pub fn day7() {
    // let input = "16,1,2,0,4,2,7,1,2,14";
    let input = include_str!("../input/day7.txt");

    let horiz_positions = input
        .trim()
        .split(",")
        .map(|s| s.parse::<i32>().unwrap())
        .collect::<Vec<_>>();

    let candidates =
        *horiz_positions.iter().min().unwrap()..=*horiz_positions.iter().max().unwrap();

    let min = candidates
        .clone()
        .min_by_key(|pos| cost_to_align_at(pos, &horiz_positions));
    println!("target position: {:?}", min);
    println!(
        "fuel cost: {:?}",
        cost_to_align_at(&min.unwrap(), &horiz_positions)
    );

    let p2_min = candidates.min_by_key(|pos| p2_cost(pos, &horiz_positions));
    println!("p2 target position: {:?}", p2_min);
    println!(
        "p2 fuel cost: {:?}",
        p2_cost(&p2_min.unwrap(), &horiz_positions)
    );
}

fn cost_to_align_at(target: &i32, horiz_positions: &[i32]) -> i32 {
    horiz_positions.iter().map(|pos| (pos - target).abs()).sum()
}

fn p2_cost(target: &i32, horiz_positions: &[i32]) -> i32 {
    horiz_positions
        .iter()
        .map(|pos| {
            let max = (pos - target).abs();
            // thanks, Gauss
            (max * (max + 1)) / 2
        })
        .sum()
}