Optimized SamplingCandidateItemsStrategy

protected FastIDSet doGetCandidateItems(long[] preferredItemIDs, DataModel dataModel) throws TasteException {
        int maxPrefsPerItemConsidered = (int) Math.min(defaultMaxPrefsPerItemConsidered, userItemCountMultiplier * Math.log(Math.max(dataModel.getNumUsers(), dataModel.getNumItems())));
        FastIDSet possibleItemsIDs = new FastIDSet();

        for (long itemID : preferredItemIDs) {

            PreferenceArray prefs = dataModel.getPreferencesForItem(itemID);
            int prefsConsidered = Math.min(prefs.length(), maxPrefsPerItemConsidered);
            Iterator<Preference> sampledPrefs = new FixedSizeSamplingIterator<Preference>(prefsConsidered, prefs.iterator());
            while (sampledPrefs.hasNext()) {
                possibleItemsIDs.addAll(dataModel.getItemIDsFromUser(sampledPrefs.next().getUserID()));
            }
        }
        possibleItemsIDs.removeAll(preferredItemIDs);

        //
        if (this.maxSampledItems > 0) {
            FastIDSet finalPossibleItemsIDs = new FastIDSet();
            Iterator<Long> sampledPossibleIDs = new FixedSizeSamplingIterator<Long>(this.maxSampledItems, possibleItemsIDs.iterator());
            while (sampledPossibleIDs.hasNext()) {
                finalPossibleItemsIDs.add(sampledPossibleIDs.next().longValue());
            }

            logger.debug("finalPossibleItemsIDs size:" + finalPossibleItemsIDs.size());

            return finalPossibleItemsIDs;
        }

        logger.debug("possibleItemsIDs size:" + possibleItemsIDs.size());
        return possibleItemsIDs;