I’m still struggling to learn Rust, and also to figure out a good solution to these problems, but here’s mine anyway:

#[allow(dead_code)]
pub fn part1(input: Lines<BufReader<File>>) {
    let mut sum = 0;
    for line in input.map_while(Result::ok) {
        let total = find_joltage(line.as_bytes(), 2, false);
        let res = str::from_utf8(total.as_slice()).unwrap();

        sum += res.parse::<u32>().unwrap();
    }

    println!("{}", sum);
}

#[allow(dead_code)]
pub fn part2(input: Lines<BufReader<File>>) {
    let mut sum = 0;
    for (_, line) in input.map_while(Result::ok).enumerate() {
        let total = find_joltage(line.as_bytes(), 12, false);
        let res = str::from_utf8(total.as_slice()).unwrap();

        sum += res.parse::<u64>().unwrap();
    }

    println!("{}", sum);
}

fn find_joltage(b: &[u8], size: usize, debug: bool) -> Vec<u8> {
    if size == 0 {
        return vec![];
    }

    let mut max: u8 = 0;
    let mut max_i = 0;
    for i in (0..=b.len() - size).rev() {
        if b[i] >= max {
            max = b[i];
            max_i = i;
        }
    }

    if debug {
        println!("max = {}, i = {}", max as char, max_i);
    }
    let mut rest = find_joltage(&b[max_i + 1..], size - 1, debug);

    rest.insert(0, max);
    rest
}