question: public Explanation idfExplain ( CollectionStatistics collectionStats , TermStatistics termStats [ ] ) { final long docCount = collectionStats . docCount ( ) == - 1 ? collectionStats . maxDoc ( ) : collectionStats . docCount ( ) ; float idf = 0.0f ; List < Explanation > details = new ArrayList < > ( ) ; for ( final TermStatistics stat : termStats ) { final long df = stat . docFreq ( ) ; final float termIdf = idf ( df , docCount ) ; details . add ( Explanation . match ( termIdf , "idf(docFreq=" + df + ", docCount=" + docCount + ")" ) ) ; idf += termIdf ; } return Explanation . match ( idf , "idf(), sum of:" , details ) ; }
target:  Computes a score factor for a phrase. <p> The default implementation sums the idf factor for each term in the phrase.