1; c.due = today + 1; }
    else                { c.state = REVIEW;   c.interval = 4; c.due = today + 4; }
  } else {
    if (g === AGAIN)     { c.ease = Math.max(1.3, c.ease - 0.2);  c.lapses++; c.state = LEARNING; c.interval = 0; c.due = today; }
    else if (g === HARD) { c.ease = Math.max(1.3, c.ease - 0.15); c.interval = Math.max(1, Math.round(c.interval * 1.2)); c.due = today + c.interval; }
    else if (g === GOOD) { c.interval = Math.max(1, Math.round(c.interval * c.ease)); c.due = today + c.interval; }
    else                 { c.ease = Math.min(3, c.ease + 0.15); c.interval = Math.max(2, Math.round(c.interval * c.ease * 1.3)); c.due = today + c.interval; }
    c.interval = Math.min(c.interval, 365);
    c.due = Math.min(c.due, today + 365);
  }
  c.reps++;
  return c;
}

export const markKnown = today => ({ state: REVIEW, interval: SECURE_INTERVAL, ease: 2.5, due: today + SECURE_INTERVAL, reps: 0, lapses: 0 });

export function statusOf(card) {
  if (!card || card.state === NEW) return "new";
  if (card.state === LEARNING) return "learning";
  return card.interval >= SECURE_INTERVAL ? "secure" : "review";
}

/** Label for the interval a grade would produce — shown on the buttons. */
export function preview(card, g, today) {
  const c = grade(card || newCard(), g, today);
  if (c.interval === 0)  return "again now";
  if (c.interval === 1)  return "1 day";
  if (c.interval < 30)   return `${c.interval} days`;
  const mo = Math.round(c.interval / 30);
  return `${mo} month${mo === 1 ? "" : "s"}`;
}

/** Local day number, DST-safe. */
export const dayNumber = (d = new Date()) =>
  Math.floor(Date.UTC(d.getFullYear(), d.getMonth(), d.getDate()) / 864e5);