* @param dao The DAO of the source file
* @return a map of users to partition numbers. Users not in a partition will return -1.
*/
protected Long2IntMap splitUsers(UserDAO dao) {
Long2IntMap userMap = new Long2IntOpenHashMap();
userMap.defaultReturnValue(-1);
switch (method) {
case PARTITION_USERS:
partitionUsers(userMap, dao.getUserIds());
break;