Session 12: i18n (10 languages, cookie-based), Africa tier .99, locale switcher, RTL Arabic (1305 tests)

web/src/middleware.ts

@@ -0,0 +1,86 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { LOCALES, DEFAULT_LOCALE, LOCALE_COOKIE, LOCALE_HEADER, isLocale, Locale } from '@/lib/locales';
+
+/**
+ * Locale-detection middleware (Session 12).
+ *
+ * Detection priority:
+ *   1. URL prefix (/es/scan, /fr/pricing) — reserved for a future
+ *      session that does the [locale] segment refactor. For now this
+ *      branch is unreachable because no route uses the prefix.
+ *   2. NEXT_LOCALE cookie — set by the locale switcher and persisted
+ *      across sessions.
+ *   3. Accept-Language header — best guess from the browser.
+ *   4. Default 'en'.
+ *
+ * The resolved locale lands on the `x-vyndr-locale` REQUEST header
+ * (NOT the response) so downstream server components can read it via
+ * `headers()` without parsing cookies themselves. The cookie is set
+ * on the response when the locale switcher fires (separate code
+ * path); the middleware itself doesn't write cookies.
+ *
+ * Skips: Next.js internals (`_next`), public files (anything with a
+ * dot in the path), and API routes (they don't render UI, no locale
+ * needed).
+ */
+
+function parseAcceptLanguage(header: string | null): Locale | null {
+  if (!header) return null;
+  // "en-US,en;q=0.9,es;q=0.8" → [{lang:'en-us', q:1}, {lang:'en', q:0.9}, ...]
+  const ranked = header
+    .split(',')
+    .map((chunk) => {
+      const [tag, ...params] = chunk.trim().split(';');
+      const qParam = params.find((p) => p.trim().startsWith('q='));
+      const q = qParam ? Number(qParam.split('=')[1]) : 1;
+      return { tag: tag.toLowerCase(), q: Number.isFinite(q) ? q : 1 };
+    })
+    .filter((entry) => entry.tag)
+    .sort((a, b) => b.q - a.q);
+
+  for (const entry of ranked) {
+    // Match the primary subtag (en-US → en).
+    const primary = entry.tag.split('-')[0];
+    if (isLocale(primary)) return primary;
+  }
+  return null;
+}
+
+function resolveLocale(req: NextRequest): Locale {
+  // 1. URL prefix — placeholder for the future [locale] refactor.
+  //    Check the first path segment against the locale registry.
+  const firstSegment = req.nextUrl.pathname.split('/')[1] || '';
+  if (isLocale(firstSegment)) return firstSegment;
+
+  // 2. Cookie.
+  const cookie = req.cookies.get(LOCALE_COOKIE)?.value;
+  if (isLocale(cookie)) return cookie;
+
+  // 3. Accept-Language.
+  const fromHeader = parseAcceptLanguage(req.headers.get('accept-language'));
+  if (fromHeader) return fromHeader;
+
+  // 4. Default.
+  return DEFAULT_LOCALE;
+}
+
+export function middleware(req: NextRequest) {
+  const locale = resolveLocale(req);
+  // Stamp the request header so server components can read locale
+  // via `headers().get('x-vyndr-locale')`. NextResponse.next() with
+  // request headers is the canonical pattern for this.
+  const requestHeaders = new Headers(req.headers);
+  requestHeaders.set(LOCALE_HEADER, locale);
+  return NextResponse.next({
+    request: { headers: requestHeaders },
+  });
+}
+
+// Skip Next.js internals, public files, and API routes (no UI).
+export const config = {
+  matcher: ['/((?!api|_next|.*\\..*).*)'],
+};
+
+// Re-export so tests can import without pulling the full middleware.
+export { resolveLocale, parseAcceptLanguage };
+export const __SUPPORTED_LOCALES = LOCALES;