extern crate itertools; // 0.8.0

use itertools::Itertools; // 0.8.0
use std::char;
use std::u64;

fn g(x: u64) -> u64 {
    let s = x.to_string();
    let s = s
        .chars()
        .map(|c| c.to_digit(10).unwrap())
        .group_by(|&k| k)
        .into_iter()
        .fold(String::new(), |mut acc, (k, group)| {
            let n = group.count().to_string();
            let k = char::from_digit(k, 10).unwrap();
            acc.push_str(&n);
            acc.push(k);
            acc
        });
    u64::from_str_radix(&s, 10).unwrap()
}

fn main() {
    assert_eq!(221311, g(2231));
    assert_eq!(1411213, g(4222222222223));
}