gt:             if (!delete(lastRet)) {
context: /*
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   * or visit www.oracle.com if you need additional information or have any
   * questions.
   */
  
  /*
   * This file is available under and governed by the GNU General Public
   * License version 2 only, as published by the Free Software Foundation.
   * However, the following notice accompanied the original version of this
   * file:
   *
   * Written by Josh Bloch of Google Inc. and released to the public domain,
   * as explained at http://creativecommons.org/publicdomain/zero/1.0/.
   */
  
  package java.util;
  
  import java.io.Serializable;
  import java.util.function.Consumer;
  import sun.misc.SharedSecrets;
  
  /**
   * Resizable-array implementation of the {@link Deque} interface.  Array
   * deques have no capacity restrictions; they grow as necessary to support
   * usage.  They are not thread-safe; in the absence of external
   * synchronization, they do not support concurrent access by multiple threads.
   * Null elements are prohibited.  This class is likely to be faster than
   * {@link Stack} when used as a stack, and faster than {@link LinkedList}
   * when used as a queue.
   *
   * <p>Most {@code ArrayDeque} operations run in amortized constant time.
   * Exceptions include {@link #remove(Object) remove}, {@link
   * #removeFirstOccurrence removeFirstOccurrence}, {@link #removeLastOccurrence
   * removeLastOccurrence}, {@link #contains contains}, {@link #iterator
   * iterator.remove()}, and the bulk operations, all of which run in linear
   * time.
   *
   * <p>The iterators returned by this class's {@code iterator} method are
   * <i>fail-fast</i>: If the deque is modified at any time after the iterator
   * is created, in any way except through the iterator's own {@code remove}
   * method, the iterator will generally throw a {@link
   * ConcurrentModificationException}.  Thus, in the face of concurrent
   * modification, the iterator fails quickly and cleanly, rather than risking
   * arbitrary, non-deterministic behavior at an undetermined time in the
   * future.
   *
   * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
   * as it is, generally speaking, impossible to make any hard guarantees in the
   * presence of unsynchronized concurrent modification.  Fail-fast iterators
   * throw {@code ConcurrentModificationException} on a best-effort basis.
   * Therefore, it would be wrong to write a program that depended on this
   * exception for its correctness: <i>the fail-fast behavior of iterators
   * should be used only to detect bugs.</i>
   *
   * <p>This class and its iterator implement all of the
   * <em>optional</em> methods of the {@link Collection} and {@link
   * Iterator} interfaces.
   *
   * <p>This class is a member of the
   * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   * Java Collections Framework</a>.
   *
   * @author  Josh Bloch and Doug Lea
   * @since   1.6
   * @param <E> the type of elements held in this collection
   */
  public class ArrayDeque<E> extends AbstractCollection<E>
                             implements Deque<E>, Cloneable, Serializable
  {
      /**
       * The array in which the elements of the deque are stored.
       * The capacity of the deque is the length of this array, which is
       * always a power of two. The array is never allowed to become
       * full, except transiently within an addX method where it is
       * resized (see doubleCapacity) immediately upon becoming full,
       * thus avoiding head and tail wrapping around to equal each
       * other.  We also guarantee that all array cells not holding
       * deque elements are always null.
       */
      transient Object[] elements; // non-private to simplify nested class access
  
      /**
       * The index of the element at the head of the deque (which is the
       * element that would be removed by remove() or pop()); or an
       * arbitrary number equal to tail if the deque is empty.
       */
      transient int head;
  
      /**
       * The index at which the next element would be added to the tail
       * of the deque (via addLast(E), add(E), or push(E)).
       */
      transient int tail;
  
      /**
       * The minimum capacity that we'll use for a newly created deque.
       * Must be a power of 2.
       */
      private static final int MIN_INITIAL_CAPACITY = 8;
  
      // ******  Array allocation and resizing utilities ******
  
      private static int calculateSize(int numElements) {
          int initialCapacity = MIN_INITIAL_CAPACITY;
          // Find the best power of two to hold elements.
          // Tests "<=" because arrays aren't kept full.
          if (numElements >= initialCapacity) {
              initialCapacity = numElements;
              initialCapacity |= (initialCapacity >>>  1);
              initialCapacity |= (initialCapacity >>>  2);
              initialCapacity |= (initialCapacity >>>  4);
              initialCapacity |= (initialCapacity >>>  8);
              initialCapacity |= (initialCapacity >>> 16);
              initialCapacity++;
  
              if (initialCapacity < 0)   // Too many elements, must back off
                  initialCapacity >>>= 1;// Good luck allocating 2 ^ 30 elements
          }
          return initialCapacity;
      }
  
      /**
       * Allocates empty array to hold the given number of elements.
       *
       * @param numElements  the number of elements to hold
       */
      private void allocateElements(int numElements) {
          elements = new Object[calculateSize(numElements)];
      }
  
      /**
       * Doubles the capacity of this deque.  Call only when full, i.e.,
       * when head and tail have wrapped around to become equal.
       */
      private void doubleCapacity() {
          assert head == tail;
          int p = head;
          int n = elements.length;
          int r = n - p; // number of elements to the right of p
          int newCapacity = n << 1;
          if (newCapacity < 0)
              throw new IllegalStateException("Sorry, deque too big");
          Object[] a = new Object[newCapacity];
          System.arraycopy(elements, p, a, 0, r);
          System.arraycopy(elements, 0, a, r, p);
          elements = a;
          head = 0;
          tail = n;
      }
  
      /**
       * Copies the elements from our element array into the specified array,
       * in order (from first to last element in the deque).  It is assumed
       * that the array is large enough to hold all elements in the deque.
       *
       * @return its argument
       */
      private <T> T[] copyElements(T[] a) {
          if (head < tail) {
              System.arraycopy(elements, head, a, 0, size());
          } else if (head > tail) {
              int headPortionLen = elements.length - head;
              System.arraycopy(elements, head, a, 0, headPortionLen);
              System.arraycopy(elements, 0, a, headPortionLen, tail);
          }
          return a;
      }
  
      /**
       * Constructs an empty array deque with an initial capacity
       * sufficient to hold 16 elements.
       */
      public ArrayDeque() {
          elements = new Object[16];
      }
  
      /**
       * Constructs an empty array deque with an initial capacity
       * sufficient to hold the specified number of elements.
       *
       * @param numElements  lower bound on initial capacity of the deque
       */
      public ArrayDeque(int numElements) {
          allocateElements(numElements);
      }
  
      /**
       * Constructs a deque containing the elements of the specified
       * collection, in the order they are returned by the collection's
       * iterator.  (The first element returned by the collection's
       * iterator becomes the first element, or <i>front</i> of the
       * deque.)
       *
       * @param c the collection whose elements are to be placed into the deque
       * @throws NullPointerException if the specified collection is null
       */
      public ArrayDeque(Collection<? extends E> c) {
          allocateElements(c.size());
          addAll(c);
      }
  
      // The main insertion and extraction methods are addFirst,
      // addLast, pollFirst, pollLast. The other methods are defined in
      // terms of these.
  
      /**
       * Inserts the specified element at the front of this deque.
       *
       * @param e the element to add
       * @throws NullPointerException if the specified element is null
       */
      public void addFirst(E e) {
          if (e == null)
              throw new NullPointerException();
          elements[head = (head - 1) & (elements.length - 1)] = e;
          if (head == tail)
              doubleCapacity();
      }
  
      /**
       * Inserts the specified element at the end of this deque.
       *
       * <p>This method is equivalent to {@link #add}.
       *
       * @param e the element to add
       * @throws NullPointerException if the specified element is null
       */
      public void addLast(E e) {
          if (e == null)
              throw new NullPointerException();
          elements[tail] = e;
          if ( (tail = (tail + 1) & (elements.length - 1)) == head)
              doubleCapacity();
      }
  
      /**
       * Inserts the specified element at the front of this deque.
       *
       * @param e the element to add
       * @return {@code true} (as specified by {@link Deque#offerFirst})
       * @throws NullPointerException if the specified element is null
       */
      public boolean offerFirst(E e) {
          addFirst(e);
          return true;
      }
  
      /**
       * Inserts the specified element at the end of this deque.
       *
       * @param e the element to add
       * @return {@code true} (as specified by {@link Deque#offerLast})
       * @throws NullPointerException if the specified element is null
       */
      public boolean offerLast(E e) {
          addLast(e);
          return true;
      }
  
      /**
       * @throws NoSuchElementException {@inheritDoc}
       */
      public E removeFirst() {
          E x = pollFirst();
          if (x == null)
              throw new NoSuchElementException();
          return x;
      }
  
      /**
       * @throws NoSuchElementException {@inheritDoc}
       */
      public E removeLast() {
          E x = pollLast();
          if (x == null)
              throw new NoSuchElementException();
          return x;
      }
  
      public E pollFirst() {
          int h = head;
          @SuppressWarnings("unchecked")
          E result = (E) elements[h];
          // Element is null if deque empty
          if (result == null)
              return null;
          elements[h] = null;     // Must null out slot
          head = (h + 1) & (elements.length - 1);
          return result;
      }
  
      public E pollLast() {
          int t = (tail - 1) & (elements.length - 1);
          @SuppressWarnings("unchecked")
          E result = (E) elements[t];
          if (result == null)
              return null;
          elements[t] = null;
          tail = t;
          return result;
      }
  
      /**
       * @throws NoSuchElementException {@inheritDoc}
       */
      public E getFirst() {
          @SuppressWarnings("unchecked")
          E result = (E) elements[head];
          if (result == null)
              throw new NoSuchElementException();
          return result;
      }
  
      /**
       * @throws NoSuchElementException {@inheritDoc}
       */
      public E getLast() {
          @SuppressWarnings("unchecked")
          E result = (E) elements[(tail - 1) & (elements.length - 1)];
          if (result == null)
              throw new NoSuchElementException();
          return result;
      }
  
      @SuppressWarnings("unchecked")
      public E peekFirst() {
          // elements[head] is null if deque empty
          return (E) elements[head];
      }
  
      @SuppressWarnings("unchecked")
      public E peekLast() {
          return (E) elements[(tail - 1) & (elements.length - 1)];
      }
  
      /**
       * Removes the first occurrence of the specified element in this
       * deque (when traversing the deque from head to tail).
       * If the deque does not contain the element, it is unchanged.
       * More formally, removes the first element {@code e} such that
       * {@code o.equals(e)} (if such an element exists).
       * Returns {@code true} if this deque contained the specified element
       * (or equivalently, if this deque changed as a result of the call).
       *
       * @param o element to be removed from this deque, if present
       * @return {@code true} if the deque contained the specified element
       */
      public boolean removeFirstOccurrence(Object o) {
          if (o == null)
              return false;
          int mask = elements.length - 1;
          int i = head;
          Object x;
          while ( (x = elements[i]) != null) {
              if (o.equals(x)) {
                  delete(i);
                  return true;
              }
              i = (i + 1) & mask;
          }
          return false;
      }
  
      /**
       * Removes the last occurrence of the specified element in this
       * deque (when traversing the deque from head to tail).
       * If the deque does not contain the element, it is unchanged.
       * More formally, removes the last element {@code e} such that
       * {@code o.equals(e)} (if such an element exists).
       * Returns {@code true} if this deque contained the specified element
       * (or equivalently, if this deque changed as a result of the call).
       *
       * @param o element to be removed from this deque, if present
       * @return {@code true} if the deque contained the specified element
       */
      public boolean removeLastOccurrence(Object o) {
          if (o == null)
              return false;
          int mask = elements.length - 1;
          int i = (tail - 1) & mask;
          Object x;
          while ( (x = elements[i]) != null) {
              if (o.equals(x)) {
                  delete(i);
                  return true;
              }
              i = (i - 1) & mask;
          }
          return false;
      }
  
      // *** Queue methods ***
  
      /**
       * Inserts the specified element at the end of this deque.
       *
       * <p>This method is equivalent to {@link #addLast}.
       *
       * @param e the element to add
       * @return {@code true} (as specified by {@link Collection#add})
       * @throws NullPointerException if the specified element is null
       */
      public boolean add(E e) {
          addLast(e);
          return true;
      }
  
      /**
       * Inserts the specified element at the end of this deque.
       *
       * <p>This method is equivalent to {@link #offerLast}.
       *
       * @param e the element to add
       * @return {@code true} (as specified by {@link Queue#offer})
       * @throws NullPointerException if the specified element is null
       */
      public boolean offer(E e) {
          return offerLast(e);
      }
  
      /**
       * Retrieves and removes the head of the queue represented by this deque.
       *
       * This method differs from {@link #poll poll} only in that it throws an
       * exception if this deque is empty.
       *
       * <p>This method is equivalent to {@link #removeFirst}.
       *
       * @return the head of the queue represented by this deque
       * @throws NoSuchElementException {@inheritDoc}
       */
      public E remove() {
          return removeFirst();
      }
  
      /**
       * Retrieves and removes the head of the queue represented by this deque
       * (in other words, the first element of this deque), or returns
       * {@code null} if this deque is empty.
       *
       * <p>This method is equivalent to {@link #pollFirst}.
       *
       * @return the head of the queue represented by this deque, or
       *         {@code null} if this deque is empty
       */
      public E poll() {
          return pollFirst();
      }
  
      /**
       * Retrieves, but does not remove, the head of the queue represented by
       * this deque.  This method differs from {@link #peek peek} only in
       * that it throws an exception if this deque is empty.
       *
       * <p>This method is equivalent to {@link #getFirst}.
       *
       * @return the head of the queue represented by this deque
       * @throws NoSuchElementException {@inheritDoc}
       */
      public E element() {
          return getFirst();
      }
  
      /**
       * Retrieves, but does not remove, the head of the queue represented by
       * this deque, or returns {@code null} if this deque is empty.
       *
       * <p>This method is equivalent to {@link #peekFirst}.
       *
       * @return the head of the queue represented by this deque, or
       *         {@code null} if this deque is empty
       */
      public E peek() {
          return peekFirst();
      }
  
      // *** Stack methods ***
  
      /**
       * Pushes an element onto the stack represented by this deque.  In other
       * words, inserts the element at the front of this deque.
       *
       * <p>This method is equivalent to {@link #addFirst}.
       *
       * @param e the element to push
       * @throws NullPointerException if the specified element is null
       */
      public void push(E e) {
          addFirst(e);
      }
  
      /**
       * Pops an element from the stack represented by this deque.  In other
       * words, removes and returns the first element of this deque.
       *
       * <p>This method is equivalent to {@link #removeFirst()}.
       *
       * @return the element at the front of this deque (which is the top
       *         of the stack represented by this deque)
       * @throws NoSuchElementException {@inheritDoc}
       */
      public E pop() {
          return removeFirst();
      }
  
      private void checkInvariants() {
          assert elements[tail] == null;
          assert head == tail ? elements[head] == null :
              (elements[head] != null &&
               elements[(tail - 1) & (elements.length - 1)] != null);
          assert elements[(head - 1) & (elements.length - 1)] == null;
      }
  
      /**
       * Removes the element at the specified position in the elements array,
       * adjusting head and tail as necessary.  This can result in motion of
       * elements backwards or forwards in the array.
       *
       * <p>This method is called delete rather than remove to emphasize
       * that its semantics differ from those of {@link List#remove(int)}.
       *
       * @return true if elements moved backwards
       */
      private boolean delete(int i) {
          checkInvariants();
          final Object[] elements = this.elements;
          final int mask = elements.length - 1;
          final int h = head;
          final int t = tail;
          final int front = (i - h) & mask;
          final int back  = (t - i) & mask;
  
          // Invariant: head <= i < tail mod circularity
          if (front >= ((t - h) & mask))
              throw new ConcurrentModificationException();
  
          // Optimize for least element motion
          if (front < back) {
              if (h <= i) {
                  System.arraycopy(elements, h, elements, h + 1, front);
              } else { // Wrap around
                  System.arraycopy(elements, 0, elements, 1, i);
                  elements[0] = elements[mask];
                  System.arraycopy(elements, h, elements, h + 1, mask - h);
              }
              elements[h] = null;
              head = (h + 1) & mask;
              return false;
          } else {
              if (i < t) { // Copy the null tail as well
                  System.arraycopy(elements, i + 1, elements, i, back);
                  tail = t - 1;
              } else { // Wrap around
                  System.arraycopy(elements, i + 1, elements, i, mask - i);
                  elements[mask] = elements[0];
                  System.arraycopy(elements, 1, elements, 0, t);
                  tail = (t - 1) & mask;
              }
              return true;
          }
      }
  
      // *** Collection Methods ***
  
      /**
       * Returns the number of elements in this deque.
       *
       * @return the number of elements in this deque
       */
      public int size() {
          return (tail - head) & (elements.length - 1);
      }
  
      /**
       * Returns {@code true} if this deque contains no elements.
       *
       * @return {@code true} if this deque contains no elements
       */
      public boolean isEmpty() {
          return head == tail;
      }
  
      /**
       * Returns an iterator over the elements in this deque.  The elements
       * will be ordered from first (head) to last (tail).  This is the same
       * order that elements would be dequeued (via successive calls to
       * {@link #remove} or popped (via successive calls to {@link #pop}).
       *
       * @return an iterator over the elements in this deque
       */
      public Iterator<E> iterator() {
          return new DeqIterator();
      }
  
      public Iterator<E> descendingIterator() {
          return new DescendingIterator();
      }
  
      private class DeqIterator implements Iterator<E> {
          /**
           * Index of element to be returned by subsequent call to next.
           */
          private int cursor = head;
  
          /**
           * Tail recorded at construction (also in remove), to stop
           * iterator and also to check for comodification.
           */
          private int fence = tail;
  
          /**
           * Index of element returned by most recent call to next.
           * Reset to -1 if element is deleted by a call to remove.
           */
          private int lastRet = -1;
  
          public boolean hasNext() {
              return cursor != fence;
          }
  
          public E next() {
              if (cursor == fence)
                  throw new NoSuchElementException();
              @SuppressWarnings("unchecked")
              E result = (E) elements[cursor];
              // This check doesn't catch all possible comodifications,
              // but does catch the ones that corrupt traversal
              if (tail != fence || result == null)
                  throw new ConcurrentModificationException();
              lastRet = cursor;
              cursor = (cursor + 1) & (elements.length - 1);
              return result;
          }
  
          public void remove() {
              if (lastRet < 0)
                  throw new IllegalStateException();
              if (delete(lastRet)) { // if left-shifted, undo increment in next()
                  cursor = (cursor - 1) & (elements.length - 1);
                  fence = tail;
              }
              lastRet = -1;
          }
  
          public void forEachRemaining(Consumer<? super E> action) {
              Objects.requireNonNull(action);
              Object[] a = elements;
              int m = a.length - 1, f = fence, i = cursor;
              cursor = f;
              while (i != f) {
                  @SuppressWarnings("unchecked") E e = (E)a[i];
                  i = (i + 1) & m;
                  if (e == null)
                      throw new ConcurrentModificationException();
                  action.accept(e);
              }
          }
      }
  
      private class DescendingIterator implements Iterator<E> {
          /*
           * This class is nearly a mirror-image of DeqIterator, using
           * tail instead of head for initial cursor, and head instead of
           * tail for fence.
           */
          private int cursor = tail;
          private int fence = head;
          private int lastRet = -1;
  
          public boolean hasNext() {
              return cursor != fence;
          }
  
          public E next() {
              if (cursor == fence)
                  throw new NoSuchElementException();
              cursor = (cursor - 1) & (elements.length - 1);
              @SuppressWarnings("unchecked")
              E result = (E) elements[cursor];
              if (head != fence || result == null)
                  throw new ConcurrentModificationException();
              lastRet = cursor;
              return result;
          }
  
          public void remove() {
              if (lastRet < 0)
                  throw new IllegalStateException();
  