Add Indices of X in List

src/main/java/ch/njol/skript/expressions/ExprIndicesOfX.java

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+package ch.njol.skript.expressions;
+
+import ch.njol.skript.Skript;
+import ch.njol.skript.doc.Description;
+import ch.njol.skript.doc.Examples;
+import ch.njol.skript.doc.Name;
+import ch.njol.skript.doc.Since;
+import ch.njol.skript.lang.Expression;
+import ch.njol.skript.lang.ExpressionType;
+import ch.njol.skript.lang.SkriptParser.ParseResult;
+import ch.njol.skript.lang.SyntaxStringBuilder;
+import ch.njol.skript.lang.Variable;
+import ch.njol.skript.lang.util.SimpleExpression;
+import ch.njol.skript.util.LiteralUtils;
+import ch.njol.util.Kleenean;
+import org.bukkit.event.Event;
+import org.jetbrains.annotations.Nullable;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+
+@Name("Indices of X in List")
+@Description("Returns all the indices of a list where their value is X.")
+@Examples({
+	"set {_list::*} to 1, 2, 3, 1, 2, 3",
+	"set {_indices::*} to the indices of the value 1 in {_list::*}",
+	"# {_indices::*} is now 1, 4",
+	"",
+	"set {_indices::*} to the indices of the value 2 in {_list::*}",
+	"# {_indices::*} is now 2, 5",
+	"",
+	"set {_otherlist::burb} to 100",
+	"set {_otherlist::burp} to 100",
+	"set {_indices::*} to the first index of the value 100 in {_otherlist::*}",
+	"# {_indices::*} is now burb",
+	"set {_indices::*} to the last index of the value 100 in {_otherlist::*}",
+	"# {_indices::*} is now burp"
+})
+@Since("INSERT VERSION")
+public class ExprIndicesOfX extends SimpleExpression<String> {
+
+	static {
+		Skript.registerExpression(ExprIndicesOfX.class, String.class, ExpressionType.COMBINED,
+			"[the] [1:first|2:last] (indices|index[es]) of [[the] value] %object% in %objects%"
+		);
+	}
+
+	private IndexType type;
+	private Expression<?> value;
+	private Variable<?> list;
+
+	@Override
+	public boolean init(Expression<?>[] exprs, int matchedPattern, Kleenean isDelayed, ParseResult parseResult) {
+		if (!(exprs[1] instanceof Variable<?> var) || !var.isList()) {
+			Skript.error("'" + exprs[1].toString(null, false) +
+				"' can only ever have one value at most, thus the 'indices of x in ...' expression is useless.");
+			return false;
+		}
+
+		list = var;
+		type = IndexType.get(parseResult.mark);
+		value = LiteralUtils.defendExpression(exprs[0]);
+
+		return LiteralUtils.canInitSafely(value);
+	}
+
+	@Override
+	protected String @Nullable [] get(Event event) {
+		Object value = this.value.getSingle(event);
+		if (value == null)
+			return null;
+
+		//noinspection unchecked
+		Map<String, Object> variable = (Map<String, Object>) list.getRaw(event);
+		if (variable == null)
+			return null;
+
+		List<String> indices = new ArrayList<>();
+
+		for (Map.Entry<String, Object> entry : variable.entrySet()) {
+			Object entryValue = entry.getValue();
+			if (entryValue instanceof Map<?, ?> map)
+				entryValue = map.get(null);
+
+			if (entryValue.equals(value))
+				indices.add(entry.getKey());
+		}
+
+		if (indices.isEmpty())
+			return null;
+
+		if (type == IndexType.FIRST)
+			return new String[]{indices.get(0)};
+		else if (type == IndexType.LAST)
+			return new String[]{indices.get(indices.size() - 1)};
+		return indices.toArray(new String[0]);
+	}
+
+	@Override
+	public boolean isSingle() {
+		return type == IndexType.FIRST || type == IndexType.LAST;
+	}
+
+	@Override
+	public Class<? extends String> getReturnType() {
+		return String.class;
+	}
+
+	@Override
+	public String toString(@Nullable Event event, boolean debug) {
+		SyntaxStringBuilder builder = new SyntaxStringBuilder(event, debug);
+
+		builder.append(type.toString());
+		if (type != IndexType.ALL)
+			builder.append("indices");
+		else
+			builder.append("index");
+		builder.append("of value", value, "in", list);
+
+		return builder.toString();
+	}
+
+	private enum IndexType {
+		FIRST, LAST, ALL;
+
+		public static IndexType get(int mark) {
+			return switch (mark) {
+				case 1 -> FIRST;
+				case 2 -> LAST;
+				default -> ALL;
+			};
+		}
+
+		@Override
+		public String toString() {
+			return name().toLowerCase();
+		}
+	}
+
+}

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

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+test "indices of x":
+	set {_list::*} to 1, 2, 3, 1, 2, 3
+	set {_indices::*} to the indices of the value 1 in {_list::*}
+	assert {_indices::*} is "1", "4" with "indices of 1 failed"
+
+	set {_indices::*} to the first index of the value 3 in {_list::*}
+	assert {_indices::*} is "3" with "first index of 3 failed"
+
+	set {_indices::*} to the last index of the value 3 in {_list::*}
+	assert {_indices::*} is "6" with "last index of 3 failed"
+
+	set {_otherlist::burb} to test-location
+	set {_otherlist::_DJ8U3f;} to test-location
+	set {_otherlist::;'w20} to test-location
+	set {_otherList::breh} to 2
+	set {_otherList::quatro} to 4
+
+	set {_indices::*} to the indices of the value test-location in {_otherlist::*}
+	assert {_indices::*} is ";'w20", "_dj8u3f;", "burb" with "indices of test-location with symbols failed"