/**
 * Detect whether the environment has Chinese language signals.
 * Used to show/hide the 中文 UI option — English-only otherwise.
 */

const ZH_TAG = /^(zh|yue|cmn)([-_]|$)/i;
export const ZH_OK_COOKIE = "cyd-zh-ok";

export function isChineseLanguageTag(tag: string | null | undefined): boolean {
  if (!tag) return false;
  const primary = tag.trim().replace("_", "-");
  return ZH_TAG.test(primary);
}

/** Parse Accept-Language / comma-separated locale list. */
export function acceptLanguageHasChinese(
  header: string | null | undefined
): boolean {
  if (!header) return false;
  return header
    .split(",")
    .map((part) => part.split(";")[0]?.trim())
    .some((tag) => isChineseLanguageTag(tag));
}

export type ChineseSignal =
  | "navigator.language"
  | "navigator.languages"
  | "intl.locale"
  | "accept-language"
  | "cookie";

export type ChineseDetection = {
  likely: boolean;
  signals: ChineseSignal[];
};

function readCookie(name: string): string | null {
  if (typeof document === "undefined") return null;
  const match = document.cookie.match(
    new RegExp(`(?:^|; )${name.replace(/[.*+?^${}()|[\]\\]/g, "\\$&")}=([^;]*)`)
  );
  return match ? decodeURIComponent(match[1]) : null;
}

function writeCookie(name: string, value: string) {
  if (typeof document === "undefined") return;
  const maxAge = 60 * 60 * 24 * 365;
  document.cookie = `${name}=${encodeURIComponent(value)}; Path=/; Max-Age=${maxAge}; SameSite=Lax`;
}

/**
 * Client-side detection (browser / OS language prefs exposed to the page).
 */
export function detectChineseClient(): ChineseDetection {
  const signals: ChineseSignal[] = [];

  if (typeof navigator !== "undefined") {
    if (isChineseLanguageTag(navigator.language)) {
      signals.push("navigator.language");
    }
    if (navigator.languages?.some((l) => isChineseLanguageTag(l))) {
      signals.push("navigator.languages");
    }
    const legacy = (navigator as Navigator & { userLanguage?: string })
      .userLanguage;
    if (isChineseLanguageTag(legacy) && !signals.includes("navigator.language")) {
      signals.push("navigator.language");
    }
  }

  try {
    const intlLocale = Intl.DateTimeFormat().resolvedOptions().locale;
    if (isChineseLanguageTag(intlLocale)) {
      signals.push("intl.locale");
    }
  } catch {
    /* ignore */
  }

  if (readCookie(ZH_OK_COOKIE) === "1") {
    signals.push("cookie");
  }

  return { likely: signals.length > 0, signals };
}

/** Persist so middleware allows /zh after client-side Chinese detection. */
export function markChineseLocaleAllowed() {
  if (typeof window === "undefined") return;
  try {
    writeCookie(ZH_OK_COOKIE, "1");
    window.localStorage.setItem(ZH_OK_COOKIE, "1");
  } catch {
    /* ignore */
  }
}