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:

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

First a 2D vector type:

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

Then a tile enum:

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

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

With a neat Debug implementation:

Rust code
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!

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

We can only implement new if T implements Default:

Rust code
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:

Rust code
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:

Rust code
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:

Rust code
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:

Rust code
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,
            })
    }
}

This seems hairy, how about a quick test?

Rust code
#[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));
    }
}
Shell session
$ cargo t -q

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

Alright, carry on.

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

Rust code
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))
    }
}

What's with the + '_?

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

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

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:

Rust code
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:

Rust code
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.

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

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:

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

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

Rust code
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:

Rust code
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);
    }
}
Shell session
$ 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 }, .)

Looking good!

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

That's new! How do we do that?

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

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

and... we just need to implement FromIterator!

Rust code
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:

Rust code
    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?

There is! Look what I found:

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

Ah, that looks like it would work.

Rust code
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:

Rust code
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:

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

Rust code
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
    }
}

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

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

Shell session
$ cargo add itertools
      Adding itertools v0.9.0 to dependencies
Rust code
fn main() {
    let maps = itertools::iterate(Map::<Tile>::parse(include_bytes!("input.txt")), Map::next);
    for map in maps.take(5) {
        println!("{:?}", map);
    }
}

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

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

Shell session
$ 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?

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

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

We could just derive PartialEq on Map?

Sounds like a plan!

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

Uhh how do we know that?

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

Rust code
#[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),
            },
        }
    }
}

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:

Rust code
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!();
    }
}

Uh oh, that doesn't work:

Rust code
$ 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...

We could just use the im crate?

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

Shell session
$ cargo add im
      Adding im v15.0.0 to dependencies
Rust code
use im::Vector;

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

Ah, we got one problem already:

Shell session
$ 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...

Rust code
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:

Rust code
#[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's hot tip

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

Rust code
pub trait Copy: Clone { }

Now we can also derive Clone for Map:

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

And finish our last method:

Rust code
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!

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

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:

Rust code
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()
    );
}
Shell session
$ 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:

Shell session
$ 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#.#
LL#L#.LLLL.LLLL..LL#.#L#L#L#LLLLLL.#L#L#L#L#L#L#L#LL.#L#LLLL.LLLL##L#L#L#LLLLL#LLLLLL.LL##LLLLLLLL
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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.

Rust code
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()
    }
}

Okay, that one definitely needs a test.

Agreed.

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

Shell session
$ cargo add indoc
      Adding indoc v1.0.3 to dependencies
Rust code
#[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);
}
Shell session
$ 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:

Rust code
#[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);
}
Shell session
$ 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

Seems bad!

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

Rust code
    // 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);
Shell session
$ 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

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

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

Or we could just skip one?

Ooh I like that.

Rust code
                // 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:

Shell session
$ 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)

Rust code
            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:

Rust code
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:

Shell session
$ 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:

Shell session
$ 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'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:

Shell session
$ cargo run --release
    Finished release [optimized] target(s) in 0.01s
     Running `target/release/day11`
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there are 2059 occupied seats

And that's it for Day 11!