use anyhow::{Context, Result};

fn extrapolate(sequence: &mut Vec<i64>) -> Option<(i64, i64)> {
    let diff = |seq: &mut Vec<i64>| {
        (0..seq.len() - 1).for_each(|i| seq[i] = seq[i + 1] - seq[i]);
        seq.pop();
    };

    let mut fwd = 0;
    let mut bwd = 0;
    let mut f = 1;

    while sequence.len() > 1 {
        fwd += sequence[sequence.len() - 1];
        bwd += f * sequence[0];
        diff(sequence);
        f *= -1;

        if sequence.iter().all(|&num| num == 0) {
            return Some((bwd, fwd));
        }
    }

    None
}

fn main() -> Result<()> {
    let filename = std::env::args()
        .nth(1)
        .context("./day09 <path to puzzle input>")?;
    let input = std::fs::read_to_string(filename)?;

    let mut part1 = 0;
    let mut part2 = 0;

    for line in input.lines() {
        let parse_i64 = |str: &str| str.parse::<i64>().unwrap();
        let mut initial: Vec<i64> = line.split_ascii_whitespace().map(parse_i64).collect();
        let (p2, p1) = extrapolate(&mut initial).context("malformed input")?;
        part1 += p1;
        part2 += p2;
    }

    println!("1) {}", part1);
    println!("2) {}", part2);

    Ok(())
}