2023 Advent of Code Day 9

main.rs

const INPUT: &str = "..snip..";

fn main() {
    let sum: i64 = INPUT.lines()
        .map(extract_digits)
        .map(|d| extrapolate_next(&d))
        .sum();
    println!("part1: {}", sum);

    let sum: i64 = INPUT.lines()
        .map(extract_digits)
        .map(|d| extrapolate_prev(&d))
        .sum();
    println!("part2: {}", sum);
}

fn extract_digits(line: &str) -> Vec<i64> {
    let mut r: Vec<i64> = vec![];
    let mut buf = String::new();
    for c in line.chars() {
        if c == ' ' && buf.len() > 0 {
            r.push(buf.parse().unwrap());
            buf.clear();
        } else if c.is_numeric() || c == '-' {
            buf.push(c);
        }
    }
    if buf.len() > 0 {
        r.push(buf.parse().unwrap());
    }

    r
}

fn extrapolate_next(n: &Vec<i64>) -> i64 {
    let mut seqs: Vec<Vec<i64>> = vec![];
    seqs.push(n.clone());

    while seqs[ seqs.len() - 1 ].iter().all(|&v| v == 0) == false {
        let next = seqs[ seqs.len() - 1]
            .windows(2)
            .map(|w| w[1] - w[0])
            .collect();
        seqs.push(next);
    }

    let mut i = seqs.len();
    while i >= 2 {
        let l = seqs[i - 1][ seqs[i-1].len() - 1 ];
        let l2 = seqs.get_mut(i - 2).unwrap();

        l2.push(
            l2[l2.len() - 1] + l
        );
        i -= 1;
    }

    seqs[0][ seqs[0].len() - 1 ]
}

fn extrapolate_prev(n: &Vec<i64>) -> i64 {
    let mut seqs: Vec<Vec<i64>> = vec![];
    seqs.push(n.clone());

    while seqs[ seqs.len() - 1 ].iter().all(|&v| v == 0) == false {
        let next = seqs[ seqs.len() - 1]
            .windows(2)
            .map(|w| w[1] - w[0])
            .collect();
        seqs.push(next);
    }

    let mut i = seqs.len();
    while i >= 2 {
        let l = seqs[i - 1][0];
        let l2 = seqs.get_mut(i - 2).unwrap();
        l2.insert(0, l2[0] - l);
        i -= 1;
    }

    seqs[0][0]
}