MapDB inverse secondary keys

InverseKeys.java

	/**
	 * Create a set of Tuple2<key in original map, value> - secondary keys, backwards.
	 */
	public static <K, V, V2> void inverseSecondaryKeys (MapWithModificationListener<K, V> map,
			final NavigableSet<Fun.Tuple2<K, V2>> set, final Function2<V2[], K, V> fun) {
		
		if (set.isEmpty()) {
			for (Map.Entry<K, V> e : map.entrySet()) {
				for (V2 val : fun.run(e.getKey(), e.getValue())) {
					set.add(new Tuple2<K, V2>(e.getKey(), val));
				}
			}
		}
		
		map.modificationListenerAdd(new MapListener<K, V> () {
			@Override
			public void update(K key, V oldVal, V newVal) {
				// clear this key
				set.subSet(new Tuple2(key, null), new Tuple2(key, Fun.HI)).clear();
				
				for (V2 val : fun.run(key, newVal)) {
					set.add(new Tuple2<K, V2>(key, val));
				}
			}
		});
	}