import type { StarlightConfig } from '../../types';
import { BuiltInDefaultLocale } from '../../utils/i18n';

/**
 * Get the BCP-47 language tag for the given locale.
 * @param locale Locale string or `undefined` for the root locale.
 */
export function localeToLang(
	config: Pick<StarlightConfig, 'defaultLocale' | 'locales'>,
	locale: string | undefined
): string {
	const lang = locale ? config.locales?.[locale]?.lang : config.locales?.root?.lang;
	const defaultLang = config.defaultLocale?.lang || config.defaultLocale?.locale;
	return lang || defaultLang || BuiltInDefaultLocale.lang;
}