navigator.language          // "zh-TW"
navigator.languages         // ["zh-TW", "en-US", "ja"](優先順序)

language[-script][-region]

zh-TW       中文-臺灣
zh-Hans-CN  中文-簡體-中國
en-US       英文-美國
ja          日文(無地區)

function getInitialLocale(): string {
  if (typeof window === 'undefined') return 'zh-TW';   // SSR fallback

  const browserLang = navigator.language;

  // 對應到我們支援的 locale
  if (browserLang.startsWith('zh')) return 'zh-TW';
  if (browserLang.startsWith('ja')) return 'ja';
  return 'en';
}

function getLocation(): Promise<GeolocationPosition> {
  return new Promise((resolve, reject) => {
    if (!navigator.geolocation) {
      reject(new Error('Geolocation not supported'));
      return;
    }

    navigator.geolocation.getCurrentPosition(
      resolve,
      reject,
      {
        enableHighAccuracy: true,
        timeout: 5000,
        maximumAge: 0,
      },
    );
  });
}

// 使用
try {
  const pos = await getLocation();
  console.log(`Lat: ${pos.coords.latitude}, Lng: ${pos.coords.longitude}`);
  // 接下來呼叫 reverse geocoding API 把座標轉成國家
} catch (err) {
  console.warn('User denied or error:', err);
}

async function getLocationWithPermission() {
  const result = await navigator.permissions.query({ name: 'geolocation' });

  switch (result.state) {
    case 'granted':
      return await getLocation();        // 已授權
    case 'prompt':
      return await getLocation();        // 觸發授權彈窗
    case 'denied':
      throw new Error('Permission denied');
  }
}

async function coordsToCountry(lat: number, lng: number): Promise<string> {
  // Nominatim(OpenStreetMap)免費
  const res = await fetch(
    `https://nominatim.openstreetmap.org/reverse?format=json&lat=${lat}&lon=${lng}`
  );
  const data = await res.json();
  return data.address?.country_code;     // "tw" / "us" / "jp"
}

async function getCountryByIP(): Promise<string> {
  const res = await fetch('https://ipapi.co/json/');
  const data = await res.json();
  return data.country_code;       // "TW" / "US"
}

// app/api/geo/route.ts(Next.js)
export async function GET(req: Request) {
  const ip = req.headers.get('x-forwarded-for') ?? '';
  const res = await fetch(`https://ipapi.co/${ip}/json/`);
  const data = await res.json();
  return Response.json({ country: data.country_code });
}

// Next.js middleware.ts
export function middleware(request: NextRequest) {
  const accept = request.headers.get('accept-language') ?? '';
  const lang = accept.split(',')[0].trim();      // "zh-TW"
  const country = lang.split('-')[1] ?? 'TW';
  // ... 用這個資料 redirect / 設 cookie
}

export function middleware(request: NextRequest) {
  const country = request.headers.get('cf-ipcountry');   // "TW"
  // 完全免費,不用打第三方 API
}

import { geolocation } from '@vercel/functions';

export function GET(req: Request) {
  const { country, city } = geolocation(req);
  return Response.json({ country, city });
}

// middleware.ts(Next.js Edge)
import { NextRequest, NextResponse } from 'next/server';

const SUPPORTED_LOCALES = ['zh-TW', 'en', 'ja'];

export function middleware(request: NextRequest) {
  const pathname = request.nextUrl.pathname;

  // 已經有 locale 前綴就不動
  if (SUPPORTED_LOCALES.some((l) => pathname.startsWith(`/${l}`))) {
    return NextResponse.next();
  }

  // 用 Cloudflare(若有)或 Accept-Language 偵測
  const country = request.headers.get('cf-ipcountry');
  const accept = request.headers.get('accept-language') ?? '';

  let locale = 'en';   // 預設
  if (country === 'TW' || country === 'HK') locale = 'zh-TW';
  else if (country === 'JP') locale = 'ja';
  else if (accept.includes('zh')) locale = 'zh-TW';
  else if (accept.includes('ja')) locale = 'ja';

  // 重導
  const url = request.nextUrl.clone();
  url.pathname = `/${locale}${pathname}`;
  return NextResponse.redirect(url);
}

export const config = {
  matcher: ['/((?!api|_next|.*\\..*).*)'],
};