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;
      }