private void countUsingBitVector(byte[] nPostingListsForDocument, int postingListBitmap) {
        for (int docId = 0; docId < nDocuments; docId++) {
            nPostingListsForDocument[docId] += (byte)Integer.bitCount(bitVector[docId] & postingListBitmap);
        }
    }
Counts the number of posting lists per document id.
Caches the most expensive posting list in a bit vector.

@author bjorncs
predicate-search/src/main/java/com/yahoo/search/predicate/index/CachedPostingListCounter.java