queue/PlaylistEdit: shuffle appended songs only within its priority group

Fixes #165.

NEWS

@@ -2,6 +2,7 @@ ver 0.20.13 (not yet released)
 * database
   - simple: don't purge mount points on update/rescan
   - upnp: work around libupnp 1.6.24 API breakage
+* queue: fix spuriously misplaced prioritized songs
 
 ver 0.20.12 (2017/11/25)
 * database

src/queue/PlaylistEdit.cxx

@@ -112,7 +112,7 @@ playlist::AppendSong(PlayerControl &pc, DetachedSong &&song)
 		else
 			start = current + 1;
 		if (start < queue.GetLength())
-			queue.ShuffleOrderLast(start, queue.GetLength());
+			queue.ShuffleOrderLastWithPriority(start, queue.GetLength());
 	}
 
 	UpdateQueuedSong(pc, queued_song);

src/queue/Queue.cxx

@@ -364,8 +364,20 @@ Queue::ShuffleOrderFirst(unsigned start, unsigned end)
 }
 
 void
-Queue::ShuffleOrderLast(unsigned start, unsigned end)
+Queue::ShuffleOrderLastWithPriority(unsigned start, unsigned end)
 {
+	assert(end <= length);
+	assert(start < end);
+
+	/* skip all items at the start which have a higher priority,
+	   because the last item shall only be shuffled within its
+	   priority group */
+	const auto last_priority = items[OrderToPosition(end - 1)].priority;
+	while (items[OrderToPosition(start)].priority != last_priority) {
+		++start;
+		assert(start < end);
+	}
+
 	rand.AutoCreate();
 
 	std::uniform_int_distribution<unsigned> distribution(start, end - 1);

src/queue/Queue.hxx

@@ -356,11 +356,12 @@ struct Queue {
 	void ShuffleOrderFirst(unsigned start, unsigned end);
 
 	/**
-	 * Shuffles the virtual order of the last song in the specified
-	 * (order) range.  This is used in random mode after a song has been
-	 * appended by queue_append().
+	 * Shuffles the virtual order of the last song in the
+	 * specified (order) range; only songs which match this song's
+	 * priority are considered.  This is used in random mode after
+	 * a song has been appended by Append().
 	 */
-	void ShuffleOrderLast(unsigned start, unsigned end);
+	void ShuffleOrderLastWithPriority(unsigned start, unsigned end);
 
 	/**
 	 * Shuffles a (position) range in the queue.  The songs are physically