Implement 2025 - Day 4 - Part 2

2025/04.rs

@@ -11,7 +11,7 @@ enum Cell {
 }
 
 fn main() {
-    let grid: Vec<_> = CONTENT.lines().map(|l| l.trim()).filter(|l| !l.is_empty()).map(|l| {
+    let mut grid: Vec<_> = CONTENT.lines().map(|l| l.trim()).filter(|l| !l.is_empty()).map(|l| {
         l.chars().map(|c| match c {
             '.' => Cell::Empty,
             '@' => Cell::PRoll,
@@ -20,23 +20,36 @@ fn main() {
     }).collect();
 
     let mut result = 0;
-    for (y, row) in grid.iter().enumerate() {
-        for (x, cell) in row.iter().enumerate() {
-            if cell != &Cell::PRoll {
-                continue;
-            }
 
-            let neighbours = GRID_OFFSETS.iter().filter_map(|(xoff, yoff)| Some((x.checked_add_signed(*xoff)?, y.checked_add_signed(*yoff)?))).filter_map(|(x, y)| {
-                let cell = grid.get(y)?.get(x)?;
-                Some((x, y, cell.clone()))
-            }).filter(|(_, _, c)| c == &Cell::PRoll).count();
-    
+    loop {
+        let mut to_remove = Vec::new();
+        for (y, row) in grid.iter().enumerate() {
+            for (x, cell) in row.iter().enumerate() {
+                if cell != &Cell::PRoll {
+                    continue;
+                }
 
-            if neighbours < 4 {
-                result += 1;
+                let neighbours = GRID_OFFSETS.iter().filter_map(|(xoff, yoff)| Some((x.checked_add_signed(*xoff)?, y.checked_add_signed(*yoff)?))).filter_map(|(x, y)| {
+                    let cell = grid.get(y)?.get(x)?;
+                    Some((x, y, cell.clone()))
+                }).filter(|(_, _, c)| c == &Cell::PRoll).count();
+        
+
+                if neighbours < 4 {
+                    result += 1;
+                    to_remove.push((x, y));
+                }
             }
         }
+        if to_remove.is_empty() {
+            break;
+        }
+
+        for (x, y) in to_remove {
+            grid[y][x] = Cell::Empty;
+        }
     }
 
+
     println!("Result: {}", result);
 }