add difficulty adjustment mods

osu.Game.Rulesets.Mania/Mods/ManiaModDifficultyAdjust.cs

@@ -0,0 +1,22 @@
+// 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 osu.Framework.Bindables;
+using osu.Game.Configuration;
+using osu.Game.Rulesets.Mods;
+
+namespace osu.Game.Rulesets.Mania.Mods
+{
+    public class ManiaModDifficultyAdjust : ModDifficultyAdjust
+    {
+        [SettingSource("Overall Difficulty", "Override the beatmap's set OD")]
+        public override BindableNumber<float> OverallDifficulty { get; } = new BindableFloat
+        {
+            MinValue = 1,
+            MaxValue = 10,
+            Default = 5,
+            Value = 5,
+            Precision = 0.1F,
+        };
+    }
+}