// Bounded async fan-out.
//
// The corpus is big enough that an unbounded `Promise.all` over a per-video or
// per-channel list is a file-descriptor bomb, not a speed-up: the largest
// channel here holds 11,224 video dirs, and opening all of them at once starves
// every other reader in the process. Every fan-out over corpus-shaped work goes
// through this instead.
//
// Order-preserving: `out[i]` is always the result for `items[i]`, regardless of
// completion order. Rejection semantics match `Promise.all` — the first
// rejection wins and already-started work keeps running to completion.
export async function mapConcurrent<T, R>(
  items: readonly T[],
  limit: number,
  fn: (item: T, index: number) => Promise<R>,
): Promise<R[]> {
  const out = new Array<R>(items.length);
  if (items.length === 0) return out;
  const width = Math.max(1, Math.min(Math.floor(limit), items.length));
  let next = 0;
  const worker = async (): Promise<void> => {
    for (;;) {
      const i = next++;
      if (i >= items.length) return;
      out[i] = await fn(items[i], i);
    }
  };
  await Promise.all(Array.from({ length: width }, worker));
  return out;
}