Advent of Code 2020 - Day 2: Password Philosophy

password_philosophy.rs

struct Line<'a> {
    lower: usize,
    upper: usize,
    letter: char,
    password: &'a str,
}

impl<'a> From<&'a str> for Line<'a> {
    fn from(l: &'a str) -> Self {
        let (policy, password) = l.split_once(": ").unwrap();
        let (range, letter) = policy.split_once(' ').unwrap();
        let (lower, upper) = range.split_once('-').unwrap();

        Self {
            lower: lower.parse().unwrap(),
            upper: upper.parse().unwrap(),
            letter: letter.chars().next().unwrap(),
            password,
        }
    }
}

fn main() {
    let list = INPUT.lines().map(Line::from).collect::<Vec<_>>();

    println!(
        "Part 1 (Sled policy): {}",
        list.iter().filter(sled_policy).count()
    );
    println!(
        "Part 2 (Toboggan policy): {}",
        list.iter().filter(toboggan_policy).count()
    );
}

fn sled_policy(line: &&Line) -> bool {
    let count = line.password.chars().filter(|&c| c == line.letter).count();
    count >= line.lower && count <= line.upper
}

fn toboggan_policy(line: &&Line) -> bool {
    (line.password.chars().nth(line.lower - 1).unwrap() == line.letter)
        ^ (line.password.chars().nth(line.upper - 1).unwrap() == line.letter)
}