refactoring

Resolve #7779

packages/backend/src/misc/cache.ts

@@ -0,0 +1,43 @@
+export class Cache<T> {
+	private cache: Map<string | null, { date: number; value: T; }>;
+	private lifetime: number;
+
+	constructor(lifetime: Cache<never>['lifetime']) {
+		this.cache = new Map();
+		this.lifetime = lifetime;
+	}
+
+	public set(key: string | null, value: T): void {
+		this.cache.set(key, {
+			date: Date.now(),
+			value
+		});
+	}
+
+	public get(key: string | null): T | undefined {
+		const cached = this.cache.get(key);
+		if (cached == null) return undefined;
+		if ((Date.now() - cached.date) > this.lifetime) {
+			this.cache.delete(key);
+			return undefined;
+		}
+		return cached.value;
+	}
+
+	public delete(key: string | null) {
+		this.cache.delete(key);
+	}
+
+	public async fetch(key: string | null, fetcher: () => Promise<T>): Promise<T> {
+		const cachedValue = this.get(key);
+		if (cachedValue !== undefined) {
+			// Cache HIT
+			return cachedValue;
+		}
+
+		// Cache MISS
+		const value = await fetcher();
+		this.set(key, value);
+		return value;
+	}
+}