fix: Implement content negotiation for markdown and html in middleware

apps/landing/src/middleware.ts

@@ -1,22 +1,48 @@
 import { NextResponse } from 'next/server';
 import type { NextRequest } from 'next/server';
 
-function acceptsMarkdown(accept: string): boolean {
-  for (const part of accept.split(',')) {
+type Negotiated = 'markdown' | 'html' | 'none';
+
+function parseAccept(accept: string): Array<{ type: string; q: number }> {
+  return accept.split(',').map(part => {
     const segments = part.trim().split(';');
-    if (segments[0]?.trim().toLowerCase() !== 'text/markdown') continue;
+    const type = segments[0]?.trim().toLowerCase() ?? '';
     const qParam = segments.slice(1).find(s => s.trim().startsWith('q='));
-    const q = qParam ? parseFloat((qParam.split('=')[1]) ?? '1') : 1;
-    return !isNaN(q) && q > 0;
-  }
-  return false;
+    const q = qParam ? parseFloat(qParam.split('=')[1] ?? '1') : 1;
+    return { type, q: isNaN(q) ? 1 : q };
+  });
+}
+
+function getQ(types: Array<{ type: string; q: number }>, target: string): number {
+  const exact = types.find(t => t.type === target);
+  if (exact) return exact.q;
+  const [main] = target.split('/');
+  const sub = types.find(t => t.type === `${main}/*`);
+  if (sub) return sub.q;
+  const wildcard = types.find(t => t.type === '*/*');
+  return wildcard ? wildcard.q : -1;
+}
+
+function negotiate(accept: string): Negotiated {
+  if (!accept) return 'html';
+  const types = parseAccept(accept);
+  const mdQ = getQ(types, 'text/markdown');
+  const htmlQ = getQ(types, 'text/html');
+  if (mdQ <= 0 && htmlQ <= 0) return 'none';
+  if (mdQ > 0 && mdQ >= htmlQ) return 'markdown';
+  return 'html';
 }
 
 export function middleware(request: NextRequest) {
   const accept = request.headers.get('accept') ?? '';
   const { pathname } = request.nextUrl;
+  const result = negotiate(accept);
 
-  if (acceptsMarkdown(accept)) {
+  if (result === 'none') {
+    return new NextResponse(null, { status: 406, headers: { 'Vary': 'Accept' } });
+  }
+
+  if (result === 'markdown') {
     const headers = new Headers(request.headers);
     headers.set('x-md-path', pathname);
     const response = NextResponse.rewrite(new URL('/api/md', request.url), { request: { headers } });