GraalJS_ProgramCounter.java

import org.graalvm.polyglot.*;

import java.lang.reflect.Array;
import java.lang.reflect.Field;
import java.util.Arrays;

public class Main {
    public static void main(String[] args) throws Exception {
        try (Context context = Context.create("js")) {
            context.eval("js", "" +
                    "function *fib() {" +
                    "yield 1;" +
                    "yield 1;" +
                    "yield 2;" +
                    "yield 3;" +
                    "yield 5;" +
                    "}");
            Value returnNext = context.eval("js", "function(gen) { return gen.next(); }");
            Value fib = context.eval("js", "" +
                    "f = fib();");
            System.out.println(returnNext.execute(fib));
            printProgramCounter(fib);
            System.out.println(returnNext.execute(fib));
            printProgramCounter(fib);
            System.out.println(returnNext.execute(fib));
            printProgramCounter(fib);
            System.out.println(returnNext.execute(fib));
            printProgramCounter(fib);
            System.out.println(returnNext.execute(fib));
            printProgramCounter(fib);
            // jump back to yield 1
            Object locals = get(fib, "receiver", "object2", "locals");
            Array.set(locals, 8, 1);
            System.out.println(returnNext.execute(fib));
            System.out.println(returnNext.execute(fib));
            System.out.println(returnNext.execute(fib));
        }
    }

    private static void printProgramCounter(Value fib) throws Exception {
        System.out.println(get(fib, "receiver", "object2", "locals", 8));
    }

    private static Object get(Object obj, Object... paths) throws Exception {
        if (paths.length == 0) {
            return obj;
        }
        if (paths.length == 1) {
            return getElementOrFieldValue(obj, paths[0]);
        }
        return get(getElementOrFieldValue(obj, paths[0]), Arrays.copyOfRange(paths, 1, paths.length));
    }

    private static Object getElementOrFieldValue(Object obj, Object indexOrFieldName) throws Exception {
        if (indexOrFieldName instanceof Integer) {
            return Array.get(obj, (Integer) indexOrFieldName);
        }
        return getFieldValue(obj, (String) indexOrFieldName);
    }

    private static Object getFieldValue(Object obj, String fieldName) throws Exception {
        Field field = getField(obj.getClass(), fieldName);
        return field.get(obj);
    }

    private static Field getField(Class clazz, String fieldName) throws Exception {
        Field field = clazz.getDeclaredField(fieldName);
        field.setAccessible(true);
        return field;
    }
}

{value: 1, done: false}
0
{value: 1, done: false}
1
{value: 2, done: false}
2
{value: 3, done: false}
3
{value: 5, done: false}
4
{value: 1, done: false}
{value: 2, done: false}
{value: 3, done: false}