struct ArrayList[T] {
mut:
    size  usize
    items []&T
}

fn new_array_list[T]() ArrayList[T] {
    return ArrayList[T]{}
}

fn (li ArrayList[T]) is_empty() bool {
    return li.size == 0
}

fn (mut li ArrayList[T]) add(item &T) {
    li.items << item
    li.size += 1
}

fn (mut li ArrayList[T]) pop() ?&T {
    if li.is_empty() {
        return none
    } else {
        item := li.items.last()
        li.items.delete_last()
        li.size -= 1
        return item
    }
}

struct Student {
    name string
mut:
    age u8
}

fn (mut s Student) grow() {
    s.age += 1
}

fn test_main() {
    mut list := new_array_list[Student]()
    assert list.is_empty() && list.size == 0

    mut stu := Student{'Tom', 16}
    list.add(&stu)
    assert !list.is_empty() && list.size == 1
    stu.grow()
    assert stu.age == 17

    tom := list.pop()?
    assert list.is_empty() && list.size == 0
    assert tom == stu
    assert tom.age == 17
}