/**
 * Limits how often something may happen, with token buckets kept in memory,
 * which a restart empties. Logins, password reset requests and the chat's
 * typing messages use it.
 *
 * @module
 */

interface Bucket {
  tokens: number;
  at: number;
}
const buckets = new Map<string, Bucket>();

/**
 * Passed to {@link takeToken} by each limited action, which declares its own
 * as a constant.
 */
export interface RateLimit {
  /** How many tokens a full bucket holds, and so the largest burst. */
  capacity: number;
  /** Tokens regained per second. */
  refill: number;
}

/**
 * Takes one token from `key`'s bucket, or returns false when it is empty. The
 * caller picks the key, such as `typing:<userId>`, and keys by user or account
 * so that more tabs or parallel requests don't buy more tokens.
 */
export function takeToken(key: string, limit: RateLimit): boolean {
  const now = Date.now();
  const bucket = buckets.get(key) ?? { tokens: limit.capacity, at: now };
  bucket.tokens = Math.min(
    limit.capacity,
    bucket.tokens + ((now - bucket.at) / 1000) * limit.refill,
  );
  bucket.at = now;
  buckets.set(key, bucket);
  if (bucket.tokens < 1) return false;
  bucket.tokens -= 1;
  return true;
}

// Every limit in use refills within the hour, so an older bucket is full and
// dropping it changes nothing.
setInterval(() => {
  const cutoff = Date.now() - 60 * 60_000;
  for (const [key, bucket] of buckets) {
    if (bucket.at < cutoff) buckets.delete(key);
  }
}, 60_000);