/**
     * Count the number of municipalities with at least a mountain hut per each
     * province.
     *
     * @return a map with the province as key and the number of municipalities as
     *         value
     */
    public Map<String, Long> countMunicipalitiesPerProvince() {
        return municipalities.values().stream().collect(groupingBy(s -> s.getProvince(), counting()));
    }

    /**
     * Count the number of mountain huts per each municipality within each province.
     *
     * @return a map with the province as key and, as value, a map with the
     *         municipality as key and the number of mountain huts as value
     */
    public Map<String, Map<String, Long>> countMountainHutsPerMunicipalityPerProvince() {
        return mountainHuts.values().stream().collect(groupingBy((s) -> s.getMunicipality().getProvince(),
                                                        groupingBy((b) -> b.getMunicipality().getName().toString(), counting())));
    }

    /**
     * Count the number of mountain huts per altitude range. If the altitude of the
     * mountain hut is not available, use the altitude of its municipality.
     *
     * @return a map with the altitude range as key and the number of mountain huts
     *         as value
     */
    public Map<String, Long> countMountainHutsPerAltitudeRange() {
        return mountainHuts.values()
                .stream()
                .collect(groupingBy((s) ->{
                    if(s.getAltitude().isPresent()) return this.getAltitudeRange(s.getAltitude().get());
                    return this.getAltitudeRange(s.getMunicipality().getAltitude());
                }, counting()));
    }

    /**
     * Compute the total number of beds available in the mountain huts per each
     * province.
     *
     * @return a map with the province as key and the total number of beds as value
     */
    public Map<String, Integer> totalBedsNumberPerProvince() {
        return mountainHuts.values().stream().collect(groupingBy((s) -> s.getMunicipality().getProvince(), summingInt((s) -> s.getBedsNumber())));
    }

    /**
     * Compute the maximum number of beds available in a single mountain hut per
     * altitude range. If the altitude of the mountain hut is not available, use the
     * altitude of its municipality.
     *
     * @return a map with the altitude range as key and the maximum number of beds
     *         as value
     */
    public Map<String, Optional<Integer>> maximumBedsNumberPerAltitudeRange() {
        return mountainHuts.values()
                .stream()
                .collect(groupingBy((s) -> {
                                            if(s.getAltitude().isPresent()) return this.getAltitudeRange(s.getAltitude().get());
                                            return this.getAltitudeRange(s.getMunicipality().getAltitude());
                                           }, mapping(s -> s.getBedsNumber(), maxBy(Integer::compareTo))));
    }

    /**
     * Compute the municipality names per number of mountain huts in a municipality.
     * The lists of municipality names must be in alphabetical order.
     *
     * @return a map with the number of mountain huts in a municipality as key and a
     *         list of municipality names as value
     */
    public Map<Long, List<String>> municipalityNamesPerCountOfMountainHuts() {
        return mountainHuts.values().stream().collect(groupingBy(s -> s.getMunicipality().getName(), counting()))
                .entrySet().stream().sorted((e1, e2) -> e1.getKey().compareTo(e2.getKey())).collect(groupingBy(e -> e.getValue(), mapping(e -> e.getKey(), toList())));
    }