feat: solve day 4 part 1

cmd/04/main.go

@@ -0,0 +1,82 @@
+package main
+
+import (
+	"github.com/StevanFreeborn/advent-of-code-2025/internal/file"
+)
+
+const PaperRollCharacter = "@"
+const EmptyCharacter = "."
+
+type move struct {
+	NumberOfRows    int
+	NumberOfColumns int
+}
+
+var (
+	up        = move{NumberOfRows: -1, NumberOfColumns: 0}
+	down      = move{NumberOfRows: 1, NumberOfColumns: 0}
+	right     = move{NumberOfRows: 0, NumberOfColumns: 1}
+	left      = move{NumberOfRows: 0, NumberOfColumns: -1}
+	upRight   = move{NumberOfRows: -1, NumberOfColumns: 1}
+	upLeft    = move{NumberOfRows: -1, NumberOfColumns: -1}
+	downRight = move{NumberOfRows: 1, NumberOfColumns: 1}
+	downLeft  = move{NumberOfRows: 1, NumberOfColumns: -1}
+)
+
+var moves = []move{
+	up,
+	down,
+	right,
+	left,
+	upRight,
+	upLeft,
+	downRight,
+	downLeft,
+}
+
+func SolvePartOne(filePath string) int {
+	input := file.ReadAllLines(filePath)
+	numberOfRows := len(input)
+	numberOfColumns := len(input[0])
+
+	total := 0
+
+	for row := range numberOfRows {
+		for column := range numberOfColumns {
+			value := string(input[row][column])
+
+			if value != PaperRollCharacter {
+				continue
+			}
+
+			numberOfPaperRolls := 0
+
+			for _, move := range moves {
+				neighborRow := row + move.NumberOfRows
+				neighborColumn := column + move.NumberOfColumns
+
+				if neighborRow > numberOfRows-1 || neighborRow < 0 {
+					continue
+				}
+
+				if neighborColumn > numberOfColumns-1 || neighborColumn < 0 {
+					continue
+				}
+
+				neighborValue := string(input[neighborRow][neighborColumn])
+
+				if neighborValue != PaperRollCharacter {
+					continue
+				}
+
+				numberOfPaperRolls++
+			}
+
+			if numberOfPaperRolls < 4 {
+				total++
+			}
+		}
+	}
+
+	return total
+}

cmd/04/main_test.go

@@ -0,0 +1,27 @@
+package main_test
+
+import (
+	"testing"
+
+	solution "github.com/StevanFreeborn/advent-of-code-2025/cmd/04"
+)
+
+func TestSolvePartOneWithExampleInput(t *testing.T) {
+	expected := 13
+
+	result := solution.SolvePartOne("EXAMPLE.txt")
+
+	if result != expected {
+		t.Errorf("got %d but wanted %d", result, expected)
+	}
+}
+
+func TestSolvePartOneWithInput(t *testing.T) {
+	expected := 1395
+
+	result := solution.SolvePartOne("INPUT.txt")
+
+	if result != expected {
+		t.Errorf("got %d but wanted %d", result, expected)
+	}
+}