pub fn day6() {
    // let input = "3,4,3,1,2";
    let input = include_str!("../input/day6.txt");

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

    count_fishes_after(&initial_fishes, 18);
    count_fishes_after(&initial_fishes, 80);
    count_fishes_after(&initial_fishes, 256);
}

fn count_fishes_after(initial_fishes: &[usize], days: u32) {
    let mut fish_batches = [0_u64; 9];

    for f in initial_fishes {
        fish_batches[*f] += 1;
    }

    for _ in 1..=days {
        let old_batches = fish_batches;
        fish_batches = [0; 9];

        for counter in 0..=7 {
            fish_batches[counter] = old_batches[counter + 1];
        }

        // each fish previously at 0 spawns a new fish with counter 8 and resets its counter to 6
        fish_batches[8] = old_batches[0];
        fish_batches[6] += old_batches[0];
    }

    println!(
        "total fishes after {} days: {}",
        days,
        fish_batches.iter().sum::<u64>()
    );
}