// Pure routine metrics shared by server and UI; ISO timestamps, UTC calendar.

const UTC = 'UTC';

/** Parse an ISO stamp, falling back when it is not a valid instant. */
function parseInstant(
	value: string,
	fallback: Temporal.Instant,
): Temporal.Instant {
	try {
		return Temporal.Instant.from(value);
	} catch {
		return fallback;
	}
}

/** Millisecond-precision ISO stamp, byte-identical to Date.toISOString(). */
function iso(instant: Temporal.Instant): string {
	return instant.toString({ smallestUnit: 'millisecond' });
}

/** Whole days as an exact millisecond count. */
function periodMs(days: number): number {
	return Temporal.Duration.from({ days }).total({ unit: 'millisecond' });
}

function maxInstant(instants: Temporal.Instant[]): Temporal.Instant | null {
	if (instants.length === 0) return null;
	return instants.reduce((a, b) =>
		Temporal.Instant.compare(a, b) >= 0 ? a : b,
	);
}

function parseCompletions(
	routineId: number,
	completions: CompletionInput[],
): Temporal.Instant[] {
	const mine: Temporal.Instant[] = [];
	for (const c of completions) {
		if (c.routineId !== routineId) continue;
		try {
			mine.push(Temporal.Instant.from(c.doneAt));
		} catch {
			// Skip corrupt stamps instead of poisoning the anchor.
		}
	}
	return mine.sort(Temporal.Instant.compare);
}

export type RoutineStatus = 'ok' | 'due' | 'late' | 'veryLate';

export interface RoutineInput {
	id: number;
	version: number;
	name: string;
	frequencyDays: number;
	createdAt: string;
}

export interface CompletionInput {
	routineId: number;
	doneAt: string;
}

export interface RoutineState {
	id: number;
	version: number;
	name: string;
	frequencyDays: number;
	lastDoneAt: string | null;
	/** When the routine should next be done. */
	dueAt: string;
	/** Days past dueAt (0 if not yet due). */
	overdueDays: number;
	/** Time past due, in units of the routine's own frequency (0 if not due). */
	overdueRatio: number;
	status: RoutineStatus;
	/** 100 on schedule, 50 one period overdue, 0 at two periods. */
	health: number;
}

export function computeRoutineState(
	routine: RoutineInput,
	completions: CompletionInput[],
	now: Temporal.Instant,
): RoutineState {
	const lastDone = maxInstant(parseCompletions(routine.id, completions));
	const reference = lastDone ?? parseInstant(routine.createdAt, now);
	const period = periodMs(routine.frequencyDays);
	// Calendar-day anchor: the period starts at the reference day's midnight.
	const due = reference
		.toZonedDateTimeISO(UTC)
		.startOfDay()
		.add({ days: lastDone === null ? 0 : routine.frequencyDays })
		.toInstant();
	const rawRatio = (now.epochMilliseconds - due.epochMilliseconds) / period;
	const overdueRatio = Math.max(0, rawRatio);
	const status: RoutineStatus =
		rawRatio < 0 && lastDone !== null
			? 'ok'
			: overdueRatio < 1
				? 'due'
				: overdueRatio < 2
					? 'late'
					: 'veryLate';

	return {
		id: routine.id,
		version: routine.version,
		name: routine.name,
		frequencyDays: routine.frequencyDays,
		lastDoneAt: lastDone !== null ? iso(lastDone) : null,
		dueAt: iso(due),
		overdueDays: Math.floor(overdueRatio * routine.frequencyDays),
		overdueRatio,
		status,
		health: Math.max(0, Math.round(100 - overdueRatio * 50)),
	};
}

/** Average health across routines; 100 when there is nothing to do. */
export function computeHealthScore(states: { health: number }[]): number {
	if (states.length === 0) return 100;
	const total = states.reduce((sum, s) => sum + s.health, 0);
	return Math.round(total / states.length);
}

export interface HealthPoint {
	/** Epoch ms of the sample. */
	at: number;
	/** Health (0-100) at that instant. */
	health: number;
}

/** Health at t given the reference anchor (creation or last completion). */
function healthAtPoint(
	reference: Temporal.Instant,
	frequencyDays: number,
	t: Temporal.Instant,
	neverDone = false,
): number {
	const due = reference
		.toZonedDateTimeISO(UTC)
		.startOfDay()
		.add({ days: neverDone ? 0 : frequencyDays })
		.toInstant();
	const rawRatio =
		(t.epochMilliseconds - due.epochMilliseconds) / periodMs(frequencyDays);
	return Math.max(0, Math.round(100 - Math.max(0, rawRatio) * 50));
}

/** Daily health samples: 100 at completion, decaying while overdue. */
export function computeHealthSeries(
	routine: RoutineInput,
	completions: CompletionInput[],
	now: Temporal.Instant,
): HealthPoint[] {
	const start = parseInstant(routine.createdAt, now);
	const doneAt = parseCompletions(routine.id, completions);

	const points: HealthPoint[] = [];
	if (Temporal.Instant.compare(now, start) <= 0) {
		return [{ at: now.epochMilliseconds, health: 100 }];
	}

	// Reference anchor: latest completion at-or-before the sampled instant.
	let reference = start;
	let nextCompletion = 0;
	const sampleAt = (t: Temporal.Instant): number => {
		while (
			nextCompletion < doneAt.length &&
			Temporal.Instant.compare(doneAt[nextCompletion], t) <= 0
		) {
			reference = doneAt[nextCompletion];
			nextCompletion++;
		}
		return healthAtPoint(
			reference,
			routine.frequencyDays,
			t,
			nextCompletion === 0,
		);
	};

	for (
		let t = start;
		Temporal.Instant.compare(t, now) <= 0;
		t = t.toZonedDateTimeISO(UTC).add({ days: 1 }).toInstant()
	) {
		points.push({ at: t.epochMilliseconds, health: sampleAt(t) });
	}

	// Final unaligned sample so the curve ends at the present instant.
	if (points[points.length - 1].at < now.epochMilliseconds) {
		points.push({ at: now.epochMilliseconds, health: sampleAt(now) });
	}
	return points;
}

/** Project health: daily mean of alive routines' health, same sawtooth. */
export function computeProjectHealthSeries(
	routines: RoutineInput[],
	completions: CompletionInput[],
	now: Temporal.Instant,
): HealthPoint[] {
	if (routines.length === 0) return [];
	const ids = new Set(routines.map((r) => r.id));
	const completionsByRoutine = new Map<number, Temporal.Instant[]>();
	for (const c of completions) {
		if (!ids.has(c.routineId)) continue;
		try {
			const t = Temporal.Instant.from(c.doneAt);
			completionsByRoutine.set(c.routineId, [
				...(completionsByRoutine.get(c.routineId) ?? []),
				t,
			]);
		} catch {
			// Skip corrupt stamps instead of poisoning the anchor.
		}
	}
	for (const times of completionsByRoutine.values())
		times.sort(Temporal.Instant.compare);

	const createdOrNow = (r: RoutineInput): Temporal.Instant =>
		parseInstant(r.createdAt, now);
	// Mean per-routine health across the routines alive at t.
	const sampleAt = (t: Temporal.Instant): number | null => {
		const alive = routines.filter((r) => {
			try {
				return (
					Temporal.Instant.compare(Temporal.Instant.from(r.createdAt), t) <= 0
				);
			} catch {
				return false;
			}
		});
		if (alive.length === 0) return null;
		const total = alive.reduce((sum, r) => {
			const doneAt = (completionsByRoutine.get(r.id) ?? []).filter(
				(d) => Temporal.Instant.compare(d, t) <= 0,
			);
			const anchor =
				doneAt.length > 0 ? (maxInstant(doneAt) ?? now) : createdOrNow(r);
			return (
				sum + healthAtPoint(anchor, r.frequencyDays, t, doneAt.length === 0)
			);
		}, 0);
		return Math.round(total / alive.length);
	};

	const start = routines
		.map(createdOrNow)
		.reduce((a, b) => (Temporal.Instant.compare(a, b) <= 0 ? a : b));
	if (Temporal.Instant.compare(now, start) <= 0)
		return [{ at: now.epochMilliseconds, health: 100 }];

	const points: HealthPoint[] = [];
	for (
		let t = start;
		Temporal.Instant.compare(t, now) <= 0;
		t = t.toZonedDateTimeISO(UTC).add({ days: 1 }).toInstant()
	) {
		const health = sampleAt(t);
		if (health !== null) points.push({ at: t.epochMilliseconds, health });
	}
	if (points.length === 0) return [{ at: now.epochMilliseconds, health: 100 }];

	// Final unaligned sample so the curve ends at the present instant.
	if (points[points.length - 1].at < now.epochMilliseconds) {
		points.push({ at: now.epochMilliseconds, health: sampleAt(now) ?? 100 });
	}
	return points;
}

export function formatFrequency(days: number): string {
	if (days === 1) return 'chaque jour';
	if (days === 7) return 'chaque semaine';
	if (days === 30) return 'chaque mois';
	if (days === 365) return 'chaque année';
	return `tous les ${days} jours`;
}

const UNIT_DAYS: Record<string, number> = { d: 1, w: 7, m: 30, y: 365 };

/** Parses "1d | 2w | 3m | 1y" into days; returns null when invalid. */
export function parseIntervalToDays(input: string): number | null {
	const match = input
		.trim()
		.toLowerCase()
		.match(/^(\d+)([dwmy])$/);
	if (!match) return null;
	const days = Number(match[1]) * UNIT_DAYS[match[2]];
	return days >= 1 ? days : null;
}

export const STATUS_LABELS: Record<RoutineStatus, string> = {
	ok: 'À jour',
	due: 'À faire',
	late: 'En retard',
	veryLate: 'Très en retard',
};