add Single Tap mod

osu.Game.Rulesets.Osu/Mods/OsuModSingleTap.cs

@@ -0,0 +1,38 @@
+// 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.
+
+#nullable disable
+
+using osu.Framework.Graphics;
+
+namespace osu.Game.Rulesets.Osu.Mods
+{
+    public class OsuModSingleTap : InputBlockingMod
+    {
+        public override string Name => @"Single Tap";
+        public override string Acronym => @"ST";
+        public override string Description => @"You must only use one key!";
+
+        protected override bool CheckCorrectAction(OsuAction action)
+        {
+            if (base.CheckCorrectAction(action))
+                return true;
+
+            if (LastActionPressed == null)
+            {
+                // First keypress, store the expected action.
+                LastActionPressed = action;
+                return true;
+            }
+
+            if (LastActionPressed == action)
+            {
+                // User singletapped correctly.
+                return true;
+            }
+
+            Ruleset.Cursor.FlashColour(Colour4.Red, FLASH_DURATION, Easing.OutQuint);
+            return false;
+        }
+    }
+}

osu.Game.Rulesets.Osu/OsuRuleset.cs

@@ -172,7 +172,7 @@ namespace osu.Game.Rulesets.Osu
                         new OsuModClassic(),
                         new OsuModRandom(),
                         new OsuModMirror(),
-                        new OsuModAlternate(),
+                        new MultiMod(new OsuModAlternate(), new OsuModSingleTap())
                     };
 
                 case ModType.Automation: