[2025] Day 4 first solution

Author: connorslade

README.md

@@ -8,6 +8,7 @@ Thank you to [Eric Wastl](http://was.tl) for running this incredible yearly even
 - [Day 01: Secret Entrance](aoc_2025/src/day_01.rs)
 - [Day 02: Gift Shop](aoc_2025/src/day_02.rs)
 - [Day 03: Lobby](aoc_2025/src/day_03.rs)
+- [Day 04: Printing Department](aoc_2025/src/day_04.rs)
 <!-- MARKER -->
 
 ## [2024](https://adventofcode.com/2024) [![aoc_2024](https://github.com/connorslade/advent-of-code/actions/workflows/aoc_2024.yml/badge.svg)](https://github.com/connorslade/advent-of-code/actions/workflows/aoc_2024.yml)

aoc_2025/src/day_03.rs

@@ -24,23 +24,21 @@ fn solve(input: &str, n: usize) -> u64 {
 }
 
 fn joltage(digits: &[u8], n: usize) -> u64 {
-    let mut num = 0;
-    let mut last_used = 0;
-
-    for bat in 0..n {
-        let size = digits.len() - (n - 1 - bat) - last_used;
-        let slice = &digits[last_used..(last_used + size).min(digits.len())];
+    let mut out = 0;
+    let mut last = 0;
 
+    for bat in 1..=n {
+        let size = digits.len() - (n - bat) - last;
         for i in (0..=9).rev() {
-            if let Some(idx) = slice.iter().position(|x| *x == i) {
-                last_used += idx + 1;
-                num = num * 10 + i as u64;
+            if let Some(idx) = digits.iter().skip(last).take(size).position(|x| *x == i) {
+                last += idx + 1;
+                out = out * 10 + i as u64;
                 break;
             }
         }
     }
 
-    num
+    out
 }
 
 #[cfg(test)]

aoc_2025/src/day_04.rs

@@ -0,0 +1,113 @@
+use aoc_lib::matrix::Grid;
+use common::{Answer, solution};
+use nd_vec::Vec2;
+
+solution!("Printing Department", 4);
+
+#[derive(Clone)]
+enum Tile {
+    Empty,
+    Roll,
+}
+
+fn part_a(input: &str) -> Answer {
+    let grid = Grid::parse(input, |chr| match chr {
+        '.' => Tile::Empty,
+        '@' => Tile::Roll,
+        _ => panic!(),
+    });
+
+    count(&grid).0.into()
+}
+
+fn part_b(input: &str) -> Answer {
+    let mut grid = Grid::parse(input, |chr| match chr {
+        '.' => Tile::Empty,
+        '@' => Tile::Roll,
+        _ => panic!(),
+    });
+
+    let mut out = 0;
+
+    loop {
+        let (removed, next) = count(&grid);
+        out += removed;
+        grid = next;
+        if removed == 0 {
+            break;
+        }
+    }
+
+    out.into()
+}
+
+fn count(grid: &Grid<Tile>) -> (u32, Grid<Tile>) {
+    let mut out = 0;
+
+    let mut next = grid.clone();
+
+    for y in 0..grid.size.y() {
+        for x in 0..grid.size().x() {
+            let pos = Vec2::new([x, y]).num_cast::<i32>().unwrap();
+            if matches!(grid.get(pos.num_cast().unwrap()).unwrap(), Tile::Empty) {
+                continue;
+            }
+
+            let mut neighbors = 0;
+            for dy in -1..=1 {
+                for dx in -1..=1 {
+                    let delta = Vec2::new([dx, dy]);
+                    let check = pos + delta;
+
+                    if delta == Vec2::zero()
+                        || check.x() < 0
+                        || check.y() < 0
+                        || check.x() >= grid.size.x() as i32
+                        || check.y() >= grid.size.y() as i32
+                    {
+                        continue;
+                    }
+
+                    if matches!(grid.get(check.num_cast().unwrap()).unwrap(), Tile::Roll) {
+                        neighbors += 1;
+                    }
+                }
+            }
+
+            if neighbors < 4 {
+                out += 1;
+                next.set(pos, Tile::Empty);
+            }
+        }
+    }
+
+    (out, next)
+}
+
+#[cfg(test)]
+mod test {
+    use indoc::indoc;
+
+    const CASE: &str = indoc! {"
+        ..@@.@@@@.
+        @@@.@.@.@@
+        @@@@@.@.@@
+        @.@@@@..@.
+        @@.@@@@.@@
+        .@@@@@@@.@
+        .@.@.@.@@@
+        @.@@@.@@@@
+        .@@@@@@@@.
+        @.@.@@@.@.
+    "};
+
+    #[test]
+    fn part_a() {
+        assert_eq!(super::part_a(CASE), 13.into());
+    }
+
+    #[test]
+    fn part_b() {
+        assert_eq!(super::part_b(CASE), 43.into());
+    }
+}

aoc_2025/src/lib.rs

@@ -3,11 +3,13 @@ use common::Solution;
 mod day_01;
 mod day_02;
 mod day_03;
+mod day_04;
 // [import_marker]
 
 pub const SOLUTIONS: &[Solution] = &[
     day_01::SOLUTION,
     day_02::SOLUTION,
     day_03::SOLUTION,
+    day_04::SOLUTION,
     // [list_marker]
 ];