import { formatLanguageString } from "./format-language-string.js";
import { parse, pick } from "./parser.js";

export type Locales = string | string[] | undefined;

/**
 * Get the client's locales from the Accept-Language header.
 * If the header is not defined returns null.
 * If the header is defined return an array of locales, sorted by the quality
 * value.
 *
 * @example
 * export let loader: LoaderFunction = async ({ request }) => {
 *   let locales = getClientLocales(request)
 *   let date = new Date().toLocaleDateString(locales, {
 *     "day": "numeric",
 *   });
 *   return json({ date })
 * }
 */
export function getClientLocales(headers: Headers): Locales;
export function getClientLocales(request: Request): Locales;
export function getClientLocales(requestOrHeaders: Request | Headers): Locales {
	let headers = getHeaders(requestOrHeaders);

	let acceptLanguage = headers.get("Accept-Language");

	// if the header is not defined, return undefined
	if (!acceptLanguage) return undefined;

	let parsedLocales = parse(acceptLanguage)
		.filter((lang) => lang.code !== "*")
		.map(formatLanguageString);

	let validLocales: string[] = [];

	for (let locale of parsedLocales) {
		try {
			// This will throw on invalid locales
			new Intl.Locale(locale);

			// If we get here, the locale is valid
			validLocales.push(locale);
		} catch {
			// We want to ignore errors here
		}
	}

	let locale = pick(Intl.DateTimeFormat.supportedLocalesOf(validLocales), acceptLanguage);

	return locale ?? undefined;
}

/**
 * Receives a Request or Headers objects.
 * If it's a Request returns the request.headers
 * If it's a Headers returns the object directly.
 */
function getHeaders(requestOrHeaders: Request | Headers): Headers {
	if (requestOrHeaders instanceof Request) return requestOrHeaders.headers;
	return requestOrHeaders;
}