From cassidoo's October 30, 2023 newsletter

lexo_next.rs

use itertools::Itertools;

pub fn lexo_next(num: usize) -> usize {
    let digits = digits(num);

    let mut perms = digits
        .iter()
        .permutations(digits.len())
        .map(number)
        .filter(|perm| *perm != num)
        .filter_map(|perm| perm.checked_sub(num).map(|diff| (perm, diff)))
        .sorted_by(|(_, d1), (_, d2)| d1.cmp(d2))
        .map(|(perm, _)| perm);

    perms.next().unwrap()
}

fn digits(mut num: usize) -> Vec<usize> {
    let mut digits = Vec::new();
    while num != 0 {
        digits.push(num % 10);
        num /= 10;
    }
    digits.reverse();
    digits
}

fn number(digits: Vec<&usize>) -> usize {
    digits.iter().rev().enumerate().fold(0, |acc, (i, e)| {
        let piece = **e * 10usize.pow(i as u32);
        acc + piece
    })
}

#[cfg(test)]
mod tests {
    use crate::lexo_next;

    #[test]
    fn test() {
        assert_eq!(lexo_next(123), 132);
        assert_eq!(lexo_next(314159), 314195);
    }
}