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Dec 23, 2020 clock icon 13 min #rust · #advent-of-code

Day 11 (Advent of Code 2020)

From the series Advent of Code 2020

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👋 This page was last updated ~4 years ago. Just so you know.

Another day, another problem.

This time the problem looks suspiciously like Conway's Game of Life, or, I guess, any old Cellular automaton.

We have a map like so:

L.LL.LL.LL
LLLLLLL.LL
L.L.L..L..
LLLL.LL.LL
L.LL.LL.LL
L.LLLLL.LL
..L.L.....
LLLLLLLLLL
L.LLLLLL.L
L.LLLLL.LL

And for each iteration:

  • L symbols turn into # if there's no # in any of the 8 adjacent cells
  • # symbols turn into L if four or more of the adjacent cells are #
  • . symbols do not change

This is a delightful problem. We can even recycle some of the stuff we did in Day 3!

First a 2D vector type:

#[derive(Debug, Clone, Copy, PartialEq)]
struct Vec2 {
    x: i64,
    y: i64,
}

Then a tile enum:

#[derive(Clone, Copy, PartialEq)]
enum Tile {
    Floor,
    EmptySeat,
    OccupiedSeat,
}

impl Default for Tile {
    fn default() -> Self {
        Self::Floor
    }
}

With a neat Debug implementation:

use std::fmt;

impl fmt::Debug for Tile {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let c = match self {
            Tile::Floor => '.',
            Tile::EmptySeat => 'L',
            Tile::OccupiedSeat => '#',
        };
        write!(f, "{}", c)
    }
}

And then, a Map - this time, let's make it generic. Just for fun!

struct Map<T> {
    size: Vec2,
    tiles: Vec<T>,
}

We can only implement new if T implements Default:

impl<T> Map<T>
where
    T: Default,
{
    fn new(size: Vec2) -> Self {
        let num_tiles = size.x * size.y;
        Self {
            size,
            tiles: (0..num_tiles)
                .into_iter()
                .map(|_| Default::default())
                .collect(),
        }
    }
}

Then Map::index, to map from 2D space to our 1D storage space:

impl<T> Map<T> {
    fn index(&self, pos: Vec2) -> Option<usize> {
        if (0..self.size.x).contains(&pos.x) && (0..self.size.y).contains(&pos.y) {
            Some((pos.x + pos.y * self.size.x) as _)
        } else {
            None
        }
    }
}

From there we can easily implement set:

impl<T> Map<T> {
    fn set(&mut self, pos: Vec2, tile: T) {
        if let Some(index) = self.index(pos) {
            self.tiles[index] = tile;
        }
    }
}

And for get, we'll require T to be copy, so we don't have to deal with references ever:

impl<T> Map<T>
where
    T: Copy,
{
    fn get(&self, pos: Vec2) -> Option<T> {
        self.index(pos).map(|index| self.tiles[index])
    }
}

Next up - we'll have to deal with neighbors, so it would be neat if we could enumerate neighbors of a given cell:

impl<T> Map<T> {
    fn neighbor_positions(&self, pos: Vec2) -> impl Iterator<Item = Vec2> {
        (-1..=1)
            .map(|dx| (-1..=1).map(move |dy| (dx, dy)))
            .flatten()
            .filter(|&(dx, dy)| !(dx == 0 && dy == 0))
            .map(move |(dx, dy)| Vec2 {
                x: pos.x + dx,
                y: pos.y + dy,
            })
    }
}
Cool bear

This seems hairy, how about a quick test?

#[test]
fn test_neighbor_positions() {
    use std::collections::HashSet;

    let map = Map::<()>::new(Vec2 { x: 3, y: 3 });
    let positions: HashSet<_> = map
        .neighbor_positions(Vec2 { x: 1, y: 1 })
        .map(|v| (v.x, v.y))
        .collect();
    for p in &[
        (0, 0),
        (0, 1),
        (0, 2),
        (1, 0),
        (2, 0),
        (1, 2),
        (2, 2),
        (2, 1),
    ] {
        assert!(positions.contains(p));
    }
}

$ cargo t -q

running 1 test
.
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Cool bear

Alright, carry on.

Then for good measures, we could have a .neighbor_tiles method that returns an iterator of Tile!

impl<T> Map<T>
where
    T: Copy,
{
    fn neighbor_tiles(&self, pos: Vec2) -> impl Iterator<Item = T> + '_ {
        self.neighbor_positions(pos)
            .filter_map(move |pos| self.get(pos))
    }
}
Cool bear

What's with the + '_?

Amos

Well, that iterator is only valid as long as &self is borrowed, because it's reading from it!

Cool bear

Ah And what would the lifetime of impl Iterator<Item = T> (without any additional annotations) be?

Amos

Just like Box<T>, it defaults to 'static, which is only true for owned types (like our neighbor_positions iterator, which owns everything it needs to yield items) or, I guess, truly 'static references, like a const, something that's leaked with Box::leak or the result of include_str!.

Okay! Now we can start implementing some of the cellular automaton logic. Let's do it as a method on Tile:

impl Tile {
    fn next<I>(self, neighbors: I) -> Self
    where
        I: Iterator<Item = Self>,
    {
        match self {
            Self::Floor => Self::Floor,
            Self::EmptySeat => match neighbors
                .filter(|t| matches!(t, Self::OccupiedSeat))
                .count()
            {
                // no one around? we can sit here!
                0 => Self::OccupiedSeat,
                // social distancing please
                _ => Self::EmptySeat,
            },
            Self::OccupiedSeat => {
                match neighbors.filter(|t| matches!(t, Self::OccupiedSeat)).count() {
                    // up to 3 neighbors: still okay for now
                    0..=3 => Self::OccupiedSeat,
                    // that's too many folks!
                    _ => Self::EmptySeat,
                }
            }
        }
    }
}

Let's see, what else do we want... ah right, a nice Debug implementation for Map too - but only if T itself is Debug. Oh, and Copy too since we need get:

impl<T> fmt::Debug for Map<T>
where
    T: fmt::Debug + Copy,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        for y in 0..self.size.y {
            for x in 0..self.size.x {
                write!(f, "{:?}", self.get(Vec2 { x, y }).unwrap())?;
            }
            writeln!(f)?;
        }
        Ok(())
    }
}

And then... you know what would be neat? Since the rules are applied to all tiles simultaneously, it would be neat if we could iterate through all tiles.

impl<T> Map<T>
where
    T: Copy,
{
    fn iter(&self) -> impl Iterator<Item = T> + '_ {
        self.tiles.iter().copied()
    }
}
Cool bear

But uhhh.. we're going to need to know their positions as well, right?

So we can check their neighbors?

Right! To do this, we can introduce another type: Positioned:

#[derive(Debug)]
struct Positioned<T>(Vec2, T);

And make our iter method returns Positioned<T> as its Item type:

impl<T> Map<T>
where
    T: Copy,
{
    // this replaces our previous `iter` method
    fn iter(&self) -> impl Iterator<Item = Positioned<T>> + '_ {
        (0..self.size.y)
            .map(move |y| {
                (0..self.size.x).map(move |x| {
                    let pos = Vec2 { x, y };
                    Positioned(pos, self.get(pos).unwrap())
                })
            })
            .flatten()
    }
}

Let's try it out:

fn main() {
    let mut m = Map::new(Vec2 { x: 3, y: 3 });
    m.set(Vec2 { x: 1, y: 1 }, Tile::OccupiedSeat);

    for tile in m.iter() {
        println!("{:?}", tile);
    }
}

$ cargo run --quiet
Positioned(Vec2 { x: 0, y: 0 }, .)
Positioned(Vec2 { x: 1, y: 0 }, .)
Positioned(Vec2 { x: 2, y: 0 }, .)
Positioned(Vec2 { x: 0, y: 1 }, .)
Positioned(Vec2 { x: 1, y: 1 }, #)
Positioned(Vec2 { x: 2, y: 1 }, .)
Positioned(Vec2 { x: 0, y: 2 }, .)
Positioned(Vec2 { x: 1, y: 2 }, .)
Positioned(Vec2 { x: 2, y: 2 }, .)
Cool bear

Looking good!

And then, finally, as a treat - we can make it possible to collect into a Map.

Cool bear

That's new! How do we do that?

All we have to do is look at the signature of Iterator::collect:

fn collect<B>(self) -> B
where
    B: FromIterator<Self::Item>,

and... we just need to implement FromIterator!

use std::iter::FromIterator;

impl<A> FromIterator<Positioned<A>> for Map<A> {
    fn from_iter<T: IntoIterator<Item = Positioned<A>>>(iter: T) -> Self {
        todo!()
    }
}

There's just, uh... one problem. Our constructor for Map takes a size:

    fn new(size: Vec2) -> Self

And we're just getting a stream of positioned tiles - we have no idea how large the resulting map should be! So I guess our problem is not well-suited to collect.

Maybe there's another method/trait? One that would let us take an existing Map?

Cool bear

There is! Look what I found:

fn extend<T>(&mut self, iter: T)
where
    T: IntoIterator<Item = A>,

Ah, that looks like it would work.

use std::iter::Extend;

impl<A> Extend<Positioned<A>> for Map<A> {
    fn extend<T: IntoIterator<Item = Positioned<A>>>(&mut self, iter: T) {
        for Positioned(pos, tile) in iter {
            self.set(pos, tile)
        }
    }
}

How neat!

Okay, I guess now we need to actually parse our map:

impl Map<Tile> {
    fn parse(input: &[u8]) -> Self {
        let mut columns = 0;
        let mut rows = 1;
        for &c in input.iter() {
            if c == b'\n' {
                rows += 1;
                columns = 0;
            } else {
                columns += 1;
            }
        }

        let mut iter = input.iter().copied();
        let mut map = Self::new(Vec2 {
            x: columns,
            y: rows,
        });
        for row in 0..map.size.y {
            for col in 0..map.size.x {
                let tile = match iter.next() {
                    Some(b'.') => Tile::Floor,
                    Some(b'L') => Tile::EmptySeat,
                    Some(b'#') => Tile::OccupiedSeat,
                    c => panic!("Expected '.', 'L' or '#', but got: {:?}", c),
                };
                map.set(Vec2 { x: col, y: row }, tile);
            }
            iter.next();
        }
        map
    }
}

Let's do a quick visual check. Our example input is:

L.LL.LL.LL
LLLLLLL.LL
L.L.L..L..
LLLL.LL.LL
L.LL.LL.LL
L.LLLLL.LL
..L.L.....
LLLLLLLLLL
L.LLLLLL.L
L.LLLLL.LL

And our parsed version is:

fn main() {
    let m = Map::<Tile>::parse(include_bytes!("input.txt"));
    dbg!(&m.size);
    println!("{:?}", m);
}

$ cargo run --quiet
[src/main.rs:230] &m.size = Vec2 {
    x: 10,
    y: 10,
}
L.LL.LL.LL
LLLLLLL.LL
L.L.L..L..
LLLL.LL.LL
L.LL.LL.LL
L.LLLLL.LL
..L.L.....
LLLLLLLLLL
L.LLLLLL.L
L.LLLLL.LL

Looks good!

Now, I guess we'll need a next method on Map as well, right? But only when T = Tile:

impl Map<Tile> {
    fn next(&self) -> Self {
        let mut res = Self::new(self.size);
        res.extend(
            self.iter()
                .map(|Positioned(pos, tile)| Positioned(pos, tile.next(self.neighbor_tiles(pos)))),
        );
        res
    }
}
Cool bear

This is really, really neat. All our foundational code paid off!

Let's print a few iterations of our example map:

$ cargo add itertools
      Adding itertools v0.9.0 to dependencies

fn main() {
    let maps = itertools::iterate(Map::<Tile>::parse(include_bytes!("input.txt")), Map::next);
    for map in maps.take(5) {
        println!("{:?}", map);
    }
}
Cool bear

You really have to turn everything into an Iterator, don't you?

Amos

It's the winter holidays, bear! We can afford it!

$ cargo run --quiet
L.LL.LL.LL
LLLLLLL.LL
L.L.L..L..
LLLL.LL.LL
L.LL.LL.LL
L.LLLLL.LL
..L.L.....
LLLLLLLLLL
L.LLLLLL.L
L.LLLLL.LL

#.##.##.##
#######.##
#.#.#..#..
####.##.##
#.##.##.##
#.#####.##
..#.#.....
##########
#.######.#
#.#####.##

#.LL.L#.##
#LLLLLL.L#
L.L.L..L..
#LLL.LL.L#
#.LL.LL.LL
#.LLLL#.##
..L.L.....
#LLLLLLLL#
#.LLLLLL.L
#.#LLLL.##

#.##.L#.##
#L###LL.L#
L.#.#..#..
#L##.##.L#
#.##.LL.LL
#.###L#.##
..#.#.....
#L######L#
#.LL###L.L
#.#L###.##

#.#L.L#.##
#LLL#LL.L#
L.L.L..#..
#LLL.##.L#
#.LL.LL.LL
#.LL#L#.##
..L.L.....
#L#LLLL#L#
#.LLLLLL.L
#.#L#L#.##

Now to answer the question:

Simulate your seating area by applying the seating rules repeatedly until no seats change state. How many seats end up occupied?

Cool bear

Mhh we're going to have to compare the previous state with the current state... sounds like a job for tuple_windows!

Amos

Why not! But I don't want to be comparing all tiles one by one...

Cool bear

We could just derive PartialEq on Map?

Amos

Sounds like a plan!

#[derive(PartialEq)]
struct Map<T> {
    size: Vec2,
    tiles: Vec<T>,
}
Cool bear

Cool bear's hot tip

Note: Vec2 already derives PartialEq. As for T, it might or it might not. Map<T> will only implement PartialEq if T itself implements PartialEq.

Amos

Uhh how do we know that?

Cool bear

Why, we can check the result of the derive macro with cargo-expand!

#[automatically_derived]
#[allow(unused_qualifications)]
impl<T: ::core::cmp::PartialEq> ::core::cmp::PartialEq for Map<T> {
    #[inline]
    fn eq(&self, other: &Map<T>) -> bool {
        match *other {
            Map {
                size: ref __self_1_0,
                tiles: ref __self_1_1,
            } => match *self {
                Map {
                    size: ref __self_0_0,
                    tiles: ref __self_0_1,
                } => (*__self_0_0) == (*__self_1_0) && (*__self_0_1) == (*__self_1_1),
            },
        }
    }
    #[inline]
    fn ne(&self, other: &Map<T>) -> bool {
        match *other {
            Map {
                size: ref __self_1_0,
                tiles: ref __self_1_1,
            } => match *self {
                Map {
                    size: ref __self_0_0,
                    tiles: ref __self_0_1,
                } => (*__self_0_0) != (*__self_1_0) || (*__self_0_1) != (*__self_1_1),
            },
        }
    }
}
Amos

Ah! Looks... generated. Which makes sense. Also it's using core:: types rather than std::, which I guess also makes sense.

Moving on - let's implement last on Map, which simulates until we reach a stable state:

impl Map<Tile> {
    fn last(self) -> Self {
        use itertools::Itertools;
        itertools::iterate(self, Map::next)
            .tuple_windows()
            .find_map(|(prev, next)| if prev == next { Some(next) } else { None });

        todo!();
    }
}
Cool bear

Uh oh, that doesn't work:

$ cargo check --quiet
error[E0277]: the trait bound `Map<Tile>: Clone` is not satisfied
   --> src/main.rs:210:14
    |
210 |             .tuple_windows()
    |              ^^^^^^^^^^^^^ the trait `Clone` is not implemented for `Map<Tile>`

error[E0277]: the trait bound `Map<Tile>: Clone` is not satisfied
   --> src/main.rs:211:14
    |
211 |             .find_map(|(prev, next)| if prev == next { Some(next) } else { None });
    |              ^^^^^^^^ the trait `Clone` is not implemented for `Map<Tile>`
    |
    = note: required because of the requirements on the impl of `Iterator` for `TupleWindows<Iterate<Map<Tile>, for<'r> fn(&'r Map<Tile>) -> Map<Tile> {Map::<Tile>::next}>, (Map<Tile>, Map<Tile>)>`

error: aborting due to 2 previous errors

Oh! We need to derive Clone on Map too. But if we're going to be cloning our Map... and we don't want a whole lot of allocations and copying to go around...

Cool bear

We could just use the im crate?

Let's see how much of a "drop-in replacement" it is:

$ cargo add im
      Adding im v15.0.0 to dependencies

use im::Vector;

#[derive(PartialEq)]
struct Map<T> {
    size: Vec2,
    tiles: Vector<T>,
}

Ah, we got one problem already:

$ cargo check --quiet
error[E0369]: binary operation `==` cannot be applied to type `Vector<T>`
  --> src/main.rs:71:5
   |
71 |     tiles: Vector<T>,
   |     ^^^^^^^^^^^^^^^^
   |
   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider further restricting this bound
   |
68 | #[derive(PartialEq + std::cmp::PartialEq)]
   |                    ^^^^^^^^^^^^^^^^^^^^^

Let's look at whether im::Vector implements PartialEq. Turns out, it does...

impl<A: Clone + PartialEq> PartialEq<Vector<A>> for Vector<A>

...but only if A is Clone.

Okay, Tile is definitely Clone, we can do that:

#[derive(PartialEq)]
struct Map<T>
where
    T: Clone,
{
    size: Vec2,
    tiles: Vector<T>,
}

Now we need to add that bound to our other impl blocks as well - this is left as an exercise to the reader.

Cool bear

Cool bear's hot tip

Wherever we have a bound of T: Copy, we don't need to add + Clone, because Copy implies Clone:

pub trait Copy: Clone { }

Now we can also derive Clone for Map:

#[derive(PartialEq, Clone)]
struct Map<T>
where
    T: Clone,
{
    size: Vec2,
    tiles: Vector<T>,
}

And finish our last method:

impl Map<Tile> {
    fn last(self) -> Self {
        use itertools::Itertools;
        itertools::iterate(self, Map::next)
            .tuple_windows()
            .find_map(|(prev, next)| if prev == next { Some(next) } else { None })
            .unwrap()
    }
}

Let's try it out!

fn main() {
    let last = Map::<Tile>::parse(include_bytes!("input.txt")).last();
    println!("{:?}", last);
}

$ cargo run --quiet
#.#L.L#.##
#LLL#LL.L#
L.#.L..#..
#L##.##.L#
#.#L.LL.LL
#.#L#L#.##
..L.L.....
#L#L##L#L#
#.LLLLLL.L
#.#L#L#.##
Cool bear

Yay, it terminates! Take that, halting problem.

And, since our answer to the question needs to be in the form of a number, let's count how many seats end up occupied:

fn main() {
    let last = Map::<Tile>::parse(include_bytes!("input.txt")).last();
    println!("{:?}", last);
    println!(
        "there are {} occupied seats",
        last.iter()
            //      👇  this is a Positioned<Tile>
            .filter(|p| matches!(p.1, Tile::OccupiedSeat))
            .count()
    );
}

$ cargo run --quiet
#.#L.L#.##
#LLL#LL.L#
L.#.L..#..
#L##.##.L#
#.#L.LL.LL
#.#L#L#.##
..L.L.....
#L#L##L#L#
#.LLLLLL.L
#.#L#L#.##

there are 37 occupied seats

That's it for the example! Let's try it with my real puzzle input:

$ cargo run --quiet
#L#L#.#LL#L##L#.#LL##L##L#.#L#L#L#.#L#L#L#L#L#L#L#L#.#L#L#L#L#.#L##L#.#L#L#L#.#L#L#L#L#LL#L#L#L#L#
LLLLL.LL.L.LLLL.LLL..LLLLL.LLLLLL#LLLLLLLLLLLLLLLLLL.LLLLLLLLL.LLLLLL.LLLLLLL.LLLLLLL.LLLLLLLLLLLL
#L#L#.L#L#.#L#L#L#L#L#L#L#L#L#L.L..#L#L#L#L#L#L#L#L#L#L#L#L#L#.#L#L#L.L#L#L##L#L#L#L#.#.L#L#L#L#L#
LLLLLLLLLL.LLLL.LLLLLLLLLL.LLLL#L#.LLLLLLLLLLLLLLLLL.LLLLLLLLL.LLLLLL#LLLLLLLLLLLLLLL.LLLLLLLLLLLL
#L#L#L#L#L#L#L#.#L##.#L#L#.#L#LLLL.##.L#L#L#..#L#L#L#L#L#L#.L#.#L#L#L#.#L#L#L#L##L#L#.L#L#L#L#L#L#
LLLLLLLLLL.LLLL.LLLL.LLLLL.LLLL#L#LLLLLLLLLL.LLLLLLLLLLLLLLLLL.LLLLLL.LLLLLL.LLLLLLLLLLLLLLLLLLLLL
#L#L#L.L##.##L#.#L##.#L#L#L##LLLL#.#L#L#L#L#.#L#L#LL.#L##L#.##.#L##.#L#L#L#.#L#LL#L##.##L#L#L#L#LL
.LL..L#.L..L.L...LL..L..L.L...L#..L...LL..LLLL.#.L.#.....L....L.LL...L.LL.LL....#...L..L....L....#
#L#L#.LL##.#L#L#L#L#.#L#L#.#L#LLL#L#L#L#L#LLL#LLL#LLL#L#L#L#L#.#L#L#L.#L#L#L#.#LLL.#L#LLL#L#L#L#LL
#LLLLL#LLL.LL#L.L#LL.LLL.L.LLLL#L#L#L#L#L#L#LLL#LLL#.L.LLLLLLLLLL.LLLLLL#L#L#.#L#LL#L.L#LLLLL.LLL#
LL#LL.#LLL.LLLL.LLL#L#L#L#.#L#LLLLLLLLLLLLLLL#LLL#LL.#L#L#L#L#.#L#L#LLLLLLLLL...LLLLLLLLL#.#L#L#LL
#LLL#L.L#L#L#L#.#L#L.LLLL#L#L.L#L#.#L#L#L#L#..L#LL.#.LLLLLL#L#.#LLLLL#.L#L#L#.#L#LL#L#L#LLL.LLL.L#
L.#...#.....LLL.L..L#...#......L.LLL........L#..L#L..#.#L#....L..#.#.L..L..L.LL..L.......LL..L.#..
#LLL#.#L#L#L#L#L#LLL#L#LLL#L#LL#L#.#L#L#L#L#.LL#LLL#LLLLLLL#L#.#LLL.L#L#L#L#L###L#L#L#L#L#L#L#LLL#
LL#L#LLLL.LLLLL.LL#L.LLL#L.LLLLLLL.LLLLLLLLLL#LLL#LL.#L#L#LLLL.LL#L#L.LL.LLLL.LLLLLLLLLL.LLLLLL#LL
#LLL###L#L.L#L#.#LLL.#LLL.##LL#L#.##L#L#L#L#.#L#LLL#.LLLLLL#L#.#LLLLL.#L#L#.#.#L#L#L#.##L#L#L#L#L#
LL#LL.LLLL#LLLL.LLL#.LL#LL.LLLLL.LLLLLLLLLLL.#LLL#LL.#L#L#LLL#LLL##L#.LLLLLLL..LLLLLL.LLLL.LLLLLLL
#LLL#.#L#L.L#L#.#LLL.#LLL#L#L#.#L#L#L##L#L#L.L##LLL#.LLLL.L#L#.#LLLLLL#L#L#L#.#L#L#L#.#L##L#L#L#L#
..#..L.L........LL.#.#..##L........L....#L..#L#.L#....#.......LL.##L#....L...L.LL........#.L......
#LLL#.#.L#.#L#L#L#L#.LLLLL.#L#L#L#L#.#LLLLLL.LLLLLL#.LLLL#L#L#.#LLLLL.#L#L#L#LLL#L#L#.#.LLL#L#L#.#
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there are 2289 occupied seats

And just like that, we're allowed to move on to the next part.

Part 2

It's a new part, and the rules, they are a a-changing.

Now we have to look for seats that may not be adjacent, but that are in any of the 8 directions.

So before, we looked at these, and we'd only see two occupied seats:

But now we look further in each direction until we see a seat - and we see eight occupied seats:

Time for a new method! visible_seats will do exactly that.

impl Map<Tile> {
    fn visible_seats(&self, pos: Vec2) -> impl Iterator<Item = Tile> + '_ {
        use itertools::Itertools;
        (-1..=1)
            .map(|dx| (-1..=1).map(move |dy| (dx, dy)))
            .flatten()
            .filter(|&(dx, dy)| !(dx == 0 && dy == 0))
            // for each direction...
            .map(move |(dx, dy)| {
                // keep moving in set direction
                itertools::iterate(pos, move |v| Vec2 {
                    x: v.x + dx,
                    y: v.y + dy,
                })
                // as long as we're on the map
                .map(move |pos| self.index(pos))
                .while_some()
                // and until we reach a seat
                .filter_map(move |index| match self.tiles[index] {
                    Tile::Floor => None,
                    seat => Some(seat),
                })
                .take(1)
            })
            .flatten()
    }
}
Cool bear

Okay, that one definitely needs a test.

Amos

Agreed.

It's a good time to show off the indoc crate!

$ cargo add indoc
      Adding indoc v1.0.3 to dependencies

#[test]
fn test_visible_seats() {
    let map = Map::<Tile>::parse(
        indoc::indoc!(
            "
            .......#.
            ...#.....
            .#.......
            .........
            ..#L....#
            ....#....
            .........
            #........
            ...#.....
            "
        )
        .trim()
        .as_bytes(),
    );
    println!("{:?}", map);
    assert_eq!(map.visible_seats(Vec2 { x: 3, y: 4 }).count(), 8);
    assert_eq!(map.visible_seats(Vec2 { x: 8, y: 0 }).count(), 2);
}

$ cargo t -q

running 2 tests
..
test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Seems good!

And a second test for good measure:

#[test]
fn test_visible_seats2() {
    let map = Map::<Tile>::parse(
        indoc::indoc!(
            "
            .##.##.
            #.#.#.#
            ##...##
            ...L...
            ##...##
            #.#.#.#
            .##.##.
            "
        )
        .trim()
        .as_bytes(),
    );

    assert_eq!(map.visible_seats(Vec2 { x: 3, y: 3 }).count(), 0);
}

$ cargo t -q

running 3 tests
.F.
failures:

---- test_visible_seats2 stdout ----
.##.##.
#.#.#.#
##...##
...L...
##...##
#.#.#.#
.##.##.

thread 'test_visible_seats2' panicked at 'assertion failed: `(left == right)`
  left: `8`,
 right: `0`', src/main.rs:224:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    test_visible_seats2

test result: FAILED. 2 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
Cool bear

Seems bad!

So uhh what's happening here? Let's try and find out what tiles it sees:

    // in our test
    dbg!(map.visible_seats(Vec2 { x: 3, y: 3 }).collect::<Vec<_>>());
    assert_eq!(map.visible_seats(Vec2 { x: 3, y: 3 }).count(), 0);

$ cargo t -q seats2

running 1 test
F
failures:

---- test_visible_seats2 stdout ----
.##.##.
#.#.#.#
##...##
...L...
##...##
#.#.#.#
.##.##.

[src/main.rs:224] map.visible_seats(Vec2{x: 3, y: 3,}).collect::<Vec<_>>() = [
    L,
    L,
    L,
    L,
    L,
    L,
    L,
    L,
]
thread 'test_visible_seats2' panicked at 'assertion failed: `(left == right)`
  left: `8`,
 right: `0`', src/main.rs:225:5
Cool bear

Ohh, it sees itself! Because we're starting at pos, not pos + (dx, dy).

Amos

Yup, seems like we got the initial value wrong in our call to itertools::iterate...

Cool bear

Or we could just skip one?

Amos

Ooh I like that.

                // keep moving in set direction
                itertools::iterate(pos, move |v| Vec2 {
                    x: v.x + dx,
                    y: v.y + dy,
                })
                // 👇 new!
                .skip(1)

Now all our tests pass:

$ cargo t -q

running 3 tests
...
test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Let's adjust our Tile::next method as specified in Part 2:

it now takes five or more visible occupied seats for an occupied seat to become empty (rather than four or more from the previous rules)

            Self::OccupiedSeat => { match neighbors
                    .filter(|t| matches!(t, Self::OccupiedSeat))
                    .count()
                {
                    // 👇 new!
                    // up to 4 neighbors: still okay for now
                    0..=4 => Self::OccupiedSeat,
                    // that's too many folks!
                    _ => Self::EmptySeat,
                }
            }

And change Map::<Tile>::next to use visible_seats instead of neighbor_tiles:

impl Map<Tile> {
    fn next(&self) -> Self {
        let mut res = Self::new(self.size);
        res.extend(
            self.iter()
                //                                                          👇👇👇
                .map(|Positioned(pos, tile)| Positioned(pos, tile.next(self.visible_seats(pos)))),
        );
        res
    }
}

And we should be good to go!

A debug build of this program takes a bit of time:

$ cargo build --quiet
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
$ time ./target/debug/day11 > /dev/null
./target/debug/day11 > /dev/null  5.68s user 0.00s system 99% cpu 5.680 total

Let's see how a release build performs:

$ cargo build --release
    Finished release [optimized] target(s) in 0.01s
$ time ./target/release/day11 > /dev/null
./target/release/day11 > /dev/null  0.24s user 0.00s system 99% cpu 0.240 total

Much better!

Cool bear

Cool bear's hot tip

If your Rust program seems slow with cargo run, don't forget to check how fast it runs with optimizations enabled, by using the --release flag!

And now let's get our actual answer:

$ cargo run --release
    Finished release [optimized] target(s) in 0.01s
     Running `target/release/day11`
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And that's it for Day 11!

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Here's another article just for you:

Declarative memory management

Sep 19, 2019
clock icon 34 min #rust · #memory

It feels like an eternity since I've started using Rust, and yet I remember vividly what it felt like to bang my head against the borrow checker for the first few times.

I'm definitely not alone in that, and there's been quite a few articles on the subject! But I want to take some time to present the borrow checker from the perspective of its , rather than as an opponent to fend with.

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