Add SortID to RulesetInfo to allow stable ordering of rulesets for display

osu.Game.Tests/NonVisual/RulesetInfoOrderingTest.cs

@@ -0,0 +1,38 @@
+// 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 System.Linq;
+using NUnit.Framework;
+using osu.Game.Rulesets;
+using osu.Game.Rulesets.Catch;
+using osu.Game.Rulesets.Osu;
+
+namespace osu.Game.Tests.NonVisual
+{
+    [TestFixture]
+    public class RulesetInfoOrderingTest
+    {
+        [Test]
+        public void TestOrdering()
+        {
+            var rulesets = new[]
+            {
+                new RulesetInfo("custom2", "Custom Ruleset 2", string.Empty, -1),
+                new OsuRuleset().RulesetInfo,
+                new RulesetInfo("custom3", "Custom Ruleset 3", string.Empty, -1),
+                new RulesetInfo("custom2", "Custom Ruleset 2", string.Empty, -1),
+                new CatchRuleset().RulesetInfo,
+                new RulesetInfo("custom3", "Custom Ruleset 3", string.Empty, -1),
+            };
+
+            var orderedRulesets = rulesets.OrderBy(r => r.SortID);
+
+            // Ensure all customs are after official.
+            Assert.That(orderedRulesets.Select(r => r.OnlineID), Is.EqualTo(new[] { 0, 2, -1, -1, -1, -1 }));
+
+            // Ensure customs are grouped next to each other (ie. stably sorted).
+            Assert.That(orderedRulesets.SkipWhile(r => r.ShortName != "custom2").Skip(1).First().ShortName, Is.EqualTo("custom2"));
+            Assert.That(orderedRulesets.SkipWhile(r => r.ShortName != "custom3").Skip(1).First().ShortName, Is.EqualTo("custom3"));
+        }
+    }
+}

osu.Game/Rulesets/RulesetInfo.cs

@@ -24,6 +24,11 @@ namespace osu.Game.Rulesets
 
         public string InstantiationInfo { get; set; } = string.Empty;
 
+        /// <summary>
+        /// A best effort sort ID which provides stable ordering and puts online rulesets before non-online rulesets.
+        /// </summary>
+        public int SortID => OnlineID >= 0 ? OnlineID : Math.Abs(ShortName.GetHashCode());
+
         public RulesetInfo(string shortName, string name, string instantiationInfo, int onlineID)
         {
             ShortName = shortName;

osu.Game/Rulesets/RulesetStore.cs

@@ -163,10 +163,7 @@ namespace osu.Game.Rulesets
                     }
                 }
 
-                // add known official rulesets first..
+                availableRulesets.AddRange(detachedRulesets.OrderBy(r => r.SortID));
-                availableRulesets.AddRange(detachedRulesets.Where(r => r.OnlineID >= 0).OrderBy(r => r.OnlineID));
-                // .. then add any customs
-                availableRulesets.AddRange(detachedRulesets.Where(r => r.OnlineID < 0).OrderBy(r => r.ShortName));
             });
         }
 

osu.Game/Screens/Select/Carousel/CarouselBeatmap.cs

@@ -89,7 +89,7 @@ namespace osu.Game.Screens.Select.Carousel
             {
                 default:
                 case SortMode.Difficulty:
-                    int ruleset = BeatmapInfo.RulesetID.CompareTo(otherBeatmap.BeatmapInfo.RulesetID);
+                    int ruleset = BeatmapInfo.Ruleset.SortID.CompareTo(otherBeatmap.BeatmapInfo.Ruleset.SortID);
                     if (ruleset != 0) return ruleset;
 
                     return BeatmapInfo.StarRating.CompareTo(otherBeatmap.BeatmapInfo.StarRating);

osu.Game/Screens/Select/Carousel/CarouselBeatmapSet.cs

@@ -39,7 +39,7 @@ namespace osu.Game.Screens.Select.Carousel
 
             beatmapSet.Beatmaps
                       .Where(b => !b.Hidden)
-                      .OrderBy(b => b.RulesetID)
+                      .OrderBy(b => b.Ruleset.SortID)
                       .ThenBy(b => b.StarRating)
                       .Select(b => new CarouselBeatmap(b))
                       .ForEach(AddChild);