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Nicky Meuleman

Posted on • Originally published at nickymeuleman.netlify.app

# Advent of Code 2022 Day 23

## Day 23: Unstable Diffusion

TL;DR: my solution in Rust

The grove that was supposed to be full of star fruit is barren.

The elves want to plant new seedlings.
The trees need space, so the elves should spread out.

Today's input is a 2D map of locations the elves are.

• A `#` represents an elf
• A `.` represents an empty space

The map is surrounded by an infinite amount of empty spaces off the sides.

An example input looks like this:

``````....#..
..###.#
#...#.#
.#...##
#.###..
##.#.##
.#..#..
``````

The map is printed so north is up.
(so if in elf moves north, it moves up)

It's an other day where indexes start at 1

## Parsing

The `Coord` struct is like Gandalf's headbobbing, it just keeps going!

``````#[derive(PartialEq, Eq, Hash, Clone, Copy)]
struct Coord {
row: i32,
col: i32,
}
``````

I'm only keeping the positions an elf is in memory.
Every elf has a `Coord`, and the result of parsing the input as a set of them.

The `Coord` struct uses numbers that can go negative.
Because the elves can move off the map in the input to the top or to the left.

``````fn parse(input: &str) -> HashSet<Coord> {
let mut elves = HashSet::new();
for (row, line) in input.lines().enumerate() {
for (col, c) in line.chars().enumerate() {
if c == '#' {
elves.insert(Coord {
row: row as i32,
col: col as i32,
});
}
}
}
elves
}
``````

## Part 1

The elves use a time-consuming method to spread out.

After 10 full rounds of this method, the elves like to check if they're on the right track.
Draw a box from the top-left elf to the bottom-right one.

The question asks how many empty spaces are in the box.

The process consists of three phases per round:

1. The consideration phase
2. The movement phase
3. The swappy-checky phase (I couldn't come up with a name, ok?)

### The consideration phase

Each Elf considers their 8 adjactent neighbours.

A quick sidebar I like to call "we're making a data type".
It's the `Direction` enum!

``````enum Direction {
North,
NorthEast,
East,
SouthEast,
South,
SouthWest,
West,
NorthWest,
}
``````

If no other Elves are in one of those eight positions, the Elf does not do anything during this round.

Otherwise, the Elf looks in each of four directions in the following order and proposes moving one step in the first valid direction:

1. If there is no Elf in the N, NE, or NW adjacent positions, the Elf proposes moving north one step.
2. If there is no Elf in the S, SE, or SW adjacent positions, the Elf proposes moving south one step.
3. If there is no Elf in the W, NW, or SW adjacent positions, the Elf proposes moving west one step.
4. If there is no Elf in the E, NE, or SE adjacent positions, the Elf proposes moving east one step.

### The movement phase

All at once, each Elf moves to their proposed destination tile if they were the only Elf to propose moving to that position.
If two or more Elves proposed moving to the same position, none of those Elves move.

### The swappy-checky phase

This is the part that changes the order of those 4 checks in the consideration phase.

The first round starts by checking the cardinal directions as they appear in the text.
First `North`, then `South`, followed by `West`, and finally `East`.

Here, the first direction in that list moves to the back.
So the second round would start by checking `South`, then `West`, followed by `East`, and finally `North`.

### Helpers

A few helpers for the `Coord` struct that let us take one step in a given `Direction`.
That's also handy to find a `Coord`'s 8 neighbours.

``````impl Coord {
fn neighbours(&self) -> [Self; 8] {
use Direction::*;
[n, ne, e, se, s, sw, w, nw]
}

fn add_dir(&self, dir: &Direction) -> Self {
use Direction::*;
match dir {
North => Coord {
row: self.row - 1,
col: self.col,
},
NorthEast => Coord {
row: self.row - 1,
col: self.col + 1,
},
East => Coord {
row: self.row,
col: self.col + 1,
},
SouthEast => Coord {
row: self.row + 1,
col: self.col + 1,
},
South => Coord {
row: self.row + 1,
col: self.col,
},
SouthWest => Coord {
row: self.row + 1,
col: self.col - 1,
},
West => Coord {
row: self.row,
col: self.col - 1,
},
NorthWest => Coord {
row: self.row - 1,
col: self.col - 1,
},
}
}
}
``````

And a helper on `Direction` that, given a list of 8 neighbour occupation booleans says if the corresponding check succeeds.

The check from the consideration phase.
There's 1 check for `North`, 1 for `South`, 1 for `West`, and 1 for `East`.

``````impl Direction {
fn check(&self, neighbours: &[bool; 8]) -> bool {
// neighbours in form [n, ne, e, se, s, sw, w, nw]
let [n, ne, e, se, s, sw, w, nw] = neighbours;
// in these 4 conditions the question says OR, but it means AND
match &self {
// If there is no Elf in the N, NE, or NW adjacent positions, the Elf proposes moving north one step.
Direction::North => !n && !ne && !nw,
// If there is no Elf in the S, SE, or SW adjacent positions, the Elf proposes moving south one step.
Direction::South => !s && !se && !sw,
// If there is no Elf in the W, NW, or SW adjacent positions, the Elf proposes moving west one step.
Direction::West => !w && !nw && !sw,
// If there is no Elf in the E, NE, or SE adjacent positions, the Elf proposes moving east one step.
Direction::East => !e && !ne && !se,
// question doesn't specify, but I assume the elf doesn't propose anything
_ => false,
}
}
}
``````

As a bonus, a function to print out the current map.

• It gets the min and max value for the `row` coordinate of every elf.
• It gets the min and max value for the `col` coordinate of every elf.

It then loops through the rows and cols. And prints something depending if an elf was found at that combination of row and col.

• `#` if an elf is there
• `.` if the space is open

Don't forget to print a newline after every row, or the output will be one big lne.

``````fn print(elves: &HashSet<Coord>) {
let (minmax_row, minmax_col) = elves.iter().fold(
((i32::MAX, i32::MIN), (i32::MAX, i32::MIN)),
|(minmax_row, minmax_col), Coord { row, col }| {
(
(minmax_row.0.min(*row), minmax_row.1.max(*row)),
(minmax_col.0.min(*col), minmax_col.1.max(*col)),
)
},
);
for row in minmax_row.0..=minmax_row.1 {
for col in minmax_col.0..=minmax_col.1 {
if elves.contains(&Coord { row, col }) {
print!("#")
} else {
print!(".")
}
}
println!();
}
}
``````

Skeleton code time!
Scratch that, with these helpers, implementation time!

### Final code

``````pub fn part_1(input: &str) -> usize {
let mut elves = parse(input);
let mut checks = [
Direction::North,
Direction::South,
Direction::West,
Direction::East,
];

for round in 1.. {
// key: proposed coordinate, val: list of old coordinates that proposed going there
let mut proposals: HashMap<Coord, Vec<Coord>> = HashMap::new();

// consideration phase
for elf in &elves {
let neighbours = elf.neighbours();
// If no other Elves are in one of those eight positions, the Elf does not do anything during this round.
if neighbours.iter().all(|coord| !elves.contains(coord)) {
continue;
}
let neighbours = neighbours
.iter()
.map(|neighbour| elves.contains(neighbour))
.collect::<Vec<_>>()
.try_into()
.unwrap();

let proposed_dir = checks.iter().find(|dir| dir.check(&neighbours));
if let Some(dir) = proposed_dir {
proposals.entry(proposal).or_default().push(*elf);
}
}

// movement phase
for (new_coord, old_coords) in proposals {
if old_coords.len() == 1 {
elves.remove(&old_coords);
elves.insert(new_coord);
}
}

// after round
// println!("== End of Round {} ==", round);
// print(&elves);
// println!();

checks.rotate_left(1);
if round == 10 {
let (minmax_row, minmax_col) = elves.iter().fold(
((i32::MAX, i32::MIN), (i32::MAX, i32::MIN)),
|(minmax_row, minmax_col), Coord { row, col }| {
(
(minmax_row.0.min(*row), minmax_row.1.max(*row)),
(minmax_col.0.min(*col), minmax_col.1.max(*col)),
)
},
);
return (minmax_row.0..=minmax_row.1)
.cartesian_product(minmax_col.0..=minmax_col.1)
.filter(|(row, col)| {
!elves.contains(&Coord {
row: *row,
col: *col,
})
})
.count();
}
}

usize::MAX
}
``````

## Part 2

Eventually, the elves come to a stop.

The question asks for the round number where no elf moves.

A `moved` boolean that starts at `false`, and flips to `true` every time an elf moves.
If it's still `false` at the end of a round, done!

### Final code

``````pub fn part_2(input: &str) -> i32 {
let mut elves = parse(input);
let mut checks = [
Direction::North,
Direction::South,
Direction::West,
Direction::East,
];

for round in 1.. {
let mut moved = false;
// key: proposed coordinate, val: list of old coordinates that proposed going there
let mut proposals: HashMap<Coord, Vec<Coord>> = HashMap::new();

// consideration phase
for elf in &elves {
let neighbours = elf.neighbours();
// If no other Elves are in one of those eight positions, the Elf does not do anything during this round.
if neighbours.iter().all(|coord| !elves.contains(coord)) {
continue;
}
let neighbours = neighbours
.iter()
.map(|neighbour| elves.contains(neighbour))
.collect::<Vec<_>>()
.try_into()
.unwrap();

let proposed_dir = checks.iter().find(|dir| dir.check(&neighbours));
if let Some(dir) = proposed_dir {
proposals.entry(proposal).or_default().push(*elf);
}
}

// movement phase
for (new_coord, old_coords) in proposals {
if old_coords.len() == 1 {
moved = true;
elves.remove(&old_coords);
elves.insert(new_coord);
}
}

// after round
if !moved {
return round;
}
checks.rotate_left(1);
}

i32::MAX
}
``````

## Final code

``````use std::collections::{HashMap, HashSet};

use itertools::Itertools;

#[derive(PartialEq, Eq, Hash, Clone, Copy)]
struct Coord {
row: i32,
col: i32,
}

enum Direction {
North,
NorthEast,
East,
SouthEast,
South,
SouthWest,
West,
NorthWest,
}

impl Direction {
fn check(&self, neighbours: &[bool; 8]) -> bool {
// neighbours in form [n, ne, e, se, s, sw, w, nw]
let [n, ne, e, se, s, sw, w, nw] = neighbours;
// in these 4 conditions the question says OR, but it means AND, right?
match &self {
// If there is no Elf in the N, NE, or NW adjacent positions, the Elf proposes moving north one step.
Direction::North => !n && !ne && !nw,
// If there is no Elf in the S, SE, or SW adjacent positions, the Elf proposes moving south one step.
Direction::South => !s && !se && !sw,
// If there is no Elf in the W, NW, or SW adjacent positions, the Elf proposes moving west one step.
Direction::West => !w && !nw && !sw,
// If there is no Elf in the E, NE, or SE adjacent positions, the Elf proposes moving east one step.
Direction::East => !e && !ne && !se,
// question doesn't specify, but I assume the elf doesn't propose anything
_ => false,
}
}
}

impl Coord {
fn neighbours(&self) -> [Self; 8] {
use Direction::*;
[n, ne, e, se, s, sw, w, nw]
}

fn add_dir(&self, dir: &Direction) -> Self {
use Direction::*;
match dir {
North => Coord {
row: self.row - 1,
col: self.col,
},
NorthEast => Coord {
row: self.row - 1,
col: self.col + 1,
},
East => Coord {
row: self.row,
col: self.col + 1,
},
SouthEast => Coord {
row: self.row + 1,
col: self.col + 1,
},
South => Coord {
row: self.row + 1,
col: self.col,
},
SouthWest => Coord {
row: self.row + 1,
col: self.col - 1,
},
West => Coord {
row: self.row,
col: self.col - 1,
},
NorthWest => Coord {
row: self.row - 1,
col: self.col - 1,
},
}
}
}

fn parse(input: &str) -> HashSet<Coord> {
let mut elves = HashSet::new();
for (row, line) in input.lines().enumerate() {
for (col, c) in line.chars().enumerate() {
if c == '#' {
elves.insert(Coord {
row: row as i32,
col: col as i32,
});
}
}
}
elves
}

#[allow(unused)]
fn print(elves: &HashSet<Coord>) {
let (minmax_row, minmax_col) = elves.iter().fold(
((i32::MAX, i32::MIN), (i32::MAX, i32::MIN)),
|(minmax_row, minmax_col), Coord { row, col }| {
(
(minmax_row.0.min(*row), minmax_row.1.max(*row)),
(minmax_col.0.min(*col), minmax_col.1.max(*col)),
)
},
);
for row in minmax_row.0..=minmax_row.1 {
for col in minmax_col.0..=minmax_col.1 {
if elves.contains(&Coord { row, col }) {
print!("#")
} else {
print!(".")
}
}
println!();
}
}

pub fn part_1(input: &str) -> usize {
let mut elves = parse(input);
let mut checks = [
Direction::North,
Direction::South,
Direction::West,
Direction::East,
];

for round in 1.. {
// key: proposed coordinate, val: list of old coordinates that proposed going there
let mut proposals: HashMap<Coord, Vec<Coord>> = HashMap::new();

// consideration phase
for elf in &elves {
let neighbours = elf.neighbours();
// If no other Elves are in one of those eight positions, the Elf does not do anything during this round.
if neighbours.iter().all(|coord| !elves.contains(coord)) {
continue;
}
let neighbours = neighbours
.iter()
.map(|neighbour| elves.contains(neighbour))
.collect::<Vec<_>>()
.try_into()
.unwrap();

let proposed_dir = checks.iter().find(|dir| dir.check(&neighbours));
if let Some(dir) = proposed_dir {
proposals.entry(proposal).or_default().push(*elf);
}
}

// movement phase
for (new_coord, old_coords) in proposals {
if old_coords.len() == 1 {
elves.remove(&old_coords);
elves.insert(new_coord);
}
}

// after round
// println!("== End of Round {} ==", round);
// print(&elves);
// println!();

checks.rotate_left(1);
if round == 10 {
let (minmax_row, minmax_col) = elves.iter().fold(
((i32::MAX, i32::MIN), (i32::MAX, i32::MIN)),
|(minmax_row, minmax_col), Coord { row, col }| {
(
(minmax_row.0.min(*row), minmax_row.1.max(*row)),
(minmax_col.0.min(*col), minmax_col.1.max(*col)),
)
},
);
return (minmax_row.0..=minmax_row.1)
.cartesian_product(minmax_col.0..=minmax_col.1)
.filter(|(row, col)| {
!elves.contains(&Coord {
row: *row,
col: *col,
})
})
.count();
}
}

usize::MAX
}

pub fn part_2(input: &str) -> i32 {
let mut elves = parse(input);
let mut checks = [
Direction::North,
Direction::South,
Direction::West,
Direction::East,
];

for round in 1.. {
let mut moved = false;
// key: proposed coordinate, val: list of old coordinates that proposed going there
let mut proposals: HashMap<Coord, Vec<Coord>> = HashMap::new();

// consideration phase
for elf in &elves {
let neighbours = elf.neighbours();
// If no other Elves are in one of those eight positions, the Elf does not do anything during this round.
if neighbours.iter().all(|coord| !elves.contains(coord)) {
continue;
}
let neighbours = neighbours
.iter()
.map(|neighbour| elves.contains(neighbour))
.collect::<Vec<_>>()
.try_into()
.unwrap();

let proposed_dir = checks.iter().find(|dir| dir.check(&neighbours));
if let Some(dir) = proposed_dir {
proposals.entry(proposal).or_default().push(*elf);
}
}

// movement phase
for (new_coord, old_coords) in proposals {
if old_coords.len() == 1 {
moved = true;
elves.remove(&old_coords);
elves.insert(new_coord);
}
}

// after round
if !moved {
return round;
}
checks.rotate_left(1);
}

i32::MAX
}
``````