day18.rs

fn parse(input: &str) -> impl Iterator<Item = (u8, i64, u32)> + '_ {
    input.lines().map(|s| {
        let mut s = s.split(&[' ', '(', '#', ')']).filter(|s| s.len() > 0);
        (
            s.next().unwrap().as_bytes()[0],
            s.next().unwrap().parse().unwrap(),
            u32::from_str_radix(s.next().unwrap(), 16).unwrap(),
        )
    })
}

#[aoc(day18, part1)]
fn part1(input: &str) -> i64 {
    part(parse(input))
}

#[aoc(day18, part2)]
fn part2(input: &str) -> i64 {
    part(parse(input).map(|(_, _, color)| (b"RDLU"[color as usize % 16], color as i64 / 16, 0)))
}

fn part(commands: impl Iterator<Item = (u8, i64, u32)>) -> i64 {
    let (mut x, mut y, mut t) = (0, 0, 2);
    let mut edges = commands
        .map(|(dir, len, ..)| {
            match dir {
                b'U' => y -= len,
                b'D' => y += len,
                b'L' => x -= len,
                _ => x += len,
            };
            t += len;
            (x, y)
        })
        .collect::<Vec<_>>();
    edges.push(edges[0]);
    (edges
        .windows(2)
        .map(|i| i[0].0 * i[1].1 - i[0].1 * i[1].0)
        .sum::<i64>()
        + t)
        / 2
}