Protocol 12: Up next (a shared queue) and where the video comes from

Up next: a server-side queue anyone edits by item id (add at end or
next, move, remove, play now, skip); each item says who added it and
how it plays. At the real end the first item swaps in after the end
card's countdown, put up by whoever can (its adder for through-me links
and files, someone with access for Jellyfin, else anyone); items that
can't play are skipped with the reason. The next item's link is
resolved ahead. Greyed "Continues with" episodes below. A chip with the
controls opens it as a popover (leaf_widgets, local path dependency for
now); "Add to queue" next to Put it up everywhere.

Delivery: the server measures the source's rate, the media's bitrate
and each member's read rate; the members card shows a source line
(who passes it on, through what, keeping up or not, source left) and
each member's route and buffer. A slow source is told once per media.
A failed server fetch is announced to everyone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/queue/queue_controller.dart

@@ -0,0 +1,549 @@
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+import 'dart:math';
+
+import 'package:flutter/foundation.dart';
+import 'package:signals/signals_flutter.dart';
+import 'package:sync_relay_core/core.dart';
+
+import '../files/name_parser.dart';
+import '../files/thumbnails.dart';
+import '../jellyfin/accounts.dart';
+import '../jellyfin/format.dart' show pickFor;
+import '../jellyfin/session_media.dart';
+import '../screens/home/picked_media.dart';
+import '../session/media_art.dart';
+import '../session/session_coordinator.dart';
+import '../state/settings.dart';
+import '../sync/avatar_cache.dart';
+import 'queue_logic.dart';
+
+// Up next on this client (see ARCHITECTURE.md "Up next"): the server's
+// queue as signals, adding to it (probed, with a picture), the end-of-media
+// countdown for its first item, putting an item up when this app is the
+// one that can, and getting that item ready shortly before.
+
+/// The end-of-media card for the first Up next item. [secondsLeft] null:
+/// no countdown here (auto-advance off, cancelled, or another app puts
+/// it up), the card offers Play now only.
+class UpNextCountdown {
+  final int mediaId;
+  final QueueItem item;
+  final int? secondsLeft;
+  const UpNextCountdown({required this.mediaId, required this.item, this.secondsLeft});
+}
+
+/// The first item's readiness, on the app that puts it up.
+enum WarmState { idle, warming, ready, failed }
+
+class QueueController {
+  final SessionCoordinator coord;
+  QueueController(this.coord);
+
+  /// Queued items' pictures by hash, from `/i/`.
+  late final PictureCache thumbs = PictureCache(coord.sync,
+      route: 'i', keep: RelayConstants.maxQueueItems + 8, live: (s) => [for (final q in s.queue) q.thumb]);
+
+  /// Up next, in order (empty when nothing is queued or not in a server).
+  late final Computed<List<QueueItem>> items =
+      computed(() => coord.serverState.value?.queue ?? const <QueueItem>[]);
+
+  final countdown = signal<UpNextCountdown?>(null);
+
+  /// The first item's id and how ready it is here (only on the app that
+  /// puts it up; null elsewhere).
+  final warm = signal<({String id, WarmState state})?>(null);
+
+  /// The natural continuation of what plays: the next episodes of the
+  /// Jellyfin series (not queued; shown greyed under Up next).
+  final continuation = signal<List<EpisodeLink>>(const []);
+
+  /// The mediaId whose continuation was cleared here.
+  final _hiddenFor = signal(-1);
+
+  /// The queue popover is open: the chip stays up.
+  final popoverOpen = signal(false);
+
+  // What adder-only items stand for, by their key (a link passed on from
+  // here, a file here). Never leaves this app.
+  final Map<String, ({PickedMedia pick, DateTime at})> _local = {};
+  final Map<String, DateTime> _performing = {};
+  final Set<String> _skipped = {};
+  int _cancelledFor = -1;
+  Timer? _timer;
+  bool _attached = false;
+  String _continuationFor = '';
+  final _random = Random.secure();
+
+  String get _me => coord.sync.memberId;
+
+  void attach() {
+    if (_attached) return;
+    _attached = true;
+    coord.sync.states.listen(_onState);
+    coord.sync.feeds.listen(_onFeed);
+    effect(() {
+      final next = coord.nextEpisode.value;
+      final plan = coord.jellyfinPlan.value;
+      unawaited(_loadContinuation(next, plan));
+    });
+  }
+
+  // ------------------------------------------------------------- adding
+
+  /// Adds [pick] to Up next (at the end, or with [next] right after what
+  /// plays). A link is probed first and refused like Put it up when it
+  /// will not play. Returns an error text, or null when sent.
+  Future<String?> add(PickedMedia pick, {bool next = false}) async {
+    coord.markInput();
+    if (coord.serverState.peek() == null) return 'not in a server';
+    if (items.peek().length >= RelayConstants.maxQueueItems) {
+      return 'Up next is full (${RelayConstants.maxQueueItems} items)';
+    }
+    final ({QueueItem item, Uint8List? thumb}) built;
+    try {
+      final b = await _build(pick);
+      if (b is String) return b;
+      built = b as ({QueueItem item, Uint8List? thumb});
+    } catch (e) {
+      return "couldn't add it: $e";
+    }
+    final item = built.item;
+    if (item.key.isNotEmpty) _local[item.key] = (pick: pick, at: DateTime.now());
+    final thumb = built.thumb;
+    coord.sync.sendQueue(QueueIntent.add(item, next: next, thumb: thumb == null ? '' : base64Encode(thumb)));
+    coord.showToast('Added ${item.title} to Up next');
+    return null;
+  }
+
+  /// A continuation episode becomes an Up next item.
+  Future<String?> promote(EpisodeLink e) => add(e.pick);
+
+  /// The item, or an error text.
+  Future<Object> _build(PickedMedia pick) async {
+    switch (pick) {
+      case JellyfinMedia():
+        final accounts = coord.jellyfin;
+        if (accounts.account(pick.serverId) == null) return 'that Jellyfin server is not linked any more';
+        if (accounts.statusOf(pick.serverId) == JellyfinAccountStatus.expired) {
+          return 'Jellyfin sign-in expired — reconnect it in Settings → Jellyfin & integrations';
+        }
+        final client = accounts.clientFor(pick.serverId);
+        final thumb = client == null ? null : await _jellyfinThumb(client, pick.itemId);
+        return (
+          item: QueueItem(
+            media: mediaForPick(pick),
+            how: QueueHow.jellyfin,
+            title: pick.title,
+            sub: pick.subtitle,
+            duration: (pick.runtime?.inMilliseconds ?? 0) / 1000,
+          ),
+          thumb: thumb,
+        );
+      case FileMedia(:final path):
+        final file = File(path);
+        if (!await file.exists()) return 'file not found — check the path';
+        final name = pick.label;
+        final parsed = parseReleaseName(name);
+        final shot = await thumbnailFor(path);
+        Uint8List? thumb;
+        if (shot != null) {
+          try {
+            final bytes = await File(shot.imagePath).readAsBytes();
+            if (bytes.length <= RelayConstants.maxPosterBytes) thumb = bytes;
+          } catch (_) {}
+        }
+        final ep = [
+          if (parsed.season != null) 'S${parsed.season}',
+          if (parsed.episode != null) 'E${parsed.episode}',
+        ].join();
+        return (
+          item: QueueItem(
+            media: HelloMedia(name: name, size: await file.length(), duration: 0),
+            how: QueueHow.file,
+            title: parsed.series.trim().isEmpty ? name : parsed.series.trim(),
+            sub: ep,
+            duration: (shot?.duration.inMilliseconds ?? 0) / 1000,
+            key: _newKey(),
+          ),
+          thumb: thumb,
+        );
+      case LinkMedia(:final url, :final anyway):
+        // Alone, a link plays direct: there is nobody to pass it on to.
+        final relay = coord.solo ? LinkRelay.direct : pick.relay;
+        final linked = await mediaForLink(coord.jellyfin, url);
+        final ref = linked.jellyfin;
+        if (ref != null) {
+          return (
+            item: QueueItem(
+                media: HelloMedia(name: linked.name, size: 0, duration: 0, jellyfin: ref),
+                how: QueueHow.jellyfin,
+                title: ref.title.isEmpty ? linked.name : ref.title,
+                sub: ref.subtitle ?? '',
+                duration: ref.runtimeSecs ?? 0),
+            thumb: null,
+          );
+        }
+        LinkProbe? probe;
+        String? probeError;
+        try {
+          probe = await coord.ytDlp.probe(url).timeout(const Duration(seconds: 45));
+        } catch (e) {
+          probeError = '$e';
+        }
+        final problem = probe == null ? playErrorText(probeError ?? '') : probe.problem;
+        if (problem != null && !anyway) return problem;
+        if (relay == LinkRelay.me) {
+          if (probe == null) return problem ?? "couldn't reach the site";
+          final relayProblem = probe.relayProblem;
+          if (relayProblem != null) return relayProblem;
+        }
+        if (relay == LinkRelay.server && !coord.serverHasPassword) return 'server fetch needs a server password';
+        if (relay == LinkRelay.server && probe != null && probe.isPage && probe.info != null) {
+          final relayProblem = probe.relayProblem;
+          if (relayProblem != null) return relayProblem;
+        }
+        final uri = Uri.tryParse(url) ?? Uri();
+        final info = probe?.info;
+        final title = probe?.title ?? linked.name;
+        final site = info == null
+            ? uri.host
+            : (info.uploader.isNotEmpty ? info.uploader : (info.extractor.isNotEmpty ? info.extractor : uri.host));
+        final thumbUrl = info?.thumbnail == null ? null : Uri.tryParse(info!.thumbnail!);
+        final thumb = thumbUrl == null ? null : (await thumbnailArt(thumbUrl))?.poster;
+        final how = switch (relay) {
+          LinkRelay.direct => QueueHow.direct,
+          LinkRelay.server => QueueHow.server,
+          LinkRelay.me => QueueHow.through,
+        };
+        return (
+          item: QueueItem(
+            media: HelloMedia(
+                name: title,
+                size: 0,
+                duration: 0,
+                url: how == QueueHow.through ? null : url,
+                relay: how == QueueHow.server ? const MediaRelay(fetch: true) : null),
+            how: how,
+            title: title,
+            sub: site,
+            duration: info?.duration ?? 0,
+            key: how == QueueHow.through ? _newKey() : '',
+          ),
+          thumb: thumb,
+        );
+    }
+  }
+
+  String _newKey() => [for (var i = 0; i < 12; i++) _random.nextInt(256).toRadixString(16).padLeft(2, '0')].join();
+
+  /// A wide picture of a Jellyfin item: an episode's own still, else the
+  /// item's (or series') thumb or backdrop, else its poster.
+  Future<Uint8List?> _jellyfinThumb(JellyfinClient client, String itemId) async {
+    try {
+      final item = await client.item(itemId).timeout(const Duration(seconds: 10));
+      final ref = (item.kind == JellyfinItemKind.episode ? item.ownImage(JellyfinImageType.primary) : null) ??
+          item.image(JellyfinImageType.thumb) ??
+          item.image(JellyfinImageType.backdrop) ??
+          item.image(JellyfinImageType.primary);
+      if (ref == null) return null;
+      return await fetchImage(
+          client.imageUrl(ref, maxWidth: 480, quality: 80), client.authHeaders, RelayConstants.maxPosterBytes);
+    } catch (e) {
+      debugPrint('queue: no Jellyfin picture for $itemId: $e');
+      return null;
+    }
+  }
+
+  // ------------------------------------------------------------ editing
+
+  void move(String id, int to) {
+    coord.markInput();
+    coord.sync.sendQueue(QueueIntent.move(id, to));
+  }
+
+  void remove(String id) {
+    coord.markInput();
+    coord.sync.sendQueue(QueueIntent.remove(id));
+  }
+
+  /// Plays [id] now: this app puts it up when it is the one that can,
+  /// else it asks the server, and the app that can puts it up.
+  void playNow(String id) {
+    coord.markInput();
+    final r = coord.serverState.peek();
+    final item = r?.queue.where((q) => q.id == id).firstOrNull;
+    if (r == null || item == null) return;
+    if (queuePerformer(item, r.members).id == _me) {
+      _perform(item);
+    } else {
+      coord.sync.sendQueue(QueueIntent.play(id));
+    }
+  }
+
+  /// Plays the first Up next item now.
+  void skip() {
+    final first = items.peek().firstOrNull;
+    if (first != null) playNow(first.id);
+  }
+
+  /// Cancel on the end-of-media card: no countdown here for this media.
+  void cancelCountdown() {
+    final c = countdown.peek();
+    _timer?.cancel();
+    _timer = null;
+    if (c == null) return;
+    _cancelledFor = c.mediaId;
+    countdown.value = UpNextCountdown(mediaId: c.mediaId, item: c.item);
+  }
+
+  /// "Clear": hides the continuation of what plays now (and its next
+  /// episode does not come up by itself).
+  void clearContinuation() {
+    final id = coord.serverState.peek()?.mediaId;
+    if (id != null) _hiddenFor.value = id;
+    if (coord.autoAdvance.peek() != null) coord.cancelAutoAdvance();
+  }
+
+  /// The continuation applies to [mediaId]: nothing is queued and it was
+  /// not cleared.
+  bool continuationShown(int mediaId) =>
+      (coord.serverState.peek()?.queue.isEmpty ?? true) && _hiddenFor.peek() != mediaId;
+
+  /// [continuation] unless cleared for the media playing.
+  late final Computed<List<EpisodeLink>> shownContinuation = computed(() {
+    final id = coord.serverState.value?.mediaId;
+    return id != null && _hiddenFor.value == id ? const <EpisodeLink>[] : continuation.value;
+  });
+
+  // --------------------------------------------------------- per state
+
+  void _onState(StateBroadcast r) {
+    _prune(r);
+    final queue = r.queue;
+    final play = r.queuePlay;
+    if (play != null) {
+      final item = queue.where((q) => q.id == play).firstOrNull;
+      if (item != null) _bringUp(item, r);
+    }
+    final finished = r.transport == 'finished';
+    if (queue.isNotEmpty && r.media == null) {
+      // Nothing up yet: the first item goes up at once.
+      _bringUp(queue.first, r);
+    } else if (finished && queue.isNotEmpty) {
+      _atEnd(r);
+    } else {
+      if (countdown.peek() != null) _stopCountdown();
+      if (!finished) _cancelledFor = -1;
+    }
+    _warm(r);
+  }
+
+  void _onFeed(FeedMessage f) {
+    if (f.action == 'queue-skip') {
+      coord.showToast('Skipped ${f.target ?? 'an item'}${f.reason.isEmpty ? '' : ': ${f.reason}'}');
+    }
+  }
+
+  /// Forgets what no longer matters: performing marks after a minute (a
+  /// swap that never landed may be tried again), local picks no longer
+  /// queued.
+  void _prune(StateBroadcast r) {
+    final now = DateTime.now();
+    _performing.removeWhere((_, at) => now.difference(at) > const Duration(minutes: 1));
+    final keys = {for (final q in r.queue) q.key};
+    _local.removeWhere((k, v) => !keys.contains(k) && now.difference(v.at) > const Duration(minutes: 1));
+    final ids = {for (final q in r.queue) q.id};
+    _skipped.removeWhere((id) => !ids.contains(id));
+  }
+
+  /// [item] should go up now: by this app when it is the one that can;
+  /// nobody can: the lowest member takes it off with the reason.
+  void _bringUp(QueueItem item, StateBroadcast r) {
+    final perf = queuePerformer(item, r.members);
+    if (perf.id == null) {
+      final why = perf.why;
+      if (why != null && queueJanitor(r.members) == _me && _skipped.add(item.id)) {
+        coord.sync.sendQueue(QueueIntent.remove(item.id, why: why));
+      }
+      return;
+    }
+    if (perf.id == _me) _perform(item);
+  }
+
+  /// The media finished with Up next waiting: the card, and with
+  /// auto-advance on the countdown, for its first item.
+  void _atEnd(StateBroadcast r) {
+    // Up next goes before the next episode, even one already counting.
+    if (coord.autoAdvance.peek() != null) coord.cancelAutoAdvance();
+    final head = r.queue.first;
+    final perf = queuePerformer(head, r.members);
+    if (perf.id == null && perf.why != null) {
+      // Taken off by [_bringUp]'s rule; the next state shows the next one.
+      _bringUp(head, r);
+      return;
+    }
+    final c = countdown.peek();
+    if (c != null && c.mediaId == r.mediaId) {
+      if (c.item.id != head.id || c.item.thumb != head.thumb) {
+        countdown.value = UpNextCountdown(mediaId: c.mediaId, item: head, secondsLeft: c.secondsLeft);
+      }
+      return;
+    }
+    final auto = AppSettings.instance.autoAdvance.peek() && _cancelledFor != r.mediaId;
+    countdown.value = UpNextCountdown(mediaId: r.mediaId, item: head, secondsLeft: auto ? autoAdvanceSecs : null);
+    _timer?.cancel();
+    _timer = auto ? Timer.periodic(const Duration(seconds: 1), (_) => _tick()) : null;
+  }
+
+  void _tick() {
+    final c = countdown.peek();
+    final r = coord.serverState.peek();
+    final left = c?.secondsLeft;
+    if (c == null || left == null || r == null || r.mediaId != c.mediaId || r.transport != 'finished') {
+      _stopCountdown();
+      return;
+    }
+    if (left > 1) {
+      countdown.value = UpNextCountdown(mediaId: c.mediaId, item: c.item, secondsLeft: left - 1);
+      return;
+    }
+    _timer?.cancel();
+    _timer = null;
+    _cancelledFor = c.mediaId;
+    countdown.value = UpNextCountdown(mediaId: c.mediaId, item: c.item);
+    // Decided on the latest state: only the app that can puts it up.
+    final head = r.queue.firstOrNull;
+    if (head != null && queuePerformer(head, r.members).id == _me) _perform(head);
+  }
+
+  void _stopCountdown() {
+    _timer?.cancel();
+    _timer = null;
+    countdown.value = null;
+  }
+
+  /// Puts [item] up from here (once per item while it is in flight).
+  /// When it cannot play after all, it is taken off with the reason.
+  void _perform(QueueItem item) {
+    if (_performing.containsKey(item.id)) return;
+    _performing[item.id] = DateTime.now();
+    unawaited(() async {
+      String? error;
+      try {
+        error = await _swapTo(item);
+      } catch (e) {
+        error = '$e';
+      }
+      if (error != null) {
+        debugPrint('queue: ${item.id} cannot go up: $error');
+        coord.sync.sendQueue(QueueIntent.remove(item.id, why: "couldn't play it ($error)"));
+      }
+    }());
+  }
+
+  Future<String?> _swapTo(QueueItem item) async {
+    switch (item.how) {
+      case QueueHow.jellyfin:
+        final ref = item.media.jellyfin;
+        final pick = ref == null ? null : pickForRef(coord.jellyfin, ref);
+        if (pick == null) return 'no Jellyfin account here for it';
+        coord.swapJellyfin(pick, queueId: item.id);
+        return null;
+      case QueueHow.direct:
+        final url = item.media.url;
+        if (url == null) return 'no link';
+        // Each viewer opens it: one computer's probe does not decide.
+        return coord.swapLink(url, relay: LinkRelay.direct, anyway: true, queueId: item.id);
+      case QueueHow.server:
+        coord.swapQueued(HelloMedia(name: item.title, size: 0, duration: 0, relay: const MediaRelay(fetch: true)),
+            item.id,
+            art: thumbs.bytesFor(item.thumb));
+        return null;
+      case QueueHow.through:
+      case QueueHow.file:
+        final local = _local[item.key];
+        if (local == null) return "it isn't on this computer any more";
+        // Only this app can pass it on, and a member sitting out has
+        // nothing open: back first.
+        if (coord.selfOut.peek()) coord.joinBack();
+        return coord.swapPicked(local.pick, queueId: item.id);
+    }
+  }
+
+  // ------------------------------------------------------------ warming
+
+  /// Near the end of what plays (or once it is buffered to its end), the
+  /// app that will put the first item up resolves its link, so the swap
+  /// does not wait for yt-dlp. A Jellyfin item, a file or a link the
+  /// server fetches has nothing to get ready here.
+  void _warm(StateBroadcast r) {
+    final head = r.queue.firstOrNull;
+    if (head == null || queuePerformer(head, r.members).id != _me) {
+      if (warm.peek() != null) warm.value = null;
+      return;
+    }
+    final w = warm.peek();
+    if (w == null || w.id != head.id) warm.value = (id: head.id, state: WarmState.idle);
+    if (warm.peek()!.state != WarmState.idle) return;
+    final duration = (r.media?['duration'] as num?)?.toDouble() ?? 0;
+    final position = coord.playback.positionSecs;
+    final due = r.media == null ||
+        queueWarmDue(
+            transport: r.transport,
+            position: position,
+            duration: duration,
+            bufferedTo: position + coord.playback.bufferedSecsAhead);
+    if (!due) return;
+    final local = _local[head.key]?.pick;
+    final url = switch (head.how) {
+      QueueHow.direct => head.media.url,
+      QueueHow.through => local is LinkMedia ? local.url : null,
+      _ => null,
+    };
+    if (url == null) {
+      warm.value = (id: head.id, state: WarmState.ready);
+      return;
+    }
+    warm.value = (id: head.id, state: WarmState.warming);
+    // The probe is kept a few minutes: the swap reads it from there.
+    unawaited(coord.ytDlp.probe(url).then((p) => p.problem == null, onError: (_) => false).then((ok) {
+      if (warm.peek()?.id != head.id) return;
+      warm.value = (id: head.id, state: ok ? WarmState.ready : WarmState.failed);
+    }));
+  }
+
+  // -------------------------------------------------------- continuation
+
+  /// The next few episodes after the one playing (own account only).
+  Future<void> _loadContinuation(EpisodeLink? next, JellyfinPlan? plan) async {
+    if (next == null || plan is! OwnPlay) {
+      _continuationFor = '';
+      if (continuation.peek().isNotEmpty) continuation.value = const [];
+      return;
+    }
+    final key = '${plan.account.serverId}/${next.item.id}';
+    if (key == _continuationFor) return;
+    _continuationFor = key;
+    continuation.value = [next];
+    final seriesId = next.item.seriesId;
+    if (seriesId == null) return;
+    try {
+      final list = await coord.jellyfin.cached(
+          plan.account.serverId, 'episodes:$seriesId', () => plan.client.episodes(seriesId));
+      if (_continuationFor != key) return;
+      final more = [next];
+      var at = next.item;
+      while (more.length < 4) {
+        final after = episodeAfter(list, at.id);
+        if (after == null) break;
+        more.add(EpisodeLink(after, pickFor(plan.account, after)));
+        at = after;
+      }
+      continuation.value = more;
+    } catch (e) {
+      debugPrint('queue: continuation of $seriesId: $e');
+    }
+  }
+}

apps/desktop/lib/src/queue/queue_logic.dart

@@ -0,0 +1,92 @@
+import 'package:sync_relay_core/core.dart';
+
+import '../jellyfin/session_media.dart' show compareMemberIds;
+
+// Who puts an Up next item up, and when to get the first one ready. Pure:
+// only what every client sees in `state` decides, so all clients agree
+// without a wire message (like the Jellyfin auto-advance swapper).
+
+/// Who puts [item] up: [id] a member id; null with [why] when nobody here
+/// can (the item is skipped), null without [why] when nobody is around
+/// right now (nothing happens).
+typedef QueuePerformer = ({String? id, String? why});
+
+/// - A link passed on by its adder, or the adder's file: only the adder
+///   (their app has the file, the cookies). Gone: skipped.
+/// - Jellyfin: the adder when it plays that server with its own account,
+///   else the lowest member id that does (`jf[serverId] == true`, not
+///   stale, not sitting out). Nobody: skipped.
+/// - A link everyone opens, or one the server fetches: the adder when
+///   present (not stale or sitting out), else the lowest such member id.
+QueuePerformer queuePerformer(QueueItem item, List<MemberView> members) {
+  MemberView? adder;
+  for (final m in members) {
+    if (m.id == item.by) adder = m;
+  }
+  bool active(MemberView m) => !m.stale && !m.out;
+  if (item.how.adderOnly) {
+    if (adder == null) return (id: null, why: '${_name(item)} left');
+    return (id: adder.id, why: null);
+  }
+  final ref = item.media.jellyfin;
+  if (item.how == QueueHow.jellyfin && ref != null) {
+    bool can(MemberView m) => active(m) && m.jf[ref.serverId] == true;
+    if (adder != null && can(adder)) return (id: adder.id, why: null);
+    final id = autoAdvanceLowest(members.where(can));
+    return id == null ? (id: null, why: 'nobody here has that Jellyfin server') : (id: id, why: null);
+  }
+  if (adder != null && active(adder)) return (id: adder.id, why: null);
+  return (id: autoAdvanceLowest(members.where(active)), why: null);
+}
+
+String _name(QueueItem item) => item.byName.isEmpty ? 'who added it' : item.byName;
+
+/// The lowest member id of [members] ("m2" before "m10"), or null.
+String? autoAdvanceLowest(Iterable<MemberView> members) {
+  final ids = [for (final m in members) m.id]..sort(compareMemberIds);
+  return ids.isEmpty ? null : ids.first;
+}
+
+/// Who takes an item nobody can put up off the queue (`remove` with why):
+/// the lowest member id that is not stale.
+String? queueJanitor(List<MemberView> members) => autoAdvanceLowest(members.where((m) => !m.stale));
+
+/// How [item] plays, in a few words for its row: "through Foxyz" (you:
+/// "through you"), "server fetches", "everyone fetches", "Jellyfin",
+/// "Foxyz's file".
+String queueHowLabel(QueueItem item, {required String selfId}) {
+  final mine = item.by == selfId && selfId.isNotEmpty;
+  final who = mine ? 'you' : _name(item);
+  return switch (item.how) {
+    QueueHow.through => 'through $who',
+    QueueHow.file => mine ? 'your file' : "$who's file",
+    QueueHow.server => 'server fetches',
+    QueueHow.direct => 'everyone fetches',
+    QueueHow.jellyfin => 'Jellyfin',
+  };
+}
+
+/// Seconds before the end at which the first Up next item is got ready.
+const queueWarmLeadSecs = 90.0;
+
+/// Whether to get the next item ready now: the current media is in its
+/// last [queueWarmLeadSecs], is buffered to its end, or has finished.
+bool queueWarmDue({
+  required String transport,
+  required double position,
+  required double duration,
+  required double bufferedTo,
+}) {
+  if (transport == 'finished') return true;
+  if (duration <= 0) return false;
+  return duration - position <= queueWarmLeadSecs || bufferedTo >= duration - 1;
+}
+
+/// "1:23:45" / "23:45" for [secs]; empty when unknown.
+String queueClock(double secs) {
+  if (secs <= 0) return '';
+  final s = secs.round();
+  final h = s ~/ 3600, m = (s % 3600) ~/ 60, r = s % 60;
+  String two(int v) => v.toString().padLeft(2, '0');
+  return h > 0 ? '$h:${two(m)}:${two(r)}' : '$m:${two(r)}';
+}