public static FeatureIndex fromInputStream(DataInputStream in) throws IOException {
            int mapSize = in.readInt();
            LongObjectHashMap<int[]> map = new LongObjectHashMap<>(mapSize);
            for (int i = 0; i < mapSize; i++) {
                long key = in.readLong();
                map.put(key, SerializationHelper.readIntArray(in));
            }
            map.compact();
            return new FeatureIndex(map);
        }
A searchable index of conjunctions (see {@link FeatureConjunction} / {@link IndexableFeatureConjunction}).
Implements the algorithm described in the paper
<a href="http://dl.acm.org/citation.cfm?id=1687633">Indexing Boolean Expressions</a>.

@author Magnar Nedland
@author bjorncs
predicate-search/src/main/java/com/yahoo/search/predicate/index/conjunction/ConjunctionIndex.java