advent-of-code-2024/src/days/d12.rs

use std::collections::HashMap;
use std::fs;

pub fn solve() {
    let path = "res/12/example";

    #[allow(unused)]
    let contents = fs::read_to_string(path).expect("Something went wrong reading the file");

    let grid: Vec<Vec<char>> = contents
        .lines()
        .map(|line| line.chars().collect())
        .collect();

    let edges: Vec<Vec<usize>> = grid
        .iter()
        .enumerate()
        .map(|(i, line)| {
            line.iter()
                .enumerate()
                .map(|(j, _)| {
                    let mut ret = 0;
                    if i == 0 || grid[i - 1][j] != grid[i][j] {
                        ret += 1;
                    }
                    if i == grid.len() - 1 || grid[i + 1][j] != grid[i][j] {
                        ret += 1;
                    }
                    if j == 0 || grid[i][j - 1] != grid[i][j] {
                        ret += 1;
                    }
                    if j == grid[0].len() - 1 || grid[i][j + 1] != grid[i][j] {
                        ret += 1;
                    }
                    ret
                })
                .collect::<Vec<usize>>()
        })
        .collect();

    let mut visited: Vec<Vec<bool>> = vec![vec![false; grid[0].len()]; grid.len()];

    let mut regions: Vec<Vec<(usize, usize)>> = vec![];

    for i in 0..grid.len() {
        for j in 0..grid[0].len() {
            if visited[i][j] {
                continue;
            }
            let mut stack: Vec<(usize, usize)> = vec![(i, j)];
            let mut region: Vec<(usize, usize)> = vec![];
            while let Some((i, j)) = stack.pop() {
                if visited[i][j] {
                    continue;
                }
                visited[i][j] = true;
                region.push((i, j));
                if i > 0 && grid[i][j] == grid[i - 1][j] && !visited[i - 1][j] {
                    stack.push((i - 1, j));
                }
                if i < grid.len() - 1 && grid[i][j] == grid[i + 1][j] && !visited[i + 1][j] {
                    stack.push((i + 1, j));
                }
                if j > 0 && grid[i][j] == grid[i][j - 1] && !visited[i][j - 1] {
                    stack.push((i, j - 1));
                }
                if j < grid[0].len() - 1 && grid[i][j] == grid[i][j + 1] && !visited[i][j + 1] {
                    stack.push((i, j + 1));
                }
            }
            regions.push(region);
        }
    }

    let result: usize = regions
        .iter()
        .map(|r| {
            let edge_count: usize = r.iter().map(|(i, j)| edges[*i][*j]).sum();
            edge_count * r.len()
        })
        .sum();

    println!("Result 1: {}", result);

    // count corners on outer side of region
    let mut result = 0;

    let regions: HashMap<_, _> = regions
        .iter()
        .enumerate()
        .map(|(i, region)| (i, region.iter().copied().collect::<Vec<_>>()))
        .collect();
    // Recolor the regions in the grid so that non-overlapping regions have different letters,
    // otherwise they could be overlapping when taking 2x2 windows over the grid.
    let mapping = regions
        .iter()
        .enumerate()
        .flat_map(|(i, (_, region))| region.iter().map(move |(x, y)| ((*x, *y), i + 1)))
        .collect::<HashMap<_, _>>();

    let grid = (0..grid.len())
        .map(|x| (0..grid[x].len()).map(|y| mapping[&(x, y)]).collect())
        .collect::<Vec<Vec<usize>>>();

    for (s, region) in regions {
        let v = *grid.get_from_coords(s.into()).unwrap();
        let subgrid = grid
            .to_minimum_spanning_subgrid(&region.iter().copied().collect::<Vec<_>>())
            .pad_into(1, 0);

        // Take 2x2 windows so that the we can use the center point of the window to check if the
        // window contains a corner (or even two corners - see comments below).
        let corners = subgrid
            .as_ndarray()
            .windows([2, 2])
            .into_iter()
            .map(|window| {
                let in_same_region = window
                    .indexed_iter()
                    .filter(|(_, &wv)| wv == v)
                    .collect_vec();
                match in_same_region.len() {
                    // A A
                    // B A
                    // (1 corner)
                    1 | 3 => 1,
                    // A A
                    // A A
                    // (no corners)
                    0 | 4 => 0,
                    // A B          A A
                    // B A    or    B B
                    // (2 corners) (no corners)
                    2 => {
                        let (x1, y1) = in_same_region[0].0;
                        let (x2, y2) = in_same_region[1].0;
                        if x1 != x2 && y1 != y2 {
                            2
                        } else {
                            0
                        }
                    }
                    _ => unreachable!(),
                }
            })
            .sum::<usize>();

        let area = region.len();
        result += area * corners;
    }

    println!("Result 2: {}", result);
}