presto-main/src/main/java/io/prestosql/type/setdigest/SetDigest.java/SetDigest/exactIntersectionCardinality
class SetDigest:    public static long exactIntersectionCardinality(SetDigest a, SetDigest b)
    {
        checkState(a.isExact(), "exact intersection cannot operate on approximate sets");
        checkArgument(b.isExact(), "exact intersection cannot operate on approximate sets");

        return Sets.intersection(a.minhash.keySet(), b.minhash.keySet()).size();
    }