pub fn next_prime(num: usize) -> Option<usize> {
    let max_next_prime = num * 2;
    let mut is_prime: Vec<bool> = vec![true; max_next_prime];

    for i in 2..max_next_prime {
        for j in (i * 2..max_next_prime).step_by(i) {
            is_prime[j] = false;
        }

        if i >= num && is_prime[i] {
            return Some(i);
        }
    }

    None
}