AoC 2021 Day 6

day6.rs

use std::io;
use std::str::FromStr;

const FISH_REPRODUCTION_CYCLE: usize = 7;
const FISH_INCUBATION_PERIOD: usize = 9;

fn calc_fish_pop(fish_pop: &Vec<u32>, max_days: usize) {
    let mut fish_by_day: [u64; FISH_REPRODUCTION_CYCLE] = [0; FISH_REPRODUCTION_CYCLE];
    let mut total_fish_count: u64 = 0;

    for fish in fish_pop {
        fish_by_day[*fish as usize] += 1;
        total_fish_count += 1;
    }

    let mut incubating_next_period: [u64; FISH_INCUBATION_PERIOD] = [0; FISH_INCUBATION_PERIOD];

    let mut days = 0;
    while days < max_days {
        let cur_day = days % FISH_REPRODUCTION_CYCLE;

        fish_by_day[cur_day] += incubating_next_period[0];

        let mut i = 0;
        while i < FISH_INCUBATION_PERIOD - 1 {
            incubating_next_period[i] = incubating_next_period[i + 1];
            i += 1;
        }

        let num_reproducing_today = fish_by_day[cur_day];

        incubating_next_period[FISH_INCUBATION_PERIOD - 1] = num_reproducing_today;
        total_fish_count += num_reproducing_today;

        days += 1;
    }

    println!("fish population after {} days: {}", max_days, total_fish_count);
}

fn main() {
    let mut line = String::new();
    let stdin = io::stdin();

    let mut fish_pop: Vec<u32> = vec![];

    while stdin.read_line(&mut line).unwrap() > 0 {
        let mut parts = line.trim().split(',');
        while let Some(part) = parts.next() {
            let cur_fish = u32::from_str(part).unwrap();
            fish_pop.push(cur_fish);
        }

        line.clear();
    }

    calc_fish_pop(&fish_pop, 80);
    calc_fish_pop(&fish_pop, 256);
}