final int windowSize = fessConfig.getRankFusionWindowSizeAsInteger();
        if (maxPageSize * 2 < windowSize) {
            logger.warn("rank.fusion.window_size is lower than paging.search.page.max.size. "
                    + "The window size should be 2x more than the page size. ({} * 2 <= {})", maxPageSize, windowSize);
            this.windowSize = 2 * maxPageSize;
        } else {

    }

    protected Map<String, Object> toLowerHyphen(final Map<String, Object> source) {
        return source.entrySet().stream()
                .collect(Collectors.toMap(e -> CaseFormat.UPPER_CAMEL.to(CaseFormat.LOWER_UNDERSCORE, e.getKey()), e -> {
                    final Object value = e.getValue();
                    if (value instanceof Map) {
                        @SuppressWarnings("unchecked")

        if (list.isEmpty()) {
            return null;
        }
        if (list.size() >= 2) {
            String msg = "The size of selected list is over 1: " + list.size();
            throw new FetchingOverSafetySizeException(msg, 1); // immediately caught by caller and translated
        }
        return list.get(0);
    }

    @Override