/*******************************************************************************
 * Breakout Cave Survey Visualizer
 *
 * Copyright (C) 2014 James Edwards
 *
 * jedwards8 at fastmail dot fm
 *
 * This program is free software; you can redistribute it and/or modify it under
 * the terms of the GNU General Public License as published by the Free Software
 * Foundation; either version 2 of the License, or (at your option) any later
 * version.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
 * details.
 *
 * You should have received a copy of the GNU General Public License along with
 * this program; if not, write to the Free Software Foundation, Inc., 51
 * Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 *******************************************************************************/
package org.andork.collect;

import java.util.Arrays;
import java.util.Iterator;

/**
 * An {@link Iterable} that concatenates the results from multiple
 * {@code Iterable}s together.
 *
 * @author andy.edwards
 *
 * @param <T>
 *            the element type.
 */
public class IterableChain<T> implements Iterable<T> {
	private static class ChainIterator<T> implements Iterator<T> {
		private final Iterator<? extends T> first;
		private final Iterator<? extends T> second;
		private Iterator<? extends T> last;

		private ChainIterator() {
			this(null, null);
		}

		private ChainIterator(Iterator<? extends T> first) {
			this(first, null);
		}

		private ChainIterator(Iterator<? extends T> first, Iterator<? extends T> second) {
			super();
			if (first == null && second != null) {
				first = second;
				second = null;
			}
			this.first = first;
			this.second = second;
		}

		@Override
		public boolean hasNext() {
			return first != null && first.hasNext() || second != null && second.hasNext();
		}

		@Override
		public T next() {
			last = first.hasNext() || second == null ? first : second;
			return last.next();
		}

		@Override
		public void remove() {
			last.remove();
		}
	}

	public static <T> Iterable<T> join(Iterable<? extends Iterable<? extends T>> chain) {
		Iterator<? extends Iterable<? extends T>> i = chain.iterator();
		if (!i.hasNext()) {
			return new IterableChain<T>();
		}
		Iterable<? extends T> first = i.next();
		if (!i.hasNext()) {
			return new IterableChain<T>(first);
		}
		Iterable<? extends T> second = i.next();

		IterableChain<T> result = new IterableChain<T>(first, second);
		while (i.hasNext()) {
			result = result.add(i.next());
		}
		return result;
	}

	public static <T> Iterable<T> join(Iterable<? extends T>... chain) {
		return join(Arrays.asList(chain));
	}

	private final Iterable<? extends T> first;

	private final Iterable<? extends T> second;

	public IterableChain() {
		this(null, null);
	}

	public IterableChain(Iterable<? extends T> first) {
		this(first, null);
	}

	public IterableChain(Iterable<? extends T> first, Iterable<? extends T> second) {
		super();
		if (first == null && second != null) {
			first = second;
			second = null;
		}
		this.first = first;
		this.second = second;
	}

	public IterableChain<T> add(Iterable<? extends T> next) {
		return new IterableChain<T>(this, next);
	}

	@Override
	public Iterator<T> iterator() {
		return new ChainIterator<T>(first == null ? null : first.iterator(), second == null ? null : second.iterator());
	}
}