Add extension method for returning next playlist item

osu.Game.Tests/OnlinePlay/PlaylistExtensionsTest.cs

@@ -0,0 +1,72 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using NUnit.Framework;
+using osu.Game.Online.Rooms;
+
+namespace osu.Game.Tests.OnlinePlay
+{
+    [TestFixture]
+    public class PlaylistExtensionsTest
+    {
+        [Test]
+        public void TestPlaylistItemsInOrder()
+        {
+            var items = new[]
+            {
+                new PlaylistItem { ID = 1, BeatmapID = 1001, PlaylistOrder = 1 },
+                new PlaylistItem { ID = 2, BeatmapID = 1002, PlaylistOrder = 2 },
+                new PlaylistItem { ID = 3, BeatmapID = 1003, PlaylistOrder = 3 },
+            };
+
+            var nextItem = items.GetNextItem();
+
+            Assert.That(nextItem, Is.EqualTo(items[0]));
+        }
+
+        [Test]
+        public void TestPlaylistItemsOutOfOrder()
+        {
+            var items = new[]
+            {
+                new PlaylistItem { ID = 2, BeatmapID = 1002, PlaylistOrder = 2 },
+                new PlaylistItem { ID = 1, BeatmapID = 1001, PlaylistOrder = 1 },
+                new PlaylistItem { ID = 3, BeatmapID = 1003, PlaylistOrder = 3 },
+            };
+
+            var nextItem = items.GetNextItem();
+
+            Assert.That(nextItem, Is.EqualTo(items[1]));
+        }
+
+        [Test]
+        public void TestExpiredPlaylistItemsSkipped()
+        {
+            var items = new[]
+            {
+                new PlaylistItem { ID = 2, BeatmapID = 1002, PlaylistOrder = 2, Expired = true },
+                new PlaylistItem { ID = 1, BeatmapID = 1001, PlaylistOrder = 1, Expired = true },
+                new PlaylistItem { ID = 3, BeatmapID = 1003, PlaylistOrder = 3 },
+            };
+
+            var nextItem = items.GetNextItem();
+
+            Assert.That(nextItem, Is.EqualTo(items[2]));
+        }
+
+        [Test]
+        public void TestAllItemsExpired()
+        {
+            var items = new[]
+            {
+                new PlaylistItem { ID = 2, BeatmapID = 1002, PlaylistOrder = 2, Expired = true },
+                new PlaylistItem { ID = 1, BeatmapID = 1001, PlaylistOrder = 1, Expired = true },
+                new PlaylistItem { ID = 3, BeatmapID = 1003, PlaylistOrder = 3, Expired = true },
+            };
+
+            var nextItem = items.GetNextItem();
+
+            Assert.That(nextItem, Is.Null);
+        }
+    }
+}

osu.Game/Online/Rooms/PlaylistExtensions.cs

@@ -1,6 +1,9 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
+#nullable enable
+
+using System.Collections.Generic;
 using System.Linq;
 using Humanizer;
 using Humanizer.Localisation;
@@ -10,6 +13,13 @@ namespace osu.Game.Online.Rooms
 {
     public static class PlaylistExtensions
     {
+        /// <summary>
+        /// Returns the next <see cref="PlaylistItem"/> to be played from the supplied <paramref name="playlist"/>,
+        /// or <see langword="null"/> if all items are expired.
+        /// </summary>
+        public static PlaylistItem? GetNextItem(this IEnumerable<PlaylistItem> playlist) =>
+            playlist.OrderBy(item => item.PlaylistOrder).FirstOrDefault(item => !item.Expired);
+
         public static string GetTotalDuration(this BindableList<PlaylistItem> playlist) =>
             playlist.Select(p => p.Beatmap.Value.Length).Sum().Milliseconds().Humanize(minUnit: TimeUnit.Second, maxUnit: TimeUnit.Hour, precision: 2);
     }