import type { Paths } from "../lib/paths";
import type { AutoQueueOrder } from "../jobs/autoQueuePolicy";
import { buildRecencyKeys, makeRecencyComparator } from "./recencyIndex";

// Recency ordering for the per-channel BATCH controllers (the digest and
// backfill lanes), as distinct from the auto-queue runners.
//
// The runners already sort by upload date through recencyIndex; the batches did
// not, so "newest first" meant one thing on two of the four pipelines and
// nothing on the other two. This is the shared half — one call site's worth of
// glue, so digest and backfill cannot drift into two different notions of what
// "newest" means.
//
// Three things are settled here rather than at each call site:
//
//   * `order: "listed"` returns null, and a null comparator means DO NOT SORT.
//     That is what reproduces today byte-for-byte: the historical order is
//     reproduced by not sorting at all, never by sorting with an identity
//     comparator, which would still perturb ties on some engines.
//   * `interpolate: false`. A batch's candidates come from a readdir of the
//     channel's data/ dir, so every one of them is downloaded and has a
//     metadata.info.json for the tail read. Playlist interpolation exists for
//     videos with nothing on disk, which by construction cannot be here.
//   * every id is owned by this channel. That is not a convenience — it is what
//     keeps layer 1 out of the wrong key space (see recencyIndex's layer-1
//     comment: candidate ids here are DIRECTORY names).
//
// Never throws. A recency lookup failing is not a reason to refuse to digest a
// channel, so a broken or mid-rebuild index degrades to the listed order.
export async function batchRecencyComparator(
  paths: Paths,
  channelSlug: string,
  ids: ReadonlyArray<string>,
  order: AutoQueueOrder,
): Promise<((a: string, b: string) => number) | null> {
  if (order === "listed" || ids.length === 0) return null;
  try {
    const owner = new Map<string, string>();
    for (const id of ids) owner.set(id, channelSlug);
    const keys = await buildRecencyKeys({
      paths,
      meta: [{ slug: channelSlug }],
      candidateIds: new Set(ids),
      owner,
      interpolate: false,
    });
    return makeRecencyComparator(keys, order);
  } catch {
    return null;
  }
}