Python

Simple brute-force is enough.

DIRECTIONS = [
    (0, 1), (1, 0), (0, -1), (-1, 0),   # cardinal
    (1, 1), (1, -1), (-1, 1), (-1, -1)  # diagonal
]
# yield all valid neighbor coordinates
def yield_neighbors(i, j, m, n):
    for di, dj in DIRECTIONS:
        ni, nj = i+di, j+dj
        if 0 <= ni < m and 0 <= nj < n:
            yield ni, nj

# build char grid from input data
def build_grid(data: str):
    return [list(row) for row in data.splitlines()]

def part1(data: str):
    grid = build_grid(data)
    m, n = len(grid), len(grid[0])

    # count accessible rolls
    accessible = 0
    for i in range(m):
        for j in range(n):
            if grid[i][j] != '@':
                continue

            neighbor_rolls = sum(grid[ni][nj] == '@' for ni, nj in yield_neighbors(i, j, m, n))
            if neighbor_rolls < 4:
                accessible += 1
    return accessible

def part2(data: str):
    grid = build_grid(data)
    m, n = len(grid), len(grid[0])

    total_accessible = 0    # total accessible rolls over all cycles
    accessible = None       # rolls accessible this cycle

    # repeat until no more rolls can be accessed
    while accessible != 0:
        accessible = 0
        for i in range(m):
            for j in range(n):
                if grid[i][j] != '@':
                    continue

                neighbor_rolls = sum(grid[ni][nj] == '@' for ni, nj in yield_neighbors(i, j, m, n))
                if neighbor_rolls < 4:
                    accessible += 1
                    # we can immediately remove this roll, no need to wait for next cycle
                    grid[i][j] = '.'
        
        # accumulate accessible rolls this cycle
        total_accessible += accessible
    return total_accessible

sample = """<paste sample here>"""
assert part1(sample) == 13
assert part2(sample) == 43

Javascript

After smashing out a functional version in 20 minutes, I converted it into a OOP approach for a more appealing solution.

const input = require('fs').readFileSync('input-day4.txt', 'utf-8');

class PrintingDepartmentMap {
    /** @param {string} input */
    constructor(input) {
        /** @type {number[][]} */
        this.map = input.split("\n").map(r => r.split('').map(v => v === '@' ? 1 : 0));
    }

    /**
     * @param {number} x
     * @param {number} y
     * @returns {number} the value of that tile, or -1 if invalid
     */
    getCoord(x, y) {
        if (x < 0 || y < 0 || x >= this.map[0].length || y >= this.map.length) {
            return -1;
        }
        return this.map[y][x];
    }

    /**
     * @param {number} x
     * @param {number} y
     * @returns {number} the number of adjacent tiles that are >= 1
     */
    countAdjacent(x, y) {
        return [
            // top-left
            this.getCoord(x - 1, y - 1) >= 1,
            // top
            this.getCoord(x, y - 1) >= 1,
            // top-right
            this.getCoord(x + 1, y - 1) >= 1,
            // right
            this.getCoord(x + 1, y) >= 1,
            // bottom-right
            this.getCoord(x + 1, y + 1) >= 1,
            // bottom
            this.getCoord(x, y + 1) >= 1,
            // bottom-left
            this.getCoord(x - 1, y + 1) >= 1,
            // left
            this.getCoord(x - 1, y) >= 1
        ].reduce((acc, v) => !!v ? acc + 1 : acc, 0);
    }

    /**
     * transform in place the map replacing any rolls (1) with (2) if they are accessible
     * @returns {PrintingDepartmentMap}
     */
    markAccessable() {
        for (let y = 0; y < this.map.length; y += 1) {
            for (let x = 0; x < this.map[0].length; x += 1) {
                if (this.map[y][x] < 1) continue;
                if (this.countAdjacent(x, y) < 4) {
                    this.map[y][x] = 2;
                }

            }
        }
        return this;
    }

    /** @returns {number} the number of removed rolls */
    removeAccessable() {
        let removed = 0;
        for (let y = 0; y < this.map.length; y += 1) {
            for (let x = 0; x < this.map[0].length; x += 1) {
                if (this.map[y][x] === 2) {
                    removed += 1;
                    this.map[y][x] = 0;
                }
            }
        }
        return removed;
    }
}

const map = new PrintingDepartmentMap(input);

let removed = 0;
while (true) {
    const justRemoved = map.markAccessable().removeAccessable();
    if (removed === 0) {
        console.log(`Part 1 Answer: ${justRemoved}`);
    }
    if (justRemoved === 0) break;
    removed += justRemoved;
}

console.log(`Part 2 Answer: ${removed}`);

Janet

• part1
• part2

Nim

type
  AOCSolution[T,U] = tuple[part1: T, part2: U]
  Vec2 = tuple[x,y: int]

proc removePaper(rolls: var seq[string]): int =
  var toRemove: seq[Vec2]
  for y, line in rolls:
    for x, c in line:
      if c != '@': continue
      var adjacent = 0
      for (dx, dy) in [(-1,-1),(0,-1),(1,-1),
                       (-1, 0),       (1, 0),
                       (-1, 1),(0, 1),(1, 1)]:
        let pos: Vec2 = (x+dx, y+dy)
        if pos.x < 0 or pos.x >= rolls[0].len or
           pos.y < 0 or pos.y >= rolls.len: continue
        if rolls[pos.y][pos.x] == '@': inc adjacent

      if adjacent < 4:
        inc result
        toRemove.add (x, y)

  for (x, y) in toRemove: rolls[y][x] = '.'

proc solve(input: string): AOCSolution[int, int] =
  var rolls = input.splitLines()
  result.part1 = rolls.removePaper()
  result.part2 = result.part1
  while (let cnt = rolls.removePaper(); result.part2 += cnt; cnt) > 0:
    discard

Today was so easy, that I decided to solve it twice, just for fun. First is a 2D traversal (see above). And then I did a node graph solution in a few minutes (in repo below). Both run in ~27 ms.

It’s a bit concerning, because a simple puzzle can only mean that tomorrow will be a nightmare. Good Luck everyone, we will need it.

Full solution is at Codeberg: solution.nim

Kotlin

Pretty simple solution, just plain count / remove the rolls until none can be removed anymore. I would’ve liked to try using imaginary numbers this year (due to this article), but sadly Kotlin doesn’t natively support them and I was too lazy to use a library.

class Day04 : Puzzle {

    val grid = mutableListOf<MutableList<Char>>()

    override fun readFile() {
        val input = readInputFromFile("src/main/resources/a2025/day04.txt")
        for ((r, line) in input.lines().filter(String::isNotBlank).withIndex()) {
            grid.add(mutableListOf())
            for (char in line) {
                grid[r].add(char)
            }
        }
    }

    override fun solvePartOne(): String {
        return countRolls(grid).toString()
    }

    override fun solvePartTwo(): String {
        var sum = 0
        var removed = -1
        while (removed != 0) {
            // main grid is modified here, not the best but doesn't really matter
            // also no idea how to clone 2D list in Kotlin
            removed = countRolls(grid, true)
            sum += removed
        }
        return sum.toString()
    }

    private fun countRolls(grid: MutableList<MutableList<Char>>, removeRolls: Boolean = false): Int {
        val dr = listOf(-1, -1, -1, 0, 0, 1, 1, 1)
        val dc = listOf(-1, 0, 1, -1, 1, -1, 0, 1)

        var sum = 0
        for (r in grid.indices) {
            for (c in grid[r].indices) {
                if (grid[r][c] != '@') continue

                var neighborCount = 0
                for (i in dr.indices) {
                    if (gridGet(grid, r + dr[i], c + dc[i]) == '@') neighborCount++
                }
                if (neighborCount < 4) {
                    sum++
                    if (removeRolls) grid[r][c] = '.'
                }
            }
        }
        return sum
    }

    private fun gridGet(grid: List<List<Char>>, r: Int, c: Int, default: Char = '.'): Char {
        return if (r in grid.indices && c in grid[r].indices) {
            grid[r][c]
        } else {
            default
        }
    }
}

full code on Codeberg

Turns out on part 2 you can remove on access rather than after a full sweep of the grid, which cuts down the number of iterations you need to do about 1/2 sometimes 1/3 (depending on input).

import itertools as it
from pathlib import Path

import numpy as np

cwd = Path(__file__).parent.resolve()


def parse_input(file_path):
  with file_path.open("r") as fp:
    grid = np.array(list(map(list, list(map(str.strip, fp.readlines())))),
                    dtype=str)
  return grid


def solve_problem(file_name, single_attempt=True):

  grid = parse_input(Path(cwd, file_name))
  nr, nc = grid.shape
  nacc_total = 0
  stop = False

  while not stop:

    nacc = 0

    for i,j in it.product(range(nr), range(nc)):

      if grid[i,j] != '@':
        continue

      if np.count_nonzero(grid[max(i-1, 0):min(i+2, nr),
                               max(j-1, 0):min(j+2, nc)] == '@')<5:
        nacc += 1

        if not single_attempt:
          grid[i,j] = '.'

    nacc_total += nacc

    if nacc==0 or single_attempt:
      stop = True

  return nacc_total

C

For loops!

#include <stdio.h>

#define GZ 144

static char g[GZ][GZ];

int
main()
{
	int p1=0,p2=0, nc=0, x,y;

	for (y=1; fgets(g[y]+1, GZ-2, stdin); y++)
		;

	for (y=1; y<GZ-1; y++)
	for (x=1; x<GZ-1; x++)
		p1 += g[y][x] == '@' &&
		      (g[y-1][x-1] == '@') +
		      (g[y-1][x  ] == '@') +
		      (g[y-1][x+1] == '@') +
		      (g[y  ][x-1] == '@') +
		      (g[y  ][x+1] == '@') +
		      (g[y+1][x-1] == '@') +
		      (g[y+1][x  ] == '@') +
		      (g[y+1][x+1] == '@') < 4;

	do {
		nc = 0;

		for (y=1; y<GZ-1; y++)
		for (x=1; x<GZ-1; x++)
			if (g[y][x] == '@' &&
			    (g[y-1][x-1] == '@') +
			    (g[y-1][x  ] == '@') +
			    (g[y-1][x+1] == '@') +
			    (g[y  ][x-1] == '@') +
			    (g[y  ][x+1] == '@') +
			    (g[y+1][x-1] == '@') +
			    (g[y+1][x  ] == '@') +
			    (g[y+1][x+1] == '@') < 4) {
				nc++;
				p2++;
				g[y][x] = '.';
			}
	} while (nc);

	printf("04: %d %d\n", p1, p2);
	return 0;
}

Repo

For my x86-16 version, the 20K input is pushing it over the 64K .COM limit, so I’ll need to implement some better compression first.

Rust

I pulled out some code from last year to make representing 2D grids as a vector easier, so this was quite straightforward. 2.5ms runtime (including reading/parsing the input twice cos of TDD).

impl Grid {
    fn neighbour(&self, i: usize, dir: Direction) -> Option<bool> {
        let width = self.width;
        let length = self.grid.len();

        use Direction::*;
        match dir {
            N if i >= width => Some(i - width),
            NE if i >= width && i % width != width - 1 => Some(i - width + 1),
            E if i % width != width - 1 => Some(i + 1),
            SE if i + width + 1 < length && i % width != width - 1 => Some(i + width + 1),
            S if i + width < length => Some(i + width),
            SW if i + width - 1 < length && !i.is_multiple_of(width) => Some(i + width - 1),
            W if !i.is_multiple_of(width) => Some(i - 1),
            NW if i >= width && !i.is_multiple_of(width) => Some(i - width - 1),
            _ => None,
        };
        .map(|i| self.grid[i])
    }

    #[rustfmt::skip]
    fn cell_accessible(&self, i: usize) -> bool {
        Direction::ALL
            .iter()
            .filter(|&&dir| self.neighbour(i, dir).unwrap_or(false))
            .count() < 4
    }

    fn num_accessible(&self) -> usize {
        self.grid
            .iter()
            .enumerate()
            .filter(|&(i, &is_occupied)| is_occupied && self.cell_accessible(i))
            .count()
    }

    fn remove_accessible(&mut self) -> Option<usize> {
        let removables = self
            .grid
            .iter()
            .enumerate()
            .filter_map(|(i, &is_occupied)| (is_occupied && self.cell_accessible(i)).then_some(i))
            .collect::<Vec<_>>();

        let count = removables.len();
        if count > 0 {
            for idx in removables {
                self.grid[idx] = false;
            }
            Some(count)
        } else {
            None
        }
    }

    fn remove_recursive(&mut self) -> usize {
        let mut total_removed = 0;
        while let Some(removed) = self.remove_accessible() {
            total_removed += removed;
        }
        total_removed
    }
}

use std::{fs, str::FromStr};

use color_eyre::eyre::{Report, Result};

#[derive(Debug, Copy, Clone)]
enum Direction {
    N, NE, E, SE, S, SW, W, NW,
}

impl Direction {
    const ALL: [Direction; 8] = [
        Direction::N,
        Direction::NE,
        Direction::E,
        Direction::SE,
        Direction::S,
        Direction::SW,
        Direction::W,
        Direction::NW,
    ];
}

#[derive(Debug, PartialEq, Eq, Clone)]
struct Grid {
    grid: Vec<bool>,
    width: usize,
    height: usize,
}

impl FromStr for Grid {
    type Err = Report;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let grid: Vec<_> = s
            .chars()
            .filter_map(|ch| match ch {
                '.' => Some(false),
                '@' => Some(true),
                '\n' => None,
                _ => panic!("Invalid input"),
            })
            .collect();
        let width = s
            .chars()
            .position(|ch| ch == '\n')
            .ok_or_else(|| Report::msg("grid width cannot be zero, or one line"))?;
        let height = grid.len() / width;
        Ok(Self {
            grid,
            width,
            height,
        })
    }
}

impl Grid {
    fn neighbour(&self, i: usize, dir: Direction) -> Option<bool> {
        let width = self.width;
        let length = self.grid.len();

        use Direction::*;
        match dir {
            N if i >= width => Some(i - width),
            NE if i >= width && i % width != width - 1 => Some(i - width + 1),
            E if i % width != width - 1 => Some(i + 1),
            SE if i + width + 1 < length && i % width != width - 1 => Some(i + width + 1),
            S if i + width < length => Some(i + width),
            SW if i + width - 1 < length && !i.is_multiple_of(width) => Some(i + width - 1),
            W if !i.is_multiple_of(width) => Some(i - 1),
            NW if i >= width && !i.is_multiple_of(width) => Some(i - width - 1),
            _ => None,
        };
        .map(|i| self.grid[i])
    }

    #[rustfmt::skip]
    fn cell_accessible(&self, i: usize) -> bool {
        Direction::ALL
            .iter()
            .filter(|&&dir| self.neighbour(i, dir).unwrap_or(false))
            .count() < 4
    }

    fn num_accessible(&self) -> usize {
        self.grid
            .iter()
            .enumerate()
            .filter(|&(i, &is_occupied)| is_occupied && self.cell_accessible(i))
            .count()
    }

    fn remove_accessible(&mut self) -> Option<usize> {
        let removables = self
            .grid
            .iter()
            .enumerate()
            .filter_map(|(i, &is_occupied)| (is_occupied && self.cell_accessible(i)).then_some(i))
            .collect::<Vec<_>>();

        let count = removables.len();
        if count > 0 {
            for idx in removables {
                self.grid[idx] = false;
            }
            Some(count)
        } else {
            None
        }
    }

    fn remove_recursive(&mut self) -> usize {
        let mut total_removed = 0;
        while let Some(removed) = self.remove_accessible() {
            total_removed += removed;
        }
        total_removed
    }
}

fn part1(filepath: &str) -> Result<usize> {
    let input = fs::read_to_string(filepath)?;
    let grid = Grid::from_str(&input)?;
    Ok(grid.num_accessible())
}

fn part2(filepath: &str) -> Result<usize> {
    let input = fs::read_to_string(filepath)?;
    let mut grid = Grid::from_str(&input)?;
    Ok(grid.remove_recursive())
}

fn main() -> Result<()> {
    color_eyre::install()?;

    println!("Part 1: {}", part1("d04/input.txt")?);
    println!("Part 2: {}", part2("d04/input.txt")?);
    Ok(())
}

This is the first day I’ve wished I were working in J, like last year, instead of Lisp. Common Lisp arrays show the age of the language design. They’re basically C arrays with less convenient syntax; if you want higher-order array iteration you have to write it yourself or use a package that provides it. This solution is also less efficient than it could be for part 2, because I recompute the full neighbors array on each iteration rather than updating it incrementally.

(defun read-inputs (filename)
  (let* ((input-lines (uiop:read-file-lines filename))
         (rows (length input-lines))
         (cols (length (car input-lines)))
         (result (make-array (list rows cols) :initial-element 0)))
    (loop for line in input-lines
          for y from 0 to (1- rows)
          do (loop for x from 0 to (1- cols)
                   if (eql (char line x) #\@)
                     do (setf (aref result y x) 1)))
    result))

(defun neighbors (grid)
  (let* ((dimensions (array-dimensions grid))
         (rows (car dimensions))
         (cols (cadr dimensions))
         (result (make-array dimensions :initial-element 0)))
    (flet ((neighbors-at (y x)
             (loop for dy from -1 to 1
                   sum (loop for dx from -1 to 1
                             sum (let ((yy (+ y dy))
                                       (xx (+ x dx)))
                                   (if (and (>= yy 0)
                                            (< yy rows)
                                            (>= xx 0)
                                            (< xx cols)
                                            (not (and (= dx 0) (= dy 0)))
                                            (> (aref grid yy xx) 0))
                                       1
                                       0))))))
      (loop for y from 0 to (1- rows)
            do (loop for x from 0 to (1- cols)
                     do (setf (aref result y x) (neighbors-at y x))))
      result)))

(defun main-1 (filename)
  (let* ((grid (read-inputs filename))
         (dimensions (array-dimensions grid))
         (neighbors-grid (neighbors grid)))
    (loop for y from 0 to (1- (car dimensions))
          sum (loop for x from 0 to (1- (cadr dimensions))
                    sum (if (and (> (aref grid y x) 0)
                                 (< (aref neighbors-grid y x) 4))
                            1
                            0)))))

(defun remove-accessible (grid)
  (let* ((dimensions (array-dimensions grid))
         (neighbors-grid (neighbors grid))
         (removed 0))
    (loop for y from 0 to (1- (car dimensions))
          do (loop for x from 0 to (1- (cadr dimensions))
                   do (if (and (> (aref grid y x) 0)
                               (< (aref neighbors-grid y x) 4))
                          (progn
                            (setf (aref grid y x) 0)
                            (incf removed)))))
    removed))

(defun main-2 (filename)
  (let ((grid (read-inputs filename))
        (removed 0))
    (loop for this-removed = (remove-accessible grid)
          until (zerop this-removed)
          do (incf removed this-removed))
    removed))

Rust

View on github

fn parse_input(input: &str) -> Vec<Vec<bool>> {
    input
        .lines()
        .map(|l| l.chars().map(|c| c == '@').collect())
        .collect()
}

fn count_adj(grid: &[Vec<bool>], (x, y): (usize, usize)) -> usize {
    let width = grid[0].len();
    let height = grid.len();
    grid.iter()
        .take((y + 2).min(height))
        .skip(y.saturating_sub(1))
        .map(|r| {
            r.iter()
                .take((x + 2).min(width))
                .skip(x.saturating_sub(1))
                .take(3)
                .filter(|e| **e)
                .count()
        })
        .sum::<usize>()
}

fn part1(input: String) {
    let grid = parse_input(&input);
    let mut count = 0u32;
    for (y, row) in grid.iter().enumerate() {
        for (x, _) in row.iter().enumerate().filter(|(_, r)| **r) {
            let n_adj = count_adj(&grid, (x, y));
            // Center roll is counted too
            if n_adj < 5 {
                count += 1;
            }
        }
    }
    println!("{count}");
}

fn part2(input: String) {
    let mut grid = parse_input(&input);
    let mut removed = 0u32;
    loop {
        let mut next_grid = grid.clone();
        let prev_removed = removed;
        for (y, row) in grid.iter().enumerate() {
            for (x, _) in row.iter().enumerate().filter(|(_, r)| **r) {
                let n_adj = count_adj(&grid, (x, y));
                // Center roll is counted too
                if n_adj < 5 {
                    next_grid[y][x] = false;
                    removed += 1;
                }
            }
        }
        if removed == prev_removed {
            break;
        }
        grid = next_grid;
    }
    println!("{}", removed);
}

util::aoc_main!();

Uiua

Quite simple this one. Part 2 still takes a few seconds because I’m essentially checking off each roll individually.

Run with example input

$ ..@@.@@@@.
$ @@@.@.@.@@
$ @@@@@.@.@@
$ @.@@@@..@.
$ @@.@@@@.@@
$ .@@@@@@@.@
$ .@.@.@.@@@
$ @.@@@.@@@@
$ .@@@@@@@@.
$ @.@.@@@.@.
⊜∘⊸≠@\n
=@@

Rolls ← ⍣(⧻⊚˙⍤)∞⊸⊡1_1
Removable ← (
  ⬚0⧈Rolls[3_3 1_1 1_1]
  ⊚≤4
)

Remove ← ⍜⊡(˜↯0⧻)

PartOne ← ⧻Removable

PartTwo ← (
  ⊙0
  ⍥(
    ⊙(⊙+⟜⧻)⟜Removable
    ˜Remove
  )∞
  ◌
)
&pf "Part One: "
&p ⊸PartOne

&pf "Part Two: "
&p PartTwo

PartTwoOld ← (
  0⊙0
  ⍢(⊙⊙◌
    ⊙⊸Removable
    +⊙⊸⧻
    ⊙⊸Remove
  | ¬≍◌)
  ⊙⋅◌
)

Go

package main

import (
	"aoc/utils"
	"fmt"
)

type rollMap [][]bool

func (rm *rollMap) addRow(line string) {
	runes := []rune(line)
	row := make([]bool, len(runes))

	for idx, r := range runes {
		row[idx] = r == '@'
	}

	*rm = append(*rm, row)
}

func (rm rollMap) getAdjacentContents(row, col int) []bool {
	contents := []bool{}
	lenrow := len(rm[0])
	lenrm := len(rm)

	for _, i := range []int{-1, 0, 1} {
		for _, j := range []int{-1, 0, 1} {
			if i == 0 && j == 0 {
				continue
			}

			r := row + i
			c := col + j
			if r >= 0 && r < lenrm && c >= 0 && c < lenrow {
				contents = append(contents, rm[r][c])
			}
		}
	}

	return contents
}

func countTrue(array []bool) int {
	count := 0
	for _, val := range array {
		if val {
			count++
		}
	}
	return count
}

func createMap(input chan string) rollMap {
	rm := rollMap{}

	for line := range input {
		rm.addRow(line)
	}

	return rm
}

func stepOne(input chan string) (int, error) {
	rm := createMap(input)
	lenrow := len(rm[0])
	accessibleRolls := 0

	for row := range rm {
		for col := range lenrow {
			if rm[row][col] {
				adj := rm.getAdjacentContents(row, col)
				numRolls := countTrue(adj)
				if numRolls < 4 {
					accessibleRolls++
				}
			}
		}
	}

	return accessibleRolls, nil
}

type position struct {
	row, col int
}

func stepTwo(input chan string) (int, error) {
	rm := createMap(input)
	lenrow := len(rm[0])
	accessibleRolls := 0

	for true {
		accessedRolls := []position{}
		for row := range rm {
			for col := range lenrow {
				if rm[row][col] {
					adj := rm.getAdjacentContents(row, col)
					numRolls := countTrue(adj)
					if numRolls < 4 {
						accessibleRolls++
						accessedRolls = append(accessedRolls, position{
							row: row,
							col: col,
						})
					}
				}
			}
		}
		if len(accessedRolls) == 0 {
			break
		} else {
			for _, pos := range accessedRolls {
				rm[pos.row][pos.col] = false
			}
		}
	}

	return accessibleRolls, nil
}

func main() {
	input := utils.FilePath("day04.txt")
	ch1, err1 := input.GetLineChannel()
	if err1 != nil {
		fmt.Errorf("step one error: %s\n", err1)
		return
	}

	val1, errStep1 := stepOne(ch1)
	if errStep1 != nil {
		fmt.Errorf("step one error: %s\n", errStep1)
		return
	}

	fmt.Printf("Step one result: %d\n", val1)

	ch2, err2 := input.GetLineChannel()
	if err2 != nil {
		fmt.Errorf("step two error: %s\n", err2)
		return
	}

	val2, errStep2 := stepTwo(ch2)
	if errStep2 != nil {
		fmt.Errorf("step two error: %s\n", errStep2)
		return
	}

	fmt.Printf("Step two result: %d\n", val2)
}

Kotlin

I’m catching up on this year’s AOC.
This one was rather easy. I already have a pretty versatile grid class that I have just iterated as often as needed.

Doing this one also lead me into the rabbit hole that is source code generation in Gradle. I used this to generate all the implementations for the primitive types of the grid class as primitive arrays are not generic in the JVM.
An Array<Int> is an array of integer references, but an IntArray is an array of primitive integers.

Code on GitHub

class Day04 : AOCSolution {
    override val year = 2025
    override val day = 4

    override fun part1(inputFile: String): String {
        val grid = readResourceLines(inputFile).map(CharSequence::toList).toCharGrid()

        var accessiblePaperRolls = 0

        // Quickly iterate the grid in top-left to bottom-right order
        for (y in 0 until grid.height) {
            for (x in 0 until grid.width) {
                // Count the neighbours of each paper roll.
                if (grid[x, y] == PAPER_ROLL &&
                    grid.countNeighbours(x, y, 1) { it == PAPER_ROLL } < 4
                ) {
                    accessiblePaperRolls++
                }
            }
        }
        return accessiblePaperRolls.toString()
    }

    override fun part2(inputFile: String): String {
        val grid = readResourceLines(inputFile).map(CharSequence::toList).toCharGrid()

        var count = 0
        while (true) {
            var iterationCount = 0

            // Quickly iterate the grid in top-left to bottom-right order
            for (y in 0 until grid.height) {
                for (x in 0 until grid.width) {
                    if (grid[x, y] == PAPER_ROLL &&
                        grid.countNeighbours(x, y, 1) { it == PAPER_ROLL } < 4
                    ) {
                        // Remove the paper roll for the next iteration
                        grid[x, y] = REMOVED_PAPER_ROLL
                        iterationCount++
                    }
                }
            }
            count += iterationCount

            // Repeat the count until no paper rolls are accessible.
            if (iterationCount == 0) {
                break
            }
        }

        return count.toString()
    }

    private companion object {
        const val PAPER_ROLL = '@'
        const val REMOVED_PAPER_ROLL = 'x'

        /**
         * Count the neighbours of the given cell in the given [radius] of cells that satisfy the given predicate.
         *
         * @param startX the horizontal position of the center of the count in the grid
         * @param startY the vertical position of the center of the count in the grid
         * @param radius the radius counted in Manhattan distance to the center
         * @param predicate the test that needs to pass for a neighbour to count.
         */
        private fun CharGrid.countNeighbours(
            startX: Int,
            startY: Int,
            radius: Int,
            predicate: Predicate<Char>,
        ): Int {
            var count = 0
            for (y in maxOf(startY - radius, 0)..minOf(startY + radius, height - 1)) {
                for (x in maxOf(startX - radius, 0)..minOf(startX + radius, width - 1)) {
                    if (y == startY && x == startX) {
                        continue
                    }
                    if (predicate.test(this[x, y])) {
                        count++
                    }
                }
            }
            return count
        }
    }
}

Haskell

Very simple, this one.

import Data.List  
import Data.Set qualified as Set  

readInput s =  
  Set.fromDistinctAscList  
    [ (i, j) :: (Int, Int)  
      | (i, l) <- zip [0 ..] (lines s),  
        (j, c) <- zip [0 ..] l,  
        c == '@'  
    ]  

accessible ps = Set.filter ((< 4) . adjacent) ps  
  where  
    adjacent (i, j) =  
      length . filter (`Set.member` ps) $  
        [ (i + di, j + dj)  
          | di <- [-1 .. 1],  
            dj <- [-1 .. 1],  
            (di, dj) /= (0, 0)  
        ]  

main = do  
  input <- readInput <$> readFile "input04"  
  let removed =  
        (`unfoldr` input) $  
          \ps ->  
            case accessible ps of  
              d  
                | Set.null d -> Nothing  
                | otherwise -> Just (Set.size d, ps Set.\\ d)  
  print $ head removed  
  print $ sum removed  

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