/** A simplified SM-2 schedule. Days are counted as local calendar days. */

export type Card = {
	due: number;
	interval: number;
	ease: number;
	reps: number;
	lapses: number;
	seen: number;
	right: number;
	last: number;
};

export function dayKey(date = new Date()): string {
	const y = date.getFullYear();
	const m = String(date.getMonth() + 1).padStart(2, '0');
	const d = String(date.getDate()).padStart(2, '0');
	return `${y}-${m}-${d}`;
}

export function dayNumber(key = dayKey()): number {
	const [y, m, d] = key.split('-').map(Number);
	return Math.round(Date.UTC(y, m - 1, d) / 86_400_000);
}

export const newCard = (today: number): Card => ({
	due: today,
	interval: 0,
	ease: 2.5,
	reps: 0,
	lapses: 0,
	seen: 0,
	right: 0,
	last: -1
});

export function review(card: Card, correct: boolean, today: number): Card {
	const next = { ...card, seen: card.seen + 1, right: card.right + (correct ? 1 : 0) };
	if (!correct) {
		return {
			...next,
			reps: 0,
			interval: 0,
			lapses: card.lapses + 1,
			ease: Math.max(1.3, card.ease - 0.2),
			due: today,
			last: today
		};
	}
	// Grinding the same card again on one day should not push it months ahead.
	if (card.last === today && card.reps > 0) return next;
	const reps = card.reps + 1;
	const interval =
		reps === 1 ? 1 : reps === 2 ? 3 : Math.round(Math.max(card.interval, 1) * card.ease);
	return {
		...next,
		reps,
		interval,
		ease: Math.min(3, card.ease + 0.05),
		due: today + interval,
		last: today
	};
}

export type Mastery = 'new' | 'learning' | 'familiar' | 'known' | 'mastered';

export function mastery(card: Card | undefined): Mastery {
	if (!card || card.seen === 0) return 'new';
	if (card.interval < 3) return 'learning';
	if (card.interval < 14) return 'familiar';
	if (card.interval < 60) return 'known';
	return 'mastered';
}

/** 0 for new cards, approaching 1 for cards reviewed months apart. */
export const strength = (card: Card | undefined) => (card ? Math.min(1, card.interval / 30) : 0);