const ISO_DATE_RE = /^\d{4}-\d{2}-\d{2}$/;

export function isValidIsoDateYmd(value) {
	if (typeof value !== "string") return false;
	const s = value.trim();

	if (!ISO_DATE_RE.test(s)) return false;

	const [y, m, d] = s.split("-").map((x) => Number(x));

	if (!Number.isInteger(y) || !Number.isInteger(m) || !Number.isInteger(d)) {
		return false;
	}

	// Keep it predictable (same policy as backend):
	// - month: 1..12
	// - day: 1..31
	// We intentionally do NOT validate month-length precisely (e.g. Feb 30),
	// because the backend currently behaves the same way.
	if (m < 1 || m > 12) return false;
	if (d < 1 || d > 31) return false;

	return true;
}

export function normalizeIsoDateYmdOrNull(value) {
	if (typeof value !== "string") return null;

	const s = value.trim();
	if (!s) return null;

	return isValidIsoDateYmd(s) ? s : null;
}

/**
 * Compare ISO dates (YYYY-MM-DD).
 *
 * Lexicographic compare is correct for this format.
 *
 * @param {string} a
 * @param {string} b
 * @returns {number} -1 | 0 | 1
 */
export function compareIsoDatesYmd(a, b) {
	const aa = String(a || "");
	const bb = String(b || "");

	if (aa === bb) return 0;
	return aa < bb ? -1 : 1;
}

/**
 * Returns true when both dates exist and the range is invalid (from > to).
 *
 * IMPORTANT:
 * - from === to is valid and represents a single day.
 */
export function isInvalidIsoDateRange(from, to) {
	const f = normalizeIsoDateYmdOrNull(from);
	const t = normalizeIsoDateYmdOrNull(to);

	if (!f || !t) return false;
	return f > t;
}

/**
 * Format ISO date (YYYY-MM-DD) as German UI date: DD.MM.YYYY
 *
 * @param {string|null} ymd
 * @returns {string|null}
 */
export function formatIsoDateDe(ymd) {
	const s = normalizeIsoDateYmdOrNull(ymd);
	if (!s) return null;

	const [y, m, d] = s.split("-");
	return `${d}.${m}.${y}`;
}

/**
 * Build a compact German label for the active date filter.
 *
 * Rules:
 * - from + to:
 *   - same day => "DD.MM.YYYY"
 *   - range    => "DD.MM.YYYY – DD.MM.YYYY"
 * - only from => "ab DD.MM.YYYY"
 * - only to   => "bis DD.MM.YYYY"
 * - none      => null
 *
 * @param {{ from?: string|null, to?: string|null }} input
 * @returns {string|null}
 */
export function formatIsoDateRangeLabelDe({ from = null, to = null } = {}) {
	const f = formatIsoDateDe(from);
	const t = formatIsoDateDe(to);

	if (f && t) {
		// Single day
		if (normalizeIsoDateYmdOrNull(from) === normalizeIsoDateYmdOrNull(to)) {
			return f;
		}
		return `${f} – ${t}`;
	}

	if (f) return `ab ${f}`;
	if (t) return `bis ${t}`;

	return null;
}