Day 6: Lanternfish

Day #6 problem description │ Problem input

At a high level, this problem requires modifying a map from ints to ints in a controlled fashion. At a low level:

This map’s keys are actually the integers from 0 to 8 (inclusive), so it can just be stored in an array.
The “controlled fashion” is merely shifting most values over. In one case we have to also add to said values.

Setup

use crate::Answer;

const N_TIMERS: usize = 9;
type Timers = [usize; N_TIMERS];

fn read_input(input: &str) -> Option<Timers> {
	let mut timers = [0; N_TIMERS];

	let nums = input
		.trim()
		.split(',')
		.map(|s| s.parse().ok())
		.collect::<Option<Vec<usize>>>()?;

	for num in nums {
		timers[num] += 1;
	}

	Some(timers)
}

fn tick_in_place(timers: &mut Timers) {
	let initial = timers[0];
	for i in 0..(N_TIMERS - 1) {
		timers[i] = timers[i + 1];
	}
	timers[8] = initial;
	timers[6] += initial;
}

// [usize; 9] implements Copy
fn tick(n_times: usize, timers: &Timers) -> Timers {
	let mut timers = *timers;
	for _ in 0..n_times {
		tick_in_place(&mut timers);
	}
	timers
}

fn ans_for_input(input: &str) -> Answer<usize, usize> {
	let timers = read_input(input).unwrap();
	(6, (pt1(&timers), pt2(&timers))).into()
}

pub fn ans() -> Answer<usize, usize> {
	ans_for_input(include_str!("sample_input.txt"))
}

Parts 1 and 2

fn pt1(timers: &Timers) -> usize {
	tick(80, timers).iter().sum()
}
fn pt2(timers: &Timers) -> usize {
	tick(256, timers).iter().sum()
}