From 442347df8eee74c903a4564ab8ec92a8b20c7964 Mon Sep 17 00:00:00 2001
From: Samuel Cattini-Schultz <syriiins@gmail.com>
Date: Fri, 19 Feb 2021 18:04:25 +1100
Subject: [PATCH] Fix clockrate adjusted difficulty calculations bug in strain
 decay

When starting a new section, the starting strain value was calculated using the unadjusted timing value, meaning decay curves were essentially being stretched or squashed according to the clockrate.

This caused incorrect strain peaks for any section where the peak occurs at the start of the section (none of the objects in the section added enough strain after decay to exceed the starting strain).

This bug caused star ratings with clockrates above 1 to be lower than they should and below 1 to be higher than they should.
---
 osu.Game.Rulesets.Mania/Difficulty/Skills/Strain.cs         | 4 ++--
 osu.Game/Rulesets/Difficulty/DifficultyCalculator.cs        | 2 +-
 .../Difficulty/Preprocessing/DifficultyHitObject.cs         | 6 ++++++
 osu.Game/Rulesets/Difficulty/Skills/Skill.cs                | 6 +++---
 4 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/osu.Game.Rulesets.Mania/Difficulty/Skills/Strain.cs b/osu.Game.Rulesets.Mania/Difficulty/Skills/Strain.cs
index 7ebc1ff752..56fb138b1f 100644
--- a/osu.Game.Rulesets.Mania/Difficulty/Skills/Strain.cs
+++ b/osu.Game.Rulesets.Mania/Difficulty/Skills/Strain.cs
@@ -71,8 +71,8 @@ namespace osu.Game.Rulesets.Mania.Difficulty.Skills
         }
 
         protected override double GetPeakStrain(double offset)
-            => applyDecay(individualStrain, offset - Previous[0].BaseObject.StartTime, individual_decay_base)
-               + applyDecay(overallStrain, offset - Previous[0].BaseObject.StartTime, overall_decay_base);
+            => applyDecay(individualStrain, offset - Previous[0].StartTime, individual_decay_base)
+               + applyDecay(overallStrain, offset - Previous[0].StartTime, overall_decay_base);
 
         private double applyDecay(double value, double deltaTime, double decayBase)
             => value * Math.Pow(decayBase, deltaTime / 1000);
diff --git a/osu.Game/Rulesets/Difficulty/DifficultyCalculator.cs b/osu.Game/Rulesets/Difficulty/DifficultyCalculator.cs
index f15e5e1df0..8c2292dcaa 100644
--- a/osu.Game/Rulesets/Difficulty/DifficultyCalculator.cs
+++ b/osu.Game/Rulesets/Difficulty/DifficultyCalculator.cs
@@ -83,7 +83,7 @@ namespace osu.Game.Rulesets.Difficulty
                     foreach (Skill s in skills)
                     {
                         s.SaveCurrentPeak();
-                        s.StartNewSectionFrom(currentSectionEnd);
+                        s.StartNewSectionFrom(currentSectionEnd / clockRate);
                     }
 
                     currentSectionEnd += sectionLength;
diff --git a/osu.Game/Rulesets/Difficulty/Preprocessing/DifficultyHitObject.cs b/osu.Game/Rulesets/Difficulty/Preprocessing/DifficultyHitObject.cs
index ebbffb5143..fa578d55f0 100644
--- a/osu.Game/Rulesets/Difficulty/Preprocessing/DifficultyHitObject.cs
+++ b/osu.Game/Rulesets/Difficulty/Preprocessing/DifficultyHitObject.cs
@@ -25,6 +25,11 @@ namespace osu.Game.Rulesets.Difficulty.Preprocessing
         /// </summary>
         public readonly double DeltaTime;
 
+        /// <summary>
+        /// Start time of <see cref="BaseObject"/>.
+        /// </summary>
+        public readonly double StartTime;
+
         /// <summary>
         /// Creates a new <see cref="DifficultyHitObject"/>.
         /// </summary>
@@ -36,6 +41,7 @@ namespace osu.Game.Rulesets.Difficulty.Preprocessing
             BaseObject = hitObject;
             LastObject = lastObject;
             DeltaTime = (hitObject.StartTime - lastObject.StartTime) / clockRate;
+            StartTime = hitObject.StartTime / clockRate;
         }
     }
 }
diff --git a/osu.Game/Rulesets/Difficulty/Skills/Skill.cs b/osu.Game/Rulesets/Difficulty/Skills/Skill.cs
index 1063a24b27..44ce78c8e3 100644
--- a/osu.Game/Rulesets/Difficulty/Skills/Skill.cs
+++ b/osu.Game/Rulesets/Difficulty/Skills/Skill.cs
@@ -75,7 +75,7 @@ namespace osu.Game.Rulesets.Difficulty.Skills
         /// <summary>
         /// Sets the initial strain level for a new section.
         /// </summary>
-        /// <param name="time">The beginning of the new section in milliseconds.</param>
+        /// <param name="time">The beginning of the new section in milliseconds, adjusted by clockrate.</param>
         public void StartNewSectionFrom(double time)
         {
             // The maximum strain of the new section is not zero by default, strain decays as usual regardless of section boundaries.
@@ -87,9 +87,9 @@ namespace osu.Game.Rulesets.Difficulty.Skills
         /// <summary>
         /// Retrieves the peak strain at a point in time.
         /// </summary>
-        /// <param name="time">The time to retrieve the peak strain at.</param>
+        /// <param name="time">The time to retrieve the peak strain at, adjusted by clockrate.</param>
         /// <returns>The peak strain.</returns>
-        protected virtual double GetPeakStrain(double time) => CurrentStrain * strainDecay(time - Previous[0].BaseObject.StartTime);
+        protected virtual double GetPeakStrain(double time) => CurrentStrain * strainDecay(time - Previous[0].StartTime);
 
         /// <summary>
         /// Returns the calculated difficulty value representing all processed <see cref="DifficultyHitObject"/>s.