From 4b51bcac08fb82828a414ca41be967be15beaef2 Mon Sep 17 00:00:00 2001
From: Keenan Tims <ktims@gotroot.ca>
Date: Wed, 3 Dec 2025 21:36:46 -0800
Subject: [PATCH] day4: part2

---
 src/day4.rs | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/src/day4.rs b/src/day4.rs
index ab98f5e..16315ac 100644
--- a/src/day4.rs
+++ b/src/day4.rs
@@ -17,7 +17,26 @@ fn part1(input: &Grid<u8>) -> u64 {
 
 #[aoc(day4, part2)]
 fn part2(input: &Grid<u8>) -> u64 {
-    input.width() as 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)]
@@ -42,6 +61,6 @@ mod tests {
 
     #[test]
     fn part2_example() {
-        assert_eq!(part2(&parse(EXAMPLE)), 0);
+        assert_eq!(part2(&parse(EXAMPLE)), 43);
     }
 }