day23.rs

#[aoc_generator(day23)]
fn input_generator(input: &str) -> Vec<usize> {
    input
        .as_bytes()
        .into_iter()
        .map(|&c| (c - b'0') as usize)
        .collect()
}

#[aoc(day23, part1)]
fn part1(data: &[usize]) -> usize {
    let next = game(data, 100, data.len());
    let (mut i, mut res) = (1, 0);
    for _ in 1..data.len() {
        i = next[i];
        res = res * 10 + i;
    }
    res
}

#[aoc(day23, part2)]
fn part2(data: &[usize]) -> usize {
    let next = game(data, 10_000_000, 1_000_000);
    next[1] * next[next[1]]
}

fn game(data: &[usize], rounds: usize, end: usize) -> Vec<usize> {
    let mut next = (0..=end).map(|i| i + 1).collect::<Vec<_>>();
    next[end] = data[0];
    for i in 0..(data.len() - 1) {
        next[data[i]] = data[i + 1];
    }
    next[data[data.len() - 1]] = if data.len() < end {
        data.len() + 1
    } else {
        data[0]
    };
    let mut sel = data[0];
    for _ in 0..rounds {
        let picked: Vec<usize> = vec![next[sel], next[next[sel]], next[next[next[sel]]]];
        next[sel] = next[picked[2]];
        let mut ins = sel;
        loop {
            ins = ((ins + end - 2) % end) + 1;
            if !picked.contains(&ins) {
                break;
            }
        }
        next[picked[2]] = next[ins];
        next[ins] = picked[0];
        sel = next[sel];
    }
    next
}