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_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)}';
+}