gistfile1.txt

fn next_letter(input: &str) -> Option<(&str, char)> {
    input
        .chars()
        .next()
        .and_then(|letter| input.strip_prefix(letter).map(|input| (input, letter)))
}

fn next_value(input: &str) -> Option<(&str, Value)> {
    next_digit(input)
        .map(|(input, value)| (input, value.pipe(Value::Digit)))
        .or_else(|| next_letter(input).map(|(input, value)| (input, value.pipe(Value::Letter))))
}

fn next_digit(input: &str) -> Option<(&str, u32)> {
    DIGITS
        .iter()
        .copied()
        .find_map(|(i, digit)| input.strip_prefix(digit).map(|input| (input, i)))
}
#[derive(Debug)]
enum Value {
    Letter(char),
    Digit(u32),
}

fn parse(mut input: &str) -> Vec<Value> {
    std::iter::once(())
        .cycle()
        .map(|_| {
            next_value(input)
                .tap_some(|(new_input, _)| input = *new_input)
                .map(|(_, value)| value)
        })
        .take_while(|v| v.is_some())
        .flatten()
        .collect()
}