/*
    * 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 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;
  
  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;
  
  /**
   * 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()}.
    *
    * @deprecated Use {@code ImmutableSet.<T>of().iterator()} instead; or for
    *     Java 7 or later, {@link Collections#emptyIterator}. This method is
    *     scheduled for removal in May 2016.
    */
   @Deprecated
   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 java.util.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 java.util.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;
   }
 }