private function getRowColumns($row)
    {
        $columns = range(0, $this->numberOfColumns - 1);
        foreach ($row as $cellKey => $cell) {
            if ($cell instanceof TableCell && $cell->getColspan() > 1) {
                // exclude grouped columns.
                $columns = array_diff($columns, range($cellKey + 1, $cellKey + $cell->getColspan() - 1));
            }
        }

        return $columns;
    }