Fix HUD unstable rate counter not including judgements before load complete

Also unifies UR calculation logic with the results screen statistic item
to reduce duplication.

osu.Game.Tests/NonVisual/Ranking/UnstableRateTest.cs

@@ -22,7 +22,8 @@ public void TestDistributedHits()
 
             var unstableRate = new UnstableRate(events);
 
-            Assert.IsTrue(Precision.AlmostEquals(unstableRate.Value, 10 * Math.Sqrt(10)));
+            Assert.IsNotNull(unstableRate.Value);
+            Assert.IsTrue(Precision.AlmostEquals(unstableRate.Value.Value, 10 * Math.Sqrt(10)));
         }
 
         [Test]

osu.Game/Rulesets/Scoring/HitEventExtensions.cs

@@ -0,0 +1,37 @@
+// 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.Rulesets.Scoring
+{
+    public static class HitEventExtensions
+    {
+        /// <summary>
+        /// Calculates the "unstable rate" for a sequence of <see cref="HitEvent"/>s.
+        /// </summary>
+        /// <returns>
+        /// A non-null <see langword="double"/> value if unstable rate could be calculated,
+        /// and <see langword="null"/> if unstable rate cannot be calculated due to <paramref name="hitEvents"/> being empty.
+        /// </returns>
+        public static double? CalculateUnstableRate(this IEnumerable<HitEvent> hitEvents)
+        {
+            double[] timeOffsets = hitEvents.Where(affectsUnstableRate).Select(ev => ev.TimeOffset).ToArray();
+            return 10 * standardDeviation(timeOffsets);
+        }
+
+        private static bool affectsUnstableRate(HitEvent e) => !(e.HitObject.HitWindows is HitWindows.EmptyHitWindows) && e.Result.IsHit();
+
+        private static double? standardDeviation(double[] timeOffsets)
+        {
+            if (timeOffsets.Length == 0)
+                return null;
+
+            double mean = timeOffsets.Average();
+            double squares = timeOffsets.Select(offset => Math.Pow(offset - mean, 2)).Sum();
+            return Math.Sqrt(squares / timeOffsets.Length);
+        }
+    }
+}

osu.Game/Screens/Play/HUD/UnstableRateCounter.cs

@@ -1,9 +1,6 @@
 // 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;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
 using osu.Framework.Graphics;
@@ -29,8 +26,6 @@ public class UnstableRateCounter : RollingCounter<int>, ISkinnableDrawable
         private const float alpha_when_invalid = 0.3f;
         private readonly Bindable<bool> valid = new Bindable<bool>();
 
-        private readonly List<double> hitOffsets = new List<double>();
-
         [Resolved]
         private ScoreProcessor scoreProcessor { get; set; }
 
@@ -54,41 +49,20 @@ protected override void LoadComplete()
         {
             base.LoadComplete();
 
-            scoreProcessor.NewJudgement += onJudgementAdded;
-            scoreProcessor.JudgementReverted += onJudgementReverted;
-        }
-
-        private void onJudgementAdded(JudgementResult judgement)
-        {
-            if (!changesUnstableRate(judgement)) return;
-
-            hitOffsets.Add(judgement.TimeOffset);
+            scoreProcessor.NewJudgement += updateDisplay;
+            scoreProcessor.JudgementReverted += updateDisplay;
             updateDisplay();
         }
 
-        private void onJudgementReverted(JudgementResult judgement)
-        {
-            if (judgement.FailedAtJudgement || !changesUnstableRate(judgement)) return;
-
-            hitOffsets.RemoveAt(hitOffsets.Count - 1);
-            updateDisplay();
-        }
+        private void updateDisplay(JudgementResult _) => Scheduler.AddOnce(updateDisplay);
 
         private void updateDisplay()
         {
-            // At Count = 0, we get NaN, While we are allowing count = 1, it will be 0 since average = offset.
-            if (hitOffsets.Count > 0)
-            {
-                double mean = hitOffsets.Average();
-                double squares = hitOffsets.Select(offset => Math.Pow(offset - mean, 2)).Sum();
-                Current.Value = (int)(Math.Sqrt(squares / hitOffsets.Count) * 10);
-                valid.Value = true;
-            }
-            else
-            {
-                Current.Value = 0;
-                valid.Value = false;
-            }
+            double? unstableRate = scoreProcessor.HitEvents.CalculateUnstableRate();
+
+            valid.Value = unstableRate != null;
+            if (unstableRate != null)
+                Current.Value = (int)unstableRate.Value;
         }
 
         protected override IHasText CreateText() => new TextComponent
@@ -102,8 +76,8 @@ protected override void Dispose(bool isDisposing)
 
             if (scoreProcessor == null) return;
 
-            scoreProcessor.NewJudgement -= onJudgementAdded;
-            scoreProcessor.JudgementReverted -= onJudgementReverted;
+            scoreProcessor.NewJudgement -= updateDisplay;
+            scoreProcessor.JudgementReverted -= updateDisplay;
         }
 
         private class TextComponent : CompositeDrawable, IHasText

osu.Game/Screens/Ranking/Statistics/UnstableRate.cs

@@ -1,9 +1,7 @@
 // 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;
 using osu.Game.Rulesets.Scoring;
 
 namespace osu.Game.Screens.Ranking.Statistics
@@ -11,7 +9,7 @@ namespace osu.Game.Screens.Ranking.Statistics
     /// <summary>
     /// Displays the unstable rate statistic for a given play.
     /// </summary>
-    public class UnstableRate : SimpleStatisticItem<double>
+    public class UnstableRate : SimpleStatisticItem<double?>
     {
         /// <summary>
         /// Creates and computes an <see cref="UnstableRate"/> statistic.
@@ -20,21 +18,9 @@ public class UnstableRate : SimpleStatisticItem<double>
         public UnstableRate(IEnumerable<HitEvent> hitEvents)
             : base("Unstable Rate")
         {
-            double[] timeOffsets = hitEvents.Where(e => !(e.HitObject.HitWindows is HitWindows.EmptyHitWindows) && e.Result.IsHit())
-                                            .Select(ev => ev.TimeOffset).ToArray();
-            Value = 10 * standardDeviation(timeOffsets);
+            Value = hitEvents.CalculateUnstableRate();
         }
 
-        private static double standardDeviation(double[] timeOffsets)
-        {
-            if (timeOffsets.Length == 0)
-                return double.NaN;
-
-            double mean = timeOffsets.Average();
-            double squares = timeOffsets.Select(offset => Math.Pow(offset - mean, 2)).Sum();
-            return Math.Sqrt(squares / timeOffsets.Length);
-        }
-
-        protected override string DisplayValue(double value) => double.IsNaN(value) ? "(not available)" : value.ToString("N2");
+        protected override string DisplayValue(double? value) => value == null ? "(not available)" : value.Value.ToString(@"N2");
     }
 }