CorrespondingIterable.java

package edu.berkeley.nlp.util;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.List;


/**
 * Iterates over a list of lists, returning the (a list containing) the correspondings element in each list at each iteration.
 * Assumes each list has the same length.
 * 
 * For example, 
 * @author adpauls
 *
 * @param <T>
 */
public class CorrespondingIterable<T> implements Iterable<List<T>>
{

	private Iterable<T>[] iterables;

	public CorrespondingIterable(Iterable<T>... iterables)
	{
		this.iterables = iterables;
	}

	private class ThisIterator implements Iterator<List<T>>
	{

		List<Iterator<T>> iters;

		public ThisIterator(Iterable<T>... iterables)
		{
			this.iters = new ArrayList<Iterator<T>>();
			for (Iterable<T> iterable : iterables)
			{
				this.iters.add(iterable.iterator());
			}
		}

		public boolean hasNext()
		{
			boolean allTrue = true;
			boolean someTrue = false;
			for (Iterator<T> iter : iters)
			{
				final boolean hasNext = iter.hasNext();
				allTrue &= hasNext;
				someTrue |= hasNext;
			}
			if (someTrue && !allTrue) throw new IllegalStateException(this.getClass().getName() +  " must have same length");
			return allTrue;
		}

		public List<T> next()
		{
			List<T> retVal = new ArrayList<T>();
			for (Iterator<T> iter : iters)
			{
				retVal.add(iter.next());
			}
			return retVal;
		}

		public void remove()
		{
			throw new UnsupportedOperationException();

		}
	}

	public Iterator<List<T>> iterator()
	{
		return new ThisIterator(this.iterables);
	}

}