1 /*
   2  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   3  *
   4  * This code is free software; you can redistribute it and/or modify it
   5  * under the terms of the GNU General Public License version 2 only, as
   6  * published by the Free Software Foundation.  Oracle designates this
   7  * particular file as subject to the "Classpath" exception as provided
   8  * by Oracle in the LICENSE file that accompanied this code.
   9  *
  10  * This code is distributed in the hope that it will be useful, but WITHOUT
  11  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  12  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  13  * version 2 for more details (a copy is included in the LICENSE file that
  14  * accompanied this code).
  15  *
  16  * You should have received a copy of the GNU General Public License version
  17  * 2 along with this work; if not, write to the Free Software Foundation,
  18  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  19  *
  20  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  21  * or visit www.oracle.com if you need additional information or have any
  22  * questions.
  23  */
  24 
  25 /*
  26  * This file is available under and governed by the GNU General Public
  27  * License version 2 only, as published by the Free Software Foundation.
  28  * However, the following notice accompanied the original version of this
  29  * file:
  30  *
  31  * Written by Josh Bloch of Google Inc. and released to the public domain,
  32  * as explained at http://creativecommons.org/publicdomain/zero/1.0/.
  33  */
  34 
  35 package java.util;
  36 
  37 import java.io.Serializable;
  38 import java.util.function.Consumer;
  39 import java.util.function.Predicate;
  40 import jdk.internal.access.SharedSecrets;
  41 
  42 /**
  43  * Resizable-array implementation of the {@link Deque} interface.  Array
  44  * deques have no capacity restrictions; they grow as necessary to support
  45  * usage.  They are not thread-safe; in the absence of external
  46  * synchronization, they do not support concurrent access by multiple threads.
  47  * Null elements are prohibited.  This class is likely to be faster than
  48  * {@link Stack} when used as a stack, and faster than {@link LinkedList}
  49  * when used as a queue.
  50  *
  51  * <p>Most {@code ArrayDeque} operations run in amortized constant time.
  52  * Exceptions include
  53  * {@link #remove(Object) remove},
  54  * {@link #removeFirstOccurrence removeFirstOccurrence},
  55  * {@link #removeLastOccurrence removeLastOccurrence},
  56  * {@link #contains contains},
  57  * {@link #iterator iterator.remove()},
  58  * and the bulk operations, all of which run in linear time.
  59  *
  60  * <p>The iterators returned by this class's {@link #iterator() iterator}
  61  * method are <em>fail-fast</em>: If the deque is modified at any time after
  62  * the iterator is created, in any way except through the iterator's own
  63  * {@code remove} method, the iterator will generally throw a {@link
  64  * ConcurrentModificationException}.  Thus, in the face of concurrent
  65  * modification, the iterator fails quickly and cleanly, rather than risking
  66  * arbitrary, non-deterministic behavior at an undetermined time in the
  67  * future.
  68  *
  69  * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
  70  * as it is, generally speaking, impossible to make any hard guarantees in the
  71  * presence of unsynchronized concurrent modification.  Fail-fast iterators
  72  * throw {@code ConcurrentModificationException} on a best-effort basis.
  73  * Therefore, it would be wrong to write a program that depended on this
  74  * exception for its correctness: <i>the fail-fast behavior of iterators
  75  * should be used only to detect bugs.</i>
  76  *
  77  * <p>This class and its iterator implement all of the
  78  * <em>optional</em> methods of the {@link Collection} and {@link
  79  * Iterator} interfaces.
  80  *
  81  * <p>This class is a member of the
  82  * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
  83  * Java Collections Framework</a>.
  84  *
  85  * @author  Josh Bloch and Doug Lea
  86  * @param <E> the type of elements held in this deque
  87  * @since   1.6
  88  */
  89 public class ArrayDeque<E> extends AbstractCollection<E>
  90                            implements Deque<E>, Cloneable, Serializable
  91 {
  92     /*
  93      * VMs excel at optimizing simple array loops where indices are
  94      * incrementing or decrementing over a valid slice, e.g.
  95      *
  96      * for (int i = start; i < end; i++) ... elements[i]
  97      *
  98      * Because in a circular array, elements are in general stored in
  99      * two disjoint such slices, we help the VM by writing unusual
 100      * nested loops for all traversals over the elements.  Having only
 101      * one hot inner loop body instead of two or three eases human
 102      * maintenance and encourages VM loop inlining into the caller.
 103      */
 104 
 105     /**
 106      * The array in which the elements of the deque are stored.
 107      * All array cells not holding deque elements are always null.
 108      * The array always has at least one null slot (at tail).
 109      */
 110     transient Object[] elements;
 111 
 112     /**
 113      * The index of the element at the head of the deque (which is the
 114      * element that would be removed by remove() or pop()); or an
 115      * arbitrary number 0 <= head < elements.length equal to tail if
 116      * the deque is empty.
 117      */
 118     transient int head;
 119 
 120     /**
 121      * The index at which the next element would be added to the tail
 122      * of the deque (via addLast(E), add(E), or push(E));
 123      * elements[tail] is always null.
 124      */
 125     transient int tail;
 126 
 127     /**
 128      * The maximum size of array to allocate.
 129      * Some VMs reserve some header words in an array.
 130      * Attempts to allocate larger arrays may result in
 131      * OutOfMemoryError: Requested array size exceeds VM limit
 132      */
 133     private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
 134 
 135     /**
 136      * Increases the capacity of this deque by at least the given amount.
 137      *
 138      * @param needed the required minimum extra capacity; must be positive
 139      */
 140     private void grow(int needed) {
 141         // overflow-conscious code
 142         final int oldCapacity = elements.length;
 143         int newCapacity;
 144         // Double capacity if small; else grow by 50%
 145         int jump = (oldCapacity < 64) ? (oldCapacity + 2) : (oldCapacity >> 1);
 146         if (jump < needed
 147             || (newCapacity = (oldCapacity + jump)) - MAX_ARRAY_SIZE > 0)
 148             newCapacity = newCapacity(needed, jump);
 149         final Object[] es = elements = Arrays.copyOf(elements, newCapacity);
 150         // Exceptionally, here tail == head needs to be disambiguated
 151         if (tail < head || (tail == head && es[head] != null)) {
 152             // wrap around; slide first leg forward to end of array
 153             int newSpace = newCapacity - oldCapacity;
 154             System.arraycopy(es, head,
 155                              es, head + newSpace,
 156                              oldCapacity - head);
 157             for (int i = head, to = (head += newSpace); i < to; i++)
 158                 es[i] = null;
 159         }
 160     }
 161 
 162     /** Capacity calculation for edge conditions, especially overflow. */
 163     private int newCapacity(int needed, int jump) {
 164         final int oldCapacity = elements.length, minCapacity;
 165         if ((minCapacity = oldCapacity + needed) - MAX_ARRAY_SIZE > 0) {
 166             if (minCapacity < 0)
 167                 throw new IllegalStateException("Sorry, deque too big");
 168             return Integer.MAX_VALUE;
 169         }
 170         if (needed > jump)
 171             return minCapacity;
 172         return (oldCapacity + jump - MAX_ARRAY_SIZE < 0)
 173             ? oldCapacity + jump
 174             : MAX_ARRAY_SIZE;
 175     }
 176 
 177     /**
 178      * Constructs an empty array deque with an initial capacity
 179      * sufficient to hold 16 elements.
 180      */
 181     public ArrayDeque() {
 182         elements = new Object[16 + 1];
 183     }
 184 
 185     /**
 186      * Constructs an empty array deque with an initial capacity
 187      * sufficient to hold the specified number of elements.
 188      *
 189      * @param numElements lower bound on initial capacity of the deque
 190      */
 191     public ArrayDeque(int numElements) {
 192         elements =
 193             new Object[(numElements < 1) ? 1 :
 194                        (numElements == Integer.MAX_VALUE) ? Integer.MAX_VALUE :
 195                        numElements + 1];
 196     }
 197 
 198     /**
 199      * Constructs a deque containing the elements of the specified
 200      * collection, in the order they are returned by the collection's
 201      * iterator.  (The first element returned by the collection's
 202      * iterator becomes the first element, or <i>front</i> of the
 203      * deque.)
 204      *
 205      * @param c the collection whose elements are to be placed into the deque
 206      * @throws NullPointerException if the specified collection is null
 207      */
 208     public ArrayDeque(Collection<? extends E> c) {
 209         this(c.size());
 210         copyElements(c);
 211     }
 212 
 213     /**
 214      * Circularly increments i, mod modulus.
 215      * Precondition and postcondition: 0 <= i < modulus.
 216      */
 217     static final int inc(int i, int modulus) {
 218         if (++i >= modulus) i = 0;
 219         return i;
 220     }
 221 
 222     /**
 223      * Circularly decrements i, mod modulus.
 224      * Precondition and postcondition: 0 <= i < modulus.
 225      */
 226     static final int dec(int i, int modulus) {
 227         if (--i < 0) i = modulus - 1;
 228         return i;
 229     }
 230 
 231     /**
 232      * Circularly adds the given distance to index i, mod modulus.
 233      * Precondition: 0 <= i < modulus, 0 <= distance <= modulus.
 234      * @return index 0 <= i < modulus
 235      */
 236     static final int inc(int i, int distance, int modulus) {
 237         if ((i += distance) - modulus >= 0) i -= modulus;
 238         return i;
 239     }
 240 
 241     /**
 242      * Subtracts j from i, mod modulus.
 243      * Index i must be logically ahead of index j.
 244      * Precondition: 0 <= i < modulus, 0 <= j < modulus.
 245      * @return the "circular distance" from j to i; corner case i == j
 246      * is disambiguated to "empty", returning 0.
 247      */
 248     static final int sub(int i, int j, int modulus) {
 249         if ((i -= j) < 0) i += modulus;
 250         return i;
 251     }
 252 
 253     /**
 254      * Returns element at array index i.
 255      * This is a slight abuse of generics, accepted by javac.
 256      */
 257     @SuppressWarnings("unchecked")
 258     static final <E> E elementAt(Object[] es, int i) {
 259         return (E) es[i];
 260     }
 261 
 262     /**
 263      * A version of elementAt that checks for null elements.
 264      * This check doesn't catch all possible comodifications,
 265      * but does catch ones that corrupt traversal.
 266      */
 267     static final <E> E nonNullElementAt(Object[] es, int i) {
 268         @SuppressWarnings("unchecked") E e = (E) es[i];
 269         if (e == null)
 270             throw new ConcurrentModificationException();
 271         return e;
 272     }
 273 
 274     // The main insertion and extraction methods are addFirst,
 275     // addLast, pollFirst, pollLast. The other methods are defined in
 276     // terms of these.
 277 
 278     /**
 279      * Inserts the specified element at the front of this deque.
 280      *
 281      * @param e the element to add
 282      * @throws NullPointerException if the specified element is null
 283      */
 284     public void addFirst(E e) {
 285         if (e == null)
 286             throw new NullPointerException();
 287         final Object[] es = elements;
 288         es[head = dec(head, es.length)] = e;
 289         if (head == tail)
 290             grow(1);
 291     }
 292 
 293     /**
 294      * Inserts the specified element at the end of this deque.
 295      *
 296      * <p>This method is equivalent to {@link #add}.
 297      *
 298      * @param e the element to add
 299      * @throws NullPointerException if the specified element is null
 300      */
 301     public void addLast(E e) {
 302         if (e == null)
 303             throw new NullPointerException();
 304         final Object[] es = elements;
 305         es[tail] = e;
 306         if (head == (tail = inc(tail, es.length)))
 307             grow(1);
 308     }
 309 
 310     /**
 311      * Adds all of the elements in the specified collection at the end
 312      * of this deque, as if by calling {@link #addLast} on each one,
 313      * in the order that they are returned by the collection's iterator.
 314      *
 315      * @param c the elements to be inserted into this deque
 316      * @return {@code true} if this deque changed as a result of the call
 317      * @throws NullPointerException if the specified collection or any
 318      *         of its elements are null
 319      */
 320     public boolean addAll(Collection<? extends E> c) {
 321         final int s, needed;
 322         if ((needed = (s = size()) + c.size() + 1 - elements.length) > 0)
 323             grow(needed);
 324         copyElements(c);
 325         return size() > s;
 326     }
 327 
 328     private void copyElements(Collection<? extends E> c) {
 329         c.forEach(this::addLast);
 330     }
 331 
 332     /**
 333      * Inserts the specified element at the front of this deque.
 334      *
 335      * @param e the element to add
 336      * @return {@code true} (as specified by {@link Deque#offerFirst})
 337      * @throws NullPointerException if the specified element is null
 338      */
 339     public boolean offerFirst(E e) {
 340         addFirst(e);
 341         return true;
 342     }
 343 
 344     /**
 345      * Inserts the specified element at the end of this deque.
 346      *
 347      * @param e the element to add
 348      * @return {@code true} (as specified by {@link Deque#offerLast})
 349      * @throws NullPointerException if the specified element is null
 350      */
 351     public boolean offerLast(E e) {
 352         addLast(e);
 353         return true;
 354     }
 355 
 356     /**
 357      * @throws NoSuchElementException {@inheritDoc}
 358      */
 359     public E removeFirst() {
 360         E e = pollFirst();
 361         if (e == null)
 362             throw new NoSuchElementException();
 363         return e;
 364     }
 365 
 366     /**
 367      * @throws NoSuchElementException {@inheritDoc}
 368      */
 369     public E removeLast() {
 370         E e = pollLast();
 371         if (e == null)
 372             throw new NoSuchElementException();
 373         return e;
 374     }
 375 
 376     public E pollFirst() {
 377         final Object[] es;
 378         final int h;
 379         E e = elementAt(es = elements, h = head);
 380         if (e != null) {
 381             es[h] = null;
 382             head = inc(h, es.length);
 383         }
 384         return e;
 385     }
 386 
 387     public E pollLast() {
 388         final Object[] es;
 389         final int t;
 390         E e = elementAt(es = elements, t = dec(tail, es.length));
 391         if (e != null)
 392             es[tail = t] = null;
 393         return e;
 394     }
 395 
 396     /**
 397      * @throws NoSuchElementException {@inheritDoc}
 398      */
 399     public E getFirst() {
 400         E e = elementAt(elements, head);
 401         if (e == null)
 402             throw new NoSuchElementException();
 403         return e;
 404     }
 405 
 406     /**
 407      * @throws NoSuchElementException {@inheritDoc}
 408      */
 409     public E getLast() {
 410         final Object[] es = elements;
 411         E e = elementAt(es, dec(tail, es.length));
 412         if (e == null)
 413             throw new NoSuchElementException();
 414         return e;
 415     }
 416 
 417     public E peekFirst() {
 418         return elementAt(elements, head);
 419     }
 420 
 421     public E peekLast() {
 422         final Object[] es;
 423         return elementAt(es = elements, dec(tail, es.length));
 424     }
 425 
 426     /**
 427      * Removes the first occurrence of the specified element in this
 428      * deque (when traversing the deque from head to tail).
 429      * If the deque does not contain the element, it is unchanged.
 430      * More formally, removes the first element {@code e} such that
 431      * {@code o.equals(e)} (if such an element exists).
 432      * Returns {@code true} if this deque contained the specified element
 433      * (or equivalently, if this deque changed as a result of the call).
 434      *
 435      * @param o element to be removed from this deque, if present
 436      * @return {@code true} if the deque contained the specified element
 437      */
 438     public boolean removeFirstOccurrence(Object o) {
 439         if (o != null) {
 440             final Object[] es = elements;
 441             for (int i = head, end = tail, to = (i <= end) ? end : es.length;
 442                  ; i = 0, to = end) {
 443                 for (; i < to; i++)
 444                     if (o.equals(es[i])) {
 445                         delete(i);
 446                         return true;
 447                     }
 448                 if (to == end) break;
 449             }
 450         }
 451         return false;
 452     }
 453 
 454     /**
 455      * Removes the last occurrence of the specified element in this
 456      * deque (when traversing the deque from head to tail).
 457      * If the deque does not contain the element, it is unchanged.
 458      * More formally, removes the last element {@code e} such that
 459      * {@code o.equals(e)} (if such an element exists).
 460      * Returns {@code true} if this deque contained the specified element
 461      * (or equivalently, if this deque changed as a result of the call).
 462      *
 463      * @param o element to be removed from this deque, if present
 464      * @return {@code true} if the deque contained the specified element
 465      */
 466     public boolean removeLastOccurrence(Object o) {
 467         if (o != null) {
 468             final Object[] es = elements;
 469             for (int i = tail, end = head, to = (i >= end) ? end : 0;
 470                  ; i = es.length, to = end) {
 471                 for (i--; i > to - 1; i--)
 472                     if (o.equals(es[i])) {
 473                         delete(i);
 474                         return true;
 475                     }
 476                 if (to == end) break;
 477             }
 478         }
 479         return false;
 480     }
 481 
 482     // *** Queue methods ***
 483 
 484     /**
 485      * Inserts the specified element at the end of this deque.
 486      *
 487      * <p>This method is equivalent to {@link #addLast}.
 488      *
 489      * @param e the element to add
 490      * @return {@code true} (as specified by {@link Collection#add})
 491      * @throws NullPointerException if the specified element is null
 492      */
 493     public boolean add(E e) {
 494         addLast(e);
 495         return true;
 496     }
 497 
 498     /**
 499      * Inserts the specified element at the end of this deque.
 500      *
 501      * <p>This method is equivalent to {@link #offerLast}.
 502      *
 503      * @param e the element to add
 504      * @return {@code true} (as specified by {@link Queue#offer})
 505      * @throws NullPointerException if the specified element is null
 506      */
 507     public boolean offer(E e) {
 508         return offerLast(e);
 509     }
 510 
 511     /**
 512      * Retrieves and removes the head of the queue represented by this deque.
 513      *
 514      * This method differs from {@link #poll() poll()} only in that it
 515      * throws an exception if this deque is empty.
 516      *
 517      * <p>This method is equivalent to {@link #removeFirst}.
 518      *
 519      * @return the head of the queue represented by this deque
 520      * @throws NoSuchElementException {@inheritDoc}
 521      */
 522     public E remove() {
 523         return removeFirst();
 524     }
 525 
 526     /**
 527      * Retrieves and removes the head of the queue represented by this deque
 528      * (in other words, the first element of this deque), or returns
 529      * {@code null} if this deque is empty.
 530      *
 531      * <p>This method is equivalent to {@link #pollFirst}.
 532      *
 533      * @return the head of the queue represented by this deque, or
 534      *         {@code null} if this deque is empty
 535      */
 536     public E poll() {
 537         return pollFirst();
 538     }
 539 
 540     /**
 541      * Retrieves, but does not remove, the head of the queue represented by
 542      * this deque.  This method differs from {@link #peek peek} only in
 543      * that it throws an exception if this deque is empty.
 544      *
 545      * <p>This method is equivalent to {@link #getFirst}.
 546      *
 547      * @return the head of the queue represented by this deque
 548      * @throws NoSuchElementException {@inheritDoc}
 549      */
 550     public E element() {
 551         return getFirst();
 552     }
 553 
 554     /**
 555      * Retrieves, but does not remove, the head of the queue represented by
 556      * this deque, or returns {@code null} if this deque is empty.
 557      *
 558      * <p>This method is equivalent to {@link #peekFirst}.
 559      *
 560      * @return the head of the queue represented by this deque, or
 561      *         {@code null} if this deque is empty
 562      */
 563     public E peek() {
 564         return peekFirst();
 565     }
 566 
 567     // *** Stack methods ***
 568 
 569     /**
 570      * Pushes an element onto the stack represented by this deque.  In other
 571      * words, inserts the element at the front of this deque.
 572      *
 573      * <p>This method is equivalent to {@link #addFirst}.
 574      *
 575      * @param e the element to push
 576      * @throws NullPointerException if the specified element is null
 577      */
 578     public void push(E e) {
 579         addFirst(e);
 580     }
 581 
 582     /**
 583      * Pops an element from the stack represented by this deque.  In other
 584      * words, removes and returns the first element of this deque.
 585      *
 586      * <p>This method is equivalent to {@link #removeFirst()}.
 587      *
 588      * @return the element at the front of this deque (which is the top
 589      *         of the stack represented by this deque)
 590      * @throws NoSuchElementException {@inheritDoc}
 591      */
 592     public E pop() {
 593         return removeFirst();
 594     }
 595 
 596     /**
 597      * Removes the element at the specified position in the elements array.
 598      * This can result in forward or backwards motion of array elements.
 599      * We optimize for least element motion.
 600      *
 601      * <p>This method is called delete rather than remove to emphasize
 602      * that its semantics differ from those of {@link List#remove(int)}.
 603      *
 604      * @return true if elements near tail moved backwards
 605      */
 606     boolean delete(int i) {
 607         final Object[] es = elements;
 608         final int capacity = es.length;
 609         final int h, t;
 610         // number of elements before to-be-deleted elt
 611         final int front = sub(i, h = head, capacity);
 612         // number of elements after to-be-deleted elt
 613         final int back = sub(t = tail, i, capacity) - 1;
 614         if (front < back) {
 615             // move front elements forwards
 616             if (h <= i) {
 617                 System.arraycopy(es, h, es, h + 1, front);
 618             } else { // Wrap around
 619                 System.arraycopy(es, 0, es, 1, i);
 620                 es[0] = es[capacity - 1];
 621                 System.arraycopy(es, h, es, h + 1, front - (i + 1));
 622             }
 623             es[h] = null;
 624             head = inc(h, capacity);
 625             return false;
 626         } else {
 627             // move back elements backwards
 628             tail = dec(t, capacity);
 629             if (i <= tail) {
 630                 System.arraycopy(es, i + 1, es, i, back);
 631             } else { // Wrap around
 632                 System.arraycopy(es, i + 1, es, i, capacity - (i + 1));
 633                 es[capacity - 1] = es[0];
 634                 System.arraycopy(es, 1, es, 0, t - 1);
 635             }
 636             es[tail] = null;
 637             return true;
 638         }
 639     }
 640 
 641     // *** Collection Methods ***
 642 
 643     /**
 644      * Returns the number of elements in this deque.
 645      *
 646      * @return the number of elements in this deque
 647      */
 648     public int size() {
 649         return sub(tail, head, elements.length);
 650     }
 651 
 652     /**
 653      * Returns {@code true} if this deque contains no elements.
 654      *
 655      * @return {@code true} if this deque contains no elements
 656      */
 657     public boolean isEmpty() {
 658         return head == tail;
 659     }
 660 
 661     /**
 662      * Returns an iterator over the elements in this deque.  The elements
 663      * will be ordered from first (head) to last (tail).  This is the same
 664      * order that elements would be dequeued (via successive calls to
 665      * {@link #remove} or popped (via successive calls to {@link #pop}).
 666      *
 667      * @return an iterator over the elements in this deque
 668      */
 669     public Iterator<E> iterator() {
 670         return new DeqIterator();
 671     }
 672 
 673     public Iterator<E> descendingIterator() {
 674         return new DescendingIterator();
 675     }
 676 
 677     private class DeqIterator implements Iterator<E> {
 678         /** Index of element to be returned by subsequent call to next. */
 679         int cursor;
 680 
 681         /** Number of elements yet to be returned. */
 682         int remaining = size();
 683 
 684         /**
 685          * Index of element returned by most recent call to next.
 686          * Reset to -1 if element is deleted by a call to remove.
 687          */
 688         int lastRet = -1;
 689 
 690         DeqIterator() { cursor = head; }
 691 
 692         public final boolean hasNext() {
 693             return remaining > 0;
 694         }
 695 
 696         public E next() {
 697             if (remaining <= 0)
 698                 throw new NoSuchElementException();
 699             final Object[] es = elements;
 700             E e = nonNullElementAt(es, cursor);
 701             cursor = inc(lastRet = cursor, es.length);
 702             remaining--;
 703             return e;
 704         }
 705 
 706         void postDelete(boolean leftShifted) {
 707             if (leftShifted)
 708                 cursor = dec(cursor, elements.length);
 709         }
 710 
 711         public final void remove() {
 712             if (lastRet < 0)
 713                 throw new IllegalStateException();
 714             postDelete(delete(lastRet));
 715             lastRet = -1;
 716         }
 717 
 718         public void forEachRemaining(Consumer<? super E> action) {
 719             Objects.requireNonNull(action);
 720             int r;
 721             if ((r = remaining) <= 0)
 722                 return;
 723             remaining = 0;
 724             final Object[] es = elements;
 725             if (es[cursor] == null || sub(tail, cursor, es.length) != r)
 726                 throw new ConcurrentModificationException();
 727             for (int i = cursor, end = tail, to = (i <= end) ? end : es.length;
 728                  ; i = 0, to = end) {
 729                 for (; i < to; i++)
 730                     action.accept(elementAt(es, i));
 731                 if (to == end) {
 732                     if (end != tail)
 733                         throw new ConcurrentModificationException();
 734                     lastRet = dec(end, es.length);
 735                     break;
 736                 }
 737             }
 738         }
 739     }
 740 
 741     private class DescendingIterator extends DeqIterator {
 742         DescendingIterator() { cursor = dec(tail, elements.length); }
 743 
 744         public final E next() {
 745             if (remaining <= 0)
 746                 throw new NoSuchElementException();
 747             final Object[] es = elements;
 748             E e = nonNullElementAt(es, cursor);
 749             cursor = dec(lastRet = cursor, es.length);
 750             remaining--;
 751             return e;
 752         }
 753 
 754         void postDelete(boolean leftShifted) {
 755             if (!leftShifted)
 756                 cursor = inc(cursor, elements.length);
 757         }
 758 
 759         public final void forEachRemaining(Consumer<? super E> action) {
 760             Objects.requireNonNull(action);
 761             int r;
 762             if ((r = remaining) <= 0)
 763                 return;
 764             remaining = 0;
 765             final Object[] es = elements;
 766             if (es[cursor] == null || sub(cursor, head, es.length) + 1 != r)
 767                 throw new ConcurrentModificationException();
 768             for (int i = cursor, end = head, to = (i >= end) ? end : 0;
 769                  ; i = es.length - 1, to = end) {
 770                 // hotspot generates faster code than for: i >= to !
 771                 for (; i > to - 1; i--)
 772                     action.accept(elementAt(es, i));
 773                 if (to == end) {
 774                     if (end != head)
 775                         throw new ConcurrentModificationException();
 776                     lastRet = end;
 777                     break;
 778                 }
 779             }
 780         }
 781     }
 782 
 783     /**
 784      * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
 785      * and <em>fail-fast</em> {@link Spliterator} over the elements in this
 786      * deque.
 787      *
 788      * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
 789      * {@link Spliterator#SUBSIZED}, {@link Spliterator#ORDERED}, and
 790      * {@link Spliterator#NONNULL}.  Overriding implementations should document
 791      * the reporting of additional characteristic values.
 792      *
 793      * @return a {@code Spliterator} over the elements in this deque
 794      * @since 1.8
 795      */
 796     public Spliterator<E> spliterator() {
 797         return new DeqSpliterator();
 798     }
 799 
 800     final class DeqSpliterator implements Spliterator<E> {
 801         private int fence;      // -1 until first use
 802         private int cursor;     // current index, modified on traverse/split
 803 
 804         /** Constructs late-binding spliterator over all elements. */
 805         DeqSpliterator() {
 806             this.fence = -1;
 807         }
 808 
 809         /** Constructs spliterator over the given range. */
 810         DeqSpliterator(int origin, int fence) {
 811             // assert 0 <= origin && origin < elements.length;
 812             // assert 0 <= fence && fence < elements.length;
 813             this.cursor = origin;
 814             this.fence = fence;
 815         }
 816 
 817         /** Ensures late-binding initialization; then returns fence. */
 818         private int getFence() { // force initialization
 819             int t;
 820             if ((t = fence) < 0) {
 821                 t = fence = tail;
 822                 cursor = head;
 823             }
 824             return t;
 825         }
 826 
 827         public DeqSpliterator trySplit() {
 828             final Object[] es = elements;
 829             final int i, n;
 830             return ((n = sub(getFence(), i = cursor, es.length) >> 1) <= 0)
 831                 ? null
 832                 : new DeqSpliterator(i, cursor = inc(i, n, es.length));
 833         }
 834 
 835         public void forEachRemaining(Consumer<? super E> action) {
 836             if (action == null)
 837                 throw new NullPointerException();
 838             final int end = getFence(), cursor = this.cursor;
 839             final Object[] es = elements;
 840             if (cursor != end) {
 841                 this.cursor = end;
 842                 // null check at both ends of range is sufficient
 843                 if (es[cursor] == null || es[dec(end, es.length)] == null)
 844                     throw new ConcurrentModificationException();
 845                 for (int i = cursor, to = (i <= end) ? end : es.length;
 846                      ; i = 0, to = end) {
 847                     for (; i < to; i++)
 848                         action.accept(elementAt(es, i));
 849                     if (to == end) break;
 850                 }
 851             }
 852         }
 853 
 854         public boolean tryAdvance(Consumer<? super E> action) {
 855             Objects.requireNonNull(action);
 856             final Object[] es = elements;
 857             if (fence < 0) { fence = tail; cursor = head; } // late-binding
 858             final int i;
 859             if ((i = cursor) == fence)
 860                 return false;
 861             E e = nonNullElementAt(es, i);
 862             cursor = inc(i, es.length);
 863             action.accept(e);
 864             return true;
 865         }
 866 
 867         public long estimateSize() {
 868             return sub(getFence(), cursor, elements.length);
 869         }
 870 
 871         public int characteristics() {
 872             return Spliterator.NONNULL
 873                 | Spliterator.ORDERED
 874                 | Spliterator.SIZED
 875                 | Spliterator.SUBSIZED;
 876         }
 877     }
 878 
 879     /**
 880      * @throws NullPointerException {@inheritDoc}
 881      */
 882     public void forEach(Consumer<? super E> action) {
 883         Objects.requireNonNull(action);
 884         final Object[] es = elements;
 885         for (int i = head, end = tail, to = (i <= end) ? end : es.length;
 886              ; i = 0, to = end) {
 887             for (; i < to; i++)
 888                 action.accept(elementAt(es, i));
 889             if (to == end) {
 890                 if (end != tail) throw new ConcurrentModificationException();
 891                 break;
 892             }
 893         }
 894     }
 895 
 896     /**
 897      * @throws NullPointerException {@inheritDoc}
 898      */
 899     public boolean removeIf(Predicate<? super E> filter) {
 900         Objects.requireNonNull(filter);
 901         return bulkRemove(filter);
 902     }
 903 
 904     /**
 905      * @throws NullPointerException {@inheritDoc}
 906      */
 907     public boolean removeAll(Collection<?> c) {
 908         Objects.requireNonNull(c);
 909         return bulkRemove(e -> c.contains(e));
 910     }
 911 
 912     /**
 913      * @throws NullPointerException {@inheritDoc}
 914      */
 915     public boolean retainAll(Collection<?> c) {
 916         Objects.requireNonNull(c);
 917         return bulkRemove(e -> !c.contains(e));
 918     }
 919 
 920     /** Implementation of bulk remove methods. */
 921     private boolean bulkRemove(Predicate<? super E> filter) {
 922         final Object[] es = elements;
 923         // Optimize for initial run of survivors
 924         for (int i = head, end = tail, to = (i <= end) ? end : es.length;
 925              ; i = 0, to = end) {
 926             for (; i < to; i++)
 927                 if (filter.test(elementAt(es, i)))
 928                     return bulkRemoveModified(filter, i);
 929             if (to == end) {
 930                 if (end != tail) throw new ConcurrentModificationException();
 931                 break;
 932             }
 933         }
 934         return false;
 935     }
 936 
 937     // A tiny bit set implementation
 938 
 939     private static long[] nBits(int n) {
 940         return new long[((n - 1) >> 6) + 1];
 941     }
 942     private static void setBit(long[] bits, int i) {
 943         bits[i >> 6] |= 1L << i;
 944     }
 945     private static boolean isClear(long[] bits, int i) {
 946         return (bits[i >> 6] & (1L << i)) == 0;
 947     }
 948 
 949     /**
 950      * Helper for bulkRemove, in case of at least one deletion.
 951      * Tolerate predicates that reentrantly access the collection for
 952      * read (but writers still get CME), so traverse once to find
 953      * elements to delete, a second pass to physically expunge.
 954      *
 955      * @param beg valid index of first element to be deleted
 956      */
 957     private boolean bulkRemoveModified(
 958         Predicate<? super E> filter, final int beg) {
 959         final Object[] es = elements;
 960         final int capacity = es.length;
 961         final int end = tail;
 962         final long[] deathRow = nBits(sub(end, beg, capacity));
 963         deathRow[0] = 1L;   // set bit 0
 964         for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg;
 965              ; i = 0, to = end, k -= capacity) {
 966             for (; i < to; i++)
 967                 if (filter.test(elementAt(es, i)))
 968                     setBit(deathRow, i - k);
 969             if (to == end) break;
 970         }
 971         // a two-finger traversal, with hare i reading, tortoise w writing
 972         int w = beg;
 973         for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg;
 974              ; w = 0) { // w rejoins i on second leg
 975             // In this loop, i and w are on the same leg, with i > w
 976             for (; i < to; i++)
 977                 if (isClear(deathRow, i - k))
 978                     es[w++] = es[i];
 979             if (to == end) break;
 980             // In this loop, w is on the first leg, i on the second
 981             for (i = 0, to = end, k -= capacity; i < to && w < capacity; i++)
 982                 if (isClear(deathRow, i - k))
 983                     es[w++] = es[i];
 984             if (i >= to) {
 985                 if (w == capacity) w = 0; // "corner" case
 986                 break;
 987             }
 988         }
 989         if (end != tail) throw new ConcurrentModificationException();
 990         circularClear(es, tail = w, end);
 991         return true;
 992     }
 993 
 994     /**
 995      * Returns {@code true} if this deque contains the specified element.
 996      * More formally, returns {@code true} if and only if this deque contains
 997      * at least one element {@code e} such that {@code o.equals(e)}.
 998      *
 999      * @param o object to be checked for containment in this deque
1000      * @return {@code true} if this deque contains the specified element
1001      */
1002     public boolean contains(Object o) {
1003         if (o != null) {
1004             final Object[] es = elements;
1005             for (int i = head, end = tail, to = (i <= end) ? end : es.length;
1006                  ; i = 0, to = end) {
1007                 for (; i < to; i++)
1008                     if (o.equals(es[i]))
1009                         return true;
1010                 if (to == end) break;
1011             }
1012         }
1013         return false;
1014     }
1015 
1016     /**
1017      * Removes a single instance of the specified element from this deque.
1018      * If the deque does not contain the element, it is unchanged.
1019      * More formally, removes the first element {@code e} such that
1020      * {@code o.equals(e)} (if such an element exists).
1021      * Returns {@code true} if this deque contained the specified element
1022      * (or equivalently, if this deque changed as a result of the call).
1023      *
1024      * <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}.
1025      *
1026      * @param o element to be removed from this deque, if present
1027      * @return {@code true} if this deque contained the specified element
1028      */
1029     public boolean remove(Object o) {
1030         return removeFirstOccurrence(o);
1031     }
1032 
1033     /**
1034      * Removes all of the elements from this deque.
1035      * The deque will be empty after this call returns.
1036      */
1037     public void clear() {
1038         circularClear(elements, head, tail);
1039         head = tail = 0;
1040     }
1041 
1042     /**
1043      * Nulls out slots starting at array index i, upto index end.
1044      * Condition i == end means "empty" - nothing to do.
1045      */
1046     private static void circularClear(Object[] es, int i, int end) {
1047         // assert 0 <= i && i < es.length;
1048         // assert 0 <= end && end < es.length;
1049         for (int to = (i <= end) ? end : es.length;
1050              ; i = 0, to = end) {
1051             for (; i < to; i++) es[i] = null;
1052             if (to == end) break;
1053         }
1054     }
1055 
1056     /**
1057      * Returns an array containing all of the elements in this deque
1058      * in proper sequence (from first to last element).
1059      *
1060      * <p>The returned array will be "safe" in that no references to it are
1061      * maintained by this deque.  (In other words, this method must allocate
1062      * a new array).  The caller is thus free to modify the returned array.
1063      *
1064      * <p>This method acts as bridge between array-based and collection-based
1065      * APIs.
1066      *
1067      * @return an array containing all of the elements in this deque
1068      */
1069     public Object[] toArray() {
1070         return toArray(Object[].class);
1071     }
1072 
1073     private <T> T[] toArray(Class<T[]> klazz) {
1074         final Object[] es = elements;
1075         final T[] a;
1076         final int head = this.head, tail = this.tail, end;
1077         if ((end = tail + ((head <= tail) ? 0 : es.length)) >= 0) {
1078             // Uses null extension feature of copyOfRange
1079             a = Arrays.copyOfRange(es, head, end, klazz);
1080         } else {
1081             // integer overflow!
1082             a = Arrays.copyOfRange(es, 0, end - head, klazz);
1083             System.arraycopy(es, head, a, 0, es.length - head);
1084         }
1085         if (end != tail)
1086             System.arraycopy(es, 0, a, es.length - head, tail);
1087         return a;
1088     }
1089 
1090     /**
1091      * Returns an array containing all of the elements in this deque in
1092      * proper sequence (from first to last element); the runtime type of the
1093      * returned array is that of the specified array.  If the deque fits in
1094      * the specified array, it is returned therein.  Otherwise, a new array
1095      * is allocated with the runtime type of the specified array and the
1096      * size of this deque.
1097      *
1098      * <p>If this deque fits in the specified array with room to spare
1099      * (i.e., the array has more elements than this deque), the element in
1100      * the array immediately following the end of the deque is set to
1101      * {@code null}.
1102      *
1103      * <p>Like the {@link #toArray()} method, this method acts as bridge between
1104      * array-based and collection-based APIs.  Further, this method allows
1105      * precise control over the runtime type of the output array, and may,
1106      * under certain circumstances, be used to save allocation costs.
1107      *
1108      * <p>Suppose {@code x} is a deque known to contain only strings.
1109      * The following code can be used to dump the deque into a newly
1110      * allocated array of {@code String}:
1111      *
1112      * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
1113      *
1114      * Note that {@code toArray(new Object[0])} is identical in function to
1115      * {@code toArray()}.
1116      *
1117      * @param a the array into which the elements of the deque are to
1118      *          be stored, if it is big enough; otherwise, a new array of the
1119      *          same runtime type is allocated for this purpose
1120      * @return an array containing all of the elements in this deque
1121      * @throws ArrayStoreException if the runtime type of the specified array
1122      *         is not a supertype of the runtime type of every element in
1123      *         this deque
1124      * @throws NullPointerException if the specified array is null
1125      */
1126     @SuppressWarnings("unchecked")
1127     public <T> T[] toArray(T[] a) {
1128         final int size;
1129         if ((size = size()) > a.length)
1130             return toArray((Class<T[]>) a.getClass());
1131         final Object[] es = elements;
1132         for (int i = head, j = 0, len = Math.min(size, es.length - i);
1133              ; i = 0, len = tail) {
1134             System.arraycopy(es, i, a, j, len);
1135             if ((j += len) == size) break;
1136         }
1137         if (size < a.length)
1138             a[size] = null;
1139         return a;
1140     }
1141 
1142     // *** Object methods ***
1143 
1144     /**
1145      * Returns a copy of this deque.
1146      *
1147      * @return a copy of this deque
1148      */
1149     public ArrayDeque<E> clone() {
1150         try {
1151             @SuppressWarnings("unchecked")
1152             ArrayDeque<E> result = (ArrayDeque<E>) super.clone();
1153             result.elements = Arrays.copyOf(elements, elements.length);
1154             return result;
1155         } catch (CloneNotSupportedException e) {
1156             throw new AssertionError();
1157         }
1158     }
1159 
1160     private static final long serialVersionUID = 2340985798034038923L;
1161 
1162     /**
1163      * Saves this deque to a stream (that is, serializes it).
1164      *
1165      * @param s the stream
1166      * @throws java.io.IOException if an I/O error occurs
1167      * @serialData The current size ({@code int}) of the deque,
1168      * followed by all of its elements (each an object reference) in
1169      * first-to-last order.
1170      */
1171     private void writeObject(java.io.ObjectOutputStream s)
1172             throws java.io.IOException {
1173         s.defaultWriteObject();
1174 
1175         // Write out size
1176         s.writeInt(size());
1177 
1178         // Write out elements in order.
1179         final Object[] es = elements;
1180         for (int i = head, end = tail, to = (i <= end) ? end : es.length;
1181              ; i = 0, to = end) {
1182             for (; i < to; i++)
1183                 s.writeObject(es[i]);
1184             if (to == end) break;
1185         }
1186     }
1187 
1188     /**
1189      * Reconstitutes this deque from a stream (that is, deserializes it).
1190      * @param s the stream
1191      * @throws ClassNotFoundException if the class of a serialized object
1192      *         could not be found
1193      * @throws java.io.IOException if an I/O error occurs
1194      */
1195     private void readObject(java.io.ObjectInputStream s)
1196             throws java.io.IOException, ClassNotFoundException {
1197         s.defaultReadObject();
1198 
1199         // Read in size and allocate array
1200         int size = s.readInt();
1201         SharedSecrets.getJavaObjectInputStreamAccess().checkArray(s, Object[].class, size + 1);
1202         elements = new Object[size + 1];
1203         this.tail = size;
1204 
1205         // Read in all elements in the proper order.
1206         for (int i = 0; i < size; i++)
1207             elements[i] = s.readObject();
1208     }
1209 
1210     /** debugging */
1211     void checkInvariants() {
1212         // Use head and tail fields with empty slot at tail strategy.
1213         // head == tail disambiguates to "empty".
1214         try {
1215             int capacity = elements.length;
1216             // assert 0 <= head && head < capacity;
1217             // assert 0 <= tail && tail < capacity;
1218             // assert capacity > 0;
1219             // assert size() < capacity;
1220             // assert head == tail || elements[head] != null;
1221             // assert elements[tail] == null;
1222             // assert head == tail || elements[dec(tail, capacity)] != null;
1223         } catch (Throwable t) {
1224             System.err.printf("head=%d tail=%d capacity=%d%n",
1225                               head, tail, elements.length);
1226             System.err.printf("elements=%s%n",
1227                               Arrays.toString(elements));
1228             throw t;
1229         }
1230     }
1231 
1232 }