feat: implement client-side caching for GitHub API responses (closes #33)

Add in-memory cache with 1-day TTL to reduce GitHub API calls.
Cache keyed by username (case-insensitive) to avoid duplicate requests.
Extracted reusable InMemoryCache class in lib/cache.ts with configurable TTL constants.

Author: Ai-chan-0411

lib/cache.ts

@@ -0,0 +1,46 @@
+interface CacheEntry<T> {
+  data: T;
+  expiresAt: number;
+}
+
+class InMemoryCache {
+  private store = new Map<string, CacheEntry<unknown>>();
+
+  set<T>(key: string, data: T, ttlMs: number): void {
+    this.store.set(key, {
+      data,
+      expiresAt: Date.now() + ttlMs,
+    });
+  }
+
+  get<T>(key: string): T | null {
+    const entry = this.store.get(key) as CacheEntry<T> | undefined;
+    if (!entry) return null;
+    if (Date.now() > entry.expiresAt) {
+      this.store.delete(key);
+      return null;
+    }
+    return entry.data;
+  }
+
+  has(key: string): boolean {
+    return this.get(key) !== null;
+  }
+
+  delete(key: string): void {
+    this.store.delete(key);
+  }
+
+  clear(): void {
+    this.store.clear();
+  }
+}
+
+// Singleton cache instance shared across API requests
+export const cache = new InMemoryCache();
+
+export const CACHE_TTL = {
+  ONE_DAY: 24 * 60 * 60 * 1000,
+  ONE_HOUR: 60 * 60 * 1000,
+  FIVE_MINUTES: 5 * 60 * 1000,
+};

lib/github.ts

@@ -1,5 +1,6 @@
 import { ContributionTotals, GitHubUserData, PullRequestNode, RepoNode } from "@/types/github";
 import { graphql } from "@octokit/graphql";
+import { cache, CACHE_TTL } from "./cache";
 
 if (!process.env.GITHUB_TOKEN) {
   throw new Error("Missing GITHUB_TOKEN");
@@ -60,15 +61,26 @@ const QUERY = /* GraphQL */ `
 export async function fetchGitHubUserData(
   username: string
 ): Promise<GitHubUserData> {
+  const cacheKey = `github:user:${username.toLowerCase()}`;
+
+  const cached = cache.get<GitHubUserData>(cacheKey);
+  if (cached) {
+    return cached;
+  }
+
   const { user } = await client<{ user: any }>(QUERY, { login: username });
 
   if (!user) {
     throw new Error("User not found");
   }
 
-  return {
+  const data: GitHubUserData = {
     repos: user.repositories.nodes as RepoNode[],
     pullRequests: user.pullRequests.nodes as PullRequestNode[],
     contributions: user.contributionsCollection as ContributionTotals,
   };
+
+  cache.set(cacheKey, data, CACHE_TTL.ONE_DAY);
+
+  return data;
 }