Add a background beatmap difficulty manager

osu.Game/Beatmaps/BeatmapDifficultyManager.cs

@@ -0,0 +1,99 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using JetBrains.Annotations;
+using osu.Framework.Allocation;
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Threading;
+using osu.Game.Rulesets;
+using osu.Game.Rulesets.Mods;
+
+namespace osu.Game.Beatmaps
+{
+    public class BeatmapDifficultyManager : CompositeDrawable
+    {
+        // Too many simultaneous updates can lead to stutters. One thread seems to work fine for song select display purposes.
+        private readonly ThreadedTaskScheduler updateScheduler = new ThreadedTaskScheduler(1, nameof(BeatmapDifficultyManager));
+
+        private readonly TimedExpiryCache<DifficultyCacheLookup, double> difficultyCache = new TimedExpiryCache<DifficultyCacheLookup, double> { ExpiryTime = 60000 };
+        private readonly BeatmapManager beatmapManager;
+
+        public BeatmapDifficultyManager(BeatmapManager beatmapManager)
+        {
+            this.beatmapManager = beatmapManager;
+        }
+
+        public Task<double> GetDifficultyAsync([NotNull] BeatmapInfo beatmapInfo, [CanBeNull] RulesetInfo rulesetInfo = null, [CanBeNull] IReadOnlyList<Mod> mods = null,
+                                               CancellationToken cancellationToken = default)
+            => Task.Factory.StartNew(() => GetDifficulty(beatmapInfo, rulesetInfo, mods), cancellationToken,
+                TaskCreationOptions.HideScheduler | TaskCreationOptions.RunContinuationsAsynchronously,
+                updateScheduler);
+
+        public double GetDifficulty([NotNull] BeatmapInfo beatmapInfo, [CanBeNull] RulesetInfo rulesetInfo = null, [CanBeNull] IReadOnlyList<Mod> mods = null)
+        {
+            // Difficulty can only be computed if the beatmap is locally available.
+            if (beatmapInfo.ID == 0)
+                return 0;
+
+            // In the case that the user hasn't given us a ruleset, use the beatmap's default ruleset.
+            rulesetInfo ??= beatmapInfo.Ruleset;
+
+            var key = new DifficultyCacheLookup(beatmapInfo, rulesetInfo, mods);
+            if (difficultyCache.TryGetValue(key, out var existing))
+                return existing;
+
+            try
+            {
+                var ruleset = rulesetInfo.CreateInstance();
+                Debug.Assert(ruleset != null);
+
+                var calculator = ruleset.CreateDifficultyCalculator(beatmapManager.GetWorkingBeatmap(beatmapInfo));
+                var attributes = calculator.Calculate(mods?.ToArray() ?? Array.Empty<Mod>());
+
+                difficultyCache.Add(key, attributes.StarRating);
+                return attributes.StarRating;
+            }
+            catch
+            {
+                difficultyCache.Add(key, 0);
+                return 0;
+            }
+        }
+
+        private readonly struct DifficultyCacheLookup : IEquatable<DifficultyCacheLookup>
+        {
+            private readonly BeatmapInfo beatmapInfo;
+            private readonly RulesetInfo rulesetInfo;
+            private readonly IReadOnlyList<Mod> mods;
+
+            public DifficultyCacheLookup(BeatmapInfo beatmapInfo, RulesetInfo rulesetInfo, IEnumerable<Mod> mods)
+            {
+                this.beatmapInfo = beatmapInfo;
+                this.rulesetInfo = rulesetInfo;
+                this.mods = mods?.OrderBy(m => m.Acronym).ToArray() ?? Array.Empty<Mod>();
+            }
+
+            public bool Equals(DifficultyCacheLookup other)
+                => beatmapInfo.Equals(other.beatmapInfo)
+                   && mods.SequenceEqual(other.mods);
+
+            public override int GetHashCode()
+            {
+                var hashCode = new HashCode();
+
+                hashCode.Add(beatmapInfo.Hash);
+                hashCode.Add(rulesetInfo.GetHashCode());
+                foreach (var mod in mods)
+                    hashCode.Add(mod.Acronym);
+
+                return hashCode.ToHashCode();
+            }
+        }
+    }
+}

osu.Game/OsuGameBase.cs

@@ -199,6 +199,7 @@ namespace osu.Game
                     ScoreManager.Undelete(getBeatmapScores(item), true);
             });
 
+            dependencies.Cache(new BeatmapDifficultyManager(BeatmapManager));
             dependencies.Cache(KeyBindingStore = new KeyBindingStore(contextFactory, RulesetStore));
             dependencies.Cache(SettingsStore = new SettingsStore(contextFactory));
             dependencies.Cache(RulesetConfigCache = new RulesetConfigCache(SettingsStore));