Discussion on: Daily Challenge #115 - Look and Say Sequence

pub struct LookAndSaySequence(String);

impl Iterator for LookAndSaySequence {
    type Item = String;

    fn next(&mut self) -> Option<Self::Item> {
        let mut result = String::new();
        let mut prev_digit = None;
        let mut counter = 0;
        let mut it = self.0.chars();
        let mut result = loop {
            let digit = it.next();
            if digit == prev_digit {
                counter += 1;
            } else {
                if let Some(prev_digit) = prev_digit {
                    use std::fmt::Write;
                    write!(&mut result, "{}{}", counter, prev_digit).unwrap();
                }
                counter = 1;
                prev_digit = digit;
            }
            if digit.is_none() {
                break result;
            }
        };
        std::mem::swap(&mut self.0, &mut result);
        Some(result)
    }
}

pub fn look_and_say_sequence(seed: &str, index_that_starts_at_one_who_does_that_its_2019: usize) -> String {
    LookAndSaySequence(seed.to_owned()).nth(index_that_starts_at_one_who_does_that_its_2019 - 1).expect("endless iterator")
}

fn main() {
    assert_eq!(look_and_say_sequence("1", 1), "1");
    assert_eq!(look_and_say_sequence("1", 3), "21");
    assert_eq!(look_and_say_sequence("1", 5), "111221");
    assert_eq!(look_and_say_sequence("22", 10), "22");
    assert_eq!(look_and_say_sequence("14", 2), "1114");
}