From e26fc98af0c8b2f6fd18af16a5bd37eb43005565 Mon Sep 17 00:00:00 2001
From: Donald Ball <donald.ball@gmail.com>
Date: Sat, 6 Dec 2025 19:41:51 -0500
Subject: [PATCH] day4 part 2

---
 day4/main.go | 43 +++++++++++++++++++++++++++++++++++--------
 1 file changed, 35 insertions(+), 8 deletions(-)

diff --git a/day4/main.go b/day4/main.go
index 9b315fe..48886f8 100644
--- a/day4/main.go
+++ b/day4/main.go
@@ -17,13 +17,15 @@ func readLines(filename string) []string {
 }
 
 type warehouse struct {
-	boxes  bitmap.Bitmap
+	boxes  *bitmap.Bitmap
 	width  uint32
 	height uint32
 }
 
 func buildWarehouse(lines []string) warehouse {
-	wh := warehouse{}
+	wh := warehouse{
+		boxes: &bitmap.Bitmap{},
+	}
 	wh.height = uint32(len(lines))
 	if wh.height == 0 {
 		return wh
@@ -35,7 +37,7 @@ func buildWarehouse(lines []string) warehouse {
 	for y, line := range lines {
 		for x, char := range line {
 			if char == '@' {
-				set(&wh.boxes, wh.width, wh.height, uint32(x), uint32(y))
+				set(wh.boxes, wh.width, wh.height, uint32(x), uint32(y))
 			}
 		}
 	}
@@ -55,7 +57,9 @@ func debug(area *bitmap.Bitmap, width uint32) {
 	})
 }
 
-func buildNeighbors(width uint32, height uint32, x uint32, y uint32) *bitmap.Bitmap {
+func buildNeighbors(width uint32, height uint32, offset uint32) *bitmap.Bitmap {
+	y := offset / width
+	x := offset % width
 	var neighbors bitmap.Bitmap
 	set(&neighbors, width, height, x+1, y)
 	set(&neighbors, width, height, x-1, y)
@@ -71,16 +75,30 @@ func buildNeighbors(width uint32, height uint32, x uint32, y uint32) *bitmap.Bit
 func (wh warehouse) countAccessibleBoxes() uint32 {
 	boxes := wh.boxes.Clone(nil)
 	boxes.Filter(func(offset uint32) bool {
-		y := offset / wh.width
-		x := offset % wh.width
-		neighbors := buildNeighbors(wh.width, wh.height, x, y)
-		neighbors.And(wh.boxes)
+		neighbors := buildNeighbors(wh.width, wh.height, offset)
+		neighbors.And(*wh.boxes)
 		accessible := neighbors.Count() < 4
 		return accessible
 	})
 	return uint32(boxes.Count())
 }
 
+func (wh *warehouse) removeAccessibleBoxes() uint32 {
+	boxes := wh.boxes.Clone(nil)
+	removed := uint32(0)
+	boxes.Filter(func(offset uint32) bool {
+		neighbors := buildNeighbors(wh.width, wh.height, offset)
+		neighbors.And(*wh.boxes)
+		accessible := neighbors.Count() < 4
+		if accessible {
+			removed++
+		}
+		return !accessible
+	})
+	wh.boxes = &boxes
+	return removed
+}
+
 func main() {
 	filename := "input0.txt"
 	if len(os.Args) > 1 {
@@ -89,4 +107,13 @@ func main() {
 	lines := readLines(filename)
 	wh := buildWarehouse(lines)
 	fmt.Printf("%v\n", wh.countAccessibleBoxes())
+	total := uint32(0)
+	for {
+		count := wh.removeAccessibleBoxes()
+		if count == 0 {
+			break
+		}
+		total += count
+	}
+	fmt.Printf("%v\n", total)
 }