2022/src/bin/day18.rs

use std::cmp::{max, min};
use std::collections::{BTreeMap, BTreeSet, VecDeque};

use aoc_2022::*;

type Coord = Vector3D<i32>;

type Input = Vec<Coord>;
type Output = usize;

lazy_static! {
    static ref TOUCHING: Vec<Coord> = vec![-1, 1]
        .iter()
        .flat_map(|&d| vec![
            Coord::new(d, 0, 0),
            Coord::new(0, d, 0),
            Coord::new(0, 0, d)
        ])
        .collect_vec();
}

fn get_bounding_box(droplets: &[Coord]) -> (Coord, Coord) {
    let min_coord =
        droplets
            .iter()
            .fold(Coord::new(i32::MAX, i32::MAX, i32::MAX), |acc, &droplet| {
                Coord::new(
                    min(acc.x(), droplet.x()),
                    min(acc.y(), droplet.y()),
                    min(acc.z(), droplet.z()),
                )
            });
    let max_coord =
        droplets
            .iter()
            .fold(Coord::new(i32::MIN, i32::MIN, i32::MIN), |acc, &droplet| {
                Coord::new(
                    max(acc.x(), droplet.x()),
                    max(acc.y(), droplet.y()),
                    max(acc.z(), droplet.z()),
                )
            });

    (min_coord, max_coord)
}

fn _check_bounding_box(droplets: &[Coord]) {
    let (min_coord, max_coord) = get_bounding_box(droplets);

    debug!("Bounding box: {:?} <-> {:?}", min_coord, max_coord);
    debug!(
        "Cubes in a bounding box: {:?}",
        (max_coord.x() - min_coord.x() + 1)
            * (max_coord.y() - min_coord.y() + 1)
            * (max_coord.z() - min_coord.z() + 1)
    );
}

#[derive(Debug, Clone, PartialEq)]
enum Cube {
    Air,
    Lava,
    Obsidian,
}

fn aerate(space: &mut BTreeMap<Coord, Cube>, start: Coord) {
    let mut queue = VecDeque::<Coord>::new();
    queue.push_back(start);
    space.insert(start, Cube::Air);

    while let Some(pos) = queue.pop_front() {
        for neighbour in TOUCHING
            .iter()
            .map(|&d| d + pos)
            .filter(|pos| *space.get(pos).unwrap_or(&Cube::Air) == Cube::Lava)
            .collect_vec()
        {
            space.insert(neighbour, Cube::Air);
            queue.push_back(neighbour);
        }
    }
}

struct Day18;
impl Solution<Input, Output> for Day18 {
    fn parse_input<P: AsRef<Path>>(pathname: P) -> Input {
        file_to_lines(pathname)
            .iter()
            .map(|l| {
                let coords = l.split(',').map(|c| c.parse().unwrap()).collect_vec();

                Coord::new(coords[0], coords[1], coords[2])
            })
            .collect_vec()
    }

    fn part_1(input: &Input) -> Output {
        let mut surface = 0;

        let mut droplets = BTreeSet::<Coord>::new();
        for &droplet in input {
            surface += 6;

            for _touching in TOUCHING
                .iter()
                .map(|&d| d + droplet)
                .filter(|d| droplets.contains(d))
            {
                surface -= 2;
            }

            droplets.insert(droplet);
        }

        surface
    }

    fn part_2(input: &Input) -> Output {
        let mut space = BTreeMap::<Coord, Cube>::new();

        _check_bounding_box(input); // for debugging purposes
        let (min_coords, max_coords) = get_bounding_box(input);

        // lava everywhere
        for cube in [
            min_coords.x()..=max_coords.x(),
            min_coords.y()..=max_coords.y(),
            min_coords.z()..=max_coords.z(),
        ]
        .into_iter()
        .multi_cartesian_product()
        .map(|coords| {
            let (x, y, z) = coords
                .into_iter()
                .collect_tuple::<(i32, i32, i32)>()
                .unwrap();

            Coord::new(x, y, z)
        }) {
            space.insert(cube, Cube::Lava);
        }

        // override with obsidian
        for &droplet in input {
            space.insert(droplet, Cube::Obsidian);
        }

        // update the outside with the air using a BFS
        aerate(
            &mut space,
            Coord::new(min_coords.x(), min_coords.y(), min_coords.z()),
        );

        // count the faces touching the air
        let mut surface = 0;
        for &droplet in input {
            surface += TOUCHING
                .iter()
                .map(|&d| space.get(&(droplet + d)).unwrap_or(&Cube::Air))
                .filter(|&cube| *cube == Cube::Air)
                .count();
        }

        surface
    }
}

fn main() -> Result<()> {
    // Day18::run("sample")
    Day18::main()
}

test_sample!(day_18, Day18, 64, 58);

#[cfg(test)]
mod day_18_additional_tests {
    use super::*;

    #[test]
    fn test_small_example() {
        assert_eq!(
            Day18::part_1(&vec![Vector3D::new(1, 1, 1), Vector3D::new(2, 1, 1)]),
            10
        )
    }
}