2021: day11: part 1 & 2
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aoc2021/input/day11.txt
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aoc2021/input/day11.txt
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10
aoc2021/input/day11_provided.txt
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aoc2021/input/day11_provided.txt
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5483143223
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2745854711
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5264556173
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6141336146
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6357385478
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2176841721
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6882881134
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236
aoc2021/src/day11.rs
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aoc2021/src/day11.rs
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use std::fmt::Write;
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use anyhow::{Context, Result};
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const INPUT: &str = include_str!("../input/day11.txt");
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pub fn run() -> Result<String> {
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let mut res = String::with_capacity(128);
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writeln!(res, "part 1: {}", part1(INPUT)?)?;
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writeln!(res, "part 2: {}", part2(INPUT)?)?;
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Ok(res)
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}
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fn part1(input: &str) -> Result<usize> {
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let mut grid: OctopusGrid = input.parse()?;
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Ok((0..100).map(|_| grid.step()).sum())
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}
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fn part2(input: &str) -> Result<usize> {
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let mut grid: OctopusGrid = input.parse()?;
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let (_, step) = (0..)
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.map(|step| (grid.step(), step))
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.find(|(flashes, _)| *flashes == 100)
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.context("never reached step where octopuses all flashed simultaneously")?;
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Ok(step + 1)
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}
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#[derive(Clone, Copy, Debug)]
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struct Octopus {
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energy_level: u8,
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has_flashed: bool,
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}
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impl Octopus {
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fn increment_energy_level(&mut self) {
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self.energy_level += 1;
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}
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fn reset_energy_level(&mut self) {
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if self.has_flashed {
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self.energy_level = 0;
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self.has_flashed = false;
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}
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}
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fn flashes(&mut self) -> bool {
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if self.energy_level > 9 && !self.has_flashed {
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self.has_flashed = true;
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return true;
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}
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false
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}
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}
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impl TryFrom<char> for Octopus {
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type Error = anyhow::Error;
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fn try_from(value: char) -> Result<Self> {
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Ok(Octopus {
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energy_level: value
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.to_digit(10)
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.with_context(|| format!("couldn't convert char `{}` to digit", value))?
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as u8,
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has_flashed: false,
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})
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}
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}
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#[derive(Clone, Copy)]
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enum Neighbour {
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Up,
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Down,
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Left,
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Right,
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UpperLeft,
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UpperRight,
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LowerLeft,
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LowerRight,
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}
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impl Neighbour {
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fn apply(&self, x: usize, y: usize, width: usize, height: usize) -> Option<(usize, usize)> {
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match self {
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Neighbour::Left if x > 0 => Some((x - 1, y)),
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Neighbour::Right if x < width - 1 => Some((x + 1, y)),
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Neighbour::Up if y > 0 => Some((x, y - 1)),
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Neighbour::Down if y < height - 1 => Some((x, y + 1)),
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Neighbour::UpperLeft => Neighbour::Left
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.apply(x, y, width, height)
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.and_then(|(x, y)| Neighbour::Up.apply(x, y, width, height)),
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Neighbour::UpperRight => Neighbour::Right
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.apply(x, y, width, height)
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.and_then(|(x, y)| Neighbour::Up.apply(x, y, width, height)),
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Neighbour::LowerLeft => Neighbour::Left
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.apply(x, y, width, height)
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.and_then(|(x, y)| Neighbour::Down.apply(x, y, width, height)),
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Neighbour::LowerRight => Neighbour::Right
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.apply(x, y, width, height)
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.and_then(|(x, y)| Neighbour::Down.apply(x, y, width, height)),
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_ => None,
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}
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}
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const ALL: &'static [Self] = &[
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Neighbour::Left,
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Neighbour::Right,
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Neighbour::Up,
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Neighbour::Down,
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Neighbour::UpperLeft,
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Neighbour::UpperRight,
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Neighbour::LowerLeft,
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Neighbour::LowerRight,
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];
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}
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#[derive(Debug)]
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struct OctopusGrid {
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octopuses: Vec<Octopus>,
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width: usize,
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height: usize,
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}
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impl OctopusGrid {
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fn neighbours(&self, x: usize, y: usize) -> impl Iterator<Item = (usize, usize)> + 'static {
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let width = self.width;
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let height = self.height;
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Neighbour::ALL
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.iter()
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.copied()
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.filter_map(move |neighbour| neighbour.apply(x, y, width, height))
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}
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fn get_mut(&mut self, x: usize, y: usize) -> &mut Octopus {
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&mut self.octopuses[y * self.width + x]
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}
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fn step(&mut self) -> usize {
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// First, the energy level of each octopus increases by 1.
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self.octopuses
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.iter_mut()
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.for_each(Octopus::increment_energy_level);
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// Then, any octopus with an energy level greater than 9 flashes. This increases the energy
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// level of all adjacent octopuses by 1, including octopuses that are diagonally adjacent.
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// If this causes an octopus to have an energy level greater than 9, it also flashes. This
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// process continues as long as new octopuses keep having their energy level increased
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// beyond 9. (An octopus can only flash at most once per step.)
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let mut flashed = true;
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let mut flashes = 0;
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while flashed {
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flashed = false;
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for index in 0..self.octopuses.len() {
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if self.octopuses[index].flashes() {
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let (x, y) = (index % self.width, index / self.width);
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for (x, y) in self.neighbours(x, y) {
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self.get_mut(x, y).increment_energy_level();
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}
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flashed = true;
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flashes += 1;
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}
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}
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}
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// Finally, any octopus that flashed during this step has its energy level set to 0, as it
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// used all of its energy to flash.
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self.octopuses
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.iter_mut()
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.for_each(Octopus::reset_energy_level);
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flashes
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}
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}
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impl std::str::FromStr for OctopusGrid {
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type Err = anyhow::Error;
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fn from_str(s: &str) -> Result<Self, Self::Err> {
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let mut octopuses = Vec::new();
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let mut height = 0;
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let mut width = None;
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for line in s.lines().map(str::trim) {
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let octopus_line = line
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.chars()
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.map(TryFrom::try_from)
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.collect::<Result<Vec<_>>>()?;
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if width.is_none() {
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width = Some(octopus_line.len());
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}
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height += 1;
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octopuses.extend_from_slice(&octopus_line);
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}
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Ok(OctopusGrid {
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octopuses,
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width: width.context("0 lines parsed, width never computed")?,
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height,
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})
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}
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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const PROVIDED: &str = include_str!("../input/day11_provided.txt");
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#[test]
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fn part1_provided() {
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assert_eq!(part1(PROVIDED).unwrap(), 1656);
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}
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#[test]
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fn part1_real() {
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assert_eq!(part1(INPUT).unwrap(), 1588);
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}
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#[test]
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fn part2_provided() {
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assert_eq!(part2(PROVIDED).unwrap(), 195);
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}
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#[test]
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fn part2_real() {
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assert_eq!(part2(INPUT).unwrap(), 517);
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}
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}
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@ -10,3 +10,4 @@ pub mod day07;
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pub mod day08;
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pub mod day09;
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pub mod day10;
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pub mod day11;
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@ -12,6 +12,7 @@ use aoc2021::day07;
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use aoc2021::day08;
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use aoc2021::day09;
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use aoc2021::day10;
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use aoc2021::day11;
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fn main() -> Result<()> {
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let days: &[DayFunc] = &[
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day08::run,
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day09::run,
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day10::run,
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day11::run,
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];
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aoc::run(days)
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