Rust

   fn count_sides(grid: &[Vec<char>], x: usize, y: usize) -> usize {
        let mut count = 0;
        for i in y.saturating_sub(1)..=y + 1 {
            for j in x.saturating_sub(1)..=x + 1 {
                if i == y && j == x {
                    continue;
                }

                if let Some(row) = grid.get(i) {
                    if let Some(col) = row.get(j) {
                        if *col == '@' {
                            count += 1;
                        }
                    }
                }
            }
        }
        count
    }

    #[test]
    fn test_y2025_day4_part1() {
        let input = std::fs::read_to_string("input/2025/day_4.txt").unwrap();
        let grid = input
            .lines()
            .map(|l| l.chars().collect::<Vec<_>>())
            .collect::<Vec<_>>();
        let mut total = 0;
        let width = grid[0].len();
        let height = grid.len();
        for y in 0..height {
            for x in 0..width {
                if grid[y][x] != '@' {
                    continue;
                }

                if count_sides(&grid, x, y) < 4 {
                    total += 1
                }
            }
        }
        println!("Total = {total}")
    }

    #[test]
    fn test_y2025_day4_part2() {
        let input = std::fs::read_to_string("input/2025/day_4.txt").unwrap();
        let grid = input
            .lines()
            .map(|l| l.chars().collect::<Vec<_>>())
            .collect::<Vec<_>>();
        let mut grid = input
            .lines()
            .map(|l| l.chars().collect::<Vec<_>>())
            .collect::<Vec<_>>();
        let mut total = 0;
        let width = grid[0].len();
        let height = grid.len();
        loop {
            let mut iter_total = 0;
            for y in 0..height {
                for x in 0..width {
                    if grid[y][x] != '@' {
                        continue;
                    }

                    if count_sides(&grid, x, y) < 4 {
                        grid[y][x] = '*';
                        iter_total += 1
                    }
                }
            }
            println!("Moved = {iter_total}");
            if iter_total == 0 {
                break;
            }
            total += iter_total;
        }
        println!("Total = {total}");
    }

Nothing really exciting here, was pretty straightforward