Refactor Reading strain to match the rest of the skills (ppy#37010)

This doesn't affect values as per the original refactor.

Co-authored-by: StanR <8269193+stanriders@users.noreply.github.com>

Author: kwotaq

osu.Game.Rulesets.Osu/Difficulty/Evaluators/ReadingEvaluator.cs

@@ -48,6 +48,9 @@ public static double EvaluateDifficultyOf(DifficultyHitObject current, bool hidd
 
             double difficulty = DifficultyCalculationUtils.Norm(1.5, preemptDifficulty, hiddenDifficulty, noteDensityDifficulty);
 
+            // Having less time to process information is harder
+            difficulty *= highBpmBonus(currObj.AdjustedDeltaTime);
+
             return difficulty;
         }
 
@@ -263,5 +266,7 @@ private static double getTimeNerfFactor(double deltaTime)
         {
             return Math.Clamp(2 - deltaTime / (reading_window_size / 2), 0, 1);
         }
+
+        private static double highBpmBonus(double ms) => 1 / (1 - Math.Pow(0.8, ms / 1000));
     }
 }

osu.Game.Rulesets.Osu/Difficulty/Skills/Reading.cs

@@ -37,9 +37,11 @@ protected override double ObjectDifficultyOf(DifficultyHitObject current)
         {
             objectList.Add(current);
 
-            currentDifficulty *= strainDecay(current.DeltaTime);
+            double decay = strainDecay(current.DeltaTime);
 
-            currentDifficulty += ReadingEvaluator.EvaluateDifficultyOf(current, hasHiddenMod) * skillMultiplier;
+            currentDifficulty *= decay;
+
+            currentDifficulty += ReadingEvaluator.EvaluateDifficultyOf(current, hasHiddenMod) * (1 - decay) * skillMultiplier;
 
             return currentDifficulty;
         }