diff --git a/internal/2015/DayOne/code_test.go b/internal/2015/DayOne/code_test.go
new file mode 100644
index 0000000..4bcc8c1
--- /dev/null
+++ b/internal/2015/DayOne/code_test.go
@@ -0,0 +1,30 @@
+package dayone
+
+import "testing"
+
+func TestPartOne(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected int
+	}{
+		{"(())", "(())", 0},
+		{"()()", "()()", 0},
+		{"(((", "(((", 3},
+		{"(()(()(", "(()(()(", 3},
+		{"))(((((", "))(((((", 3},
+		{"())", "())", -1},
+		{"))(", "))(", -1},
+		{")))", ")))", -3},
+		{")())())", ")())())", -3},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := PartOne(tt.input)
+			if got != tt.expected {
+				t.Errorf("PartOne(%q) = %d, want %d", tt.input, got, tt.expected)
+			}
+		})
+	}
+}