daysix.rs

use std::fs::read_to_string;

const DAYS: usize = 256;

type Fish = Vec<u64>;

fn main() {
    let mut fish = vec![0; 9];
    let mut scratch;
    parse_input(&mut fish);

    for _ in 0..DAYS {
        scratch = vec![0; 9];
        let mut pos = 8;
        while pos != 0 {
            scratch[pos - 1] = fish[pos];
            pos -= 1;
        }
        scratch[8] = fish[0];
        scratch[6] += fish[0];
        fish = scratch;
    }

    println!("Total: {}", fish.iter().sum::<u64>());
}

fn parse_input(fish: &mut Fish) {
    let s = read_to_string("/tmp/input").expect("File I/O");

    for entry in s.trim_end().split(",") {
        match entry.parse::<usize>() {
            Ok(n) => fish[n] += 1,
            Err(_) => println!("err on: {}", entry),
        }
    }
}