public function columns() : array
    {
        if ($this->numRows() > 1) {
            return array_map(null, ...$this->samples);
        }

        $n = $this->numColumns();

        $columns = [];

        for ($i = 0; $i < $n; $i++) {
            $columns[] = array_column($this->samples, $i);
        }

        return $columns;
    }