/**
* Creates a new {@link PairSet} instance with only non-empty transactions and items.
*
* @return the compacted {@link PairSet} instance
*/
public PairSet<T, I> compacted() {
// trivial case
if (isEmpty()) return empty();
// compute the new universe
final Set<T> newAllTransactions = new LinkedHashSet<T>(involvedTransactions());
final Set<I> newAllItems = new LinkedHashSet<I>(involvedItems());
if (newAllTransactions.size() == allTransactions.size()
&& newAllItems.size() == allItems.size()) return clone();
// compute the union of pairs
PairSet<T, I> res = new PairSet<T, I>(matrix.empty(), newAllTransactions, newAllItems);
res.addAll(this);
return res;
}
/** {@inheritDoc} */
@Override
public PairSet<T, I> empty() {
return createFromIndices(matrix.empty());
}