Empty systems run once per phase

Previously they didn't run at all
since they don't match any entities.

src/gaemstone/Utility/IL/IterActionGenerator.cs

@@ -24,10 +24,12 @@ public unsafe class IterActionGenerator
 	private static readonly PropertyInfo _handleTargetProp     = typeof(GCHandle).GetProperty(nameof(GCHandle.Target))!;
 
 	private static readonly ConditionalWeakTable<MethodInfo, IterActionGenerator> _cache = new();
+	private static readonly Dictionary<Type, Action<ILGeneratorWrapper, IArgument<Iterator>>> _globalUniqueParameters = new() {
+		[typeof(Universe)] = (IL, iter) => { IL.Load(iter, _iteratorUniverseProp); },
+		[typeof(TimeSpan)] = (IL, iter) => { IL.Load(iter, _iteratorDeltaTimeProp); },
+	};
 	private static readonly Dictionary<Type, Action<ILGeneratorWrapper, IArgument<Iterator>, ILocal<int>>> _uniqueParameters = new() {
 		[typeof(Iterator)]  = (IL, iter, i) => { IL.Load(iter); },
-		[typeof(Universe)]  = (IL, iter, i) => { IL.Load(iter, _iteratorUniverseProp); },
-		[typeof(TimeSpan)]  = (IL, iter, i) => { IL.Load(iter, _iteratorDeltaTimeProp); },
 		[typeof(EntityRef)] = (IL, iter, i) => { IL.Load(iter); IL.Load(i); IL.Call(_iteratorEntityMethod); },
 	};
 
@@ -61,7 +63,6 @@ public unsafe class IterActionGenerator
 		// if (!Parameters.Any(c => c.IsRequired && (c.Kind != ParamKind.Unique)))
 		// 	throw new ArgumentException($"At least one parameter in {method} is required");
 
-		var terms = new List<Term>();
 		var name  = "<>Query_" + string.Join("_", Parameters.Select(p => p.UnderlyingType.Name));
 		var genMethod = new DynamicMethod(name, null, new[] { typeof(object), typeof(Iterator) });
 		var IL = new ILGeneratorWrapper(genMethod);
@@ -69,12 +70,32 @@ public unsafe class IterActionGenerator
 		var instanceArg = IL.Argument<object?>(0);
 		var iteratorArg = IL.Argument<Iterator>(1);
 
+		// If parameters only contains global unique paremeters (such as
+		// Universe or TimeSpan), or is empty, just run the system, since
+		// without terms it won't match any entities anyway.
+		if (Parameters.All(p => p.Kind == ParamKind.GlobalUnique)) {
+			if (!Method.IsStatic) IL.Load(instanceArg);
+			foreach (var p in Parameters) {
+				IL.Comment($"Global unique parameter {p.ParameterType.GetFriendlyName()}");
+				_globalUniqueParameters[p.ParameterType](IL, iteratorArg);
+			}
+			IL.Call(Method);
+			IL.Return();
+
+			Terms = Array.Empty<Term>();
+			GeneratedAction = genMethod.CreateDelegate<Action<object?, Iterator>>();
+			ReadableString  = IL.ToReadableString();
+
+			return;
+		}
+
+		var terms = new List<Term>();
 		var fieldLocals = new ILocal[Parameters.Length];
 		var tempLocals  = new ILocal[Parameters.Length];
 
 		for (var i = 0; i < Parameters.Length; i++) {
 			var p = Parameters[i];
-			if (p.Kind == ParamKind.Unique) continue;
+			if (p.Kind <= ParamKind.Unique) continue;
 
 			// Add an entry to the terms to look for this type.
 			terms.Add(new(universe.LookupOrThrow(p.UnderlyingType)) {
@@ -128,16 +149,18 @@ public unsafe class IterActionGenerator
 
 		// If there's any reference type parameters, we need to define a GCHandle local.
 		var hasReferenceType = Parameters
-			.Where(p => p.Kind != ParamKind.Unique)
+			.Where(p => p.Kind > ParamKind.Unique)
 			.Any(p => !p.UnderlyingType.IsValueType);
 		var handleLocal = hasReferenceType ? IL.Local<GCHandle>() : null;
 
 		using (IL.For(() => IL.Load(iteratorArg, _iteratorCountProp), out var currentLocal)) {
-			if (!Method.IsStatic)
-				IL.Load(instanceArg);
+			if (!Method.IsStatic) IL.Load(instanceArg);
 			for (var i = 0; i < Parameters.Length; i++) {
 				var p = Parameters[i];
-				if (p.Kind == ParamKind.Unique) {
+				if (p.Kind == ParamKind.GlobalUnique) {
+					IL.Comment($"Global unique parameter {p.ParameterType.GetFriendlyName()}");
+					_globalUniqueParameters[p.ParameterType](IL, iteratorArg);
+				} else if (p.Kind == ParamKind.Unique) {
 					IL.Comment($"Unique parameter {p.ParameterType.GetFriendlyName()}");
 					_uniqueParameters[p.ParameterType](IL, iteratorArg, currentLocal);
 				} else if (p.Kind is ParamKind.Has or ParamKind.Not) {
@@ -242,6 +265,8 @@ public unsafe class IterActionGenerator
 			if (info.ParameterType.IsArray)   throw new ArgumentException("Arrays are not supported\nParameter: " + info);
 			if (info.ParameterType.IsPointer) throw new ArgumentException("Pointers are not supported\nParameter: " + info);
 
+			if (_globalUniqueParameters.ContainsKey(info.ParameterType))
+				return new(info, index, ParamKind.GlobalUnique, info.ParameterType, info.ParameterType);
 			if (_uniqueParameters.ContainsKey(info.ParameterType))
 				return new(info, index, ParamKind.Unique, info.ParameterType, info.ParameterType);
 
@@ -289,7 +314,9 @@ public unsafe class IterActionGenerator
 
 	public enum ParamKind
 	{
-		/// <summary> Parameter is not part of terms, handled uniquely, such as Universe and Entity. </summary>
+		/// <summary> Parameter is not part of terms, handled uniquely, such as Universe. </summary>
+		GlobalUnique,
+		/// <summary> Parameter is unique per matched entity, such as EntityRef. </summary>
 		Unique,
 		/// <summary> Passed by value. </summary>
 		Normal,