// Cache for Set conversions to avoid repeated array->Set conversions
const availableLocalesSetCache = new WeakMap();
/**
* https://tc39.es/ecma402/#sec-bestavailablelocale
* @param availableLocales
* @param locale
*/
export function BestAvailableLocale(availableLocales, locale) {
	// Fast path: use Set for O(1) lookups instead of O(n) indexOf
	let availableSet = availableLocalesSetCache.get(availableLocales);
	if (!availableSet) {
		availableSet = new Set(availableLocales);
		availableLocalesSetCache.set(availableLocales, availableSet);
	}
	let candidate = locale;
	while (true) {
		if (availableSet.has(candidate)) {
			return candidate;
		}
		let pos = candidate.lastIndexOf("-");
		if (!~pos) {
			return undefined;
		}
		if (pos >= 2 && candidate[pos - 2] === "-") {
			pos -= 2;
		}
		candidate = candidate.slice(0, pos);
	}
}