feat: solve day 4 part 2

README.md

@@ -29,7 +29,7 @@ go test ./cmd/01
 | 01  | [Problem](https://adventofcode.com/2025/day/1)  | [Solution](./cmd/01/) | Part 1 seemed straight forward enough. Part 2 completely stumped me. The trickiest part was not counting zero's twice when you ended and then started from there.                                                     |
 | 02  | [Problem](https://adventofcode.com/2025/day/2)  | [Solution](./cmd/02/) | Part 1 should have been super straight-forward, but some whitespace screwed up my parsing of the final range. I some how was able to stumble over a solution to part 2 rather blindly.                                |
 | 03  | [Problem](https://adventofcode.com/2025/day/3)  | [Solution](./cmd/03/) | Part 1 intuitively makes you think you only care about highest nums, but then you need to consider placement. I once again tripped over the solution using a stack. However chat morally supported me the whole time. |
-| 04  | [Problem](https://adventofcode.com/2025/day/4)  | [Solution](./cmd/04/) | Part 1 was by far the most straight-forward challenge so far.                                                                                                                                                         |
+| 04  | [Problem](https://adventofcode.com/2025/day/4)  | [Solution](./cmd/04/) | Part 1 was by far the most straight-forward challenge so far. This was easiest day of AoC this year by far.                                                                                                           |
 | 05  | [Problem](https://adventofcode.com/2025/day/5)  | [Solution](./cmd/05/) |                                                                                                                                                                                                                       |
 | 06  | [Problem](https://adventofcode.com/2025/day/6)  | [Solution](./cmd/06/) |                                                                                                                                                                                                                       |
 | 07  | [Problem](https://adventofcode.com/2025/day/7)  | [Solution](./cmd/07/) |                                                                                                                                                                                                                       |

cmd/04/main.go

@@ -4,9 +4,11 @@ import (
 	"github.com/StevanFreeborn/advent-of-code-2025/internal/file"
 	"github.com/StevanFreeborn/advent-of-code-2025/internal/grid"
 	"github.com/StevanFreeborn/advent-of-code-2025/internal/move"
+	"github.com/StevanFreeborn/advent-of-code-2025/internal/position"
 )
 
 const PaperRollCharacter = "@"
+const EmptySpaceCharacter = "."
 
 func SolvePartOne(filePath string) int {
 	input := file.ReadAllLines(filePath)
@@ -27,5 +29,40 @@ func SolvePartOne(filePath string) int {
 			total++
 		}
 	}
+
+	return total
+}
+
+func SolvePartTwo(filePath string) int {
+	input := file.ReadAllLines(filePath)
+	g := grid.From(input)
+
+	total := 0
+
+	for {
+		positionsToRemove := []position.Position{}
+
+		for position := range g.Positions() {
+			value := g.GetValueAt(position)
+
+			if value != PaperRollCharacter {
+				continue
+			}
+
+			sameNeighbors := g.GetSameNeighborsOf(position, move.AllDirections)
+
+			if len(sameNeighbors) < 4 {
+				positionsToRemove = append(positionsToRemove, position)
+				total++
+			}
+		}
+
+		if len(positionsToRemove) == 0 {
+			break
+		}
+
+		g.SetValuesAt(positionsToRemove, EmptySpaceCharacter)
+	}
+
 	return total
 }

cmd/04/main_test.go

@@ -25,3 +25,23 @@ func TestSolvePartOneWithInput(t *testing.T) {
 		t.Errorf("got %d but wanted %d", result, expected)
 	}
 }
+
+func TestSolvePartTwoWithExampleInput(t *testing.T) {
+	expected := 43
+
+	result := solution.SolvePartTwo("EXAMPLE.txt")
+
+	if result != expected {
+		t.Errorf("got %d but wanted %d", result, expected)
+	}
+}
+
+func TestSolvePartTwoWithInput(t *testing.T) {
+	expected := 8451
+
+	result := solution.SolvePartTwo("INPUT.txt")
+
+	if result != expected {
+		t.Errorf("got %d but wanted %d", result, expected)
+	}
+}

internal/grid/grid.go

@@ -1,6 +1,8 @@
 package grid
 
 import (
+	"strings"
+
 	"github.com/StevanFreeborn/advent-of-code-2025/internal/move"
 	"github.com/StevanFreeborn/advent-of-code-2025/internal/position"
 )
@@ -12,6 +14,7 @@ type Grid interface {
 	GetValueAt(position.Position) string
 	GetSameNeighborsOf(position.Position, []move.Move) []position.Position
 	Positions() map[position.Position]string
+	SetValuesAt(positions []position.Position, value string)
 }
 
 type grid struct {
@@ -20,6 +23,10 @@ type grid struct {
 	positions       map[position.Position]string
 }
 
+func New(positions map[position.Position]string) Grid {
+	return grid{positions: positions}
+}
+
 func From(input []string) Grid {
 	numberOfRows := len(input)
 	numberOfColumns := len(input[0])
@@ -95,3 +102,27 @@ func (g grid) Positions() map[position.Position]string {
 
 	return positions
 }
+
+func (g grid) SetValuesAt(positions []position.Position, value string) {
+	for _, pos := range positions {
+		g.positions[pos] = value
+	}
+}
+
+func (g grid) String() string {
+	var rowString strings.Builder
+
+	for row := range g.numberOfRows {
+		var colString strings.Builder
+
+		for col := range g.numberOfColumns {
+			pos := position.From(row, col)
+			value := g.positions[pos]
+			colString.WriteString(value)
+		}
+
+		rowString.WriteString(colString.String() + "\n")
+	}
+
+	return rowString.String()
+}