Encapsulate mod hotkey selection logic in strategy pattern

osu.Game/Overlays/Mods/Input/SequentialModHotkeyHandler.cs

@@ -0,0 +1,58 @@
+// 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.Linq;
+using osu.Game.Rulesets.Mods;
+using osuTK.Input;
+
+namespace osu.Game.Overlays.Mods.Input
+{
+    /// <summary>
+    /// This implementation of <see cref="IModHotkeyHandler"/> receives a sequence of <see cref="Key"/>s,
+    /// and maps the sequence of keys onto the items it is provided in <see cref="HandleHotkeyPressed"/>.
+    /// In this case, particular mods are not bound to particular keys, the hotkeys are a byproduct of mod ordering.
+    /// </summary>
+    public class SequentialModHotkeyHandler : IModHotkeyHandler
+    {
+        public static SequentialModHotkeyHandler Create(ModType modType)
+        {
+            switch (modType)
+            {
+                case ModType.DifficultyReduction:
+                    return new SequentialModHotkeyHandler(new[] { Key.Q, Key.W, Key.E, Key.R, Key.T, Key.Y, Key.U, Key.I, Key.O, Key.P });
+
+                case ModType.DifficultyIncrease:
+                    return new SequentialModHotkeyHandler(new[] { Key.A, Key.S, Key.D, Key.F, Key.G, Key.H, Key.J, Key.K, Key.L });
+
+                case ModType.Automation:
+                    return new SequentialModHotkeyHandler(new[] { Key.Z, Key.X, Key.C, Key.V, Key.B, Key.N, Key.M });
+
+                default:
+                    throw new ArgumentOutOfRangeException(nameof(modType), modType, $"Cannot create {nameof(SequentialModHotkeyHandler)} for provided mod type");
+            }
+        }
+
+        private readonly Key[] toggleKeys;
+
+        private SequentialModHotkeyHandler(Key[] keys)
+        {
+            toggleKeys = keys;
+        }
+
+        public bool HandleHotkeyPressed(Key hotkey, IEnumerable<ModState> availableMods)
+        {
+            int index = Array.IndexOf(toggleKeys, hotkey);
+            if (index < 0)
+                return false;
+
+            var modState = availableMods.ElementAtOrDefault(index);
+            if (modState == null || modState.Filtered.Value)
+                return false;
+
+            modState.Active.Toggle();
+            return true;
+        }
+    }
+}