From 7564a3627fe766f5490292a36cb050b84d4bab78 Mon Sep 17 00:00:00 2001
From: Amatsugu <khamraj@kaisei.app>
Date: Tue, 30 Dec 2025 18:56:24 -0500
Subject: [PATCH] day 4 part 2

---
 .../AOC2025/Day4/PrintingDeparment.cs         | 69 ++++++++++++++-----
 1 file changed, 50 insertions(+), 19 deletions(-)

diff --git a/AdventOfCode/Problems/AOC2025/Day4/PrintingDeparment.cs b/AdventOfCode/Problems/AOC2025/Day4/PrintingDeparment.cs
index 18a56de..9f3edd1 100644
--- a/AdventOfCode/Problems/AOC2025/Day4/PrintingDeparment.cs
+++ b/AdventOfCode/Problems/AOC2025/Day4/PrintingDeparment.cs
@@ -1,6 +1,7 @@
 using AdventOfCode.Utils.Models;
 
 using System;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Text;
 
@@ -13,36 +14,23 @@ internal class PrintingDeparment: Problem<int, int>
 
 	public override void CalculatePart1()
 	{
-		var c = 0;
-		for (int y = 0; y < _size.Y; y++)
-		{
-			for (int x = 0; x < _size.X; x++)
-			{
-				var pos = new Vec2<int>(x, y);
-				if (_data[pos.Y][pos.X] != '@')
-					continue;
-				var n = CountNeighbors(pos);
-				if (n < 4)
-					c++;
-			}
-		}
-		Part1 = c;
+		Part1 = GetAccessableRolls(_data, _size).Count;
 	}
 
-	public int CountNeighbors(Vec2<int> pos)
+	public static int CountNeighbors(string[] data, Vec2<int> size, Vec2<int> pos)
 	{
 		var c = 0;
 		for (int y = pos.Y-1; y <= pos.Y + 1; y++)
 		{
-			if (y < 0 || y >= _size.Y)
+			if (y < 0 || y >= size.Y)
 				continue;
 			for (int x = pos.X - 1; x <= pos.X + 1; x++)
 			{
-				if (x < 0 || x >= _size.X)
+				if (x < 0 || x >= size.X)
 					continue;
 				if (pos.X == x && pos.Y == y)
 					continue;
-				if (_data[y][x] == '@')
+				if (data[y][x] == '@')
 					c++;
 			}
 		}
@@ -51,7 +39,50 @@ internal class PrintingDeparment: Problem<int, int>
 
 	public override void CalculatePart2()
 	{
-		throw new NotImplementedException();
+		var data = _data;
+		var rolls = GetAccessableRolls(data, _size);
+		Part2 += rolls.Count;
+		while(rolls.Count > 0)
+		{
+			data = RemoveRolls(data, _size, rolls);
+			rolls = GetAccessableRolls(data, _size);
+			Part2 += rolls.Count;
+		}
+	}
+
+	public static List<Vec2<int>> GetAccessableRolls(string[] data, Vec2<int> size)
+	{
+		var results = new List<Vec2<int>>();
+		for (int y = 0; y < size.Y; y++)
+		{
+			for (int x = 0; x < size.X; x++)
+			{
+				var pos = new Vec2<int>(x, y);
+				if (data[pos.Y][pos.X] != '@')
+					continue;
+				var n = CountNeighbors(data, size, pos);
+				if (n < 4)
+					results.Add(pos);
+			}
+		}
+		return results;
+	}
+
+	public static string[] RemoveRolls(string[] data, Vec2<int> size, List<Vec2<int>> rolls)
+	{
+		var positions = rolls.ToFrozenSet();
+		return data.Select((row, y) =>
+		{
+			var updatedRow = row.Select((col, x) =>
+			{
+				var pos = new Vec2<int>(x, y);
+				if (positions.Contains(pos))
+					return '.';
+				else
+					return col;
+			}).ToArray();
+			return new string(updatedRow);
+		}).ToArray();
 	}
 
 	public override void LoadInput()