Rust

My first version worked with numbers, but since I was still sick of yesterday's pow(10) calls, I changed it to use ascii for the second half - the nice thing is that means it can work with hex input with no modification!

Clippy was complaining about "needless_range_loops", but it's way more readable this way.

struct PowerSource(Vec<Bank>);

impl FromStr for PowerSource {
    type Err = Report;

    fn from_str(s: &str) -> Result<Self> {
        let banks = s.lines().map(|l| Bank(l.to_owned())).collect();
        Ok(Self(banks))
    }
}

impl PowerSource {
    fn max_joltage(&self, num_digits: usize) -> usize {
        self.0.iter().map(|b| b.max_joltage(num_digits)).sum()
    }
}

struct Bank(String);

impl Bank {
    fn max_joltage(&self, num_digits: usize) -> usize {
        let batts = self.0.as_bytes();

        let mut digits = vec![b'0'; num_digits];
        let mut start = 0;
        for d in 0..num_digits {
            for i in start..=batts.len() - num_digits + d {
                if batts[i] > digits[d] {
                    digits[d] = batts[i];
                    start = i + 1;
                }
            }
        }

        usize::from_str(str::from_utf8(&digits).unwrap()).unwrap()
    }
}