Begin Hnefatafl from the game kit

server/src/ratelimit.ts

@@ -0,0 +1,31 @@
+// A sliding-window count per key, for the doors a stranger can knock on.
+
+export class RateLimit {
+	private hits = new Map<string, number[]>();
+
+	constructor(
+		private limit: number,
+		private windowMs: number
+	) {}
+
+	/** True if the key may act now; the act is recorded when it is allowed. */
+	allow(key: string, now = Date.now()): boolean {
+		const since = now - this.windowMs;
+		const recent = (this.hits.get(key) ?? []).filter((t) => t > since);
+		if (recent.length >= this.limit) {
+			this.hits.set(key, recent);
+			return false;
+		}
+		recent.push(now);
+		this.hits.set(key, recent);
+		if (this.hits.size > 10000) this.prune(now);
+		return true;
+	}
+
+	prune(now = Date.now()): void {
+		const since = now - this.windowMs;
+		for (const [key, times] of this.hits) {
+			if (!times.some((t) => t > since)) this.hits.delete(key);
+		}
+	}
+}