diff --git a/internal/2015/DayThirteen/code_test.go b/internal/2015/DayThirteen/code_test.go
new file mode 100644
index 0000000..c2cfd7c
--- /dev/null
+++ b/internal/2015/DayThirteen/code_test.go
@@ -0,0 +1,26 @@
+package daythirteen
+
+import "testing"
+
+var testInput = []string{
+	"Alice would gain 54 happiness units by sitting next to Bob",
+	"Alice would lose 79 happiness units by sitting next to Carol",
+	"Alice would lose 2 happiness units by sitting next to David",
+	"Bob would gain 83 happiness units by sitting next to Alice",
+	"Bob would lose 7 happiness units by sitting next to Carol",
+	"Bob would lose 63 happiness units by sitting next to David",
+	"Carol would lose 62 happiness units by sitting next to Alice",
+	"Carol would gain 60 happiness units by sitting next to Bob",
+	"Carol would gain 55 happiness units by sitting next to David",
+	"David would gain 46 happiness units by sitting next to Alice",
+	"David would lose 7 happiness units by sitting next to Bob",
+	"David would gain 41 happiness units by sitting next to Carol",
+}
+
+func TestPartOne(t *testing.T) {
+	expected := 330
+	got := PartOne(testInput)
+	if got != expected {
+		t.Errorf("PartOne() = %d, want %d", got, expected)
+	}
+}