import { useEffect, useRef } from 'react';
import { useAppDispatch, useAppSelector } from '../../hooks/useAppDispatch';
import { useRadioNextMutation } from '../../api/endpoints/radio';
import { appendToQueue } from '../../store/slices/queue';
import { pickToEntry } from '../../lib/radio';

// Prefetch more radio tracks once the current position is within this many of
// the end of the queue.
const LOOKAHEAD = 3;

/**
 * Headless controller: while a radio drives the queue, top it up as it drains
 * (§6.5 / B6 infinite feed). Mounted once in AppShell. A failed prefetch just
 * stops the feed — playback of what's already queued is unaffected.
 */
export function RadioController() {
  const dispatch = useAppDispatch();
  const radio = useAppSelector((s) => s.queue.radio);
  const currentIndex = useAppSelector((s) => s.queue.currentIndex);
  const entries = useAppSelector((s) => s.queue.entries);
  const [radioNext] = useRadioNextMutation();
  const loadingRef = useRef(false);

  useEffect(() => {
    if (!radio) return;
    const nearEnd = currentIndex >= entries.length - LOOKAHEAD;
    if (!nearEnd || loadingRef.current) return;

    loadingRef.current = true;
    void (async () => {
      try {
        const res = await radioNext({
          seedTrackId: radio.seedTrackId ?? undefined,
          fromLikes: radio.fromLikes,
          exploration: radio.exploration,
          count: 10,
          excludeIds: entries.map((e) => e.trackId),
        }).unwrap();
        if (res.picks.length > 0) {
          dispatch(appendToQueue(res.picks.map(pickToEntry)));
        }
      } catch {
        // best-effort — leave the queue as-is
      } finally {
        loadingRef.current = false;
      }
    })();
  }, [radio, currentIndex, entries, radioNext, dispatch]);

  return null;
}