aoc 2020 day 9

main.rs

fn part1(data: &[u32], preamble_len: usize) -> u32 {
    use itertools::Itertools;
    data.iter()
        .skip(preamble_len)
        .enumerate()
        .map(|(i, &datum)| {
            (
                data.iter()
                    .skip(i)
                    .take(preamble_len)
                    .copied()
                    .collect::<Vec<u32>>(),
                datum,
            )
        })
        .find(|(previous, datum)| {
            !previous
                .iter()
                .cartesian_product(previous.iter())
                .any(|(left, right)| left + right == *datum)
        })
        .unwrap()
        .1
}

fn part2(data: &[u32], preamble_len: usize) -> u32 {
    let target = part1(data, preamble_len);
    let windows = (2..)
        .find(|&windows| {
            data.windows(windows)
                .find(|it| it.iter().map(|&it| it as u64).sum::<u64>() == target as u64)
                .is_some()
        })
        .unwrap();

    if let Some(r) = data
        .windows(windows)
        .find(|it| it.iter().sum::<u32>() == target)
    {
        r.iter().min().unwrap() + r.iter().max().unwrap()
    } else {
        0
    }
}

fn main() {
    let data: Vec<u32> = include_str!("data.txt")
        .lines()
        .filter_map(|it| it.parse().ok())
        .collect();
    println!("first mismatch is {}", part1(&data, 25));
    println!("weakness is {}", part2(&data, 25))
}