/*
 * 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,
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/*
 * 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 java.util.function.Predicate;
import java.util.function.UnaryOperator;
import jdk.internal.misc.SharedSecrets;

Resizable-array implementation of the 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 Stack when used as a stack, and faster than LinkedList when used as a queue. 
Most ArrayDeque operations run in amortized constant time. Exceptions include remove, removeFirstOccurrence, removeLastOccurrence, contains, iterator.remove(), and the bulk operations, all of which run in linear time. 
The iterators returned by this class's iterator method are fail-fast: If the deque is modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will generally throw a 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. 
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 ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators
should be used only to detect bugs.
This class and its iterator implement all of the
optional methods of the Collection and Iterator interfaces. 
This class is a member of the

Java Collections Framework.
Author:
 Josh Bloch and Doug Lea
Type parameters:
<E> – the type of elements held in this deque
Since:
  1.6/**
 * 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).
/**
     * 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.
/**
     * 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.
/**
     * 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
/**
     * 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.
Params:
needed – the required minimum extra capacity; must be positive/**
     * 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. /** 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.
/**
     * 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.
Params:
numElements – lower bound on initial capacity of the deque/**
     * 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 front of the
deque.)
Params:
c – the collection whose elements are to be placed into the deque
Throws:
NullPointerException – if the specified collection is null/**
     * 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.
/**
     * 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.
/**
     * 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.
Returns:
index 0 <= i < modulus/**
     * 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.
Returns:
the "circular distance" from j to i; corner case i == j
is disambiguated to "empty", returning 0./**
     * 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.
/**
     * 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.
/**
     * 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.
Params:
e – the element to add
Throws:
NullPointerException – if the specified element is null/**
     * 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.
This method is equivalent to add. 
Params:
e – the element to add
Throws:
NullPointerException – if the specified element is null/**
     * 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 addLast on each one, in the order that they are returned by the collection's iterator. 
Params:
c – the elements to be inserted into this deque
Throws:
NullPointerException – if the specified collection or any
        of its elements are null
Returns:
true if this deque changed as a result of the call/**
     * 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.
Params:
e – the element to add
Throws:
NullPointerException – if the specified element is null
Returns:
true (as specified by Deque.offerFirst)/**
     * 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.
Params:
e – the element to add
Throws:
NullPointerException – if the specified element is null
Returns:
true (as specified by Deque.offerLast)/**
     * 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}/**
     * @throws NoSuchElementException {@inheritDoc}
     */
    public E removeFirst() {
        E e = pollFirst();
        if (e == null)
            throw new NoSuchElementException();
        return e;
    }

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

    
Throws:
NoSuchElementException – {@inheritDoc}/**
     * @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 e such that o.equals(e) (if such an element exists). Returns true if this deque contained the specified element (or equivalently, if this deque changed as a result of the call). 
Params:
o – element to be removed from this deque, if present
Returns:
true if the deque contained the specified element/**
     * 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 e such that o.equals(e) (if such an element exists). Returns true if this deque contained the specified element (or equivalently, if this deque changed as a result of the call). 
Params:
o – element to be removed from this deque, if present
Returns:
true if the deque contained the specified element/**
     * 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.
This method is equivalent to addLast. 
Params:
e – the element to add
Throws:
NullPointerException – if the specified element is null
Returns:
true (as specified by Collection.add)/**
     * 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.
This method is equivalent to offerLast. 
Params:
e – the element to add
Throws:
NullPointerException – if the specified element is null
Returns:
true (as specified by Queue.offer)/**
     * 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 poll() only in that it throws an exception if this deque is empty. 
This method is equivalent to removeFirst. 
Throws:
NoSuchElementException – {@inheritDoc}
Returns:
the head of the queue represented by this deque/**
     * 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 null if this deque is empty. 
This method is equivalent to pollFirst. 
Returns:
the head of the queue represented by this deque, or null if this deque is empty/**
     * 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 peek only in that it throws an exception if this deque is empty. 
This method is equivalent to getFirst. 
Throws:
NoSuchElementException – {@inheritDoc}
Returns:
the head of the queue represented by this deque/**
     * 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 null if this deque is empty. 
This method is equivalent to peekFirst. 
Returns:
the head of the queue represented by this deque, or null if this deque is empty/**
     * 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.
This method is equivalent to addFirst. 
Params:
e – the element to push
Throws:
NullPointerException – if the specified element is null/**
     * 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.
This method is equivalent to removeFirst(). 
Throws:
NoSuchElementException – {@inheritDoc}
Returns:
the element at the front of this deque (which is the top
        of the stack represented by this deque)/**
     * 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.
This method is called delete rather than remove to emphasize that its semantics differ from those of List.remove(int). 
Returns:
true if elements near tail moved backwards/**
     * 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.
Returns:
the number of elements in this deque/**
     * 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 true if this deque contains no elements. 
Returns:
true if this deque contains no elements/**
     * 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 remove or popped (via successive calls to pop). 
Returns:
an iterator over the elements in this deque/**
     * 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. /** Index of element to be returned by subsequent call to next. */
        int cursor;

        
Number of elements yet to be returned. /** 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.
/**
         * 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 late-binding
and fail-fast Spliterator over the elements in this deque. 
The Spliterator reports Spliterator.SIZED, Spliterator.SUBSIZED, Spliterator.ORDERED, and Spliterator.NONNULL. Overriding implementations should document the reporting of additional characteristic values. 
Returns:
a Spliterator over the elements in this deque
Since:
1.8/**
     * 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. /** Constructs late-binding spliterator over all elements. */
        DeqSpliterator() {
            this.fence = -1;
        }

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

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

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

    
Implementation of bulk remove methods. /** 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.
Params:
beg – valid index of first element to be deleted/**
     * 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 true if this deque contains the specified element. More formally, returns true if and only if this deque contains at least one element e such that o.equals(e). 
Params:
o – object to be checked for containment in this deque
Returns:
true if this deque contains the specified element/**
     * 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 e such that o.equals(e) (if such an element exists). Returns true if this deque contained the specified element (or equivalently, if this deque changed as a result of the call). 
This method is equivalent to removeFirstOccurrence(Object). 
Params:
o – element to be removed from this deque, if present
Returns:
true if this deque contained the specified element/**
     * 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.
/**
     * 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.
/**
     * 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).
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.
This method acts as bridge between array-based and collection-based
APIs.
Returns:
an array containing all of the elements in this deque/**
     * 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.
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 null. 
Like the 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. 
Suppose 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 String: 
 String[] y = x.toArray(new String[0]);
 Note that toArray(new Object[0]) is identical in function to toArray(). 
Params:
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
Throws:
ArrayStoreException – if the runtime type of the specified array
        is not a supertype of the runtime type of every element in
        this deque
NullPointerException – if the specified array is null
Returns:
an array containing all of the elements in this deque/**
     * 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.
Returns:
a copy of this deque/**
     * 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).
Params:
s – the stream
Throws:
IOException – if an I/O error occurs
@serialData
The current size (int) of the deque, followed by all of its elements (each an object reference) in first-to-last order./**
     * 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).
Params:
s – the stream
Throws:
ClassNotFoundException – if the class of a serialized object
        could not be found
IOException – if an I/O error occurs/**
     * 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 /** 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;
        }
    }

}