Add ModUtils class for validating mod usages

osu.Game.Tests.Android/osu.Game.Tests.Android.csproj

@@ -69,5 +69,9 @@
       <Name>osu.Game</Name>
     </ProjectReference>
   </ItemGroup>
+  <ItemGroup Label="Package References">
+    <PackageReference Include="DeepEqual" Version="2.0.0" />
+    <PackageReference Include="Moq" Version="4.16.0" />
+  </ItemGroup>
   <Import Project="$(MSBuildExtensionsPath)\Xamarin\Android\Xamarin.Android.CSharp.targets" />
 </Project>

osu.Game.Tests.iOS/osu.Game.Tests.iOS.csproj

@@ -45,6 +45,7 @@
   </ItemGroup>
   <ItemGroup Label="Package References">
     <PackageReference Include="DeepEqual" Version="2.0.0" />
+    <PackageReference Include="Moq" Version="4.16.0" />
   </ItemGroup>
   <Import Project="$(MSBuildExtensionsPath)\Xamarin\iOS\Xamarin.iOS.CSharp.targets" />
 </Project>

osu.Game.Tests/Mods/ModUtilsTest.cs

@@ -0,0 +1,64 @@
+// 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 Moq;
+using NUnit.Framework;
+using osu.Game.Rulesets.Mods;
+using osu.Game.Utils;
+
+namespace osu.Game.Tests.Mods
+{
+    [TestFixture]
+    public class ModUtilsTest
+    {
+        [Test]
+        public void TestModIsCompatibleByItself()
+        {
+            var mod = new Mock<Mod>();
+            Assert.That(ModUtils.CheckCompatibleSet(new[] { mod.Object }));
+        }
+
+        [Test]
+        public void TestIncompatibleThroughTopLevel()
+        {
+            var mod1 = new Mock<Mod>();
+            var mod2 = new Mock<Mod>();
+
+            mod1.Setup(m => m.IncompatibleMods).Returns(new[] { mod2.Object.GetType() });
+
+            // Test both orderings.
+            Assert.That(ModUtils.CheckCompatibleSet(new[] { mod1.Object, mod2.Object }), Is.False);
+            Assert.That(ModUtils.CheckCompatibleSet(new[] { mod2.Object, mod1.Object }), Is.False);
+        }
+
+        [Test]
+        public void TestIncompatibleThroughMultiMod()
+        {
+            var mod1 = new Mock<Mod>();
+
+            // The nested mod.
+            var mod2 = new Mock<Mod>();
+            mod2.Setup(m => m.IncompatibleMods).Returns(new[] { mod1.Object.GetType() });
+
+            var multiMod = new MultiMod(new MultiMod(mod2.Object));
+
+            // Test both orderings.
+            Assert.That(ModUtils.CheckCompatibleSet(new[] { multiMod, mod1.Object }), Is.False);
+            Assert.That(ModUtils.CheckCompatibleSet(new[] { mod1.Object, multiMod }), Is.False);
+        }
+
+        [Test]
+        public void TestAllowedThroughMostDerivedType()
+        {
+            var mod = new Mock<Mod>();
+            Assert.That(ModUtils.CheckAllowed(new[] { mod.Object }, new[] { mod.Object.GetType() }));
+        }
+
+        [Test]
+        public void TestNotAllowedThroughBaseType()
+        {
+            var mod = new Mock<Mod>();
+            Assert.That(ModUtils.CheckAllowed(new[] { mod.Object }, new[] { typeof(Mod) }), Is.False);
+        }
+    }
+}

osu.Game.Tests/osu.Game.Tests.csproj

@@ -7,6 +7,7 @@
     <PackageReference Include="NUnit" Version="3.12.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="3.17.0" />
     <PackageReference Update="Microsoft.EntityFrameworkCore.Sqlite" Version="2.1.4" />
+    <PackageReference Include="Moq" Version="4.16.0" />
   </ItemGroup>
   <PropertyGroup Label="Project">
     <OutputType>WinExe</OutputType>

osu.Game/Utils/ModUtils.cs

@@ -0,0 +1,109 @@
+// 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.Framework.Extensions.TypeExtensions;
+using osu.Game.Rulesets.Mods;
+
+#nullable enable
+
+namespace osu.Game.Utils
+{
+    /// <summary>
+    /// A set of utilities to handle <see cref="Mod"/> combinations.
+    /// </summary>
+    public static class ModUtils
+    {
+        /// <summary>
+        /// Checks that all <see cref="Mod"/>s are compatible with each-other, and that all appear within a set of allowed types.
+        /// </summary>
+        /// <remarks>
+        /// The allowed types must contain exact <see cref="Mod"/> types for the respective <see cref="Mod"/>s to be allowed.
+        /// </remarks>
+        /// <param name="combination">The <see cref="Mod"/>s to check.</param>
+        /// <param name="allowedTypes">The set of allowed <see cref="Mod"/> types.</param>
+        /// <returns>Whether all <see cref="Mod"/>s are compatible with each-other and appear in the set of allowed types.</returns>
+        public static bool CheckCompatibleSetAndAllowed(IEnumerable<Mod> combination, IEnumerable<Type> allowedTypes)
+        {
+            // Prevent multiple-enumeration.
+            var combinationList = combination as ICollection<Mod> ?? combination.ToArray();
+            return CheckCompatibleSet(combinationList) && CheckAllowed(combinationList, allowedTypes);
+        }
+
+        /// <summary>
+        /// Checks that all <see cref="Mod"/>s in a combination are compatible with each-other.
+        /// </summary>
+        /// <param name="combination">The <see cref="Mod"/> combination to check.</param>
+        /// <returns>Whether all <see cref="Mod"/>s in the combination are compatible with each-other.</returns>
+        public static bool CheckCompatibleSet(IEnumerable<Mod> combination)
+        {
+            var incompatibleTypes = new HashSet<Type>();
+            var incomingTypes = new HashSet<Type>();
+
+            foreach (var mod in combination.SelectMany(FlattenMod))
+            {
+                // Add the new mod incompatibilities, checking whether any match the existing mod types.
+                foreach (var t in mod.IncompatibleMods)
+                {
+                    if (incomingTypes.Contains(t))
+                        return false;
+
+                    incompatibleTypes.Add(t);
+                }
+
+                // Add the new mod types, checking whether any match the incompatible types.
+                foreach (var t in mod.GetType().EnumerateBaseTypes())
+                {
+                    if (incomingTypes.Contains(t))
+                        return false;
+
+                    incomingTypes.Add(t);
+                }
+            }
+
+            return true;
+        }
+
+        /// <summary>
+        /// Checks that all <see cref="Mod"/>s in a combination appear within a set of allowed types.
+        /// </summary>
+        /// <remarks>
+        /// The set of allowed types must contain exact <see cref="Mod"/> types for the respective <see cref="Mod"/>s to be allowed.
+        /// </remarks>
+        /// <param name="combination">The <see cref="Mod"/> combination to check.</param>
+        /// <param name="allowedTypes">The set of allowed <see cref="Mod"/> types.</param>
+        /// <returns>Whether all <see cref="Mod"/>s in the combination are allowed.</returns>
+        public static bool CheckAllowed(IEnumerable<Mod> combination, IEnumerable<Type> allowedTypes)
+        {
+            var allowedSet = new HashSet<Type>(allowedTypes);
+
+            return combination.SelectMany(FlattenMod)
+                              .All(m => allowedSet.Contains(m.GetType()));
+        }
+
+        /// <summary>
+        /// Flattens a set of <see cref="Mod"/>s, returning a new set with all <see cref="MultiMod"/>s removed.
+        /// </summary>
+        /// <param name="mods">The set of <see cref="Mod"/>s to flatten.</param>
+        /// <returns>The new set, containing all <see cref="Mod"/>s in <paramref name="mods"/> recursively with all <see cref="MultiMod"/>s removed.</returns>
+        public static IEnumerable<Mod> FlattenMods(IEnumerable<Mod> mods) => mods.SelectMany(FlattenMod);
+
+        /// <summary>
+        /// Flattens a <see cref="Mod"/>, returning a set of <see cref="Mod"/>s in-place of any <see cref="MultiMod"/>s.
+        /// </summary>
+        /// <param name="mod">The <see cref="Mod"/> to flatten.</param>
+        /// <returns>A set of singular "flattened" <see cref="Mod"/>s</returns>
+        public static IEnumerable<Mod> FlattenMod(Mod mod)
+        {
+            if (mod is MultiMod multi)
+            {
+                foreach (var m in multi.Mods.SelectMany(FlattenMod))
+                    yield return m;
+            }
+            else
+                yield return mod;
+        }
+    }
+}