Advent of Code 2025 - Day 4Printing Department

Rust | Problem statement | Source code | Tags: Cellular automata

Part 1

There are 8 neighbor positions to test, which can be iterated by enumerating the relative coordinates for row and column. For each position, check if it's in bounds and contains a @.

fn neighbors(data: &Vec<Vec<char>>, i: isize, j: isize, w: isize, h: isize) -> usize {
    let mut count = 0;
    for di in -1..=1 {
        for dj in -1..=1 {
            if di == 0 && dj == 0 {
                continue;
            }
            let ni = i + di;
            let nj = j + dj;
            if ni >= 0 && ni < h && nj >= 0 && nj < w && data[ni as usize][nj as usize] == '@' {
                count += 1;
            }
        }
    }
    count
}
rust

Part 1 just counts the cells that contain @ and have fewer than 4 @ neighbors.

Part 2

I guess, the only pitfall here is to make sure to update the grid simultaneously because earlier updates can affect tests for later cells. I save a list of cells to update, and then apply the updates after iterating through the whole grid. I stop when there are no more updates to apply.

loop {
    let mut positions: Vec<(usize, usize)> = vec![];
    for (i, line) in data.iter().enumerate() {
        for (j, c) in line.iter().enumerate() {
            if *c != '@' {
                continue;
            }
            if neighbors(&data, i as isize, j as isize, w, h) < 4 {
                positions.push((i, j));
            }
        }
    }
    if positions.is_empty() {
        break;
    }
    for (i, j) in positions.iter() {
        data[*i][*j] = '.';
    }
    total += positions.len();
}
rust