perf: Add SHA1 ETag caching and 304 Status response for Compare API

app/api/compare/route.test.ts

@@ -160,4 +160,48 @@ describe('GET /api/compare', () => {
 
     expect(res.status).toBe(500);
   });
+
+  // ── ETag & Caching ─────────────────────────────────────────────────────────
+
+  it('adds ETag and Cache-Control headers to 200 response', async () => {
+    const res = await GET(makeRequest('user1=alice&user2=bob'));
+    expect(res.status).toBe(200);
+    expect(res.headers.get('ETag')).toBeTruthy();
+    expect(res.headers.get('Cache-Control')).toBe('public, s-maxage=3600');
+  });
+
+  it('returns 304 Not Modified when If-None-Match matches weak ETag', async () => {
+    const res1 = await GET(makeRequest('user1=alice&user2=bob'));
+    const etag = res1.headers.get('ETag');
+    expect(etag).toBeTruthy();
+
+    const requestWithEtag = new Request('http://localhost:3000/api/compare?user1=alice&user2=bob', {
+      headers: {
+        'if-none-match': etag!,
+      },
+    });
+
+    const res2 = await GET(requestWithEtag);
+    expect(res2.status).toBe(304);
+    expect(res2.body).toBeNull();
+    expect(res2.headers.get('ETag')).toBe(etag);
+    expect(res2.headers.get('Cache-Control')).toBe('public, s-maxage=3600');
+  });
+
+  it('returns 304 Not Modified when If-None-Match matches strong ETag', async () => {
+    const res1 = await GET(makeRequest('user1=alice&user2=bob'));
+    const etag = res1.headers.get('ETag');
+    expect(etag).toBeTruthy();
+
+    const strongEtag = etag!.replace(/^W\//, '');
+
+    const requestWithEtag = new Request('http://localhost:3000/api/compare?user1=alice&user2=bob', {
+      headers: {
+        'if-none-match': strongEtag,
+      },
+    });
+
+    const res2 = await GET(requestWithEtag);
+    expect(res2.status).toBe(304);
+  });
 });

app/api/compare/route.ts

@@ -1,6 +1,7 @@
 import { NextResponse } from 'next/server';
 import { getFullDashboardData } from '@/lib/github';
 import { compareParamsSchema } from '@/lib/validations';
+import crypto from 'crypto';
 
 export const revalidate = 3600;
 
@@ -77,10 +78,36 @@ export async function GET(request: Request) {
       return buildCompareFetchErrorResponse(user2, result2.reason);
     }
 
-    return NextResponse.json({
+    const jsonPayload = JSON.stringify({
       user1: result1.value,
       user2: result2.value,
     });
+
+    const etag = crypto.createHash('sha1').update(jsonPayload).digest('hex');
+    const weakEtag = `W/"${etag}"`;
+    const ifNoneMatch = request.headers.get('if-none-match');
+    const cacheControl = 'public, s-maxage=3600';
+
+    if (ifNoneMatch) {
+      const etags = ifNoneMatch.split(',').map((e) => e.trim());
+      if (etags.includes(weakEtag) || etags.includes(`"${etag}"`)) {
+        return new NextResponse(null, {
+          status: 304,
+          headers: {
+            'Cache-Control': cacheControl,
+            ETag: weakEtag,
+          },
+        });
+      }
+    }
+
+    return new NextResponse(jsonPayload, {
+      headers: {
+        'Content-Type': 'application/json',
+        'Cache-Control': cacheControl,
+        ETag: weakEtag,
+      },
+    });
   } catch (error) {
     const message = error instanceof Error ? error.message : 'Internal server error';
     return NextResponse.json({ error: message }, { status: 500 });