// inverse document frequency quotient between the number of docs in corpus and number of docs the
// term appears Normalize the value in case the number of appearances is 0.
double idf = Math.log10((double) numberOfDocumentsInCorpus /
(double) ((numberOfDocumentsInCorpusWhereKeyAppears == 0 ? 1 : 0) +
numberOfDocumentsInCorpusWhereKeyAppears));