/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
 *
 *
 *
 *
 * 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 java.util.function.Predicate;
import java.util.function.UnaryOperator;
import jdk.internal.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 {@link #iterator() iterator}
 * method are <em>fail-fast</em>: 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}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @author Josh Bloch and Doug Lea
 * @param <E> the type of elements held in this deque
 * @since 1.6
 */
public class ArrayDeque<E> extends AbstractCollection<E>
 implements Deque<E>, Cloneable, Serializable
{
 /*
 * VMs excel at optimizing simple array loops where indices are
 * incrementing or decrementing over a valid slice, e.g.
 *
 * for (int i = start; i < end; i++) ... elements[i]
 *
 * Because in a circular array, elements are in general stored in
 * two disjoint such slices, we help the VM by writing unusual
 * nested loops for all traversals over the elements. Having only
 * one hot inner loop body instead of two or three eases human
 * maintenance and encourages VM loop inlining into the caller.
 */

 /**
 * The array in which the elements of the deque are stored.
 * All array cells not holding deque elements are always null.
 * The array always has at least one null slot (at tail).
 */
 transient Object[] elements;

 /**
 * 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 0 <= head < elements.length 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));
 * elements[tail] is always null.
 */
 transient int tail;

 /**
 * The maximum size of array to allocate.
 * Some VMs reserve some header words in an array.
 * Attempts to allocate larger arrays may result in
 * OutOfMemoryError: Requested array size exceeds VM limit
 */
 private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

 /**
 * Increases the capacity of this deque by at least the given amount.
 *
 * @param needed the required minimum extra capacity; must be positive
 */
 private void grow(int needed) {
 // overflow-conscious code
 final int oldCapacity = elements.length;
 int newCapacity;
 // Double capacity if small; else grow by 50%
 int jump = (oldCapacity < 64) ? (oldCapacity + 2) : (oldCapacity >> 1);
 if (jump < needed
 || (newCapacity = (oldCapacity + jump)) - MAX_ARRAY_SIZE > 0)
 newCapacity = newCapacity(needed, jump);
 final Object[] es = elements = Arrays.copyOf(elements, newCapacity);
 // Exceptionally, here tail == head needs to be disambiguated
 if (tail < head || (tail == head && es[head] != null)) {
 // wrap around; slide first leg forward to end of array
 int newSpace = newCapacity - oldCapacity;
 System.arraycopy(es, head,
 es, head + newSpace,
 oldCapacity - head);
 for (int i = head, to = (head += newSpace); i < to; i++)
 es[i] = null;
 }
 }

 /** Capacity calculation for edge conditions, especially overflow. */
 private int newCapacity(int needed, int jump) {
 final int oldCapacity = elements.length, minCapacity;
 if ((minCapacity = oldCapacity + needed) - MAX_ARRAY_SIZE > 0) {
 if (minCapacity < 0)
 throw new IllegalStateException("Sorry, deque too big");
 return Integer.MAX_VALUE;
 }
 if (needed > jump)
 return minCapacity;
 return (oldCapacity + jump - MAX_ARRAY_SIZE < 0)
 ? oldCapacity + jump
 : MAX_ARRAY_SIZE;
 }

 /**
 * 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) {
 elements =
 new Object[(numElements < 1) ? 1 :
 (numElements == Integer.MAX_VALUE) ? Integer.MAX_VALUE :
 numElements + 1];
 }

 /**
 * 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) {
 this(c.size());
 copyElements(c);
 }

 /**
 * Circularly increments i, mod modulus.
 * Precondition and postcondition: 0 <= i < modulus.
 */
 static final int inc(int i, int modulus) {
 if (++i >= modulus) i = 0;
 return i;
 }

 /**
 * Circularly decrements i, mod modulus.
 * Precondition and postcondition: 0 <= i < modulus.
 */
 static final int dec(int i, int modulus) {
 if (--i < 0) i = modulus - 1;
 return i;
 }

 /**
 * Circularly adds the given distance to index i, mod modulus.
 * Precondition: 0 <= i < modulus, 0 <= distance <= modulus.
 * @return index 0 <= i < modulus
 */
 static final int inc(int i, int distance, int modulus) {
 if ((i += distance) - modulus >= 0) i -= modulus;
 return i;
 }

 /**
 * Subtracts j from i, mod modulus.
 * Index i must be logically ahead of index j.
 * Precondition: 0 <= i < modulus, 0 <= j < modulus.
 * @return the "circular distance" from j to i; corner case i == j
 * is disambiguated to "empty", returning 0.
 */
 static final int sub(int i, int j, int modulus) {
 if ((i -= j) < 0) i += modulus;
 return i;
 }

 /**
 * Returns element at array index i.
 * This is a slight abuse of generics, accepted by javac.
 */
 @SuppressWarnings("unchecked")
 static final <E> E elementAt(Object[] es, int i) {
 return (E) es[i];
 }

 /**
 * A version of elementAt that checks for null elements.
 * This check doesn't catch all possible comodifications,
 * but does catch ones that corrupt traversal.
 */
 static final <E> E nonNullElementAt(Object[] es, int i) {
 @SuppressWarnings("unchecked") E e = (E) es[i];
 if (e == null)
 throw new ConcurrentModificationException();
 return e;
 }

 // 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();
 final Object[] es = elements;
 es[head = dec(head, es.length)] = e;
 if (head == tail)
 grow(1);
 }

 /**
 * 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();
 final Object[] es = elements;
 es[tail] = e;
 if (head == (tail = inc(tail, es.length)))
 grow(1);
 }

 /**
 * Adds all of the elements in the specified collection at the end
 * of this deque, as if by calling {@link #addLast} on each one,
 * in the order that they are returned by the collection's iterator.
 *
 * @param c the elements to be inserted into this deque
 * @return {@code true} if this deque changed as a result of the call
 * @throws NullPointerException if the specified collection or any
 * of its elements are null
 */
 public boolean addAll(Collection<? extends E> c) {
 final int s, needed;
 if ((needed = (s = size()) + c.size() + 1 - elements.length) > 0)
 grow(needed);
 copyElements(c);
 return size() > s;
 }

 private void copyElements(Collection<? extends E> c) {
 c.forEach(this::addLast);
 }

 /**
 * 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 e = pollFirst();
 if (e == null)
 throw new NoSuchElementException();
 return e;
 }

 /**
 * @throws NoSuchElementException {@inheritDoc}
 */
 public E removeLast() {
 E e = pollLast();
 if (e == null)
 throw new NoSuchElementException();
 return e;
 }

 public E pollFirst() {
 final Object[] es;
 final int h;
 E e = elementAt(es = elements, h = head);
 if (e != null) {
 es[h] = null;
 head = inc(h, es.length);
 }
 return e;
 }

 public E pollLast() {
 final Object[] es;
 final int t;
 E e = elementAt(es = elements, t = dec(tail, es.length));
 if (e != null)
 es[tail = t] = null;
 return e;
 }

 /**
 * @throws NoSuchElementException {@inheritDoc}
 */
 public E getFirst() {
 E e = elementAt(elements, head);
 if (e == null)
 throw new NoSuchElementException();
 return e;
 }

 /**
 * @throws NoSuchElementException {@inheritDoc}
 */
 public E getLast() {
 final Object[] es = elements;
 E e = elementAt(es, dec(tail, es.length));
 if (e == null)
 throw new NoSuchElementException();
 return e;
 }

 public E peekFirst() {
 return elementAt(elements, head);
 }

 public E peekLast() {
 final Object[] es;
 return elementAt(es = elements, dec(tail, es.length));
 }

 /**
 * 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) {
 final Object[] es = elements;
 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
 ; i = 0, to = end) {
 for (; i < to; i++)
 if (o.equals(es[i])) {
 delete(i);
 return true;
 }
 if (to == end) break;
 }
 }
 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) {
 final Object[] es = elements;
 for (int i = tail, end = head, to = (i >= end) ? end : 0;
 ; i = es.length, to = end) {
 for (i--; i > to - 1; i--)
 if (o.equals(es[i])) {
 delete(i);
 return true;
 }
 if (to == end) break;
 }
 }
 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();
 }

 /**
 * Removes the element at the specified position in the elements array.
 * This can result in forward or backwards motion of array elements.
 * We optimize for least element motion.
 *
 * <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 near tail moved backwards
 */
 boolean delete(int i) {
 final Object[] es = elements;
 final int capacity = es.length;
 final int h, t;
 // number of elements before to-be-deleted elt
 final int front = sub(i, h = head, capacity);
 // number of elements after to-be-deleted elt
 final int back = sub(t = tail, i, capacity) - 1;
 if (front < back) {
 // move front elements forwards
 if (h <= i) {
 System.arraycopy(es, h, es, h + 1, front);
 } else { // Wrap around
 System.arraycopy(es, 0, es, 1, i);
 es[0] = es[capacity - 1];
 System.arraycopy(es, h, es, h + 1, front - (i + 1));
 }
 es[h] = null;
 head = inc(h, capacity);
 return false;
 } else {
 // move back elements backwards
 tail = dec(t, capacity);
 if (i <= tail) {
 System.arraycopy(es, i + 1, es, i, back);
 } else { // Wrap around
 System.arraycopy(es, i + 1, es, i, capacity - (i + 1));
 es[capacity - 1] = es[0];
 System.arraycopy(es, 1, es, 0, t - 1);
 }
 es[tail] = null;
 return true;
 }
 }

 // *** Collection Methods ***

 /**
 * Returns the number of elements in this deque.
 *
 * @return the number of elements in this deque
 */
 public int size() {
 return sub(tail, head, elements.length);
 }

 /**
 * 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. */
 int cursor;

 /** Number of elements yet to be returned. */
 int remaining = size();

 /**
 * Index of element returned by most recent call to next.
 * Reset to -1 if element is deleted by a call to remove.
 */
 int lastRet = -1;

 DeqIterator() { cursor = head; }

 public final boolean hasNext() {
 return remaining > 0;
 }

 public E next() {
 if (remaining <= 0)
 throw new NoSuchElementException();
 final Object[] es = elements;
 E e = nonNullElementAt(es, cursor);
 cursor = inc(lastRet = cursor, es.length);
 remaining--;
 return e;
 }

 void postDelete(boolean leftShifted) {
 if (leftShifted)
 cursor = dec(cursor, elements.length);
 }

 public final void remove() {
 if (lastRet < 0)
 throw new IllegalStateException();
 postDelete(delete(lastRet));
 lastRet = -1;
 }

 public void forEachRemaining(Consumer<? super E> action) {
 Objects.requireNonNull(action);
 int r;
 if ((r = remaining) <= 0)
 return;
 remaining = 0;
 final Object[] es = elements;
 if (es[cursor] == null || sub(tail, cursor, es.length) != r)
 throw new ConcurrentModificationException();
 for (int i = cursor, end = tail, to = (i <= end) ? end : es.length;
 ; i = 0, to = end) {
 for (; i < to; i++)
 action.accept(elementAt(es, i));
 if (to == end) {
 if (end != tail)
 throw new ConcurrentModificationException();
 lastRet = dec(end, es.length);
 break;
 }
 }
 }
 }

 private class DescendingIterator extends DeqIterator {
 DescendingIterator() { cursor = dec(tail, elements.length); }

 public final E next() {
 if (remaining <= 0)
 throw new NoSuchElementException();
 final Object[] es = elements;
 E e = nonNullElementAt(es, cursor);
 cursor = dec(lastRet = cursor, es.length);
 remaining--;
 return e;
 }

 void postDelete(boolean leftShifted) {
 if (!leftShifted)
 cursor = inc(cursor, elements.length);
 }

 public final void forEachRemaining(Consumer<? super E> action) {
 Objects.requireNonNull(action);
 int r;
 if ((r = remaining) <= 0)
 return;
 remaining = 0;
 final Object[] es = elements;
 if (es[cursor] == null || sub(cursor, head, es.length) + 1 != r)
 throw new ConcurrentModificationException();
 for (int i = cursor, end = head, to = (i >= end) ? end : 0;
 ; i = es.length - 1, to = end) {
 // hotspot generates faster code than for: i >= to !
 for (; i > to - 1; i--)
 action.accept(elementAt(es, i));
 if (to == end) {
 if (end != head)
 throw new ConcurrentModificationException();
 lastRet = end;
 break;
 }
 }
 }
 }

 /**
 * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
 * and <em>fail-fast</em> {@link Spliterator} over the elements in this
 * deque.
 *
 * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
 * {@link Spliterator#SUBSIZED}, {@link Spliterator#ORDERED}, and
 * {@link Spliterator#NONNULL}. Overriding implementations should document
 * the reporting of additional characteristic values.
 *
 * @return a {@code Spliterator} over the elements in this deque
 * @since 1.8
 */
 public Spliterator<E> spliterator() {
 return new DeqSpliterator();
 }

 final class DeqSpliterator implements Spliterator<E> {
 private int fence; // -1 until first use
 private int cursor; // current index, modified on traverse/split

 /** Constructs late-binding spliterator over all elements. */
 DeqSpliterator() {
 this.fence = -1;
 }

 /** Constructs spliterator over the given range. */
 DeqSpliterator(int origin, int fence) {
 // assert 0 <= origin && origin < elements.length;
 // assert 0 <= fence && fence < elements.length;
 this.cursor = origin;
 this.fence = fence;
 }

 /** Ensures late-binding initialization; then returns fence. */
 private int getFence() { // force initialization
 int t;
 if ((t = fence) < 0) {
 t = fence = tail;
 cursor = head;
 }
 return t;
 }

 public DeqSpliterator trySplit() {
 final Object[] es = elements;
 final int i, n;
 return ((n = sub(getFence(), i = cursor, es.length) >> 1) <= 0)
 ? null
 : new DeqSpliterator(i, cursor = inc(i, n, es.length));
 }

 public void forEachRemaining(Consumer<? super E> action) {
 if (action == null)
 throw new NullPointerException();
 final int end = getFence(), cursor = this.cursor;
 final Object[] es = elements;
 if (cursor != end) {
 this.cursor = end;
 // null check at both ends of range is sufficient
 if (es[cursor] == null || es[dec(end, es.length)] == null)
 throw new ConcurrentModificationException();
 for (int i = cursor, to = (i <= end) ? end : es.length;
 ; i = 0, to = end) {
 for (; i < to; i++)
 action.accept(elementAt(es, i));
 if (to == end) break;
 }
 }
 }

 public boolean tryAdvance(Consumer<? super E> action) {
 Objects.requireNonNull(action);
 final Object[] es = elements;
 if (fence < 0) { fence = tail; cursor = head; } // late-binding
 final int i;
 if ((i = cursor) == fence)
 return false;
 E e = nonNullElementAt(es, i);
 cursor = inc(i, es.length);
 action.accept(e);
 return true;
 }

 public long estimateSize() {
 return sub(getFence(), cursor, elements.length);
 }

 public int characteristics() {
 return Spliterator.NONNULL
 | Spliterator.ORDERED
 | Spliterator.SIZED
 | Spliterator.SUBSIZED;
 }
 }

 /**
 * @throws NullPointerException {@inheritDoc}
 */
 public void forEach(Consumer<? super E> action) {
 Objects.requireNonNull(action);
 final Object[] es = elements;
 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
 ; i = 0, to = end) {
 for (; i < to; i++)
 action.accept(elementAt(es, i));
 if (to == end) {
 if (end != tail) throw new ConcurrentModificationException();
 break;
 }
 }
 }

 /**
 * @throws NullPointerException {@inheritDoc}
 */
 public boolean removeIf(Predicate<? super E> filter) {
 Objects.requireNonNull(filter);
 return bulkRemove(filter);
 }

 /**
 * @throws NullPointerException {@inheritDoc}
 */
 public boolean removeAll(Collection<?> c) {
 Objects.requireNonNull(c);
 return bulkRemove(e -> c.contains(e));
 }

 /**
 * @throws NullPointerException {@inheritDoc}
 */
 public boolean retainAll(Collection<?> c) {
 Objects.requireNonNull(c);
 return bulkRemove(e -> !c.contains(e));
 }

 /** Implementation of bulk remove methods. */
 private boolean bulkRemove(Predicate<? super E> filter) {
 final Object[] es = elements;
 // Optimize for initial run of survivors
 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
 ; i = 0, to = end) {
 for (; i < to; i++)
 if (filter.test(elementAt(es, i)))
 return bulkRemoveModified(filter, i);
 if (to == end) {
 if (end != tail) throw new ConcurrentModificationException();
 break;
 }
 }
 return false;
 }

 // A tiny bit set implementation

 private static long[] nBits(int n) {
 return new long[((n - 1) >> 6) + 1];
 }
 private static void setBit(long[] bits, int i) {
 bits[i >> 6] |= 1L << i;
 }
 private static boolean isClear(long[] bits, int i) {
 return (bits[i >> 6] & (1L << i)) == 0;
 }

 /**
 * Helper for bulkRemove, in case of at least one deletion.
 * Tolerate predicates that reentrantly access the collection for
 * read (but writers still get CME), so traverse once to find
 * elements to delete, a second pass to physically expunge.
 *
 * @param beg valid index of first element to be deleted
 */
 private boolean bulkRemoveModified(
 Predicate<? super E> filter, final int beg) {
 final Object[] es = elements;
 final int capacity = es.length;
 final int end = tail;
 final long[] deathRow = nBits(sub(end, beg, capacity));
 deathRow[0] = 1L; // set bit 0
 for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg;
 ; i = 0, to = end, k -= capacity) {
 for (; i < to; i++)
 if (filter.test(elementAt(es, i)))
 setBit(deathRow, i - k);
 if (to == end) break;
 }
 // a two-finger traversal, with hare i reading, tortoise w writing
 int w = beg;
 for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg;
 ; w = 0) { // w rejoins i on second leg
 // In this loop, i and w are on the same leg, with i > w
 for (; i < to; i++)
 if (isClear(deathRow, i - k))
 es[w++] = es[i];
 if (to == end) break;
 // In this loop, w is on the first leg, i on the second
 for (i = 0, to = end, k -= capacity; i < to && w < capacity; i++)
 if (isClear(deathRow, i - k))
 es[w++] = es[i];
 if (i >= to) {
 if (w == capacity) w = 0; // "corner" case
 break;
 }
 }
 if (end != tail) throw new ConcurrentModificationException();
 circularClear(es, tail = w, end);
 return true;
 }

 /**
 * Returns {@code true} if this deque contains the specified element.
 * More formally, returns {@code true} if and only if this deque contains
 * at least one element {@code e} such that {@code o.equals(e)}.
 *
 * @param o object to be checked for containment in this deque
 * @return {@code true} if this deque contains the specified element
 */
 public boolean contains(Object o) {
 if (o != null) {
 final Object[] es = elements;
 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
 ; i = 0, to = end) {
 for (; i < to; i++)
 if (o.equals(es[i]))
 return true;
 if (to == end) break;
 }
 }
 return false;
 }

 /**
 * Removes a single instance of the specified element from this deque.
 * 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).
 *
 * <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}.
 *
 * @param o element to be removed from this deque, if present
 * @return {@code true} if this deque contained the specified element
 */
 public boolean remove(Object o) {
 return removeFirstOccurrence(o);
 }

 /**
 * Removes all of the elements from this deque.
 * The deque will be empty after this call returns.
 */
 public void clear() {
 circularClear(elements, head, tail);
 head = tail = 0;
 }

 /**
 * Nulls out slots starting at array index i, upto index end.
 * Condition i == end means "empty" - nothing to do.
 */
 private static void circularClear(Object[] es, int i, int end) {
 // assert 0 <= i && i < es.length;
 // assert 0 <= end && end < es.length;
 for (int to = (i <= end) ? end : es.length;
 ; i = 0, to = end) {
 for (; i < to; i++) es[i] = null;
 if (to == end) break;
 }
 }

 /**
 * Returns an array containing all of the elements in this deque
 * in proper sequence (from first to last element).
 *
 * <p>The returned array will be "safe" in that no references to it are
 * maintained by this deque. (In other words, this method must allocate
 * a new array). The caller is thus free to modify the returned array.
 *
 * <p>This method acts as bridge between array-based and collection-based
 * APIs.
 *
 * @return an array containing all of the elements in this deque
 */
 public Object[] toArray() {
 return toArray(Object[].class);
 }

 private <T> T[] toArray(Class<T[]> klazz) {
 final Object[] es = elements;
 final T[] a;
 final int head = this.head, tail = this.tail, end;
 if ((end = tail + ((head <= tail) ? 0 : es.length)) >= 0) {
 // Uses null extension feature of copyOfRange
 a = Arrays.copyOfRange(es, head, end, klazz);
 } else {
 // integer overflow!
 a = Arrays.copyOfRange(es, 0, end - head, klazz);
 System.arraycopy(es, head, a, 0, es.length - head);
 }
 if (end != tail)
 System.arraycopy(es, 0, a, es.length - head, tail);
 return a;
 }

 /**
 * Returns an array containing all of the elements in this deque in
 * proper sequence (from first to last element); the runtime type of the
 * returned array is that of the specified array. If the deque fits in
 * the specified array, it is returned therein. Otherwise, a new array
 * is allocated with the runtime type of the specified array and the
 * size of this deque.
 *
 * <p>If this deque fits in the specified array with room to spare
 * (i.e., the array has more elements than this deque), the element in
 * the array immediately following the end of the deque is set to
 * {@code null}.
 *
 * <p>Like the {@link #toArray()} method, this method acts as bridge between
 * array-based and collection-based APIs. Further, this method allows
 * precise control over the runtime type of the output array, and may,
 * under certain circumstances, be used to save allocation costs.
 *
 * <p>Suppose {@code x} is a deque known to contain only strings.
 * The following code can be used to dump the deque into a newly
 * allocated array of {@code String}:
 *
 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
 *
 * Note that {@code toArray(new Object[0])} is identical in function to
 * {@code toArray()}.
 *
 * @param a the array into which the elements of the deque are to
 * be stored, if it is big enough; otherwise, a new array of the
 * same runtime type is allocated for this purpose
 * @return an array containing all of the elements in this deque
 * @throws ArrayStoreException if the runtime type of the specified array
 * is not a supertype of the runtime type of every element in
 * this deque
 * @throws NullPointerException if the specified array is null
 */
 @SuppressWarnings("unchecked")
 public <T> T[] toArray(T[] a) {
 final int size;
 if ((size = size()) > a.length)
 return toArray((Class<T[]>) a.getClass());
 final Object[] es = elements;
 for (int i = head, j = 0, len = Math.min(size, es.length - i);
 ; i = 0, len = tail) {
 System.arraycopy(es, i, a, j, len);
 if ((j += len) == size) break;
 }
 if (size < a.length)
 a[size] = null;
 return a;
 }

 // *** Object methods ***

 /**
 * Returns a copy of this deque.
 *
 * @return a copy of this deque
 */
 public ArrayDeque<E> clone() {
 try {
 @SuppressWarnings("unchecked")
 ArrayDeque<E> result = (ArrayDeque<E>) super.clone();
 result.elements = Arrays.copyOf(elements, elements.length);
 return result;
 } catch (CloneNotSupportedException e) {
 throw new AssertionError();
 }
 }

 private static final long serialVersionUID = 2340985798034038923L;

 /**
 * Saves this deque to a stream (that is, serializes it).
 *
 * @param s the stream
 * @throws java.io.IOException if an I/O error occurs
 * @serialData The current size ({@code int}) of the deque,
 * followed by all of its elements (each an object reference) in
 * first-to-last order.
 */
 private void writeObject(java.io.ObjectOutputStream s)
 throws java.io.IOException {
 s.defaultWriteObject();

 // Write out size
 s.writeInt(size());

 // Write out elements in order.
 final Object[] es = elements;
 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
 ; i = 0, to = end) {
 for (; i < to; i++)
 s.writeObject(es[i]);
 if (to == end) break;
 }
 }

 /**
 * Reconstitutes this deque from a stream (that is, deserializes it).
 * @param s the stream
 * @throws ClassNotFoundException if the class of a serialized object
 * could not be found
 * @throws java.io.IOException if an I/O error occurs
 */
 private void readObject(java.io.ObjectInputStream s)
 throws java.io.IOException, ClassNotFoundException {
 s.defaultReadObject();

 // Read in size and allocate array
 int size = s.readInt();
 SharedSecrets.getJavaObjectInputStreamAccess().checkArray(s, Object[].class, size + 1);
 elements = new Object[size + 1];
 this.tail = size;

 // Read in all elements in the proper order.
 for (int i = 0; i < size; i++)
 elements[i] = s.readObject();
 }

 /** debugging */
 void checkInvariants() {
 // Use head and tail fields with empty slot at tail strategy.
 // head == tail disambiguates to "empty".
 try {
 int capacity = elements.length;
 // assert 0 <= head && head < capacity;
 // assert 0 <= tail && tail < capacity;
 // assert capacity > 0;
 // assert size() < capacity;
 // assert head == tail || elements[head] != null;
 // assert elements[tail] == null;
 // assert head == tail || elements[dec(tail, capacity)] != null;
 } catch (Throwable t) {
 System.err.printf("head=%d tail=%d capacity=%d%n",
 head, tail, elements.length);
 System.err.printf("elements=%s%n",
 Arrays.toString(elements));
 throw t;
 }
 }

}