From c8d3dd0e5a62e126eb9e2d7701aec7e731a5effb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bart=C5=82omiej=20Dach?= <dach.bartlomiej@gmail.com>
Date: Sun, 27 Oct 2019 23:14:14 +0100
Subject: [PATCH] Make carousel item sorting stable

Migrate beatmap carousel item sorting from List<T>.Sort()
to IEnumerable<T>.OrderBy(), as the second variant is documented to be
a stable sorting algorithm. This allows for eliminating unnecessary
movement of carousel items occurring whenever any set of items is tied
when changing sorting criteria.
---
 .../Screens/Select/Carousel/CarouselGroup.cs  | 29 +++++++++++++++----
 1 file changed, 23 insertions(+), 6 deletions(-)

diff --git a/osu.Game/Screens/Select/Carousel/CarouselGroup.cs b/osu.Game/Screens/Select/Carousel/CarouselGroup.cs
index 09b728abeb..b32561eb88 100644
--- a/osu.Game/Screens/Select/Carousel/CarouselGroup.cs
+++ b/osu.Game/Screens/Select/Carousel/CarouselGroup.cs
@@ -1,7 +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.
 
+using System;
 using System.Collections.Generic;
+using System.Linq;
 
 namespace osu.Game.Screens.Select.Carousel
 {
@@ -81,12 +83,9 @@ namespace osu.Game.Screens.Select.Carousel
         {
             base.Filter(criteria);
 
-            var children = new List<CarouselItem>(InternalChildren);
-
-            children.ForEach(c => c.Filter(criteria));
-            children.Sort((x, y) => x.CompareTo(criteria, y));
-
-            InternalChildren = children;
+            InternalChildren.ForEach(c => c.Filter(criteria));
+            // IEnumerable<T>.OrderBy() is used instead of List<T>.Sort() to ensure sorting stability
+            InternalChildren = InternalChildren.OrderBy(c => c, new CriteriaComparer(criteria)).ToList();
         }
 
         protected virtual void ChildItemStateChanged(CarouselItem item, CarouselItemState value)
@@ -104,5 +103,23 @@ namespace osu.Game.Screens.Select.Carousel
                 State.Value = CarouselItemState.Selected;
             }
         }
+
+        private class CriteriaComparer : IComparer<CarouselItem>
+        {
+            private readonly FilterCriteria criteria;
+
+            public CriteriaComparer(FilterCriteria criteria)
+            {
+                this.criteria = criteria;
+            }
+
+            public int Compare(CarouselItem x, CarouselItem y)
+            {
+                if (x != null && y != null)
+                    return x.CompareTo(criteria, y);
+
+                throw new ArgumentNullException();
+            }
+        }
     }
 }