extern crate itertools; // 0.8.0
use itertools::Itertools; // 0.8.0

fn g(x: u32) -> u32 {
    let s = x.to_string();
    s.chars()
        .map(|c| c.to_digit(10).unwrap())
        .into_iter()
        .group_by(|&x| x)
        .into_iter()
        .map(|(k, group)| (group.count() as u32, k))
        .fold(0, |acc, (repeats, digit)| acc * 100 + repeats * 10 + digit)
}

fn main() {
    println!("{:?} should be 221311", g(2231));
}