/**
 * CLIENT-SAFE half of region resolution. Nothing here may reach
 * `next/headers`, because `<StoreProvider>` and the region switcher import it.
 * The server half lives in `region.server.ts`.
 *
 * ## Why regions exist even when the merchant has never heard of them
 *
 * A store can split the world into regions, each with its own currency and
 * payment providers. Every other optional feature in this project auto-hides
 * when it is off, so forgetting one costs nothing until the merchant switches
 * it on. Regions are the exception: a storefront that never sends `regionId`
 * does not hide anything, it silently renders the DEFAULT region's prices to
 * every shopper on earth. The page looks perfect and the prices are wrong.
 *
 * A single-region store is a complete no-op. `getStoreRegions()` returns one
 * row, `regions.length <= 1`, the switcher renders nothing, and the `regionId`
 * we attach to reads resolves to the default the backend would have used
 * anyway. There is no empty dropdown and no extra chrome. The whole point is
 * that the day a merchant adds a second region, this storefront is already
 * correct instead of quietly overcharging half its customers.
 */

import type { PublicRegion } from 'brainerce';

/**
 * Manual region choice, set by the switcher. A cookie rather than React state
 * on purpose: the server renders product pages and has to pick the same region
 * the shopper chose, or SSR paints the geo-guessed prices and the client flips
 * them a moment later.
 *
 * Not `httpOnly`, because the switcher is a client component and has to write
 * it. It holds a region id, which is public data already present in the page.
 */
export const REGION_COOKIE = 'brainerce_region';

/** One year. A region preference is not session state; it is where you live. */
const REGION_COOKIE_MAX_AGE = 60 * 60 * 24 * 365;

/**
 * Request headers that carry a buyer country, in priority order.
 *
 * Brainerce cannot derive this itself: the storefront SERVER is what reaches
 * the Brainerce API, so the backend sees this machine's IP, not the shopper's.
 * The country has to be extracted at the edge and forwarded, which is exactly
 * what these headers are.
 *
 * `x-country` is last and is the manual escape hatch: a host with no geo
 * header of its own can be made to set it in a proxy rule.
 */
export const COUNTRY_HEADERS = [
  'cf-ipcountry', // Cloudflare
  'x-vercel-ip-country', // Vercel
  'client-geo-country', // Fastly
  'x-country', // manual / self-hosted proxies
] as const;

/**
 * Accept only a plausible ISO-3166-1 alpha-2 code. Cloudflare sends `XX` for
 * unknown clients and `T1` for Tor, and both would otherwise be forwarded to
 * the API as if they were countries.
 */
export function normalizeCountry(raw: string | null | undefined): string | null {
  if (!raw) return null;
  const code = raw.trim().toUpperCase();
  if (!/^[A-Z]{2}$/.test(code)) return null;
  if (code === 'XX' || code === 'T1') return null;
  return code;
}

/**
 * Pick the region for a country from an already-fetched list. Mirrors the
 * SDK's `client.detectRegion(country, regions)` without needing a client
 * instance, and keeps the return type narrowed to `PublicRegion`.
 *
 * Order: explicit country match → the default region → the first row. The last
 * two fallbacks are what make a misconfigured store render prices instead of
 * nothing.
 */
export function pickRegion(
  regions: PublicRegion[],
  country: string | null | undefined
): PublicRegion | null {
  if (regions.length === 0) return null;
  const code = normalizeCountry(country);
  if (code) {
    const matched = regions.find((r) => r.countries.includes(code));
    if (matched) return matched;
  }
  return regions.find((r) => r.isDefault) ?? regions[0] ?? null;
}

/** Read the manual region choice in the browser. Returns null during SSR. */
export function readRegionCookie(): string | null {
  if (typeof document === 'undefined') return null;
  const match = document.cookie.match(
    new RegExp(`(?:^|;\\s*)${REGION_COOKIE}=([^;]*)`)
  );
  return match ? decodeURIComponent(match[1]) : null;
}

/**
 * Persist the manual region choice. `SameSite=Lax` so a link from an email or
 * an ad still arrives with the shopper's chosen currency; `Secure` only off
 * localhost, where there is no https to attach it to.
 */
export function writeRegionCookie(regionId: string): void {
  if (typeof document === 'undefined') return;
  const secure = typeof location !== 'undefined' && location.protocol === 'https:';
  document.cookie =
    `${REGION_COOKIE}=${encodeURIComponent(regionId)}; path=/; max-age=${REGION_COOKIE_MAX_AGE}; SameSite=Lax` +
    (secure ? '; Secure' : '');
}