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Multirator : an iterator that iterates over other iterators

See original GitHub issue

Original issue created by thierryler on 2014-04-15 at 11:29 AM

Hello,

Would you like to add a multirator in Guava ? A multirator (named choosen in 5 minutes) is an iterator that encapsulates other iterators. I gives the next element based on the next elements of the other iterators.

Here is what i looks like :

public interface Multirator<E> extends Iterator<E>, Iterable<E> { void add(final Iterator<E>… iterators); void add(final Iterable<E>… iterables); }

I wrote a first implementation who choose the next element as the smallest available :

public class SortedPeekingMultirator<E extends Comparable<E>> implements Multirator<E> {

private final List<PeekingIterator<E>> iterators = new ArrayList<>();

/* Constructors */

@SafeVarargs
public SortedPeekingMultirator(final Iterator<E>... iterators) {
    add(iterators);
}

@SafeVarargs
public SortedPeekingMultirator(final Iterable<E>... iterables) {
    add(iterables);
}

/* From Multirator.java */

@SuppressWarnings("unchecked")
@Override
public void add(final Iterator<E>... iterators) {
    for (final Iterator<E> iter : iterators) {
        final PeekingIterator<E> pi = Iterators.peekingIterator(iter);
        this.iterators.add(pi);
    }
}

@SuppressWarnings("unchecked")
@Override
public void add(final Iterable<E>... iterables) {
    for (final Iterable<E> iterable : iterables) {
        add(iterable.iterator());
    }
}

/* From Iterator.java */


@Override
public boolean hasNext() {

    for (final PeekingIterator<E> pi : iterators) {
        if (pi.hasNext()) {
            return true;
        }
    }

    return false;
}


@Override
public E next() {

    if (!hasNext()) {
        throw new NoSuchElementException();
    }

    E smallest = null;
    int pos = 0;
    int i = 0;

    for (final PeekingIterator<E> pi : iterators) {

        if (!pi.hasNext()) {
            i++;
            continue;
        }

        final E elt = pi.peek();

        if (smallest == null) {
            smallest = elt;
            pos = i++;
            continue;
        }

        if (smallest.compareTo(elt) > 0) {
            smallest = elt;
            pos = i++;
        }
    }

    // next
    iterators.get(pos).next();

    return smallest;
}

}

Please have a look at the attached files for complete source code and tests.

What do you think about this multirator ?

Thierry