Year 2025 Day 4

Author: maneatingape

README.md

@@ -80,6 +80,7 @@ Performance is reasonable even on older hardware, for example a 2011 MacBook Pro
 | 1 | [Secret Entrance](https://adventofcode.com/2025/day/1) | [Source](src/year2025/day01.rs) | 25 |
 | 2 | [Gift Shop](https://adventofcode.com/2025/day/2) | [Source](src/year2025/day02.rs) | 1 |
 | 3 | [Lobby](https://adventofcode.com/2025/day/3) | [Source](src/year2025/day03.rs) | 18 |
+| 4 | [Printing Department](https://adventofcode.com/2025/day/4) | [Source](src/year2025/day04.rs) | 244 |
 
 ## 2024
 

benches/benchmark.rs

@@ -95,5 +95,5 @@ benchmark!(year2024
 );
 
 benchmark!(year2025
-    day01, day02, day03
+    day01, day02, day03, day04
 );

src/lib.rs

@@ -74,5 +74,5 @@ library!(year2024 "Locate the Chief Historian in time for the big Christmas slei
 );
 
 library!(year2025 "Finish the North Pole decorations in time for Christmas."
-    day01, day02, day03
+    day01, day02, day03, day04
 );

src/main.rs

@@ -141,5 +141,5 @@ run!(year2024
 );
 
 run!(year2025
-    day01, day02, day03
+    day01, day02, day03, day04
 );

src/year2025/day04.rs

@@ -0,0 +1,55 @@
+//! # Printing Department
+use crate::util::grid::*;
+use crate::util::point::*;
+
+type Input = (usize, usize);
+
+pub fn parse(input: &str) -> Input {
+    let grid = Grid::parse(input);
+    let offset = Point::new(1, 1);
+    let mut todo = Vec::new();
+    let mut padded = Grid::new(grid.width + 2, grid.height + 2, u8::MAX);
+
+    for y in 0..grid.height {
+        for x in 0..grid.width {
+            let point = Point::new(x, y);
+
+            if grid[point] == b'@' {
+                let count = DIAGONAL
+                    .iter()
+                    .map(|&d| point + d)
+                    .filter(|&next| grid.contains(next) && grid[next] == b'@')
+                    .count();
+
+                if count < 4 {
+                    todo.push(point + offset);
+                }
+                padded[point + offset] = count as u8;
+            }
+        }
+    }
+
+    let part_one = todo.len();
+    let mut part_two = 0;
+
+    while let Some(point) = todo.pop() {
+        part_two += 1;
+
+        for next in DIAGONAL.map(|d| point + d) {
+            if padded[next] == 4 {
+                todo.push(next);
+            }
+            padded[next] -= 1;
+        }
+    }
+
+    (part_one, part_two)
+}
+
+pub fn part1(input: &Input) -> usize {
+    input.0
+}
+
+pub fn part2(input: &Input) -> usize {
+    input.1
+}

tests/test.rs

@@ -87,5 +87,5 @@ test!(year2024
 );
 
 test!(year2025
-    day01, day02, day03
+    day01, day02, day03, day04
 );

tests/year2025/day04.rs

@@ -0,0 +1,25 @@
+use aoc::year2025::day04::*;
+
+const EXAMPLE: &str = "\
+..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.";
+
+#[test]
+fn part1_test() {
+    let input = parse(EXAMPLE);
+    assert_eq!(part1(&input), 13);
+}
+
+#[test]
+fn part2_test() {
+    let input = parse(EXAMPLE);
+    assert_eq!(part2(&input), 43);
+}