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+/**
+ * The music player's queue, as a reducer over `{ tracks, order, pos }`.
+ *
+ * `tracks` is every entry the queue holds, in the order they were added.
+ * `order` is indices into it — the *play* order, which is what shuffle
+ * rewrites without disturbing `tracks`. `pos` walks `order`. The track playing
+ * is `tracks[order[pos]]`, and `docs/playlists.md` §9 is why these three live
+ * here rather than as three pieces of component state.
+ *
+ * The queue used to be replaceable and nothing else: every call to
+ * `onPlayQueue` reset all three together, which is all an album needs. "Play
+ * next" and "add to queue" do not replace, and building them on three separate
+ * `useState`s ships a defect — `setOrder` needs the length `setTracks` is about
+ * to produce and cannot see it, so two enqueues batched into one tick both read
+ * the stale length and write indices past the end. A reducer sees one state.
+ *
+ * No imports, and there must be none: the whole module is executed standalone
+ * by `test_queue_ops.py`, the way `source-merge.js` is, and the queue is
+ * untested from the moment it cannot be.
+ */
+
+/**
+ * A play order over `n` tracks, shuffled, with `keepFirst` moved to the front
+ * so turning shuffle on mid-album does not interrupt what is already playing.
+ *
+ * `rand` is injectable for the tests alone — callers pass nothing.
+ */
+function shuffledOrder(n, keepFirst, rand) {
+  const rnd = rand || Math.random;
+  const order = Array.from({ length: n }, (_, i) => i);
+  for (let i = order.length - 1; i > 0; i--) {
+    const j = Math.floor(rnd() * (i + 1));
+    [order[i], order[j]] = [order[j], order[i]];
+  }
+  if (keepFirst != null) {
+    const at = order.indexOf(keepFirst);
+    if (at > 0) { order.splice(at, 1); order.unshift(keepFirst); }
+  }
+  return order;
+}
+
+function identityOrder(n) {
+  return Array.from({ length: n }, (_, i) => i);
+}
+
+function emptyQueue() {
+  return { tracks: [], order: [], pos: 0 };
+}
+
+/**
+ * `{ tracks, order, pos }` after `action`. Pure, and never mutates `state`:
+ * the player holds this in `useReducer`, and a mutated array would not
+ * re-render the queue panel that is looking at it.
+ *
+ *   replace    { tracks, startIndex, shuffle }  the whole queue, as before
+ *   append     { tracks }                       at the end of the play order
+ *   insertNext { tracks }                       right after what is playing
+ *   removeAt   { at }                           `at` indexes `order`, not `tracks`
+ *   skipTo     { pos }
+ *   reshuffle  { shuffle }                      keeping the current track playing
+ */
+function queueReducer(state, action) {
+  switch (action.type) {
+    case 'replace': {
+      const incoming = action.tracks || [];
+      const n = incoming.length;
+      const start = action.startIndex || 0;
+      return {
+        tracks: incoming,
+        // Identical to what the player did inline before this module existed:
+        // shuffled with the requested track pulled to the front and played from
+        // position 0, or the plain order played from the requested index.
+        order: action.shuffle
+          ? shuffledOrder(n, start, action.rand)
+          : identityOrder(n),
+        pos: action.shuffle ? 0 : start,
+      };
+    }
+
+    case 'append':
+    case 'insertNext': {
+      const incoming = action.tracks || [];
+      if (!incoming.length) return state;
+      // The one line this module exists for: the new indices are derived from
+      // the length in `state`, which is the length after every action already
+      // applied — not from a closure captured when the click handler was built.
+      const added = incoming.map((_, i) => state.tracks.length + i);
+      const tracks = [...state.tracks, ...incoming];
+      if (action.type === 'append') {
+        return { tracks, order: [...state.order, ...added], pos: state.pos };
+      }
+      // Appended tracks are *not* reshuffled into the middle when shuffle is
+      // on: "add to queue" means at the end, and "play next" means next, in
+      // the order given. Reshuffling either would be the player deciding it
+      // knew better than the verb the user pressed.
+      const at = state.order.length ? state.pos + 1 : 0;
+      return {
+        tracks,
+        order: [...state.order.slice(0, at), ...added, ...state.order.slice(at)],
+        pos: state.pos,
+      };
+    }
+
+    case 'removeAt': {
+      const { at } = action;
+      if (at < 0 || at >= state.order.length) return state;
+      const order = [...state.order.slice(0, at), ...state.order.slice(at + 1)];
+      // Dropped from the play order only. Reindexing `tracks` would mean
+      // rewriting every entry of `order` for one removal, and the orphan costs
+      // a pointer — `tracks` is bounded by the queue, not by the session.
+      //
+      // Removing what is playing leaves `pos` where it is, so the next track
+      // slides into that slot and the player's load effect starts it: that is
+      // what removing the current track is expected to do.
+      let pos = state.pos;
+      if (at < state.pos) pos -= 1;
+      if (pos > order.length - 1) pos = Math.max(0, order.length - 1);
+      return { tracks: state.tracks, order, pos };
+    }
+
+    case 'skipTo':
+      return { ...state, pos: action.pos };
+
+    case 'reshuffle': {
+      // The track playing is `order[pos]` — an index into `tracks` — so that
+      // *is* the index to keep. The first version of this recovered it by
+      // looking up the current entry's id and searching `tracks` for it, which
+      // agreed with `order[pos]` only while the queue held no duplicates. A
+      // queue can now hold duplicates (enqueue the same track twice, add an
+      // album that overlaps a playlist), and the id search then jumps playback
+      // to the first copy.
+      const currentIdx = state.order.length ? state.order[state.pos] : null;
+      return {
+        tracks: state.tracks,
+        order: action.shuffle
+          ? shuffledOrder(state.tracks.length, currentIdx, action.rand)
+          : identityOrder(state.tracks.length),
+        pos: action.shuffle ? 0 : (currentIdx == null ? 0 : currentIdx),
+      };
+    }
+
+    default:
+      return state;
+  }
+}
+
+export { queueReducer, shuffledOrder, identityOrder, emptyQueue };