## DEV Community

Nicky Meuleman

Posted on • Originally published at nickymeuleman.netlify.app

# Advent of Code 2023 Day 17

## Day 17: Clumsy Crucible

TL;DR: my solution in Rust

The lava starts flowing down to Metal Island.

There, elves are busy taking the lava to the factory in big crucibles (cups for molten stuff).

They want to get the crucibles to the factory as fast as possible to lose the least amount of heat.

Figuring out the ideal route to take is your job.

The elves prepared a map that list how much heat is lost if you enter a tile.
The starting location is in the top-left, the ending location in the bottom-right.

An example input looks like this:

``````2413432311323
3215453535623
3255245654254
3446585845452
4546657867536
1438598798454
4457876987766
3637877979653
4654967986887
4564679986453
1224686865563
2546548887735
4322674655533
``````

## Parsing

A 2D list of digits:

``````fn parse(input: &str) -> Vec<Vec<u8>> {
input
.lines()
.map(|line| {
line.chars()
.map(|c| c.to_digit(10).unwrap() as u8)
.collect()
})
.collect()
}
``````

## Part 1

The crucibles are unwieldy, pretty hard to "drive" (is that the right word? Hard to move.)

• They cannot move in a straight line for more than 3 tiles.
• They cannot reverse direction

The question asks for the minimum amount of heatloss when you reach the destination.

This is a shortest path problem, time to pull out the Dijkstra.

### Helpers

To do some Dijkstra's algorithm, I need a priority queue.
In Rust, I'll use a `BinaryHeap` for this.

Some skeleton/pseudo-code I want to work towards:

``````pub fn part_1(input: &str) -> u32 {
let grid = parse(input);
let goal = Coord {
row: grid.len() - 1,
col: grid[0].len() - 1,
};

let mut pq = BinaryHeap::new();

// push the initial 2 directions the crucible can move onto the pq
pq.push(initial_crucibles);

// loop until the goal is reached, or the pq is empty
while let Some(crucible) = pq.pop() {
if crucible.pos == goal {
return crucible.cost;
}
// determine the next possible crucibles and push them onto the pq
for successor in node.successors(&grid) {
pq.push(successor);
}
}
}
``````

The things I'll push onto that queue and pop off it are my own data structures, `Crucible`:

``````struct Crucible {
cost: u32,
pos: Coord,
dir: Direction,
moves_in_dir: u8,
}
``````

The position it has is tracked by a `Coord`:

``````struct Coord {
row: usize,
col: usize,
}
``````

The direction the crucible is moving in is a `Direction`:

``````enum Direction {
Up,
Down,
Left,
Right,
}
``````

Now, to handle the soring of the `Crucible`s in that priority queue, I specified how they should be sorted, by cost, from low to high.
That way, every time I pop from the queue, I get the crucible with the lowest cost.

``````// The priority queue holds Crucible
// We define an ordering trait so the one with the lowest cost gets popped from the pq first.
// We do this by flipping the ordering on cost (comparing "other to self" instead of "self to other")
// that way, nodes with a lower cost will compare as Ordering::Greater, and get sent to the front of the pq
impl Ord for Crucible {
fn cmp(&self, other: &Self) -> Ordering {
other.cost.cmp(&self.cost)
}
}

// Ensure partialOrd is consistent with Ord. From the docs:
// > If Ord is also implemented for Self and Rhs, it must also be consistent with partial_cmp (see the documentation of that trait for the exact requirements).
// > Itβs easy to accidentally make them disagree by deriving some of the traits and manually implementing others.
// tl;dr: implement PartialOrd in terms of the Ord we just specified to ensure correctness. Letting it be automatically determined is wrong sometimes.
impl PartialOrd for Crucible {
fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
Some(self.cmp(other))
}
}
``````

The next step is the function that determines the successors (a fancy word for the next possible crucibles).
It tries to move in every direction, and only does so if the rules from the question pass:

``````impl Crucible {
fn successors(&self, grid: &Vec<Vec<u8>>) -> Vec<Self> {
let rows = grid.len();
let cols = grid[0].len();

let mut successors = Vec::new();
for dir in [
Direction::Up,
Direction::Down,
Direction::Left,
Direction::Right,
] {
if self.dir == dir && self.moves_in_dir == 3 {
// already moved 3 tiles in a straight line, can't move further
continue;
}
if self.dir.opposite() == dir {
// can't move in opposite direction
continue;
}
// simulate a move inside the bounds
if let Some(pos) = self.pos.forward(&dir, rows, cols) {
// calculate the total cost to get to that neighbour
// it's the total cost to get to the current node + the cost to travel to the neighbour
let cost = self.cost + grid[pos.row][pos.col] as u32;

// increment straight_moves if we went straight, else we moved 1 tile in the current direction
let moves_in_dir = if self.dir == dir {
self.moves_in_dir + 1
} else {
1
};

successors.push(Crucible {
pos,
cost,
dir,
moves_in_dir,
})
}
}

successors
}
}
``````

This handles some of the rules crucibles must adhere to, like:

• Not moving more than 3 tiles in a straight line
• Not reversing direction

The not reversing direction rule uses another helper:

``````impl Direction {
fn opposite(&self) -> Self {
match self {
Direction::Up => Direction::Down,
Direction::Down => Direction::Up,
Direction::Left => Direction::Right,
Direction::Right => Direction::Left,
}
}
}
``````

It also makes sure a crucible doesn't go off the map with that `forward` helper, it only adds a new crucible if it's inside the grid.
That's because that `forward` helper only returns a valid coordinate if it's inside the grid:

``````impl Coord {
fn forward(&self, dir: &Direction, rows: usize, cols: usize) -> Option<Self> {
let coord = match dir {
Direction::Up if self.row > 0 => Coord {
row: self.row - 1,
col: self.col,
},
Direction::Down if self.row < (rows - 1) => Coord {
row: self.row + 1,
col: self.col,
},
Direction::Left if self.col > 0 => Coord {
row: self.row,
col: self.col - 1,
},
Direction::Right if self.col < (cols - 1) => Coord {
row: self.row,
col: self.col + 1,
},
_ => return None,
};
Some(coord)
}
}
``````

### Code

This worked, but went very slow, so I kept track of previous crucibles in a `HashSet`.
Only tracking the `Crucible` position is not enough, as it matters which direction the crucible is travelling in, and how far it already has in a straight line.

So every time I want to add a successor to the priority queue, I check if that `(coord, dir, moves_in_dir)` triplet has already been seen.
If it hasn't, I add that crucible to the queue, and that triplet to the `seen` cache.

``````pub fn part_1(input: &str) -> u32 {
let grid = parse(input);
let goal = Coord {
row: grid.len() - 1,
col: grid[0].len() - 1,
};

let mut pq = BinaryHeap::new();
let mut seen = HashSet::new();

let right = Crucible {
cost: grid[0][1] as u32,
dir: Direction::Right,
pos: Coord { row: 0, col: 1 },
moves_in_dir: 1,
};
pq.push(right);

let down = Crucible {
cost: grid[1][0] as u32,
dir: Direction::Down,
pos: Coord { row: 1, col: 0 },
moves_in_dir: 1,
};
pq.push(down);

while let Some(crucible) = pq.pop() {
if crucible.pos == goal {
return crucible.cost;
}
for crucible in crucible.successors(&grid) {
if seen.insert((crucible.pos, crucible.dir, crucible.moves_in_dir)) {
pq.push(crucible);
}
}
}

panic!("No path found")
}
``````

## Part 2

It's not going fast enough, to remedy this, the elves brought bigger crucibles, ultracrucibles.

They can move more lava, but are also even more unwieldy.

They need to move a minimum of 4 tiles in a single direction before they can turn or stop.

They can't go further than 10 tiles in a single direction before they have to turn.

The question asks for the minimum amount of heatloss when you reach the destination.

### Helpers

The nodes that go into the priority queue now are `UltraCrucible`s:

``````struct UltraCrucible {
cost: u32,
pos: Coord,
dir: Direction,
moves_in_dir: u8,
}
``````

The same logic is used to order them:

``````impl Ord for UltraCrucible {
fn cmp(&self, other: &Self) -> Ordering {
other.cost.cmp(&self.cost)
}
}

impl PartialOrd for UltraCrucible {
fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
Some(self.cmp(other))
}
}
``````

And now, for the part that is different between part1 and part2, the `successors`:

``````impl UltraCrucible {
fn successors(&self, grid: &Vec<Vec<u8>>) -> Vec<Self> {
let rows = grid.len();
let cols = grid[0].len();

let mut successors = Vec::new();
for dir in [
Direction::Up,
Direction::Down,
Direction::Left,
Direction::Right,
] {
// Once an ultra crucible starts moving in a direction, it needs to move a minimum of four blocks in that direction before it can turn
if self.moves_in_dir < 4 && dir != self.dir {
continue;
}
// an ultra crucible can move a maximum of ten consecutive blocks without turning.
if self.dir == dir && self.moves_in_dir == 10 {
// already moved 3 tiles in a straight line, can't move further
continue;
}
if self.dir.opposite() == dir {
// can't move in opposite direction
continue;
}
// simulate a move inside the bounds
if let Some(pos) = self.pos.forward(&dir, rows, cols) {
// calculate the total cost to get to that neighbour
// it's the total cost to get to the current node + the cost to travel to the neighbour
let cost = self.cost + grid[pos.row][pos.col] as u32;

// increment straight_moves if we went straight, else we moved 1 tile in the current direction
let moves_in_dir = if self.dir == dir {
self.moves_in_dir + 1
} else {
1
};

successors.push(UltraCrucible {
pos,
cost,
dir,
moves_in_dir,
})
}
}

successors
}
}
``````

This function encodes the movement rules laid out by the question.

The only other minor change is the check to see if the destination has been reached in the loop.
Since an `UltraCrucible` can't stop immediately, I added a check to see if it has been moving for at least 4 tiles.

### Code

``````pub fn part_2(input: &str) -> u32 {
let grid = parse(input);
let goal = Coord {
row: grid.len() - 1,
col: grid[0].len() - 1,
};

let mut pq = BinaryHeap::new();
let mut seen = HashSet::new();

let right = UltraCrucible {
cost: grid[0][1] as u32,
dir: Direction::Right,
pos: Coord { row: 0, col: 1 },
moves_in_dir: 1,
};
pq.push(right);

let down = UltraCrucible {
cost: grid[1][0] as u32,
dir: Direction::Down,
pos: Coord { row: 1, col: 0 },
moves_in_dir: 1,
};
pq.push(down);

while let Some(ultra_crucible) = pq.pop() {
if ultra_crucible.pos == goal && ultra_crucible.moves_in_dir >= 4 {
return ultra_crucible.cost;
}
for ultra_crucible in ultra_crucible.successors(&grid) {
if seen.insert((
ultra_crucible.pos,
ultra_crucible.dir,
ultra_crucible.moves_in_dir,
)) {
pq.push(ultra_crucible);
}
}
}

panic!("No path found")
}
``````

## Final code

``````use std::{
cmp::Ordering,
collections::{BinaryHeap, HashSet},
};

#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy)]
struct Coord {
row: usize,
col: usize,
}

impl Coord {
fn forward(&self, dir: &Direction, rows: usize, cols: usize) -> Option<Self> {
let coord = match dir {
Direction::Up if self.row > 0 => Coord {
row: self.row - 1,
col: self.col,
},
Direction::Down if self.row < (rows - 1) => Coord {
row: self.row + 1,
col: self.col,
},
Direction::Left if self.col > 0 => Coord {
row: self.row,
col: self.col - 1,
},
Direction::Right if self.col < (cols - 1) => Coord {
row: self.row,
col: self.col + 1,
},
_ => return None,
};
Some(coord)
}
}

#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy)]
enum Direction {
Up,
Down,
Left,
Right,
}

impl Direction {
fn opposite(&self) -> Self {
match self {
Direction::Up => Direction::Down,
Direction::Down => Direction::Up,
Direction::Left => Direction::Right,
Direction::Right => Direction::Left,
}
}
}

#[derive(Debug, PartialEq, Eq, Hash, Clone, Copy)]
struct Crucible {
cost: u32,
pos: Coord,
dir: Direction,
moves_in_dir: u8,
}

// The priority queue holds Nodes
// We define an ordering trait so the one with the lowest cost gets popped from the pq first.
// We do this by flipping the ordering on cost (comparing "other to self" instead of "self to other")
// that way, nodes with a lower cost will compare as Ordering::Greater, and get sent to the front of the pq
impl Ord for Crucible {
fn cmp(&self, other: &Self) -> Ordering {
other.cost.cmp(&self.cost)
}
}

// Ensure partialOrd is consistent with Ord. If you #[derive(PartialOrd)] this it might not be the same as that implementation uses a top-down ordering on the Node struct fields
// in this case, it would order by idx first (as that field occurs first in the source code where Node is defined) and would not be consistent.
// From the docs:
// > If Ord is also implemented for Self and Rhs, it must also be consistent with partial_cmp (see the documentation of that trait for the exact requirements).
// > Itβs easy to accidentally make them disagree by deriving some of the traits and manually implementing others.
impl PartialOrd for Crucible {
fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
Some(self.cmp(other))
}
}

impl Crucible {
fn successors(&self, grid: &Vec<Vec<u8>>) -> Vec<Self> {
let rows = grid.len();
let cols = grid[0].len();

let mut successors = Vec::new();
for dir in [
Direction::Up,
Direction::Down,
Direction::Left,
Direction::Right,
] {
if self.dir == dir && self.moves_in_dir == 3 {
// already moved 3 tiles in a straight line, can't move further
continue;
}
if self.dir.opposite() == dir {
// can't move in opposite direction
continue;
}
// simulate a move inside the bounds
if let Some(pos) = self.pos.forward(&dir, rows, cols) {
// calculate the total cost to get to that neighbour
// it's the total cost to get to the current node + the cost to travel to the neighbour
let cost = self.cost + grid[pos.row][pos.col] as u32;

// increment straight_moves if we went straight, else we moved 1 tile in the current direction
let moves_in_dir = if self.dir == dir {
self.moves_in_dir + 1
} else {
1
};

successors.push(Crucible {
pos,
cost,
dir,
moves_in_dir,
})
}
}

successors
}
}

fn parse(input: &str) -> Vec<Vec<u8>> {
input
.lines()
.map(|line| {
line.chars()
.map(|c| c.to_digit(10).unwrap() as u8)
.collect()
})
.collect()
}

pub fn part_1(input: &str) -> u32 {
let grid = parse(input);
let goal = Coord {
row: grid.len() - 1,
col: grid[0].len() - 1,
};

let mut pq = BinaryHeap::new();
let mut seen = HashSet::new();

let right = Crucible {
cost: grid[0][1] as u32,
dir: Direction::Right,
pos: Coord { row: 0, col: 1 },
moves_in_dir: 1,
};
pq.push(right);

let down = Crucible {
cost: grid[1][0] as u32,
dir: Direction::Down,
pos: Coord { row: 1, col: 0 },
moves_in_dir: 1,
};
pq.push(down);

while let Some(crucible) = pq.pop() {
if crucible.pos == goal {
return crucible.cost;
}
for crucible in crucible.successors(&grid) {
if seen.insert((crucible.pos, crucible.dir, crucible.moves_in_dir)) {
pq.push(crucible);
}
}
}

panic!("No path found")
}

#[derive(Debug, PartialEq, Eq, Hash, Clone, Copy)]
struct UltraCrucible {
cost: u32,
pos: Coord,
dir: Direction,
moves_in_dir: u8,
}

impl Ord for UltraCrucible {
fn cmp(&self, other: &Self) -> Ordering {
other.cost.cmp(&self.cost)
}
}

impl PartialOrd for UltraCrucible {
fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
Some(self.cmp(other))
}
}

impl UltraCrucible {
fn successors(&self, grid: &Vec<Vec<u8>>) -> Vec<Self> {
let rows = grid.len();
let cols = grid[0].len();

let mut successors = Vec::new();
for dir in [
Direction::Up,
Direction::Down,
Direction::Left,
Direction::Right,
] {
// Once an ultra crucible starts moving in a direction, it needs to move a minimum of four blocks in that direction before it can turn
if self.moves_in_dir < 4 && dir != self.dir {
continue;
}
// an ultra crucible can move a maximum of ten consecutive blocks without turning.
if self.dir == dir && self.moves_in_dir == 10 {
// already moved 3 tiles in a straight line, can't move further
continue;
}
if self.dir.opposite() == dir {
// can't move in opposite direction
continue;
}
// simulate a move inside the bounds
if let Some(pos) = self.pos.forward(&dir, rows, cols) {
// calculate the total cost to get to that neighbour
// it's the total cost to get to the current node + the cost to travel to the neighbour
let cost = self.cost + grid[pos.row][pos.col] as u32;

// increment straight_moves if we went straight, else we moved 1 tile in the current direction
let moves_in_dir = if self.dir == dir {
self.moves_in_dir + 1
} else {
1
};

successors.push(UltraCrucible {
pos,
cost,
dir,
moves_in_dir,
})
}
}

successors
}
}

pub fn part_2(input: &str) -> u32 {
let grid = parse(input);
let goal = Coord {
row: grid.len() - 1,
col: grid[0].len() - 1,
};

let mut pq = BinaryHeap::new();
let mut seen = HashSet::new();

let right = UltraCrucible {
cost: grid[0][1] as u32,
dir: Direction::Right,
pos: Coord { row: 0, col: 1 },
moves_in_dir: 1,
};
pq.push(right);

let down = UltraCrucible {
cost: grid[1][0] as u32,
dir: Direction::Down,
pos: Coord { row: 1, col: 0 },
moves_in_dir: 1,
};
pq.push(down);

while let Some(ultra_crucible) = pq.pop() {
if ultra_crucible.pos == goal && ultra_crucible.moves_in_dir >= 4 {
return ultra_crucible.cost;
}
for ultra_crucible in ultra_crucible.successors(&grid) {
if seen.insert((
ultra_crucible.pos,
ultra_crucible.dir,
ultra_crucible.moves_in_dir,
)) {
pq.push(ultra_crucible);
}
}
}

panic!("No path found")
}
``````