export type Lang = 'zh' | 'en' | 'ja' | 'fr';

export const LANGS: Lang[] = ['zh', 'en', 'ja', 'fr'];

export const DEFAULT_LANG: Lang = 'zh';

export const htmlLang: Record<Lang, string> = {
  zh: 'zh-CN',
  en: 'en',
  ja: 'ja',
  fr: 'fr',
};

export const langLabels: Record<Lang, string> = {
  zh: '中文',
  en: 'English',
  ja: '日本語',
  fr: 'Français',
};

export function isLang(value: string | undefined): value is Lang {
  return value !== undefined && (LANGS as string[]).includes(value);
}

/** 站内路径（不含语言前缀），首页为空字符串 */
export type RouteKey = '' | 'teaching-philosophy' | 'curriculum' | 'faculty' | 'about';

export function hrefFor(lang: Lang, segment: RouteKey): string {
  const prefix = lang === DEFAULT_LANG ? '' : `/${lang}`;
  if (!segment) return prefix || '/';
  return `${prefix}/${segment}`;
}

/** 从 [...lang] rest 参数解析语言，undefined 时返回默认语言 */
export function resolveLang(param: string | undefined): Lang {
  return isLang(param) ? param : DEFAULT_LANG;
}