copperhead

export interface RetryOpts {
  retries?: number;
  baseMs?: number;
  isRetryable?: (err: unknown) => boolean;
  onRetry?: (attempt: number, err: unknown) => void;
  sleep?: (ms: number) => Promise<void>;
}

export function isRateLimit(err: unknown): boolean {
  const status = (err as { status?: number; statusCode?: number })?.status
    ?? (err as { statusCode?: number })?.statusCode;
  return status === 429;
}

/** Exponential backoff ×N for rate limits (SPEC §4.5). */
export async function withRetry<T>(fn: () => Promise<T>, opts: RetryOpts = {}): Promise<T> {
  const retries = opts.retries ?? 3;
  const baseMs = opts.baseMs ?? 1000;
  const isRetryable = opts.isRetryable ?? isRateLimit;
  const sleep = opts.sleep ?? ((ms) => new Promise((r) => setTimeout(r, ms)));
  let lastErr: unknown;
  for (let attempt = 0; attempt <= retries; attempt++) {
    try {
      return await fn();
    } catch (err) {
      lastErr = err;
      if (!isRetryable(err) || attempt === retries) throw err;
      opts.onRetry?.(attempt + 1, err);
      await sleep(baseMs * 2 ** attempt);
    }
  }
  throw lastErr;
}