/**
 * Like Promise.all but returns [result, error] tuples (never throws).
 */
export async function allSettledTuples(promises) {
  const results = await Promise.allSettled(promises);
  return results.map(r =>
    r.status === 'fulfilled' ? [r.value, null] : [null, r.reason]
  );
}

/**
 * Retry an async function up to `times` times with exponential back-off.
 */
export async function retry(fn, { times = 3, baseDelay = 200 } = {}) {
  let lastErr;
  for (let i = 0; i < times; i++) {
    try { return await fn(); }
    catch (e) {
      lastErr = e;
      if (i < times - 1) await sleep(baseDelay * 2 ** i);
    }
  }
  throw lastErr;
}

/**
 * Run `fn` on items in chunks of `concurrency`.
 */
export async function pMap(items, fn, { concurrency = 5 } = {}) {
  const results = [];
  for (let i = 0; i < items.length; i += concurrency) {
    const chunk = items.slice(i, i + concurrency);
    results.push(...await Promise.all(chunk.map(fn)));
  }
  return results;
}

const sleep = (ms) => new Promise(r => setTimeout(r, ms));