better arithmetic changer handling

Author: sovdeeth

src/main/java/ch/njol/skript/classes/Changer.java

@@ -6,6 +6,12 @@
 
 import ch.njol.skript.classes.data.DefaultChangers;
 import ch.njol.skript.lang.Expression;
+import org.skriptlang.skript.lang.arithmetic.Arithmetics;
+import org.skriptlang.skript.lang.arithmetic.OperationInfo;
+import org.skriptlang.skript.lang.arithmetic.Operator;
+
+import java.util.List;
+import java.util.function.Predicate;
 
 /**
  * An interface to declare changeable values. All Expressions implement something similar like this by default, but refuse any change if {@link Expression#acceptChange(ChangeMode)}
@@ -94,6 +100,29 @@ public static boolean acceptsChangeTypes(Class<?>[] validTypes, Class<?> @NotNul
 			return false;
 		}
 
+		/**
+		 * Gets the types that can be added/removed via arithmetic for the given type.
+		 * This is used to determine accepted change types for add/remove when no changer is present.
+		 * @param type The type to get arithmetic change types for.
+		 * @param mode Whether to get addition or subtraction types.
+		 * @param filter A filter to apply to the available operations. Used for custom constraints on the operations, like
+		 *               ensuring the return type matches the left type.
+		 * @return The types that can be added/removed via arithmetic for the given type and mode, after applying the filter.
+		 * @param <T> The type to get arithmetic change types for.
+		 */
+		public static <T> Class<?>[] getArithmeticChangeTypes(Class<T> type, ChangeMode mode, Predicate<OperationInfo<T, ? ,?>> filter) {
+			List<OperationInfo<T, ?, ?>> opInfos = List.of();
+			if (mode == ChangeMode.ADD) {
+				opInfos = Arithmetics.getOperations(Operator.ADDITION, type);
+			} else if (mode == ChangeMode.REMOVE) {
+				opInfos = Arithmetics.getOperations(Operator.SUBTRACTION, type);
+			}
+			return opInfos.stream()
+				.filter(filter)
+				.map(OperationInfo::right)
+				.toArray(Class[]::new);
+		}
+
 	}
 
 }

src/main/java/org/skriptlang/skript/bukkit/pdc/expressions/ExprPersistentData.java

@@ -41,6 +41,8 @@
 import java.util.*;
 import java.util.function.Consumer;
 
+import static ch.njol.skript.classes.Changer.ChangerUtils.getArithmeticChangeTypes;
+
 @Name("Persistent Data Value")
 @Description("""
 	Provides access to the 'persistent data container' Bukkit provides on many objects. These values are stored on the \
@@ -190,6 +192,7 @@ protected Object[] get(Event event, Object[] source) {
 				}
 
 				if (parsedType != null) {
+					// we have a specific type to aim for
 					ClassInfo<?> classInfo = parsedType.getClassInfo();
 
 					if (plural) {
@@ -251,8 +254,11 @@ public boolean isSingle() {
 					if (changer != null) {
 						yield changer.acceptChange(mode);
 					}
-					if (mode == ChangeMode.SET)
+					if (mode == ChangeMode.SET) {
 						yield CollectionUtils.array(type.getC());
+					} else if (mode == ChangeMode.ADD || mode == ChangeMode.REMOVE) {
+						yield getArithmeticChangeTypes(type.getC(), mode, operation -> type.getC().isAssignableFrom(operation.returnType()));
+					}
 					yield null;
 				}
 				yield CollectionUtils.array(plural ? Object[].class : Object.class);

src/test/skript/tests/syntaxes/expressions/ExprPersistentData.sk

@@ -233,6 +233,16 @@ test "persistent data add to singular value":
 	assert {_val} is 12 with "removing from number should work"
 	delete persistent data tag "skript:add_single" of {_item}
 
+test "persistent data add to singular value of differing type":
+	set {_item} to a diamond sword
+	set {_now} to now
+	set date data tag "date" of {_item} to {_now}
+	add 1 day to date data tag "date" of {_item}
+	assert date data tag "date" of {_item} is {_now} + 1 day with "adding to date should work"
+	parse:
+		add "hello" to date data tag "date" of {_item}
+	assert last parse logs contains """hello"" can't be added to date data tag ""date"" of {_item} because the former is not a time span" with "adding incompatible type should error"
+
 test "persistent data add to list":
 	set {_item} to a diamond sword
 	# Start with a list, then add to it