Fix GetRateAdjustedDisplayDifficulty() (partially incorrectly) locally reimplementing difficulty range calculations

osu.Game.Rulesets.Catch.Tests/CatchRateAdjustedDisplayDifficultyTest.cs

@@ -0,0 +1,52 @@
+// 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.Collections.Generic;
+using NUnit.Framework;
+using osu.Game.Beatmaps;
+
+namespace osu.Game.Rulesets.Catch.Tests
+{
+    [TestFixture]
+    public class CatchRateAdjustedDisplayDifficultyTest
+    {
+        private static IEnumerable<float> difficultyValuesToTest()
+        {
+            for (float i = 0; i <= 10; i += 0.5f)
+                yield return i;
+        }
+
+        [TestCaseSource(nameof(difficultyValuesToTest))]
+        public void TestApproachRateIsUnchangedWithRateEqualToOne(float originalApproachRate)
+        {
+            var ruleset = new CatchRuleset();
+            var difficulty = new BeatmapDifficulty { ApproachRate = originalApproachRate };
+
+            var adjustedDifficulty = ruleset.GetRateAdjustedDisplayDifficulty(difficulty, 1);
+
+            Assert.That(adjustedDifficulty.ApproachRate, Is.EqualTo(originalApproachRate));
+        }
+
+        [Test]
+        public void TestRateBelowOne()
+        {
+            var ruleset = new CatchRuleset();
+            var difficulty = new BeatmapDifficulty();
+
+            var adjustedDifficulty = ruleset.GetRateAdjustedDisplayDifficulty(difficulty, 0.75);
+
+            Assert.That(adjustedDifficulty.ApproachRate, Is.EqualTo(1.67).Within(0.01));
+        }
+
+        [Test]
+        public void TestRateAboveOne()
+        {
+            var ruleset = new CatchRuleset();
+            var difficulty = new BeatmapDifficulty();
+
+            var adjustedDifficulty = ruleset.GetRateAdjustedDisplayDifficulty(difficulty, 1.5);
+
+            Assert.That(adjustedDifficulty.ApproachRate, Is.EqualTo(7.67).Within(0.01));
+        }
+    }
+}

osu.Game.Rulesets.Catch/CatchRuleset.cs

@@ -15,6 +15,7 @@
 using osu.Game.Rulesets.Catch.Difficulty;
 using osu.Game.Rulesets.Catch.Edit;
 using osu.Game.Rulesets.Catch.Mods;
+using osu.Game.Rulesets.Catch.Objects;
 using osu.Game.Rulesets.Catch.Replays;
 using osu.Game.Rulesets.Catch.Scoring;
 using osu.Game.Rulesets.Catch.Skinning.Argon;
@@ -236,17 +237,14 @@ public override StatisticItem[] CreateStatisticsForScore(ScoreInfo score, IBeatm
             };
         }
 
+        /// <seealso cref="CatchHitObject.ApplyDefaultsToSelf"/>
         public override BeatmapDifficulty GetRateAdjustedDisplayDifficulty(IBeatmapDifficultyInfo difficulty, double rate)
         {
             BeatmapDifficulty adjustedDifficulty = new BeatmapDifficulty(difficulty);
 
-            double preempt = adjustedDifficulty.ApproachRate < 6
-                ? 1200.0 + 600.0 * (5 - adjustedDifficulty.ApproachRate) / 5
-                : 1200.0 - 750.0 * (adjustedDifficulty.ApproachRate - 5) / 5;
-
+            double preempt = IBeatmapDifficultyInfo.DifficultyRange(adjustedDifficulty.ApproachRate, CatchHitObject.PREEMPT_MAX, CatchHitObject.PREEMPT_MID, CatchHitObject.PREEMPT_MIN);
             preempt /= rate;
-
-            adjustedDifficulty.ApproachRate = (float)(preempt > 1200 ? (1800 - preempt) / 120 : (1200 - preempt) / 150 + 5);
+            adjustedDifficulty.ApproachRate = (float)IBeatmapDifficultyInfo.InverseDifficultyRange(preempt, CatchHitObject.PREEMPT_MAX, CatchHitObject.PREEMPT_MID, CatchHitObject.PREEMPT_MIN);
 
             return adjustedDifficulty;
         }

osu.Game.Rulesets.Catch/Objects/CatchHitObject.cs

@@ -150,7 +150,7 @@ protected override void ApplyDefaultsToSelf(ControlPointInfo controlPointInfo, I
         {
             base.ApplyDefaultsToSelf(controlPointInfo, difficulty);
 
-            TimePreempt = (float)IBeatmapDifficultyInfo.DifficultyRange(difficulty.ApproachRate, 1800, 1200, 450);
+            TimePreempt = (float)IBeatmapDifficultyInfo.DifficultyRange(difficulty.ApproachRate, PREEMPT_MAX, PREEMPT_MID, PREEMPT_MIN);
 
             Scale = LegacyRulesetExtensions.CalculateScaleFromCircleSize(difficulty.CircleSize);
         }
@@ -189,6 +189,21 @@ public void UpdateComboInformation(IHasComboInformation? lastObj)
         // The half of the height of the osu! playfield.
         public const float DEFAULT_LEGACY_CONVERT_Y = 192;
 
+        /// <summary>
+        /// Minimum preempt time at AR=10.
+        /// </summary>
+        public const double PREEMPT_MIN = 450;
+
+        /// <summary>
+        /// Median preempt time at AR=5.
+        /// </summary>
+        public const double PREEMPT_MID = 1200;
+
+        /// <summary>
+        /// Maximum preempt time at AR=0.
+        /// </summary>
+        public const double PREEMPT_MAX = 1800;
+
         /// <summary>
         /// The Y position of the hit object is not used in the normal osu!catch gameplay.
         /// It is preserved to maximize the backward compatibility with the legacy editor, in which the mappers use the Y position to organize the patterns.

osu.Game.Rulesets.Osu.Tests/OsuRateAdjustedDisplayDifficultyTest.cs

@@ -0,0 +1,65 @@
+// 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.Collections.Generic;
+using NUnit.Framework;
+using osu.Game.Beatmaps;
+
+namespace osu.Game.Rulesets.Osu.Tests
+{
+    [TestFixture]
+    public class OsuRateAdjustedDisplayDifficultyTest
+    {
+        private static IEnumerable<float> difficultyValuesToTest()
+        {
+            for (float i = 0; i <= 10; i += 0.5f)
+                yield return i;
+        }
+
+        [TestCaseSource(nameof(difficultyValuesToTest))]
+        public void TestApproachRateIsUnchangedWithRateEqualToOne(float originalApproachRate)
+        {
+            var ruleset = new OsuRuleset();
+            var difficulty = new BeatmapDifficulty { ApproachRate = originalApproachRate };
+
+            var adjustedDifficulty = ruleset.GetRateAdjustedDisplayDifficulty(difficulty, 1);
+
+            Assert.That(adjustedDifficulty.ApproachRate, Is.EqualTo(originalApproachRate));
+        }
+
+        [TestCaseSource(nameof(difficultyValuesToTest))]
+        public void TestOverallDifficultyIsUnchangedWithRateEqualToOne(float originalOverallDifficulty)
+        {
+            var ruleset = new OsuRuleset();
+            var difficulty = new BeatmapDifficulty { OverallDifficulty = originalOverallDifficulty };
+
+            var adjustedDifficulty = ruleset.GetRateAdjustedDisplayDifficulty(difficulty, 1);
+
+            Assert.That(adjustedDifficulty.OverallDifficulty, Is.EqualTo(originalOverallDifficulty));
+        }
+
+        [Test]
+        public void TestRateBelowOne()
+        {
+            var ruleset = new OsuRuleset();
+            var difficulty = new BeatmapDifficulty();
+
+            var adjustedDifficulty = ruleset.GetRateAdjustedDisplayDifficulty(difficulty, 0.75);
+
+            Assert.That(adjustedDifficulty.ApproachRate, Is.EqualTo(1.67).Within(0.01));
+            Assert.That(adjustedDifficulty.OverallDifficulty, Is.EqualTo(2.22).Within(0.01));
+        }
+
+        [Test]
+        public void TestRateAboveOne()
+        {
+            var ruleset = new OsuRuleset();
+            var difficulty = new BeatmapDifficulty();
+
+            var adjustedDifficulty = ruleset.GetRateAdjustedDisplayDifficulty(difficulty, 1.5);
+
+            Assert.That(adjustedDifficulty.ApproachRate, Is.EqualTo(7.67).Within(0.01));
+            Assert.That(adjustedDifficulty.OverallDifficulty, Is.EqualTo(7.77).Within(0.01));
+        }
+    }
+}

osu.Game.Rulesets.Osu/Objects/OsuHitObject.cs

@@ -37,6 +37,16 @@ public abstract class OsuHitObject : HitObject, IHasComboInformation, IHasPositi
         /// </summary>
         public const double PREEMPT_MIN = 450;
 
+        /// <summary>
+        /// Median preempt time at AR=5.
+        /// </summary>
+        public const double PREEMPT_MID = 1200;
+
+        /// <summary>
+        /// Maximum preempt time at AR=0.
+        /// </summary>
+        public const double PREEMPT_MAX = 1800;
+
         public double TimePreempt = 600;
         public double TimeFadeIn = 400;
 
@@ -148,7 +158,7 @@ protected override void ApplyDefaultsToSelf(ControlPointInfo controlPointInfo, I
         {
             base.ApplyDefaultsToSelf(controlPointInfo, difficulty);
 
-            TimePreempt = (float)IBeatmapDifficultyInfo.DifficultyRange(difficulty.ApproachRate, 1800, 1200, PREEMPT_MIN);
+            TimePreempt = (float)IBeatmapDifficultyInfo.DifficultyRange(difficulty.ApproachRate, PREEMPT_MAX, PREEMPT_MID, PREEMPT_MIN);
 
             // Preempt time can go below 450ms. Normally, this is achieved via the DT mod which uniformly speeds up all animations game wide regardless of AR.
             // This uniform speedup is hard to match 1:1, however we can at least make AR>10 (via mods) feel good by extending the upper linear function above.

osu.Game.Rulesets.Osu/OsuRuleset.cs

@@ -332,23 +332,20 @@ public override StatisticItem[] CreateStatisticsForScore(ScoreInfo score, IBeatm
 
         public override RulesetSetupSection CreateEditorSetupSection() => new OsuSetupSection();
 
+        /// <seealso cref="OsuHitObject.ApplyDefaultsToSelf"/>
+        /// <seealso cref="OsuHitWindows"/>
         public override BeatmapDifficulty GetRateAdjustedDisplayDifficulty(IBeatmapDifficultyInfo difficulty, double rate)
         {
             BeatmapDifficulty adjustedDifficulty = new BeatmapDifficulty(difficulty);
 
-            double preempt = adjustedDifficulty.ApproachRate < 5
-                ? 1200.0 + 600.0 * (5 - adjustedDifficulty.ApproachRate) / 5
-                : 1200.0 - 750.0 * (adjustedDifficulty.ApproachRate - 5) / 5;
-
+            double preempt = IBeatmapDifficultyInfo.DifficultyRange(adjustedDifficulty.ApproachRate, OsuHitObject.PREEMPT_MAX, OsuHitObject.PREEMPT_MID, OsuHitObject.PREEMPT_MIN);
             preempt /= rate;
+            adjustedDifficulty.ApproachRate = (float)IBeatmapDifficultyInfo.InverseDifficultyRange(preempt, OsuHitObject.PREEMPT_MAX, OsuHitObject.PREEMPT_MID, OsuHitObject.PREEMPT_MIN);
 
-            adjustedDifficulty.ApproachRate = (float)(preempt > 1200 ? (1800 - preempt) / 120 : (1200 - preempt) / 150 + 5);
-
-            double hitwindow = 80.0 - 6 * adjustedDifficulty.OverallDifficulty;
-
-            hitwindow /= rate;
-
-            adjustedDifficulty.OverallDifficulty = (float)(80.0 - hitwindow) / 6;
+            var greatHitWindowRange = OsuHitWindows.OSU_RANGES.Single(range => range.Result == HitResult.Great);
+            double greatHitWindow = IBeatmapDifficultyInfo.DifficultyRange(adjustedDifficulty.OverallDifficulty, greatHitWindowRange.Min, greatHitWindowRange.Average, greatHitWindowRange.Max);
+            greatHitWindow /= rate;
+            adjustedDifficulty.OverallDifficulty = (float)IBeatmapDifficultyInfo.InverseDifficultyRange(greatHitWindow, greatHitWindowRange.Min, greatHitWindowRange.Average, greatHitWindowRange.Max);
 
             return adjustedDifficulty;
         }

osu.Game.Rulesets.Osu/Scoring/OsuHitWindows.cs

@@ -12,7 +12,7 @@ public class OsuHitWindows : HitWindows
         /// </summary>
         public const double MISS_WINDOW = 400;
 
-        private static readonly DifficultyRange[] osu_ranges =
+        internal static readonly DifficultyRange[] OSU_RANGES =
         {
             new DifficultyRange(HitResult.Great, 80, 50, 20),
             new DifficultyRange(HitResult.Ok, 140, 100, 60),
@@ -34,6 +34,6 @@ public override bool IsHitResultAllowed(HitResult result)
             return false;
         }
 
-        protected override DifficultyRange[] GetRanges() => osu_ranges;
+        protected override DifficultyRange[] GetRanges() => OSU_RANGES;
     }
 }

osu.Game.Rulesets.Taiko.Tests/TaikoRateAdjustedDisplayDifficultyTest.cs

@@ -0,0 +1,52 @@
+// 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.Collections.Generic;
+using NUnit.Framework;
+using osu.Game.Beatmaps;
+
+namespace osu.Game.Rulesets.Taiko.Tests
+{
+    [TestFixture]
+    public class TaikoRateAdjustedDisplayDifficultyTest
+    {
+        private static IEnumerable<float> difficultyValuesToTest()
+        {
+            for (float i = 0; i <= 10; i += 0.5f)
+                yield return i;
+        }
+
+        [TestCaseSource(nameof(difficultyValuesToTest))]
+        public void TestOverallDifficultyIsUnchangedWithRateEqualToOne(float originalOverallDifficulty)
+        {
+            var ruleset = new TaikoRuleset();
+            var difficulty = new BeatmapDifficulty { OverallDifficulty = originalOverallDifficulty };
+
+            var adjustedDifficulty = ruleset.GetRateAdjustedDisplayDifficulty(difficulty, 1);
+
+            Assert.That(adjustedDifficulty.OverallDifficulty, Is.EqualTo(originalOverallDifficulty));
+        }
+
+        [Test]
+        public void TestRateBelowOne()
+        {
+            var ruleset = new TaikoRuleset();
+            var difficulty = new BeatmapDifficulty();
+
+            var adjustedDifficulty = ruleset.GetRateAdjustedDisplayDifficulty(difficulty, 0.75);
+
+            Assert.That(adjustedDifficulty.OverallDifficulty, Is.EqualTo(1.11).Within(0.01));
+        }
+
+        [Test]
+        public void TestRateAboveOne()
+        {
+            var ruleset = new TaikoRuleset();
+            var difficulty = new BeatmapDifficulty();
+
+            var adjustedDifficulty = ruleset.GetRateAdjustedDisplayDifficulty(difficulty, 1.5);
+
+            Assert.That(adjustedDifficulty.OverallDifficulty, Is.EqualTo(8.89).Within(0.01));
+        }
+    }
+}

osu.Game.Rulesets.Taiko/Scoring/TaikoHitWindows.cs

@@ -7,7 +7,7 @@ namespace osu.Game.Rulesets.Taiko.Scoring
 {
     public class TaikoHitWindows : HitWindows
     {
-        private static readonly DifficultyRange[] taiko_ranges =
+        internal static readonly DifficultyRange[] TAIKO_RANGES =
         {
             new DifficultyRange(HitResult.Great, 50, 35, 20),
             new DifficultyRange(HitResult.Ok, 120, 80, 50),
@@ -27,6 +27,6 @@ public override bool IsHitResultAllowed(HitResult result)
             return false;
         }
 
-        protected override DifficultyRange[] GetRanges() => taiko_ranges;
+        protected override DifficultyRange[] GetRanges() => TAIKO_RANGES;
     }
 }

osu.Game.Rulesets.Taiko/TaikoRuleset.cs

@@ -265,15 +265,15 @@ public override StatisticItem[] CreateStatisticsForScore(ScoreInfo score, IBeatm
             };
         }
 
+        /// <seealso cref="TaikoHitWindows"/>
         public override BeatmapDifficulty GetRateAdjustedDisplayDifficulty(IBeatmapDifficultyInfo difficulty, double rate)
         {
             BeatmapDifficulty adjustedDifficulty = new BeatmapDifficulty(difficulty);
 
-            double hitWindow = 35.0 - 15.0 * (adjustedDifficulty.OverallDifficulty - 5) / 5;
-
-            hitWindow /= rate;
-
-            adjustedDifficulty.OverallDifficulty = (float)(5 * (35 - hitWindow) / 15 + 5);
+            var greatHitWindowRange = TaikoHitWindows.TAIKO_RANGES.Single(range => range.Result == HitResult.Great);
+            double greatHitWindow = IBeatmapDifficultyInfo.DifficultyRange(adjustedDifficulty.OverallDifficulty, greatHitWindowRange.Min, greatHitWindowRange.Average, greatHitWindowRange.Max);
+            greatHitWindow /= rate;
+            adjustedDifficulty.OverallDifficulty = (float)IBeatmapDifficultyInfo.InverseDifficultyRange(greatHitWindow, greatHitWindowRange.Min, greatHitWindowRange.Average, greatHitWindowRange.Max);
 
             return adjustedDifficulty;
         }

osu.Game/Beatmaps/IBeatmapDifficultyInfo.cs

@@ -1,6 +1,8 @@
 // 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;
+
 namespace osu.Game.Beatmaps
 {
     /// <summary>
@@ -92,5 +94,21 @@ static double DifficultyRange(double difficulty, double min, double mid, double
         /// <returns>Value to which the difficulty value maps in the specified range.</returns>
         static double DifficultyRange(double difficulty, (double od0, double od5, double od10) range)
             => DifficultyRange(difficulty, range.od0, range.od5, range.od10);
+
+        /// <summary>
+        /// Inverse function to <see cref="DifficultyRange(double,double,double,double)"/>.
+        /// Maps a value returned by the function above back to the difficulty that produced it.
+        /// </summary>
+        /// <param name="difficultyValue">The difficulty-dependent value to be unmapped.</param>
+        /// <param name="diff0">Minimum of the resulting range which will be achieved by a difficulty value of 0.</param>
+        /// <param name="diff5">Midpoint of the resulting range which will be achieved by a difficulty value of 5.</param>
+        /// <param name="diff10">Maximum of the resulting range which will be achieved by a difficulty value of 10.</param>
+        /// <returns>Value to which the difficulty value maps in the specified range.</returns>
+        static double InverseDifficultyRange(double difficultyValue, double diff0, double diff5, double diff10)
+        {
+            return Math.Sign(difficultyValue - diff5) == Math.Sign(diff10 - diff5)
+                ? (difficultyValue - diff5) / (diff10 - diff5) * 5 + 5
+                : (difficultyValue - diff5) / (diff5 - diff0) * 5 + 5;
+        }
     }
 }