public function idf($input, Zend_Search_Lucene_Interface $reader)
    {
        if (!is_array($input)) {
            return $this->idfFreq($reader->docFreq($input), $reader->count());
        } else {
            $idf = 0.0;
            foreach ($input as $term) {
                $idf += $this->idfFreq($reader->docFreq($term), $reader->count());
            }
            return $idf;
        }
    }