Use console IPC

osu.Desktop/Program.cs

@@ -3,13 +3,22 @@
 
 using System;
 using System.IO;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using osu.Framework;
 using osu.Framework.Development;
 using osu.Framework.Logging;
 using osu.Framework.Platform;
+using osu.Game.Beatmaps;
+using osu.Game.Beatmaps.Legacy;
 using osu.Game.IPC;
+using osu.Game.Rulesets;
+using osu.Game.Rulesets.Catch;
+using osu.Game.Rulesets.Mania;
+using osu.Game.Rulesets.Mods;
+using osu.Game.Rulesets.Osu;
+using osu.Game.Rulesets.Taiko;
 using osu.Game.Tournament;
 
 namespace osu.Desktop
@@ -19,7 +28,7 @@ namespace osu.Desktop
         private const string base_game_name = @"osu";
 
         [STAThread]
-        public static int Main(string[] args)
+        public static void Main(string[] args)
         {
             // Back up the cwd before DesktopGameHost changes it
             string cwd = Environment.CurrentDirectory;
@@ -49,6 +58,34 @@ namespace osu.Desktop
 
                         gameName = $"{base_game_name}-{clientID}";
                         break;
+
+                    case "--osu-stable-difficulty-stream":
+                        while (true)
+                        {
+                            try
+                            {
+                                string beatmapFile = Console.ReadLine() ?? string.Empty;
+                                int rulesetId = int.Parse(Console.ReadLine() ?? string.Empty);
+                                LegacyMods legacyMods = (LegacyMods)int.Parse(Console.ReadLine() ?? string.Empty);
+
+                                Ruleset ruleset = rulesetId switch
+                                {
+                                    0 => new OsuRuleset(),
+                                    1 => new TaikoRuleset(),
+                                    2 => new CatchRuleset(),
+                                    3 => new ManiaRuleset(),
+                                    _ => throw new ArgumentException("Invalid ruleset id")
+                                };
+
+                                Mod[] mods = ruleset.ConvertFromLegacyMods(legacyMods).ToArray();
+                                WorkingBeatmap beatmap = new FlatFileWorkingBeatmap(beatmapFile, _ => ruleset);
+                                Console.WriteLine(ruleset.CreateDifficultyCalculator(beatmap).Calculate(mods).StarRating);
+                            }
+                            catch
+                            {
+                                Console.WriteLine(0);
+                            }
+                        }
                 }
             }
 
@@ -69,14 +106,14 @@ namespace osu.Desktop
                                 throw new TimeoutException(@"IPC took too long to send");
                         }
 
-                        return 0;
+                        return;
                     }
 
                     // we want to allow multiple instances to be started when in debug.
                     if (!DebugUtils.IsDebugBuild)
                     {
                         Logger.Log(@"osu! does not support multiple running instances.", LoggingTarget.Runtime, LogLevel.Error);
-                        return 0;
+                        return;
                     }
                 }
 
@@ -84,8 +121,6 @@ namespace osu.Desktop
                     host.Run(new TournamentGame());
                 else
                     host.Run(new OsuGameDesktop(args));
-
-                return 0;
             }
         }
 
@@ -107,4 +142,12 @@ namespace osu.Desktop
             return continueExecution;
         }
     }
+
+    // Note: Keep in osu.Desktop namespace, or update osu!stable also.
+    public class DifficultyCalculationMessage
+    {
+        public string BeatmapFile { get; set; }
+        public int RulesetId { get; set; }
+        public int Mods { get; set; }
+    }
 }

osu.Game/Beatmaps/FlatFileWorkingBeatmap.cs

@@ -0,0 +1,52 @@
+// 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.IO;
+using osu.Framework.Audio.Track;
+using osu.Framework.Graphics.Textures;
+using osu.Game.Beatmaps.Formats;
+using osu.Game.IO;
+using osu.Game.Rulesets;
+using osu.Game.Skinning;
+
+namespace osu.Game.Beatmaps
+{
+    /// <summary>
+    /// A <see cref="WorkingBeatmap"/> which can be constructed directly from a .osu file, providing an implementation for
+    /// <see cref="WorkingBeatmap.GetPlayableBeatmap(osu.Game.Rulesets.IRulesetInfo,System.Collections.Generic.IReadOnlyList{osu.Game.Rulesets.Mods.Mod})"/>.
+    /// </summary>
+    public class FlatFileWorkingBeatmap : WorkingBeatmap
+    {
+        private readonly Beatmap beatmap;
+
+        public FlatFileWorkingBeatmap(string file, Func<int, Ruleset> rulesetProvider, int? beatmapId = null)
+            : this(readFromFile(file), rulesetProvider, beatmapId)
+        {
+        }
+
+        private FlatFileWorkingBeatmap(Beatmap beatmap, Func<int, Ruleset> rulesetProvider, int? beatmapId = null)
+            : base(beatmap.BeatmapInfo, null)
+        {
+            this.beatmap = beatmap;
+
+            beatmap.BeatmapInfo.Ruleset = rulesetProvider(beatmap.BeatmapInfo.RulesetID).RulesetInfo;
+
+            if (beatmapId.HasValue)
+                beatmap.BeatmapInfo.OnlineID = beatmapId;
+        }
+
+        private static Beatmap readFromFile(string filename)
+        {
+            using (var stream = File.OpenRead(filename))
+            using (var reader = new LineBufferedReader(stream))
+                return Decoder.GetDecoder<Beatmap>(reader).Decode(reader);
+        }
+
+        protected override IBeatmap GetBeatmap() => beatmap;
+        protected override Texture GetBackground() => throw new NotImplementedException();
+        protected override Track GetBeatmapTrack() => throw new NotImplementedException();
+        protected internal override ISkin GetSkin() => throw new NotImplementedException();
+        public override Stream GetStream(string storagePath) => throw new NotImplementedException();
+    }
+}