day09.rs

use std::fs;
use itertools::Itertools;

pub(crate) fn day09() {
    println!("{}", part_a(fs::read_to_string("input/2023/day09/input.txt").unwrap()));
    println!("{}", part_b(fs::read_to_string("input/2023/day09/input.txt").unwrap()));
}

fn part_a(input: String) -> i32 {
    input.lines().map(|line| {
        let mut nums: Vec<i32> = line.split_whitespace().map(|s| s.parse::<i32>().unwrap()).collect();
        let mut last = vec!{nums.last().unwrap().clone()};
        while nums.iter().any(|a| *a > 0) {
            let new_nums: Vec<i32> = nums.into_iter().tuple_windows().map(|(a, b)| b - a).collect();
            last.push(new_nums.last().unwrap().clone());
            nums = new_nums;
        }

        let mut ans = 0;
        for i in (0..last.len() - 1).rev() {
            ans = ans + last[i];
        }
        ans
    }).sum()
}

fn part_b(input: String) -> i32 {
    input.lines().map(|line| {
        let mut nums: Vec<i32> = line.split_whitespace().map(|s| s.parse::<i32>().unwrap()).collect();
        let mut first = vec!{nums.first().unwrap().clone()};
        while nums.iter().any(|a| *a > 0) {
            let new_nums: Vec<i32> = nums.into_iter().tuple_windows().map(|(a, b)| b - a).collect();
            first.push(new_nums.first().unwrap().clone());
            nums = new_nums;
        }

        let mut ans = 0;
        for i in (0..first.len() - 1).rev() {
            ans = first[i] - ans;
        }
        ans
    }).sum()
}

#[cfg(test)]
mod day09_tests {
    use std::fs;

    use crate::y2023::day09::{part_a, part_b};

    #[test]
    fn test_works() {
        assert_eq!(114, part_a(fs::read_to_string("input/2023/day09/test.txt").unwrap()));
        assert_eq!(2, part_b(fs::read_to_string("input/2023/day09/test.txt").unwrap()));
    }

    #[test]
    fn input_works() {
        assert_eq!(1974913025, part_a(fs::read_to_string("input/2023/day09/input.txt").unwrap()));
        assert_eq!(884, part_b(fs::read_to_string("input/2023/day09/input.txt").unwrap()));
    }
}