day4: part2

README.md

@@ -1,6 +1,6 @@
 <!-- AOC TILES BEGIN -->
 <h1 align="center">
-  2025 - 6 ⭐ - Rust
+  2025 - 7 ⭐ - Rust
 </h1>
 <a href="src/day1.rs">
   <img src=".aoc_tiles/tiles/2025/01.png" width="161px">
@@ -11,4 +11,7 @@
 <a href="src/day3.rs">
   <img src=".aoc_tiles/tiles/2025/03.png" width="161px">
 </a>
+<a href="src/day4.rs">
+  <img src=".aoc_tiles/tiles/2025/04.png" width="161px">
+</a>
 <!-- AOC TILES END -->

src/day4.rs

@@ -0,0 +1,66 @@
+use aoc_runner_derive::{aoc, aoc_generator};
+use grid::Grid;
+
+#[aoc_generator(day4)]
+fn parse(input: &str) -> Grid<u8> {
+    input.parse().unwrap()
+}
+
+#[aoc(day4, part1)]
+fn part1(input: &Grid<u8>) -> u64 {
+    (0..input.height() * input.width())
+        .filter(|i| *input.get(&input.coord(*i as i64).unwrap()).unwrap() == b'@')
+        .map(|i| input.neighbours_count(input.coord(i as i64).unwrap(), |c| *c == b'@'))
+        .filter(|n| *n < 4)
+        .count() as u64
+}
+
+#[aoc(day4, part2)]
+fn part2(input: &Grid<u8>) -> u64 {
+    let mut grid = input.clone();
+    let mut removed = 0;
+
+    loop {
+        let mut removed_iteration = 0;
+        for i in 0..grid.width() * grid.height() {
+            let pos = grid.coord(i as i64).unwrap();
+            if grid.get(&pos).is_some_and(|c| *c == b'@')
+                && grid.neighbours_count(pos, |c| *c == b'@') < 4
+            {
+                // remove the roll
+                grid.set(&pos, b'.');
+                removed_iteration += 1;
+            }
+        }
+        if removed_iteration == 0 {
+            return removed;
+        }
+        removed += removed_iteration
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const EXAMPLE: &str = "..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.";
+
+    #[test]
+    fn part1_example() {
+        assert_eq!(part1(&parse(EXAMPLE)), 13);
+    }
+
+    #[test]
+    fn part2_example() {
+        assert_eq!(part2(&parse(EXAMPLE)), 43);
+    }
+}

src/lib.rs

@@ -1,6 +1,7 @@
 mod day1;
 mod day2;
 mod day3;
+mod day4;
 
 use aoc_runner_derive::aoc_lib;
 

utils/grid/lib.rs

@@ -352,6 +352,28 @@ impl<T: Clone + Eq + PartialEq + Debug> Grid<T> {
         }
     }
 
+    /// Return the count of neighbours (8 directions) matching predicate p
+    pub fn neighbours_count<C: AsCoord2d, P>(&self, c: C, mut p: P) -> usize
+    where
+        P: FnMut(&T) -> bool,
+    {
+        const DIRECTIONS: [(i64, i64); 8] = [
+            (-1, -1),
+            (0, -1),
+            (1, -1),
+            (-1, 0),
+            (1, 0),
+            (1, 1),
+            (0, 1),
+            (-1, 1),
+        ];
+        DIRECTIONS
+            .iter()
+            .map(|d| (c.x() + d.0, c.y() + d.1))
+            .filter(|c| self.get(c).is_some_and(|x| p(x)))
+            .count()
+    }
+
     // fn window_compare_impl<const REV: bool>(&self, needle: &[T]) -> Vec<(i64, i64)> {
     //     if (self.width as usize) < needle.len() {
     //         return Vec::new();