fn fib(n: i64) -> Option<i64> {
    // A function declared inside another function does not pollute the outer namespace.
    fn actual_fib(n: i64) -> i64 {
        if n < 2 {
            n
        } else {
            actual_fib(n - 1) + actual_fib(n - 2)
        }
    }

    if n < 0 {
        None
    } else {
        Some(actual_fib(n))
    }
}

fn main() {
    println!("Fib(-1) = {:?}", fib(-1));
    println!("Fib(0) = {:?}", fib(0));
    println!("Fib(1) = {:?}", fib(1));
    println!("Fib(2) = {:?}", fib(2));
    println!("Fib(3) = {:?}", fib(3));
    println!("Fib(4) = {:?}", fib(4));
    println!("Fib(5) = {:?}", fib(5));
    println!("Fib(10) = {:?}", fib(10));
}

#[test]
fn test_fib() {
    assert_eq!(fib(0).unwrap(), 0);
    assert_eq!(fib(1).unwrap(), 1);
    assert_eq!(fib(2).unwrap(), 1);
    assert_eq!(fib(3).unwrap(), 2);
    assert_eq!(fib(4).unwrap(), 3);
    assert_eq!(fib(5).unwrap(), 5);
    assert_eq!(fib(10).unwrap(), 55);
}

#[test]
fn test_invalid_argument() {
    assert_eq!(fib(-1), None);
}