AoC2021/src/bin/day6.rs

use std::error::Error;
use std::fs::File;
use std::io::{self, BufRead};
use std::vec::Vec;

fn main() -> Result<(), Box<dyn Error>> {
    let file = File::open("inputs/day6.txt")?;
    let fishes: Vec<u64> = io::BufReader::new(file)
        .lines()
        .nth(0)
        .unwrap()
        .unwrap()
        .split(",")
        .map(|f| f.parse().unwrap())
        .collect();

    let mut answer1 = 0;

    let mut population: Vec<u64> = Vec::new();
    population.resize(9, 0);
    for fish in fishes {
        population[fish as usize] += 1;
    }

    for day in 0..256 {
        let mut next_population: Vec<u64> = Vec::new();
        next_population.resize(9, 0);

        next_population[6] = population[0];
        next_population[8] = population[0];

        for i in 1..9 {
            next_population[i - 1] += population[i];
        }

        population = next_population;

        if day == 18 {
            answer1 = population.iter().fold(0, |a, b| a + b);
        }
    }

    let answer2 = population.iter().fold(0, |a, b| a + b);

    println!("Answer1: {}", answer1);

    println!("Answer2: {}", answer2);

    Ok(())
}