Iterables.java

/*
 * @(#)Iterables.java 2012-8-1 下午10:00:00
 *
 * Copyright (c) 2011-2012 Makersoft.org all rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 *
 */
package org.makersoft.core.collect;

import java.util.Collections;
import java.util.Iterator;
import java.util.List;

/**
 * Helper methods related to {@link Iterable}s.
 * 
 * @author maxr@google.com (Max Ross)
 */
public abstract class Iterables {

	private Iterables() {
	}

	/**
	 * @return an Iterable that allows you to iterate over the contents of all
	 *         the iterables passed in.
	 */
	public static <T> Iterable<T> concat(Iterable<? extends T>... iterables) {
		List<T> list = Lists.newArrayList();
		for (Iterable<? extends T> subIterable : iterables) {
			for (T obj : subIterable) {
				list.add(obj);
			}
		}
		return list;
	}

	/**
	 * @return an Iterable that allows you to iterate over the contents of all
	 *         the iterables passed in.
	 */
	public static <T> Iterable<T> concat(
			Iterable<? extends Iterable<? extends T>> iterable) {
		List<T> list = Lists.newArrayList();
		for (Iterable<? extends T> subIterable : iterable) {
			for (T obj : subIterable) {
				list.add(obj);
			}
		}
		return list;
	}

	/**
	 * Provides a rotated view of a list. Differs from
	 * {@link Collections#rotate} only in that it leaves the underlying list
	 * unchanged. Note that this is a "live" view of the list that will change
	 * as the list changes. However, the behavior of an {@link Iterator}
	 * constructed from a rotated view of the list is undefined if the list is
	 * changed after the Iterator is constructed.
	 * 
	 * @param list
	 *            the list to return a rotated view of.
	 * @param distance
	 *            the distance to rotate the list. There are no constraints on
	 *            this value; it may be zero, negative, or greater than
	 *            {@code list.size()}.
	 * @return a rotated view of the given list
	 */
	public static <T> Iterable<T> rotate(final List<T> list, final int distance) {
		if(list == null){
			throw new RuntimeException("list can not be null.");
		}

		/*
		 * If no rotation is requested or there is nothing to rotate (list of
		 * size 0 or 1), just return the original list
		 */
		if (distance == 0 || list.size() <= 1) {
			return list;
		}

		return new Iterable<T>() {
			/**
			 * Determines the actual distance we need to rotate (distance
			 * provided might be larger than the size of the list or negative).
			 */
			private int calcActualDistance(int size) {
				// we already know distance and size are non-zero
				int actualDistance = distance % size;
				if (actualDistance < 0) {
					// distance must have been negative
					actualDistance += size;
				}
				return actualDistance;
			}

			public Iterator<T> iterator() {
				int size = list.size();
				int actualDistance = calcActualDistance(size);
				// optimization:
				// lists of a size that go into the distance evenly don't need
				// rotation
				if (actualDistance == 0) {
					return list.iterator();
				}

				@SuppressWarnings("unchecked")
				Iterable<T> rotated = concat(
						list.subList(actualDistance, size),
						list.subList(0, actualDistance));
				return rotated.iterator();
			}
		};
	}

}