Add component for unstable rate statistic

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

@@ -0,0 +1,39 @@
+// 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
+{
+    /// <summary>
+    /// Displays the unstable rate statistic for a given play.
+    /// </summary>
+    public class UnstableRate : SimpleStatisticItem<double>
+    {
+        /// <summary>
+        /// Creates and computes an <see cref="UnstableRate"/> statistic.
+        /// </summary>
+        /// <param name="hitEvents">Sequence of <see cref="HitEvent"/>s to calculate the unstable rate based on.</param>
+        public UnstableRate(IEnumerable<HitEvent> hitEvents)
+            : base("Unstable Rate")
+        {
+            var timeOffsets = hitEvents.Select(ev => ev.TimeOffset).ToArray();
+            Value = 10 * standardDeviation(timeOffsets);
+        }
+
+        private static double standardDeviation(double[] timeOffsets)
+        {
+            if (timeOffsets.Length == 0)
+                return double.NaN;
+
+            var mean = timeOffsets.Average();
+            var 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");
+    }
+}