Merge pull request #18067 from smoogipoo/reduce-diffcalc-sort-complexity

Reduce sorting worst-case time complexity in difficulty calculation

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

@@ -38,7 +38,11 @@ public override double DifficultyValue()
             double difficulty = 0;
             double weight = 1;
 
-            List<double> strains = GetCurrentStrainPeaks().OrderByDescending(d => d).ToList();
+            // Sections with 0 strain are excluded to avoid worst-case time complexity of the following sort (e.g. /b/2351871).
+            // These sections will not contribute to the difficulty.
+            var peaks = GetCurrentStrainPeaks().Where(p => p > 0);
+
+            List<double> strains = peaks.OrderByDescending(d => d).ToList();
 
             // We are reducing the highest strains first to account for extreme difficulty spikes
             for (int i = 0; i < Math.Min(strains.Count, ReducedSectionCount); i++)

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

@@ -141,7 +141,13 @@ private double locallyCombinedDifficulty(Colour colour, Rhythm rhythm, Stamina s
                 double colourPeak = colourPeaks[i] * colour_skill_multiplier;
                 double rhythmPeak = rhythmPeaks[i] * rhythm_skill_multiplier;
                 double staminaPeak = (staminaRightPeaks[i] + staminaLeftPeaks[i]) * stamina_skill_multiplier * staminaPenalty;
-                peaks.Add(norm(2, colourPeak, rhythmPeak, staminaPeak));
+
+                double peak = norm(2, colourPeak, rhythmPeak, staminaPeak);
+
+                // Sections with 0 strain are excluded to avoid worst-case time complexity of the following sort (e.g. /b/2351871).
+                // These sections will not contribute to the difficulty.
+                if (peak > 0)
+                    peaks.Add(peak);
             }
 
             double difficulty = 0;

osu.Game/Rulesets/Difficulty/Skills/StrainSkill.cs

@@ -100,9 +100,13 @@ public override double DifficultyValue()
             double difficulty = 0;
             double weight = 1;
 
+            // Sections with 0 strain are excluded to avoid worst-case time complexity of the following sort (e.g. /b/2351871).
+            // These sections will not contribute to the difficulty.
+            var peaks = GetCurrentStrainPeaks().Where(p => p > 0);
+
             // Difficulty is the weighted sum of the highest strains from every section.
             // We're sorting from highest to lowest strain.
-            foreach (double strain in GetCurrentStrainPeaks().OrderByDescending(d => d))
+            foreach (double strain in peaks.OrderByDescending(d => d))
             {
                 difficulty += strain * weight;
                 weight *= DecayWeight;