Add diffcalc beatmap decoder

osu.Game.Rulesets.Osu/Difficulty/Preprocessing/OsuDifficultyHitObject.cs

@@ -3,7 +3,6 @@
 
 using System;
 using System.Linq;
-using osu.Game.Rulesets.Objects.Types;
 using osu.Game.Rulesets.Osu.Objects;
 using OpenTK;
 

osu.Game/Beatmaps/Formats/LegacyDifficultyCalculatorBeatmapDecoder.cs

@@ -0,0 +1,36 @@
+// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System.Linq;
+using osu.Game.Beatmaps.ControlPoints;
+
+namespace osu.Game.Beatmaps.Formats
+{
+    /// <summary>
+    /// A <see cref="LegacyBeatmapDecoder"/> built for difficulty calculation of legacy <see cref="Beatmap"/>s
+    /// <remarks>
+    /// To use this, the decoder must be registered by the application through <see cref="LegacyDifficultyCalculatorBeatmapDecoder.Register"/>.
+    /// Doing so will override any existing <see cref="Beatmap"/> decoders.
+    /// </remarks>
+    /// </summary>
+    public class LegacyDifficultyCalculatorBeatmapDecoder : LegacyBeatmapDecoder
+    {
+        public LegacyDifficultyCalculatorBeatmapDecoder(int version = LATEST_VERSION)
+            : base(version)
+        {
+        }
+
+        public new static void Register()
+        {
+            AddDecoder<Beatmap>(@"osu file format v", m => new LegacyDifficultyCalculatorBeatmapDecoder(int.Parse(m.Split('v').Last())));
+        }
+
+        protected override TimingControlPoint CreateTimingControlPoint()
+            => new LegacyDifficultyCalculatorControlPoint();
+
+        private class LegacyDifficultyCalculatorControlPoint : TimingControlPoint
+        {
+            public override double BeatLength { get; set; } = 1000;
+        }
+    }
+}