AOC 2023 Day 9

use std::env;
use std::io::{self, prelude::*, BufReader};
use std::fs::File;

#[derive(Debug)]
struct History {
    values: Vec<isize>,
}
impl From<&String> for History {
    fn from(s: &String) -> Self {
        Self {
            values: s.split_whitespace().map(|n| n.parse::<isize>().unwrap()).collect(),
        }
    }
}
impl History {
    pub fn solve(&self) -> (isize,isize) {
        // Initializations
        let mut values:       Vec<isize> = self.values.clone();
        let mut first_values: Vec<isize> = vec![*values.first().unwrap()];
        let mut last_values:  Vec<isize> = vec![*values.last().unwrap()];
        // Do diffs
        loop {
            let diffs: Vec<_> = values.windows(2).map(|v| v[1] - v[0]).collect();
            values = diffs.clone();
            first_values.push(*diffs.first().unwrap());
            last_values.push( *diffs.last().unwrap());
            if diffs.iter().all(|v| *v == 0isize) { break; }
        }
        // Identify first value
        let first = first_values.iter().rev().fold(0, |acc,x| x - acc);
        (last_values.iter().sum(),first)
    }
}

fn solve(input: &str) -> io::Result<()> {
    let file = File::open(input).expect("Input file not found.");
    let reader = BufReader::new(file);

    // Input
    let input: Vec<String> = match reader.lines().collect() {
            Err(err) => panic!("Unknown error reading input: {err}"),
            Ok(result) => result,
    };
    let input: Vec<_> = input.iter().map(History::from).collect();

    let part1: isize = input.iter().map(|x| x.solve().0).sum();
    println!("Part 1: {part1}"); // 1684566095

    let part2: isize = input.iter().map(|x| x.solve().1).sum();
    println!("Part 2: {part2}"); // 1136

    Ok(())
}


fn main() {
    let args: Vec<String> = env::args().collect();
    let filename = &args[1];
    solve(&filename).unwrap();
}