fn persistence(num: u64) -> u64 {
    match num {
        0..=9 => 0,
        _ => {
            let res = num
                .to_string()
                .chars()
                .map(|c| c.to_digit(10).unwrap() as u64)
                .product();
            1 + persistence(res)
        }
    }
}

#[cfg(test)]
mod tests {
    #[test]
    fn sample_tests() {
        assert_eq!(super::persistence(39), 3);
        assert_eq!(super::persistence(4), 0);
        assert_eq!(super::persistence(25), 2);
        assert_eq!(super::persistence(999), 4);
    }
}