public GenericBooleanPrefDataModel(FastByIDMap<FastIDSet> userData, FastByIDMap<FastByIDMap<Long>> timestamps) {
	    Preconditions.checkArgument(userData != null, "userData is null");

	    this.preferenceFromUsers = userData;
	    this.preferenceForItems = new FastByIDMap<FastIDSet>();
	    FastIDSet itemIDSet = new FastIDSet();
	    for (Map.Entry<Long, FastIDSet> entry : preferenceFromUsers.entrySet()) {
	      long userID = entry.getKey();
	      FastIDSet itemIDs = entry.getValue();
	      itemIDSet.addAll(itemIDs);
	      if (itemIDs.size() > 1) {
		      LongPrimitiveIterator it = itemIDs.iterator();
		      while (it.hasNext()) {
		        long itemID = it.nextLong();
		        FastIDSet userIDs = preferenceForItems.get(itemID);
		        if (userIDs == null) {
		          userIDs = new FastIDSet(2);
		          preferenceForItems.put(itemID, userIDs);
		        }
		        userIDs.add(userID);
		      }
	      }
	    }

	    this.itemIDs = itemIDSet.toArray();
	    itemIDSet = null; // Might help GC -- this is big
	    Arrays.sort(itemIDs);

	    this.userIDs = new long[userData.size()];
	    int i = 0;
	    LongPrimitiveIterator it = userData.keySetIterator();
	    while (it.hasNext()) {
	      userIDs[i++] = it.next();
	    }
	    Arrays.sort(userIDs);

	    this.timestamps = timestamps;
	  }