Added CompleteCode function.

serial/serial_test.go

@@ -192,6 +192,38 @@ func TestEncodeRejectsNonDigitInNumber(t *testing.T) {
 	}
 }
 
+func TestCompleteCodeRoundtrip(t *testing.T) {
+	original, err := serial.Decode(123456)
+	if err != nil {
+		t.Fatalf("Decode failed: %v", err)
+	}
+
+	withoutChecksum := original[:len(original)-1]
+	full, idx, err := serial.CompleteCode(withoutChecksum)
+	if err != nil {
+		t.Fatalf("CompleteCode failed: %v", err)
+	}
+
+	if full != original {
+		t.Fatalf("completed code mismatch: got %q, want %q", full, original)
+	}
+
+	idx2, err := serial.Encode(full)
+	if err != nil {
+		t.Fatalf("Encode failed for completed code %q: %v", full, err)
+	}
+	if idx2 != idx {
+		t.Fatalf("idx mismatch: got %d, want %d", idx, idx2)
+	}
+}
+
+func TestCompleteCodeRejectsInvalidFormat(t *testing.T) {
+	_, _, err := serial.CompleteCode("AAA-10-A")
+	if !errors.Is(err, serial.ErrInvalidFormatNoChecksum) {
+		t.Fatalf("expected invalid no-checksum format error, got: %v", err)
+	}
+}
+
 func TestRandomCodeRoundtrip(t *testing.T) {
 	for i := 0; i < 100; i++ {
 		code, idx, err := serial.RandomCode()