From 02fe79f702bbede053b1f27afba27c5726fc3fa0 Mon Sep 17 00:00:00 2001
From: C4 Patino <c4patino@gmail.com>
Date: Wed, 3 Dec 2025 23:09:17 -0600
Subject: [PATCH] feat: completed day 4 for advent of code 2025

---
 2025/cmd/root.go         |   2 +
 2025/day04/day04.go      | 118 +++++++++++++++++++++++++++++++++++++++
 2025/day04/day04_test.go |  34 +++++++++++
 2025/day04/test.txt      |  10 ++++
 4 files changed, 164 insertions(+)
 create mode 100755 2025/day04/day04.go
 create mode 100755 2025/day04/day04_test.go
 create mode 100755 2025/day04/test.txt

diff --git a/2025/cmd/root.go b/2025/cmd/root.go
index f919404..301897b 100644
--- a/2025/cmd/root.go
+++ b/2025/cmd/root.go
@@ -12,12 +12,14 @@ import (
 	"cpatino.com/advent-of-code/2025/day01"
 	"cpatino.com/advent-of-code/2025/day02"
 	"cpatino.com/advent-of-code/2025/day03"
+	"cpatino.com/advent-of-code/2025/day04"
 )
 
 var days = map[int]func(string) (any, any){
 	1: day01.Run,
 	2: day02.Run,
 	3: day03.Run,
+	4: day04.Run,
 }
 
 var rootCmd = &cobra.Command{
diff --git a/2025/day04/day04.go b/2025/day04/day04.go
new file mode 100755
index 0000000..f50c4f8
--- /dev/null
+++ b/2025/day04/day04.go
@@ -0,0 +1,118 @@
+package day04
+
+import (
+	"bufio"
+	"os"
+)
+
+var dirs = [8][2]int{
+	{-1, -1}, {-1, 0}, {-1, 1},
+	{0, -1}, {0, 1},
+	{1, -1}, {1, 0}, {1, 1},
+}
+
+func copyGrid(src [][]rune) [][]rune {
+	res := make([][]rune, len(src))
+	for i := range src {
+		res[i] = make([]rune, len(src[i]))
+		copy(res[i], src[i])
+	}
+	return res
+}
+
+func Part1(grid [][]rune) int {
+	rows := len(grid)
+	if rows == 0 {
+		return 0
+	}
+
+	cols := len(grid[0])
+	accessible := 0
+
+	for r := range rows {
+		for c := range cols {
+			if grid[r][c] != '@' {
+				continue
+			}
+
+			adj := 0
+			for _, d := range dirs {
+				nr, nc := r+d[0], c+d[1]
+				if nr >= 0 && nr < rows && nc >= 0 && nc < cols && grid[nr][nc] == '@' {
+					adj++
+				}
+			}
+
+			if adj < 4 {
+				accessible++
+			}
+		}
+	}
+
+	return accessible
+}
+
+func Part2(grid [][]rune) int {
+	rows, cols := len(grid), len(grid[0])
+	totalRemoved := 0
+	for {
+		toRemove := make([][2]int, 0)
+		for r := range rows {
+			for c := range cols {
+				if grid[r][c] != '@' {
+					continue
+				}
+
+				adj := 0
+				for _, d := range dirs {
+					nr, nc := r+d[0], c+d[1]
+					if nr >= 0 && nr < rows && nc >= 0 && nc < cols && grid[nr][nc] == '@' {
+						adj++
+					}
+				}
+
+				if adj < 4 {
+					toRemove = append(toRemove, [2]int{r, c})
+				}
+			}
+		}
+
+		if len(toRemove) == 0 {
+			break
+		}
+
+		for _, pos := range toRemove {
+			r, c := pos[0], pos[1]
+			grid[r][c] = '.'
+		}
+
+		totalRemoved += len(toRemove)
+	}
+
+	return totalRemoved
+}
+
+func Run(filename string) (any, any) {
+	file, err := os.Open(filename)
+	if err != nil {
+		panic(err)
+	}
+	defer file.Close()
+
+	scanner := bufio.NewScanner(file)
+	var grid [][]rune
+	for scanner.Scan() {
+		line := scanner.Text()
+		if len(line) == 0 {
+			continue
+		}
+		grid = append(grid, []rune(line))
+	}
+	if err := scanner.Err(); err != nil {
+		panic(err)
+	}
+
+	part1 := Part1(copyGrid(grid))
+	part2 := Part2(copyGrid(grid))
+	return part1, part2
+}
diff --git a/2025/day04/day04_test.go b/2025/day04/day04_test.go
new file mode 100755
index 0000000..c40d49a
--- /dev/null
+++ b/2025/day04/day04_test.go
@@ -0,0 +1,34 @@
+package day04
+
+import (
+	"os"
+	"testing"
+)
+
+type TestCase struct {
+	FileName string
+	Part1    any
+	Part2    any
+}
+
+func TestDay04(t *testing.T) {
+	tests := []TestCase{
+		{"test.txt", 13, 43},
+		{"input.txt", 1523, 9290},
+	}
+
+	for _, test := range tests {
+		if _, err := os.Stat(test.FileName); os.IsNotExist(err) {
+			t.Fatalf("test file does not exist: %v", test.FileName)
+		}
+
+		part1, part2 := Run(test.FileName)
+		if test.Part1 != nil && part1 != test.Part1 {
+			t.Fatalf("%s: unexpected part1:\nwant:\t%v\ngot:\t%v", test.FileName, test.Part1, part1)
+		}
+
+		if test.Part2 != nil && part2 != test.Part2 {
+			t.Fatalf("%s: unexpected part2:\nwant:\t%v\ngot:\t%v", test.FileName, test.Part2, part2)
+		}
+	}
+}
diff --git a/2025/day04/test.txt b/2025/day04/test.txt
new file mode 100755
index 0000000..8209399
--- /dev/null
+++ b/2025/day04/test.txt
@@ -0,0 +1,10 @@
+..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.