Reimplement as extension method on IBeatmap

Implementation has changed slightly to support arbitrary levels of
nested hitobjects.

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

@@ -9,7 +9,6 @@ using osu.Game.Rulesets.Difficulty;
 using osu.Game.Rulesets.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Difficulty.Skills;
 using osu.Game.Rulesets.Mods;
-using osu.Game.Rulesets.Objects;
 using osu.Game.Rulesets.Osu.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Osu.Difficulty.Skills;
 using osu.Game.Rulesets.Osu.Mods;
@@ -62,21 +61,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty
 
             double preempt = IBeatmapDifficultyInfo.DifficultyRange(beatmap.Difficulty.ApproachRate, 1800, 1200, 450) / clockRate;
             double drainRate = beatmap.Difficulty.DrainRate;
-
-            int maxCombo = 0;
-
-            void countCombo(HitObject ho)
-            {
-                if (ho.CreateJudgement().MaxResult.AffectsCombo())
-                    maxCombo++;
-            }
-
-            foreach (HitObject ho in beatmap.HitObjects)
-            {
-                countCombo(ho);
-                foreach (HitObject nested in ho.NestedHitObjects)
-                    countCombo(nested);
-            }
+            int maxCombo = beatmap.GetMaxCombo();
 
             int hitCirclesCount = beatmap.HitObjects.Count(h => h is HitCircle);
             int sliderCount = beatmap.HitObjects.Count(h => h is Slider);

osu.Game/Beatmaps/IBeatmap.cs

@@ -5,6 +5,7 @@ using System.Collections.Generic;
 using osu.Game.Beatmaps.ControlPoints;
 using osu.Game.Beatmaps.Timing;
 using osu.Game.Rulesets.Objects;
+using osu.Game.Rulesets.Scoring;
 
 namespace osu.Game.Beatmaps
 {
@@ -70,4 +71,27 @@ namespace osu.Game.Beatmaps
         /// </summary>
         new IReadOnlyList<T> HitObjects { get; }
     }
+
+    public static class BeatmapExtensions
+    {
+        /// <summary>
+        /// Finds the maximum achievable combo by hitting all <see cref="HitObject"/>s in a beatmap.
+        /// </summary>
+        public static int GetMaxCombo(this IBeatmap beatmap)
+        {
+            int combo = 0;
+            foreach (var h in beatmap.HitObjects)
+                addCombo(h, ref combo);
+            return combo;
+
+            static void addCombo(HitObject hitObject, ref int combo)
+            {
+                if (hitObject.CreateJudgement().MaxResult.AffectsCombo())
+                    combo++;
+
+                foreach (var nested in hitObject.NestedHitObjects)
+                    addCombo(nested, ref combo);
+            }
+        }
+    }
 }