Make virtual beatmap tracks approximate beatmap length

osu.Game/Beatmaps/WorkingBeatmap_VirtualBeatmapTrack.cs

@@ -0,0 +1,46 @@
+// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System.Linq;
+using osu.Framework.Audio.Track;
+using osu.Game.Rulesets.Objects.Types;
+
+namespace osu.Game.Beatmaps
+{
+    public partial class WorkingBeatmap
+    {
+        private class VirtualBeatmapTrack : TrackVirtual
+        {
+            private readonly IBeatmap beatmap;
+
+            public VirtualBeatmapTrack(IBeatmap beatmap)
+            {
+                this.beatmap = beatmap;
+            }
+
+            protected override void UpdateState()
+            {
+                updateVirtualLength();
+                base.UpdateState();
+            }
+
+            private void updateVirtualLength()
+            {
+                var lastObject = beatmap.HitObjects.LastOrDefault();
+
+                switch (lastObject)
+                {
+                    case null:
+                        Length = 1000;
+                        break;
+                    case IHasEndTime endTime:
+                        Length = endTime.EndTime + 1000;
+                        break;
+                    default:
+                        Length = lastObject.StartTime + 1000;
+                        break;
+                }
+            }
+        }
+    }
+}