private static ConjunctionIndex.FeatureIndex buildFeatureIndex(FeatureIndexBuilder featureIndexBuilder) {
        LongObjectHashMap<int[]> map = new LongObjectHashMap<>();
        for (Map.Entry<Long, Set<Integer>> featureEntry : featureIndexBuilder.map.entrySet()) {
            int[] conjunctionIds = Ints.toArray(featureEntry.getValue());
            map.put(featureEntry.getKey(), conjunctionIds);
        }
        map.compact();
        return new ConjunctionIndex.FeatureIndex(map);
    }
@return The number of unique conjunctions indexed.
predicate-search/src/main/java/com/yahoo/search/predicate/index/conjunction/ConjunctionIndexBuilder.java