/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import static com.google.common.base.Predicates.equalTo;
import static com.google.common.base.Predicates.in;
import static com.google.common.base.Predicates.instanceOf;
import static com.google.common.base.Predicates.not;
import static com.google.common.collect.CollectPreconditions.checkRemove;

import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.util.PriorityQueue;
import java.util.Queue;

import javax.annotation.Nullable;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Function;
import com.google.common.base.Objects;
import com.google.common.base.Optional;
import com.google.common.base.Preconditions;
import com.google.common.base.Predicate;

/**
 * This class contains static utility methods that operate on or return objects
 * of type {@link Iterator}. Except as noted, each method has a corresponding
 * {@link Iterable}-based method in the {@link Iterables} class.
 *
 * <p>
 * <i>Performance notes:</i> Unless otherwise noted, all of the iterators
 * produced in this class are <i>lazy</i>, which means that they only advance
 * the backing iteration when absolutely necessary.
 *
 * <p>
 * See the Guava User Guide section on <a href=
 * "http://code.google.com/p/guava-libraries/wiki/CollectionUtilitiesExplained#Iterables">
 * {@code Iterators}</a>.
 *
 * @author Kevin Bourrillion
 * @author Jared Levy
 * @since 2.0 (imported from Google Collections Library)
 */
@GwtCompatible(emulated = true)
public final class Iterators {
	private Iterators() {
	}

	static final UnmodifiableListIterator<Object> EMPTY_LIST_ITERATOR = new UnmodifiableListIterator<Object>() {
		@Override
		public boolean hasNext() {
			return false;
		}

		@Override
		public Object next() {
			throw new NoSuchElementException();
		}

		@Override
		public boolean hasPrevious() {
			return false;
		}

		@Override
		public Object previous() {
			throw new NoSuchElementException();
		}

		@Override
		public int nextIndex() {
			return 0;
		}

		@Override
		public int previousIndex() {
			return -1;
		}
	};

	/**
	 * Returns the empty iterator.
	 *
	 * <p>
	 * The {@link Iterable} equivalent of this method is {@link ImmutableSet#of()}.
	 */
	public static <T> UnmodifiableIterator<T> emptyIterator() {
		return emptyListIterator();
	}

	/**
	 * Returns the empty iterator.
	 *
	 * <p>
	 * The {@link Iterable} equivalent of this method is {@link ImmutableSet#of()}.
	 */
	// Casting to any type is safe since there are no actual elements.
	@SuppressWarnings("unchecked")
	static <T> UnmodifiableListIterator<T> emptyListIterator() {
		return (UnmodifiableListIterator<T>) EMPTY_LIST_ITERATOR;
	}

	private static final Iterator<Object> EMPTY_MODIFIABLE_ITERATOR = new Iterator<Object>() {
		@Override
		public boolean hasNext() {
			return false;
		}

		@Override
		public Object next() {
			throw new NoSuchElementException();
		}

		@Override
		public void remove() {
			checkRemove(false);
		}
	};

	/**
	 * Returns the empty {@code Iterator} that throws {@link IllegalStateException}
	 * instead of {@link UnsupportedOperationException} on a call to
	 * {@link Iterator#remove()}.
	 */
	// Casting to any type is safe since there are no actual elements.
	@SuppressWarnings("unchecked")
	static <T> Iterator<T> emptyModifiableIterator() {
		return (Iterator<T>) EMPTY_MODIFIABLE_ITERATOR;
	}

	/** Returns an unmodifiable view of {@code iterator}. */
	public static <T> UnmodifiableIterator<T> unmodifiableIterator(final Iterator<T> iterator) {
		checkNotNull(iterator);
		if (iterator instanceof UnmodifiableIterator) {
			return (UnmodifiableIterator<T>) iterator;
		}
		return new UnmodifiableIterator<T>() {
			@Override
			public boolean hasNext() {
				return iterator.hasNext();
			}

			@Override
			public T next() {
				return iterator.next();
			}
		};
	}

	/**
	 * Simply returns its argument.
	 *
	 * @deprecated no need to use this
	 * @since 10.0
	 */
	@Deprecated
	public static <T> UnmodifiableIterator<T> unmodifiableIterator(UnmodifiableIterator<T> iterator) {
		return checkNotNull(iterator);
	}

	/**
	 * Returns the number of elements remaining in {@code iterator}. The iterator
	 * will be left exhausted: its {@code hasNext()} method will return
	 * {@code false}.
	 */
	public static int size(Iterator<?> iterator) {
		int count = 0;
		while (iterator.hasNext()) {
			iterator.next();
			count++;
		}
		return count;
	}

	/**
	 * Returns {@code true} if {@code iterator} contains {@code element}.
	 */
	public static boolean contains(Iterator<?> iterator, @Nullable Object element) {
		return any(iterator, equalTo(element));
	}

	/**
	 * Traverses an iterator and removes every element that belongs to the provided
	 * collection. The iterator will be left exhausted: its {@code hasNext()} method
	 * will return {@code false}.
	 *
	 * @param removeFrom       the iterator to (potentially) remove elements from
	 * @param elementsToRemove the elements to remove
	 * @return {@code true} if any element was removed from {@code iterator}
	 */
	public static boolean removeAll(Iterator<?> removeFrom, Collection<?> elementsToRemove) {
		return removeIf(removeFrom, in(elementsToRemove));
	}

	/**
	 * Removes every element that satisfies the provided predicate from the
	 * iterator. The iterator will be left exhausted: its {@code hasNext()} method
	 * will return {@code false}.
	 *
	 * @param removeFrom the iterator to (potentially) remove elements from
	 * @param predicate  a predicate that determines whether an element should be
	 *                   removed
	 * @return {@code true} if any elements were removed from the iterator
	 * @since 2.0
	 */
	public static <T> boolean removeIf(Iterator<T> removeFrom, Predicate<? super T> predicate) {
		checkNotNull(predicate);
		boolean modified = false;
		while (removeFrom.hasNext()) {
			if (predicate.apply(removeFrom.next())) {
				removeFrom.remove();
				modified = true;
			}
		}
		return modified;
	}

	/**
	 * Traverses an iterator and removes every element that does not belong to the
	 * provided collection. The iterator will be left exhausted: its
	 * {@code hasNext()} method will return {@code false}.
	 *
	 * @param removeFrom       the iterator to (potentially) remove elements from
	 * @param elementsToRetain the elements to retain
	 * @return {@code true} if any element was removed from {@code iterator}
	 */
	public static boolean retainAll(Iterator<?> removeFrom, Collection<?> elementsToRetain) {
		return removeIf(removeFrom, not(in(elementsToRetain)));
	}

	/**
	 * Determines whether two iterators contain equal elements in the same order.
	 * More specifically, this method returns {@code true} if {@code iterator1} and
	 * {@code iterator2} contain the same number of elements and every element of
	 * {@code iterator1} is equal to the corresponding element of {@code iterator2}.
	 *
	 * <p>
	 * Note that this will modify the supplied iterators, since they will have been
	 * advanced some number of elements forward.
	 */
	public static boolean elementsEqual(Iterator<?> iterator1, Iterator<?> iterator2) {
		while (iterator1.hasNext()) {
			if (!iterator2.hasNext()) {
				return false;
			}
			Object o1 = iterator1.next();
			Object o2 = iterator2.next();
			if (!Objects.equal(o1, o2)) {
				return false;
			}
		}
		return !iterator2.hasNext();
	}

	/**
	 * Returns a string representation of {@code iterator}, with the format
	 * {@code [e1, e2, ..., en]}. The iterator will be left exhausted: its
	 * {@code hasNext()} method will return {@code false}.
	 */
	public static String toString(Iterator<?> iterator) {
		return Collections2.STANDARD_JOINER.appendTo(new StringBuilder().append('['), iterator).append(']').toString();
	}

	/**
	 * Returns the single element contained in {@code iterator}.
	 *
	 * @throws NoSuchElementException   if the iterator is empty
	 * @throws IllegalArgumentException if the iterator contains multiple elements.
	 *                                  The state of the iterator is unspecified.
	 */
	public static <T> T getOnlyElement(Iterator<T> iterator) {
		T first = iterator.next();
		if (!iterator.hasNext()) {
			return first;
		}

		StringBuilder sb = new StringBuilder();
		sb.append("expected one element but was: <" + first);
		for (int i = 0; i < 4 && iterator.hasNext(); i++) {
			sb.append(", " + iterator.next());
		}
		if (iterator.hasNext()) {
			sb.append(", ...");
		}
		sb.append('>');

		throw new IllegalArgumentException(sb.toString());
	}

	/**
	 * Returns the single element contained in {@code iterator}, or {@code
	 * defaultValue} if the iterator is empty.
	 *
	 * @throws IllegalArgumentException if the iterator contains multiple elements.
	 *                                  The state of the iterator is unspecified.
	 */
	@Nullable
	public static <T> T getOnlyElement(Iterator<? extends T> iterator, @Nullable T defaultValue) {
		return iterator.hasNext() ? getOnlyElement(iterator) : defaultValue;
	}

	/**
	 * Copies an iterator's elements into an array. The iterator will be left
	 * exhausted: its {@code hasNext()} method will return {@code false}.
	 *
	 * @param iterator the iterator to copy
	 * @param type     the type of the elements
	 * @return a newly-allocated array into which all the elements of the iterator
	 *         have been copied
	 */
	@GwtIncompatible("Array.newInstance(Class, int)")
	public static <T> T[] toArray(Iterator<? extends T> iterator, Class<T> type) {
		List<T> list = Lists.newArrayList(iterator);
		return Iterables.toArray(list, type);
	}

	/**
	 * Adds all elements in {@code iterator} to {@code collection}. The iterator
	 * will be left exhausted: its {@code hasNext()} method will return
	 * {@code false}.
	 *
	 * @return {@code true} if {@code collection} was modified as a result of this
	 *         operation
	 */
	public static <T> boolean addAll(Collection<T> addTo, Iterator<? extends T> iterator) {
		checkNotNull(addTo);
		checkNotNull(iterator);
		boolean wasModified = false;
		while (iterator.hasNext()) {
			wasModified |= addTo.add(iterator.next());
		}
		return wasModified;
	}

	/**
	 * Returns the number of elements in the specified iterator that equal the
	 * specified object. The iterator will be left exhausted: its {@code hasNext()}
	 * method will return {@code false}.
	 *
	 * @see Collections#frequency
	 */
	public static int frequency(Iterator<?> iterator, @Nullable Object element) {
		return size(filter(iterator, equalTo(element)));
	}

	/**
	 * Returns an iterator that cycles indefinitely over the elements of {@code
	 * iterable}.
	 *
	 * <p>
	 * The returned iterator supports {@code remove()} if the provided iterator
	 * does. After {@code remove()} is called, subsequent cycles omit the removed
	 * element, which is no longer in {@code iterable}. The iterator's
	 * {@code hasNext()} method returns {@code true} until {@code iterable} is
	 * empty.
	 *
	 * <p>
	 * <b>Warning:</b> Typical uses of the resulting iterator may produce an
	 * infinite loop. You should use an explicit {@code break} or be certain that
	 * you will eventually remove all the elements.
	 */
	public static <T> Iterator<T> cycle(final Iterable<T> iterable) {
		checkNotNull(iterable);
		return new Iterator<T>() {
			Iterator<T> iterator = emptyIterator();
			Iterator<T> removeFrom;

			@Override
			public boolean hasNext() {
				if (!iterator.hasNext()) {
					iterator = iterable.iterator();
				}
				return iterator.hasNext();
			}

			@Override
			public T next() {
				if (!hasNext()) {
					throw new NoSuchElementException();
				}
				removeFrom = iterator;
				return iterator.next();
			}

			@Override
			public void remove() {
				checkRemove(removeFrom != null);
				removeFrom.remove();
				removeFrom = null;
			}
		};
	}

	/**
	 * Returns an iterator that cycles indefinitely over the provided elements.
	 *
	 * <p>
	 * The returned iterator supports {@code remove()}. After {@code remove()} is
	 * called, subsequent cycles omit the removed element, but {@code elements} does
	 * not change. The iterator's {@code hasNext()} method returns {@code true}
	 * until all of the original elements have been removed.
	 *
	 * <p>
	 * <b>Warning:</b> Typical uses of the resulting iterator may produce an
	 * infinite loop. You should use an explicit {@code break} or be certain that
	 * you will eventually remove all the elements.
	 */
	public static <T> Iterator<T> cycle(T... elements) {
		return cycle(Lists.newArrayList(elements));
	}

	/**
	 * Combines two iterators into a single iterator. The returned iterator iterates
	 * across the elements in {@code a}, followed by the elements in {@code b}. The
	 * source iterators are not polled until necessary.
	 *
	 * <p>
	 * The returned iterator supports {@code remove()} when the corresponding input
	 * iterator supports it.
	 *
	 * <p>
	 * <b>Note:</b> the current implementation is not suitable for nested
	 * concatenated iterators, i.e. the following should be avoided when in a loop:
	 * {@code iterator = Iterators.concat(iterator, suffix);}, since iteration over
	 * the resulting iterator has a cubic complexity to the depth of the nesting.
	 */
	public static <T> Iterator<T> concat(Iterator<? extends T> a, Iterator<? extends T> b) {
		return concat(ImmutableList.of(a, b).iterator());
	}

	/**
	 * Combines three iterators into a single iterator. The returned iterator
	 * iterates across the elements in {@code a}, followed by the elements in
	 * {@code b}, followed by the elements in {@code c}. The source iterators are
	 * not polled until necessary.
	 *
	 * <p>
	 * The returned iterator supports {@code remove()} when the corresponding input
	 * iterator supports it.
	 *
	 * <p>
	 * <b>Note:</b> the current implementation is not suitable for nested
	 * concatenated iterators, i.e. the following should be avoided when in a loop:
	 * {@code iterator = Iterators.concat(iterator, suffix);}, since iteration over
	 * the resulting iterator has a cubic complexity to the depth of the nesting.
	 */
	public static <T> Iterator<T> concat(Iterator<? extends T> a, Iterator<? extends T> b, Iterator<? extends T> c) {
		return concat(ImmutableList.of(a, b, c).iterator());
	}

	/**
	 * Combines four iterators into a single iterator. The returned iterator
	 * iterates across the elements in {@code a}, followed by the elements in
	 * {@code b}, followed by the elements in {@code c}, followed by the elements in
	 * {@code d}. The source iterators are not polled until necessary.
	 *
	 * <p>
	 * The returned iterator supports {@code remove()} when the corresponding input
	 * iterator supports it.
	 *
	 * <p>
	 * <b>Note:</b> the current implementation is not suitable for nested
	 * concatenated iterators, i.e. the following should be avoided when in a loop:
	 * {@code iterator = Iterators.concat(iterator, suffix);}, since iteration over
	 * the resulting iterator has a cubic complexity to the depth of the nesting.
	 */
	public static <T> Iterator<T> concat(Iterator<? extends T> a, Iterator<? extends T> b, Iterator<? extends T> c,
			Iterator<? extends T> d) {
		return concat(ImmutableList.of(a, b, c, d).iterator());
	}

	/**
	 * Combines multiple iterators into a single iterator. The returned iterator
	 * iterates across the elements of each iterator in {@code inputs}. The input
	 * iterators are not polled until necessary.
	 *
	 * <p>
	 * The returned iterator supports {@code remove()} when the corresponding input
	 * iterator supports it.
	 *
	 * <p>
	 * <b>Note:</b> the current implementation is not suitable for nested
	 * concatenated iterators, i.e. the following should be avoided when in a loop:
	 * {@code iterator = Iterators.concat(iterator, suffix);}, since iteration over
	 * the resulting iterator has a cubic complexity to the depth of the nesting.
	 *
	 * @throws NullPointerException if any of the provided iterators is null
	 */
	public static <T> Iterator<T> concat(Iterator<? extends T>... inputs) {
		return concat(ImmutableList.copyOf(inputs).iterator());
	}

	/**
	 * Combines multiple iterators into a single iterator. The returned iterator
	 * iterates across the elements of each iterator in {@code inputs}. The input
	 * iterators are not polled until necessary.
	 *
	 * <p>
	 * The returned iterator supports {@code remove()} when the corresponding input
	 * iterator supports it. The methods of the returned iterator may throw
	 * {@code NullPointerException} if any of the input iterators is null.
	 *
	 * <p>
	 * <b>Note:</b> the current implementation is not suitable for nested
	 * concatenated iterators, i.e. the following should be avoided when in a loop:
	 * {@code iterator = Iterators.concat(iterator, suffix);}, since iteration over
	 * the resulting iterator has a cubic complexity to the depth of the nesting.
	 */
	public static <T> Iterator<T> concat(final Iterator<? extends Iterator<? extends T>> inputs) {
		checkNotNull(inputs);
		return new Iterator<T>() {
			Iterator<? extends T> current = emptyIterator();
			Iterator<? extends T> removeFrom;

			@Override
			public boolean hasNext() {
				// http://code.google.com/p/google-collections/issues/detail?id=151
				// current.hasNext() might be relatively expensive, worth minimizing.
				boolean currentHasNext;
				// checkNotNull eager for GWT
				// note: it must be here & not where 'current' is assigned,
				// because otherwise we'll have called inputs.next() before throwing
				// the first NPE, and the next time around we'll call inputs.next()
				// again, incorrectly moving beyond the error.
				while (!(currentHasNext = checkNotNull(current).hasNext()) && inputs.hasNext()) {
					current = inputs.next();
				}
				return currentHasNext;
			}

			@Override
			public T next() {
				if (!hasNext()) {
					throw new NoSuchElementException();
				}
				removeFrom = current;
				return current.next();
			}

			@Override
			public void remove() {
				checkRemove(removeFrom != null);
				removeFrom.remove();
				removeFrom = null;
			}
		};
	}

	/**
	 * Divides an iterator into unmodifiable sublists of the given size (the final
	 * list may be smaller). For example, partitioning an iterator containing
	 * {@code [a, b, c, d, e]} with a partition size of 3 yields {@code
	 * [[a, b, c], [d, e]]} -- an outer iterator containing two inner lists of three
	 * and two elements, all in the original order.
	 *
	 * <p>
	 * The returned lists implement {@link net.lax1dude.eaglercraft.v1_8.RandomAccess}.
	 *
	 * @param iterator the iterator to return a partitioned view of
	 * @param size     the desired size of each partition (the last may be smaller)
	 * @return an iterator of immutable lists containing the elements of {@code
	 *     iterator} divided into partitions
	 * @throws IllegalArgumentException if {@code size} is nonpositive
	 */
	public static <T> UnmodifiableIterator<List<T>> partition(Iterator<T> iterator, int size) {
		return partitionImpl(iterator, size, false);
	}

	/**
	 * Divides an iterator into unmodifiable sublists of the given size, padding the
	 * final iterator with null values if necessary. For example, partitioning an
	 * iterator containing {@code [a, b, c, d, e]} with a partition size of 3 yields
	 * {@code [[a, b, c], [d, e, null]]} -- an outer iterator containing two inner
	 * lists of three elements each, all in the original order.
	 *
	 * <p>
	 * The returned lists implement {@link net.lax1dude.eaglercraft.v1_8.RandomAccess}.
	 *
	 * @param iterator the iterator to return a partitioned view of
	 * @param size     the desired size of each partition
	 * @return an iterator of immutable lists containing the elements of {@code
	 *     iterator} divided into partitions (the final iterable may have trailing
	 *         null elements)
	 * @throws IllegalArgumentException if {@code size} is nonpositive
	 */
	public static <T> UnmodifiableIterator<List<T>> paddedPartition(Iterator<T> iterator, int size) {
		return partitionImpl(iterator, size, true);
	}

	private static <T> UnmodifiableIterator<List<T>> partitionImpl(final Iterator<T> iterator, final int size,
			final boolean pad) {
		checkNotNull(iterator);
		checkArgument(size > 0);
		return new UnmodifiableIterator<List<T>>() {
			@Override
			public boolean hasNext() {
				return iterator.hasNext();
			}

			@Override
			public List<T> next() {
				if (!hasNext()) {
					throw new NoSuchElementException();
				}
				Object[] array = new Object[size];
				int count = 0;
				for (; count < size && iterator.hasNext(); count++) {
					array[count] = iterator.next();
				}
				for (int i = count; i < size; i++) {
					array[i] = null; // for GWT
				}

				@SuppressWarnings("unchecked") // we only put Ts in it
				List<T> list = Collections.unmodifiableList((List<T>) Arrays.asList(array));
				return (pad || count == size) ? list : list.subList(0, count);
			}
		};
	}

	/**
	 * Returns the elements of {@code unfiltered} that satisfy a predicate.
	 */
	public static <T> UnmodifiableIterator<T> filter(final Iterator<T> unfiltered,
			final Predicate<? super T> predicate) {
		checkNotNull(unfiltered);
		checkNotNull(predicate);
		return new AbstractIterator<T>() {
			@Override
			protected T computeNext() {
				while (unfiltered.hasNext()) {
					T element = unfiltered.next();
					if (predicate.apply(element)) {
						return element;
					}
				}
				return endOfData();
			}
		};
	}

	/**
	 * Returns all instances of class {@code type} in {@code unfiltered}. The
	 * returned iterator has elements whose class is {@code type} or a subclass of
	 * {@code type}.
	 *
	 * @param unfiltered an iterator containing objects of any type
	 * @param type       the type of elements desired
	 * @return an unmodifiable iterator containing all elements of the original
	 *         iterator that were of the requested type
	 */
	@SuppressWarnings("unchecked") // can cast to <T> because non-Ts are removed
	@GwtIncompatible("Class.isInstance")
	public static <T> UnmodifiableIterator<T> filter(Iterator<?> unfiltered, Class<T> type) {
		return (UnmodifiableIterator<T>) filter(unfiltered, instanceOf(type));
	}

	/**
	 * Returns {@code true} if one or more elements returned by {@code iterator}
	 * satisfy the given predicate.
	 */
	public static <T> boolean any(Iterator<T> iterator, Predicate<? super T> predicate) {
		return indexOf(iterator, predicate) != -1;
	}

	/**
	 * Returns {@code true} if every element returned by {@code iterator} satisfies
	 * the given predicate. If {@code iterator} is empty, {@code true} is returned.
	 */
	public static <T> boolean all(Iterator<T> iterator, Predicate<? super T> predicate) {
		checkNotNull(predicate);
		while (iterator.hasNext()) {
			T element = iterator.next();
			if (!predicate.apply(element)) {
				return false;
			}
		}
		return true;
	}

	/**
	 * Returns the first element in {@code iterator} that satisfies the given
	 * predicate; use this method only when such an element is known to exist. If no
	 * such element is found, the iterator will be left exhausted: its {@code
	 * hasNext()} method will return {@code false}. If it is possible that <i>no</i>
	 * element will match, use {@link #tryFind} or
	 * {@link #find(Iterator, Predicate, Object)} instead.
	 *
	 * @throws NoSuchElementException if no element in {@code iterator} matches the
	 *                                given predicate
	 */
	public static <T> T find(Iterator<T> iterator, Predicate<? super T> predicate) {
		return filter(iterator, predicate).next();
	}

	/**
	 * Returns the first element in {@code iterator} that satisfies the given
	 * predicate. If no such element is found, {@code defaultValue} will be returned
	 * from this method and the iterator will be left exhausted: its
	 * {@code hasNext()} method will return {@code false}. Note that this can
	 * usually be handled more naturally using {@code
	 * tryFind(iterator, predicate).or(defaultValue)}.
	 *
	 * @since 7.0
	 */
	@Nullable
	public static <T> T find(Iterator<? extends T> iterator, Predicate<? super T> predicate, @Nullable T defaultValue) {
		return getNext(filter(iterator, predicate), defaultValue);
	}

	/**
	 * Returns an {@link Optional} containing the first element in {@code
	 * iterator} that satisfies the given predicate, if such an element exists. If
	 * no such element is found, an empty {@link Optional} will be returned from
	 * this method and the iterator will be left exhausted: its {@code
	 * hasNext()} method will return {@code false}.
	 *
	 * <p>
	 * <b>Warning:</b> avoid using a {@code predicate} that matches {@code
	 * null}. If {@code null} is matched in {@code iterator}, a NullPointerException
	 * will be thrown.
	 *
	 * @since 11.0
	 */
	public static <T> Optional<T> tryFind(Iterator<T> iterator, Predicate<? super T> predicate) {
		UnmodifiableIterator<T> filteredIterator = filter(iterator, predicate);
		return filteredIterator.hasNext() ? Optional.of(filteredIterator.next()) : Optional.<T>absent();
	}

	/**
	 * Returns the index in {@code iterator} of the first element that satisfies the
	 * provided {@code predicate}, or {@code -1} if the Iterator has no such
	 * elements.
	 *
	 * <p>
	 * More formally, returns the lowest index {@code i} such that
	 * {@code predicate.apply(Iterators.get(iterator, i))} returns {@code true}, or
	 * {@code -1} if there is no such index.
	 *
	 * <p>
	 * If -1 is returned, the iterator will be left exhausted: its {@code hasNext()}
	 * method will return {@code false}. Otherwise, the iterator will be set to the
	 * element which satisfies the {@code predicate}.
	 *
	 * @since 2.0
	 */
	public static <T> int indexOf(Iterator<T> iterator, Predicate<? super T> predicate) {
		checkNotNull(predicate, "predicate");
		for (int i = 0; iterator.hasNext(); i++) {
			T current = iterator.next();
			if (predicate.apply(current)) {
				return i;
			}
		}
		return -1;
	}

	/**
	 * Returns an iterator that applies {@code function} to each element of {@code
	 * fromIterator}.
	 *
	 * <p>
	 * The returned iterator supports {@code remove()} if the provided iterator
	 * does. After a successful {@code remove()} call, {@code fromIterator} no
	 * longer contains the corresponding element.
	 */
	public static <F, T> Iterator<T> transform(final Iterator<F> fromIterator,
			final Function<? super F, ? extends T> function) {
		checkNotNull(function);
		return new TransformedIterator<F, T>(fromIterator) {
			@Override
			T transform(F from) {
				return function.apply(from);
			}
		};
	}

	/**
	 * Advances {@code iterator} {@code position + 1} times, returning the element
	 * at the {@code position}th position.
	 *
	 * @param position position of the element to return
	 * @return the element at the specified position in {@code iterator}
	 * @throws IndexOutOfBoundsException if {@code position} is negative or greater
	 *                                   than or equal to the number of elements
	 *                                   remaining in {@code iterator}
	 */
	public static <T> T get(Iterator<T> iterator, int position) {
		checkNonnegative(position);
		int skipped = advance(iterator, position);
		if (!iterator.hasNext()) {
			throw new IndexOutOfBoundsException("position (" + position
					+ ") must be less than the number of elements that remained (" + skipped + ")");
		}
		return iterator.next();
	}

	static void checkNonnegative(int position) {
		if (position < 0) {
			throw new IndexOutOfBoundsException("position (" + position + ") must not be negative");
		}
	}

	/**
	 * Advances {@code iterator} {@code position + 1} times, returning the element
	 * at the {@code position}th position or {@code defaultValue} otherwise.
	 *
	 * @param position     position of the element to return
	 * @param defaultValue the default value to return if the iterator is empty or
	 *                     if {@code position} is greater than the number of
	 *                     elements remaining in {@code iterator}
	 * @return the element at the specified position in {@code iterator} or
	 *         {@code defaultValue} if {@code iterator} produces fewer than
	 *         {@code position + 1} elements.
	 * @throws IndexOutOfBoundsException if {@code position} is negative
	 * @since 4.0
	 */
	@Nullable
	public static <T> T get(Iterator<? extends T> iterator, int position, @Nullable T defaultValue) {
		checkNonnegative(position);
		advance(iterator, position);
		return getNext(iterator, defaultValue);
	}

	/**
	 * Returns the next element in {@code iterator} or {@code defaultValue} if the
	 * iterator is empty. The {@link Iterables} analog to this method is
	 * {@link Iterables#getFirst}.
	 *
	 * @param defaultValue the default value to return if the iterator is empty
	 * @return the next element of {@code iterator} or the default value
	 * @since 7.0
	 */
	@Nullable
	public static <T> T getNext(Iterator<? extends T> iterator, @Nullable T defaultValue) {
		return iterator.hasNext() ? iterator.next() : defaultValue;
	}

	/**
	 * Advances {@code iterator} to the end, returning the last element.
	 *
	 * @return the last element of {@code iterator}
	 * @throws NoSuchElementException if the iterator is empty
	 */
	public static <T> T getLast(Iterator<T> iterator) {
		while (true) {
			T current = iterator.next();
			if (!iterator.hasNext()) {
				return current;
			}
		}
	}

	/**
	 * Advances {@code iterator} to the end, returning the last element or
	 * {@code defaultValue} if the iterator is empty.
	 *
	 * @param defaultValue the default value to return if the iterator is empty
	 * @return the last element of {@code iterator}
	 * @since 3.0
	 */
	@Nullable
	public static <T> T getLast(Iterator<? extends T> iterator, @Nullable T defaultValue) {
		return iterator.hasNext() ? getLast(iterator) : defaultValue;
	}

	/**
	 * Calls {@code next()} on {@code iterator}, either {@code numberToAdvance}
	 * times or until {@code hasNext()} returns {@code false}, whichever comes
	 * first.
	 *
	 * @return the number of elements the iterator was advanced
	 * @since 13.0 (since 3.0 as {@code Iterators.skip})
	 */
	public static int advance(Iterator<?> iterator, int numberToAdvance) {
		checkNotNull(iterator);
		checkArgument(numberToAdvance >= 0, "numberToAdvance must be nonnegative");

		int i;
		for (i = 0; i < numberToAdvance && iterator.hasNext(); i++) {
			iterator.next();
		}
		return i;
	}

	/**
	 * Creates an iterator returning the first {@code limitSize} elements of the
	 * given iterator. If the original iterator does not contain that many elements,
	 * the returned iterator will have the same behavior as the original iterator.
	 * The returned iterator supports {@code remove()} if the original iterator
	 * does.
	 *
	 * @param iterator  the iterator to limit
	 * @param limitSize the maximum number of elements in the returned iterator
	 * @throws IllegalArgumentException if {@code limitSize} is negative
	 * @since 3.0
	 */
	public static <T> Iterator<T> limit(final Iterator<T> iterator, final int limitSize) {
		checkNotNull(iterator);
		checkArgument(limitSize >= 0, "limit is negative");
		return new Iterator<T>() {
			private int count;

			@Override
			public boolean hasNext() {
				return count < limitSize && iterator.hasNext();
			}

			@Override
			public T next() {
				if (!hasNext()) {
					throw new NoSuchElementException();
				}
				count++;
				return iterator.next();
			}

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

	/**
	 * Returns a view of the supplied {@code iterator} that removes each element
	 * from the supplied {@code iterator} as it is returned.
	 *
	 * <p>
	 * The provided iterator must support {@link Iterator#remove()} or else the
	 * returned iterator will fail on the first call to {@code
	 * next}.
	 *
	 * @param iterator the iterator to remove and return elements from
	 * @return an iterator that removes and returns elements from the supplied
	 *         iterator
	 * @since 2.0
	 */
	public static <T> Iterator<T> consumingIterator(final Iterator<T> iterator) {
		checkNotNull(iterator);
		return new UnmodifiableIterator<T>() {
			@Override
			public boolean hasNext() {
				return iterator.hasNext();
			}

			@Override
			public T next() {
				T next = iterator.next();
				iterator.remove();
				return next;
			}

			@Override
			public String toString() {
				return "Iterators.consumingIterator(...)";
			}
		};
	}

	/**
	 * Deletes and returns the next value from the iterator, or returns {@code null}
	 * if there is no such value.
	 */
	@Nullable
	static <T> T pollNext(Iterator<T> iterator) {
		if (iterator.hasNext()) {
			T result = iterator.next();
			iterator.remove();
			return result;
		} else {
			return null;
		}
	}

	// Methods only in Iterators, not in Iterables

	/**
	 * Clears the iterator using its remove method.
	 */
	static void clear(Iterator<?> iterator) {
		checkNotNull(iterator);
		while (iterator.hasNext()) {
			iterator.next();
			iterator.remove();
		}
	}

	/**
	 * Returns an iterator containing the elements of {@code array} in order. The
	 * returned iterator is a view of the array; subsequent changes to the array
	 * will be reflected in the iterator.
	 *
	 * <p>
	 * <b>Note:</b> It is often preferable to represent your data using a collection
	 * type, for example using {@link Arrays#asList(Object[])}, making this method
	 * unnecessary.
	 *
	 * <p>
	 * The {@code Iterable} equivalent of this method is either
	 * {@link Arrays#asList(Object[])}, {@link ImmutableList#copyOf(Object[])}}, or
	 * {@link ImmutableList#of}.
	 */
	public static <T> UnmodifiableIterator<T> forArray(final T... array) {
		return forArray(array, 0, array.length, 0);
	}

	/**
	 * Returns a list iterator containing the elements in the specified range of
	 * {@code array} in order, starting at the specified index.
	 *
	 * <p>
	 * The {@code Iterable} equivalent of this method is {@code
	 * Arrays.asList(array).subList(offset, offset + length).listIterator(index)}.
	 */
	static <T> UnmodifiableListIterator<T> forArray(final T[] array, final int offset, int length, int index) {
		checkArgument(length >= 0);
		int end = offset + length;

		// Technically we should give a slightly more descriptive error on overflow
		Preconditions.checkPositionIndexes(offset, end, array.length);
		Preconditions.checkPositionIndex(index, length);
		if (length == 0) {
			return emptyListIterator();
		}

		/*
		 * We can't use call the two-arg constructor with arguments (offset, end)
		 * because the returned Iterator is a ListIterator that may be moved back past
		 * the beginning of the iteration.
		 */
		return new AbstractIndexedListIterator<T>(length, index) {
			@Override
			protected T get(int index) {
				return array[offset + index];
			}
		};
	}

	/**
	 * Returns an iterator containing only {@code value}.
	 *
	 * <p>
	 * The {@link Iterable} equivalent of this method is
	 * {@link Collections#singleton}.
	 */
	public static <T> UnmodifiableIterator<T> singletonIterator(@Nullable final T value) {
		return new UnmodifiableIterator<T>() {
			boolean done;

			@Override
			public boolean hasNext() {
				return !done;
			}

			@Override
			public T next() {
				if (done) {
					throw new NoSuchElementException();
				}
				done = true;
				return value;
			}
		};
	}

	/**
	 * Adapts an {@code Enumeration} to the {@code Iterator} interface.
	 *
	 * <p>
	 * This method has no equivalent in {@link Iterables} because viewing an
	 * {@code Enumeration} as an {@code Iterable} is impossible. However, the
	 * contents can be <i>copied</i> into a collection using
	 * {@link Collections#list}.
	 */
	public static <T> UnmodifiableIterator<T> forEnumeration(final Enumeration<T> enumeration) {
		checkNotNull(enumeration);
		return new UnmodifiableIterator<T>() {
			@Override
			public boolean hasNext() {
				return enumeration.hasMoreElements();
			}

			@Override
			public T next() {
				return enumeration.nextElement();
			}
		};
	}

	/**
	 * Adapts an {@code Iterator} to the {@code Enumeration} interface.
	 *
	 * <p>
	 * The {@code Iterable} equivalent of this method is either
	 * {@link Collections#enumeration} (if you have a {@link Collection}), or
	 * {@code Iterators.asEnumeration(collection.iterator())}.
	 */
	public static <T> Enumeration<T> asEnumeration(final Iterator<T> iterator) {
		checkNotNull(iterator);
		return new Enumeration<T>() {
			@Override
			public boolean hasMoreElements() {
				return iterator.hasNext();
			}

			@Override
			public T nextElement() {
				return iterator.next();
			}
		};
	}

	/**
	 * Implementation of PeekingIterator that avoids peeking unless necessary.
	 */
	private static class PeekingImpl<E> implements PeekingIterator<E> {

		private final Iterator<? extends E> iterator;
		private boolean hasPeeked;
		private E peekedElement;

		public PeekingImpl(Iterator<? extends E> iterator) {
			this.iterator = checkNotNull(iterator);
		}

		@Override
		public boolean hasNext() {
			return hasPeeked || iterator.hasNext();
		}

		@Override
		public E next() {
			if (!hasPeeked) {
				return iterator.next();
			}
			E result = peekedElement;
			hasPeeked = false;
			peekedElement = null;
			return result;
		}

		@Override
		public void remove() {
			checkState(!hasPeeked, "Can't remove after you've peeked at next");
			iterator.remove();
		}

		@Override
		public E peek() {
			if (!hasPeeked) {
				peekedElement = iterator.next();
				hasPeeked = true;
			}
			return peekedElement;
		}
	}

	/**
	 * Returns a {@code PeekingIterator} backed by the given iterator.
	 *
	 * <p>
	 * Calls to the {@code peek} method with no intervening calls to {@code
	 * next} do not affect the iteration, and hence return the same object each
	 * time. A subsequent call to {@code next} is guaranteed to return the same
	 * object again. For example:
	 * 
	 * <pre>
	 *    {@code
	 *
	 *   PeekingIterator<String> peekingIterator =
	 *       Iterators.peekingIterator(Iterators.forArray("a", "b"));
	 *   String a1 = peekingIterator.peek(); // returns "a"
	 *   String a2 = peekingIterator.peek(); // also returns "a"
	 *   String a3 = peekingIterator.next(); // also returns "a"}
	 * </pre>
	 *
	 * <p>
	 * Any structural changes to the underlying iteration (aside from those
	 * performed by the iterator's own {@link PeekingIterator#remove()} method) will
	 * leave the iterator in an undefined state.
	 *
	 * <p>
	 * The returned iterator does not support removal after peeking, as explained by
	 * {@link PeekingIterator#remove()}.
	 *
	 * <p>
	 * Note: If the given iterator is already a {@code PeekingIterator}, it
	 * <i>might</i> be returned to the caller, although this is neither guaranteed
	 * to occur nor required to be consistent. For example, this method <i>might</i>
	 * choose to pass through recognized implementations of {@code PeekingIterator}
	 * when the behavior of the implementation is known to meet the contract
	 * guaranteed by this method.
	 *
	 * <p>
	 * There is no {@link Iterable} equivalent to this method, so use this method to
	 * wrap each individual iterator as it is generated.
	 *
	 * @param iterator the backing iterator. The {@link PeekingIterator} assumes
	 *                 ownership of this iterator, so users should cease making
	 *                 direct calls to it after calling this method.
	 * @return a peeking iterator backed by that iterator. Apart from the additional
	 *         {@link PeekingIterator#peek()} method, this iterator behaves exactly
	 *         the same as {@code iterator}.
	 */
	public static <T> PeekingIterator<T> peekingIterator(Iterator<? extends T> iterator) {
		if (iterator instanceof PeekingImpl) {
			// Safe to cast <? extends T> to <T> because PeekingImpl only uses T
			// covariantly (and cannot be subclassed to add non-covariant uses).
			@SuppressWarnings("unchecked")
			PeekingImpl<T> peeking = (PeekingImpl<T>) iterator;
			return peeking;
		}
		return new PeekingImpl<T>(iterator);
	}

	/**
	 * Simply returns its argument.
	 *
	 * @deprecated no need to use this
	 * @since 10.0
	 */
	@Deprecated
	public static <T> PeekingIterator<T> peekingIterator(PeekingIterator<T> iterator) {
		return checkNotNull(iterator);
	}

	/**
	 * Returns an iterator over the merged contents of all given {@code iterators},
	 * traversing every element of the input iterators. Equivalent entries will not
	 * be de-duplicated.
	 *
	 * <p>
	 * Callers must ensure that the source {@code iterators} are in non-descending
	 * order as this method does not sort its input.
	 *
	 * <p>
	 * For any equivalent elements across all {@code iterators}, it is undefined
	 * which element is returned first.
	 *
	 * @since 11.0
	 */
	@Beta
	public static <T> UnmodifiableIterator<T> mergeSorted(Iterable<? extends Iterator<? extends T>> iterators,
			Comparator<? super T> comparator) {
		checkNotNull(iterators, "iterators");
		checkNotNull(comparator, "comparator");

		return new MergingIterator<T>(iterators, comparator);
	}

	/**
	 * An iterator that performs a lazy N-way merge, calculating the next value each
	 * time the iterator is polled. This amortizes the sorting cost over the
	 * iteration and requires less memory than sorting all elements at once.
	 *
	 * <p>
	 * Retrieving a single element takes approximately O(log(M)) time, where M is
	 * the number of iterators. (Retrieving all elements takes approximately
	 * O(N*log(M)) time, where N is the total number of elements.)
	 */
	private static class MergingIterator<T> extends UnmodifiableIterator<T> {
		final Queue<PeekingIterator<T>> queue;

		public MergingIterator(Iterable<? extends Iterator<? extends T>> iterators,
				final Comparator<? super T> itemComparator) {
			// A comparator that's used by the heap, allowing the heap
			// to be sorted based on the top of each iterator.
			Comparator<PeekingIterator<T>> heapComparator = new Comparator<PeekingIterator<T>>() {
				@Override
				public int compare(PeekingIterator<T> o1, PeekingIterator<T> o2) {
					return itemComparator.compare(o1.peek(), o2.peek());
				}
			};

			queue = new PriorityQueue<PeekingIterator<T>>(2, heapComparator);

			for (Iterator<? extends T> iterator : iterators) {
				if (iterator.hasNext()) {
					queue.add(Iterators.peekingIterator(iterator));
				}
			}
		}

		@Override
		public boolean hasNext() {
			return !queue.isEmpty();
		}

		@Override
		public T next() {
			PeekingIterator<T> nextIter = queue.remove();
			T next = nextIter.next();
			if (nextIter.hasNext()) {
				queue.add(nextIter);
			}
			return next;
		}
	}

	/**
	 * Used to avoid http://bugs.sun.com/view_bug.do?bug_id=6558557
	 */
	static <T> ListIterator<T> cast(Iterator<T> iterator) {
		return (ListIterator<T>) iterator;
	}
}