Solve Day 4: Printing Department

README.md

@@ -14,8 +14,9 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 1](./src/bin/01.rs) | `102.5µs` | `123.5µs` |
 | [Day 2](./src/bin/02.rs) | `45.0ms` | `69.6ms` |
 | [Day 3](./src/bin/03.rs) | `85.1µs` | `121.6µs` |
+| [Day 4](./src/bin/04.rs) | `260.1µs` | `4.0ms` |
 
-**Total: 115.03ms**
+**Total: 119.29ms**
 <!--- benchmarking table --->
 
 ---

data/examples/04.txt

@@ -0,0 +1,11 @@
+..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.
+

src/bin/04.rs

@@ -0,0 +1,103 @@
+advent_of_code::solution!(4);
+
+/// Count adjacent paper rolls (8 directions) for a given position
+fn count_neighbors(grid: &[Vec<char>], row: usize, col: usize) -> usize {
+    let rows = grid.len() as i32;
+    let cols = grid[0].len() as i32;
+    let mut count = 0;
+
+    for dr in -1..=1 {
+        for dc in -1..=1 {
+            if dr == 0 && dc == 0 {
+                continue;
+            }
+            let nr = row as i32 + dr;
+            let nc = col as i32 + dc;
+            if nr >= 0 && nr < rows && nc >= 0 && nc < cols {
+                if grid[nr as usize][nc as usize] == '@' {
+                    count += 1;
+                }
+            }
+        }
+    }
+
+    count
+}
+
+pub fn part_one(input: &str) -> Option<u64> {
+    let grid: Vec<Vec<char>> = input
+        .lines()
+        .filter(|line| !line.trim().is_empty())
+        .map(|line| line.chars().collect())
+        .collect();
+
+    let mut accessible = 0u64;
+
+    for (row, line) in grid.iter().enumerate() {
+        for (col, &ch) in line.iter().enumerate() {
+            // Only check paper rolls
+            if ch == '@' {
+                // Accessible if fewer than 4 neighbors
+                if count_neighbors(&grid, row, col) < 4 {
+                    accessible += 1;
+                }
+            }
+        }
+    }
+
+    Some(accessible)
+}
+
+pub fn part_two(input: &str) -> Option<u64> {
+    let mut grid: Vec<Vec<char>> = input
+        .lines()
+        .filter(|line| !line.trim().is_empty())
+        .map(|line| line.chars().collect())
+        .collect();
+
+    let mut total_removed = 0u64;
+
+    loop {
+        // Find all accessible rolls (fewer than 4 neighbors)
+        let mut to_remove: Vec<(usize, usize)> = Vec::new();
+
+        for (row, line) in grid.iter().enumerate() {
+            for (col, &ch) in line.iter().enumerate() {
+                if ch == '@' && count_neighbors(&grid, row, col) < 4 {
+                    to_remove.push((row, col));
+                }
+            }
+        }
+
+        // If no rolls are accessible, we're done
+        if to_remove.is_empty() {
+            break;
+        }
+
+        // Remove all accessible rolls
+        for (row, col) in &to_remove {
+            grid[*row][*col] = '.';
+        }
+
+        total_removed += to_remove.len() as u64;
+    }
+
+    Some(total_removed)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part_one() {
+        let result = part_one(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(13));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(43));
+    }
+}