// Pure logic for the "More Episodes" strip in EpisodeFocusView.
//
// Extracted from the component so it can be unit-tested: the strip must never
// collapse to just the current episode while real siblings exist, and it must
// not mistake number-less episodes for the current one.
//
// TRACES: UR-048 | DR-062
import type { MediaItem } from "$lib/api/types";

/**
 * Does `ep` refer to the same episode as `current`?
 *
 * Matches by id first. Falls back to season+episode number, but ONLY when both
 * numbers are known on both sides — otherwise `undefined === undefined` would
 * mark every number-less episode as the current one (the bug that made the
 * whole strip look like the current episode).
 */
export function isCurrentEpisode(ep: MediaItem, current: MediaItem): boolean {
  if (ep.id === current.id) return true;
  if (
    ep.indexNumber == null || current.indexNumber == null ||
    ep.parentIndexNumber == null || current.parentIndexNumber == null
  ) {
    return false;
  }
  return (
    ep.parentIndexNumber === current.parentIndexNumber &&
    ep.indexNumber === current.indexNumber
  );
}

/**
 * The window of episodes shown under the hero: up to 3 before and 6 after the
 * current episode. Degrades gracefully:
 *  - prefers the current season, falling back to the full list when the season
 *    is unknown (e.g. the episode was fetched directly on an API-ID mismatch);
 *  - splices the current episode into the pool at its numeric position when it
 *    isn't present, so it still anchors the window;
 *  - returns just `[current]` only when there genuinely are no other episodes.
 */
export function adjacentEpisodes(current: MediaItem, allEpisodes: MediaItem[]): MediaItem[] {
  const seasonMatches = allEpisodes.filter(
    (e) => current.parentIndexNumber != null && e.parentIndexNumber === current.parentIndexNumber
  );
  const pool = (seasonMatches.length > 0 ? seasonMatches : allEpisodes)
    .slice()
    .sort((a, b) => (a.indexNumber ?? 0) - (b.indexNumber ?? 0));

  let idx = pool.findIndex((e) => isCurrentEpisode(e, current));

  if (idx === -1) {
    const epNum = current.indexNumber ?? 0;
    const insertAt = pool.findIndex((e) => (e.indexNumber ?? 0) > epNum);
    idx = insertAt === -1 ? pool.length : insertAt;
    pool.splice(idx, 0, current);
  }

  const start = Math.max(0, idx - 3);
  const end = Math.min(pool.length, idx + 7);
  return pool.slice(start, end);
}