Optimise mod comparison function for most common case

Author: peppy

osu.Server.Queues.ScoreStatisticsProcessor/Commands/Maintenance/MarkNonPreservedScoresCommand.cs

@@ -10,6 +10,7 @@
 using Dapper;
 using McMaster.Extensions.CommandLineUtils;
 using MySqlConnector;
+using osu.Game.Extensions;
 using osu.Server.QueueProcessor;
 using osu.Server.Queues.ScoreStatisticsProcessor.Helpers;
 using osu.Server.Queues.ScoreStatisticsProcessor.Models;
@@ -155,6 +156,9 @@ private static void formatOutput(SoloScore score, bool delete, string reason)
             );
         }
 
+        private static readonly HashSet<string> a_hash_set = new HashSet<string>();
+        private static readonly HashSet<string> b_hash_set = new HashSet<string>();
+
         private static bool checkIsUserHigh(IEnumerable<SoloScore> userScores, SoloScore candidate, out HashSet<SoloScore> preservedAlternatives)
         {
             var scores = userScores.Where(s =>
@@ -200,11 +204,22 @@ private static bool checkIsUserHigh(IEnumerable<SoloScore> userScores, SoloScore
 
             static bool compareMods(SoloScore a, SoloScore b)
             {
-                // Compare non-ordered mods, ignoring any settings applied.
-                var aMods = new HashSet<string>(a.ScoreData.Mods.Select(m => m.Acronym));
-                var bMods = new HashSet<string>(b.ScoreData.Mods.Select(m => m.Acronym));
+                var aMods = a.ScoreData.Mods;
+                var bMods = b.ScoreData.Mods;
+
+                if (aMods.Length == 0 && bMods.Length == 0)
+                    return true;
+
+                if (aMods.Length != bMods.Length)
+                    return false;
+
+                a_hash_set.Clear();
+                a_hash_set.AddRange(aMods.Select(m => m.Acronym));
+
+                b_hash_set.Clear();
+                b_hash_set.AddRange(bMods.Select(m => m.Acronym));
 
-                return aMods.SetEquals(bMods);
+                return a_hash_set.SetEquals(b_hash_set);
             }
         }
     }