Fix incorrect score conversion on selected beatmaps due to incorrect difficultyPeppyStars rounding

osu.Game.Rulesets.Catch/Difficulty/CatchLegacyScoreSimulator.cs

@@ -10,6 +10,7 @@
 using osu.Game.Rulesets.Catch.Scoring;
 using osu.Game.Rulesets.Mods;
 using osu.Game.Rulesets.Objects;
+using osu.Game.Rulesets.Objects.Legacy;
 using osu.Game.Rulesets.Objects.Types;
 using osu.Game.Rulesets.Scoring;
 using osu.Game.Rulesets.Scoring.Legacy;
@@ -62,13 +63,7 @@ public LegacyScoreAttributes Simulate(IWorkingBeatmap workingBeatmap, IBeatmap p
                 drainLength = ((int)Math.Round(baseBeatmap.HitObjects[^1].StartTime) - (int)Math.Round(baseBeatmap.HitObjects[0].StartTime) - breakLength) / 1000;
             }
 
-            int difficultyPeppyStars = (int)Math.Round(
-                (baseBeatmap.Difficulty.DrainRate
-                 + baseBeatmap.Difficulty.OverallDifficulty
-                 + baseBeatmap.Difficulty.CircleSize
-                 + Math.Clamp((float)objectCount / drainLength * 8, 0, 16)) / 38 * 5);
-
-            scoreMultiplier = difficultyPeppyStars;
+            scoreMultiplier = LegacyRulesetExtensions.CalculateDifficultyPeppyStars(baseBeatmap.Difficulty, objectCount, drainLength);
 
             LegacyScoreAttributes attributes = new LegacyScoreAttributes();
 

osu.Game.Rulesets.Osu/Difficulty/OsuLegacyScoreSimulator.cs

@@ -7,6 +7,7 @@
 using osu.Game.Beatmaps;
 using osu.Game.Rulesets.Mods;
 using osu.Game.Rulesets.Objects;
+using osu.Game.Rulesets.Objects.Legacy;
 using osu.Game.Rulesets.Objects.Types;
 using osu.Game.Rulesets.Osu.Mods;
 using osu.Game.Rulesets.Osu.Objects;
@@ -62,13 +63,7 @@ public LegacyScoreAttributes Simulate(IWorkingBeatmap workingBeatmap, IBeatmap p
                 drainLength = ((int)Math.Round(baseBeatmap.HitObjects[^1].StartTime) - (int)Math.Round(baseBeatmap.HitObjects[0].StartTime) - breakLength) / 1000;
             }
 
-            int difficultyPeppyStars = (int)Math.Round(
-                (baseBeatmap.Difficulty.DrainRate
-                 + baseBeatmap.Difficulty.OverallDifficulty
-                 + baseBeatmap.Difficulty.CircleSize
-                 + Math.Clamp((float)objectCount / drainLength * 8, 0, 16)) / 38 * 5);
-
-            scoreMultiplier = difficultyPeppyStars;
+            scoreMultiplier = LegacyRulesetExtensions.CalculateDifficultyPeppyStars(baseBeatmap.Difficulty, objectCount, drainLength);
 
             LegacyScoreAttributes attributes = new LegacyScoreAttributes();
 

osu.Game.Rulesets.Taiko/Difficulty/TaikoLegacyScoreSimulator.cs

@@ -7,6 +7,7 @@
 using osu.Game.Beatmaps;
 using osu.Game.Rulesets.Mods;
 using osu.Game.Rulesets.Objects;
+using osu.Game.Rulesets.Objects.Legacy;
 using osu.Game.Rulesets.Objects.Types;
 using osu.Game.Rulesets.Scoring;
 using osu.Game.Rulesets.Scoring.Legacy;
@@ -65,11 +66,7 @@ public LegacyScoreAttributes Simulate(IWorkingBeatmap workingBeatmap, IBeatmap p
                 drainLength = ((int)Math.Round(baseBeatmap.HitObjects[^1].StartTime) - (int)Math.Round(baseBeatmap.HitObjects[0].StartTime) - breakLength) / 1000;
             }
 
-            difficultyPeppyStars = (int)Math.Round(
-                (baseBeatmap.Difficulty.DrainRate
-                 + baseBeatmap.Difficulty.OverallDifficulty
-                 + baseBeatmap.Difficulty.CircleSize
-                 + Math.Clamp((float)objectCount / drainLength * 8, 0, 16)) / 38 * 5);
+            difficultyPeppyStars = LegacyRulesetExtensions.CalculateDifficultyPeppyStars(baseBeatmap.Difficulty, objectCount, drainLength);
 
             LegacyScoreAttributes attributes = new LegacyScoreAttributes();
 

osu.Game/Rulesets/Objects/Legacy/LegacyRulesetExtensions.cs

@@ -57,5 +57,40 @@ public static float CalculateScaleFromCircleSize(float circleSize, bool applyFud
 
             return (float)(1.0f - 0.7f * IBeatmapDifficultyInfo.DifficultyRange(circleSize)) / 2 * (applyFudge ? broken_gamefield_rounding_allowance : 1);
         }
+
+        public static int CalculateDifficultyPeppyStars(BeatmapDifficulty difficulty, int objectCount, int drainLength)
+        {
+            /*
+             * WARNING: DO NOT TOUCH IF YOU DO NOT KNOW WHAT YOU ARE DOING
+             *
+             * It so happens that in stable, due to .NET Framework internals, float math would be performed
+             * using x87 registers and opcodes.
+             * .NET (Core) however uses SSE instructions on 32- and 64-bit words.
+             * x87 registers are _80 bits_ wide. Which is notably wider than _both_ float and double.
+             * Therefore, on a significant number of beatmaps, the rounding would not produce correct values.
+             *
+             * Thus, to crudely - but, seemingly *mostly* accurately, after checking across all ranked maps - emulate this,
+             * use `decimal`, which is slow, but has bigger precision than `double`.
+             * At the time of writing, there is _one_ ranked exception to this - namely https://osu.ppy.sh/beatmapsets/1156087#osu/2625853 -
+             * but it is considered an "acceptable casualty", since in that case scores aren't inflated by _that_ much compared to others.
+             */
+
+            decimal objectToDrainRatio = drainLength != 0
+                ? Math.Clamp((decimal)objectCount / drainLength * 8, 0, 16)
+                : 16;
+
+            /*
+             * Notably, THE `double` CASTS BELOW ARE IMPORTANT AND MUST REMAIN.
+             * Their goal is to trick the compiler / runtime into NOT promoting from single-precision float, as doing so would prompt it
+             * to attempt to "silently" fix the single-precision values when converting to decimal,
+             * which is NOT what the x87 FPU does.
+             */
+
+            decimal drainRate = (decimal)(double)difficulty.DrainRate;
+            decimal overallDifficulty = (decimal)(double)difficulty.OverallDifficulty;
+            decimal circleSize = (decimal)(double)difficulty.CircleSize;
+
+            return (int)Math.Round((drainRate + overallDifficulty + circleSize + objectToDrainRatio) / 38 * 5);
+        }
     }
 }

osu.Game/Scoring/Legacy/LegacyScoreEncoder.cs

@@ -13,6 +13,7 @@
 using osu.Game.IO.Legacy;
 using osu.Game.IO.Serialization;
 using osu.Game.Replays.Legacy;
+using osu.Game.Rulesets.Objects.Legacy;
 using osu.Game.Rulesets.Replays;
 using osu.Game.Rulesets.Replays.Types;
 using SharpCompress.Compressors.LZMA;
@@ -38,9 +39,13 @@ public class LegacyScoreEncoder
         /// <item><description>30000009: Fix edge cases in conversion for scores which have 0.0x mod multiplier on stable. Reconvert all scores.</description></item>
         /// <item><description>30000010: Fix mania score V1 conversion using score V1 accuracy rather than V2 accuracy. Reconvert all scores.</description></item>
         /// <item><description>30000011: Re-do catch scoring to mirror stable Score V2 as closely as feasible. Reconvert all scores.</description></item>
+        /// <item><description>
+        /// 30000012: Fix incorrect total score conversion on selected beatmaps after implementing the more correct
+        /// <see cref="LegacyRulesetExtensions.CalculateDifficultyPeppyStars"/> method. Reconvert all scores.
+        /// </description></item>
         /// </list>
         /// </remarks>
-        public const int LATEST_VERSION = 30000011;
+        public const int LATEST_VERSION = 30000012;
 
         /// <summary>
         /// The first stable-compatible YYYYMMDD format version given to lazer usage of replays.