/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Written by Josh Bloch of Google Inc. and released to the public domain,
 * as explained at http://creativecommons.org/publicdomain/zero/1.0/.
 */

package java.util;

import java.io.Serializable;
import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;
import jdk.internal.misc.SharedSecrets;

/**
 * Resizable-array implementation of the {@link Deque} interface. This class
 * also implements all of the methods of the {@link java.util.List} interface
 * (except for equals, hashCode and remove(index) (use removeAt(index) instead))
 * but does not actually implement java.util.List because the original
 * implementation did not and doing so now would break compatibility. There is,
 * however, a subclass {@link ArrayDeque.List} which implements java.util.List
 * provided. 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, but a subclass {@link ArrayDeque.WithNulls} which
 * allows nulls is provided. 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 #add(int, Object)},
 * {@link #remove(Object)},
 * {@link #remove(int)},
 * {@link #indexOf},
 * {@link #lastIndexOf},
 * {@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;

    /**
     * Shared arrays for empty instances
     */
    private static final Object[] EMPTY = new Object[1], DEFAULT = new Object[1];

    /**
     * Increases the capacity of this deque.  Call only when full, i.e.,
     * when head and tail have wrapped around to become equal.
     */
    Object[] grow() {
        assert head == tail;
        Object[] e = elements;
        int n = e.length, p = head, r = n - p;
        Object[] a = new Object[newCapacity(1)];
        System.arraycopy(e, p, a, 0, r);
        System.arraycopy(e, 0, a, r, p);
        head = 0;
        tail = n;
        return elements = a;
    }

    /** Capacity calculation for edge conditions, especially overflow. */
    private int newCapacity(int needed) {
        final int oldCapacity = elements.length, minCapacity, newCapacity;
        if (oldCapacity == 1 && elements == DEFAULT && needed <= 16)
            return 16 + 1;
        // 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)
            return newCapacity;
        if ((minCapacity = oldCapacity + needed) + (1 << 31) > MAX_ARRAY_SIZE + (1 << 31)) {
            if (minCapacity < 0)
                throw new IllegalStateException("Sorry, deque too big");
            return Integer.MAX_VALUE;
        }
        return jump >= needed ? MAX_ARRAY_SIZE : minCapacity;
    }

    /**
     * Copies the elements from our element array into the specified array,
     * in order (from first to last element in the deque).  It is assumed
     * that the array is large enough to hold all elements in the deque.
     *
     * @return its argument
     */
    <T> T[] copyElements(T[] a, int from, int to) {
        if (from < to) {
            System.arraycopy(elements, from, a, 0, to - from);
        } else if (from > to) {
            Object[] es = elements;
            int headPortionLen = es.length - from;
            System.arraycopy(es, from, a, 0, headPortionLen);
            System.arraycopy(es, 0, a, headPortionLen, to);
        }
        return a;
    }

    /**
     * Attempts to reallocate the backing array so that is has space for as close as
     * possible to, but not less than, minSize elements unless it already has space
     * for between minSize and maxSize elements. If the size() of this ArrayDeque is
     * greater than minSize then size() will be treated as minSize.
     *
     * <p>This method can be used to achieve the same result as {@link ArrayList#trimToSize}
     * by calling {@code reallocate(0, 0)} or {@link ArrayList#ensureCapacity(n)} by calling
     * {@code reallocate(n, Integer.MAX_VALUE)}.
     *
     * @param minSize the requested minimum capacity
     * @param maxSize the requested maximum capacity
     * @return the new (or unchanged) capacity
     * @throws IllegalArgumentException if {@code minSize > maxSize} (size() may be greater
     * than maxSize without throwing an exception)
     * @throws OutOfMemoryError if there is not enough memory available to allocate
     * the array or the array would be larger than an implementation defined limit
     */
    public int reallocate(int minSize, int maxSize) {
        if (minSize > maxSize) throw new IllegalArgumentException(minSize + " > " + maxSize);
        int c = elements.length - 1, s = size();
        if (c < minSize) {
            if (minSize + 1 < 0) throw new OutOfMemoryError();
        } else {
            if (c <= maxSize) return c;
            if (s > minSize) {
                if (c == (minSize = s)) return c;
            }
        }
        elements = minSize == 0 ? EMPTY : copyElements(new Object[minSize + 1], head, tail);
        head = 0;
        tail = s;
        return minSize;
    }

    /**
     * Constructs an empty array deque with an initial capacity
     * sufficient to hold 16 elements.
     */
    public ArrayDeque() {
        elements = DEFAULT;
    }

    /**
     * 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 = numElements < 1 ? EMPTY :
            new Object[(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) {
        Object[] a = c.toArray();
        if (a.length == 0) {
            elements = EMPTY;
            return;
        }
        checkArray(a);
        elements = Arrays.copyOf(a, a.length + 1, Object[].class);
        tail = a.length;
    }

    /**
     * A subclass of ArrayDeque that implements {@link java.util.List}. This
     * class overrides {@link #equals} and {@link #hashCode} to provide the
     * behavior specified by the List interface.
     */
    public static class List<E> extends ArrayDeque<E> implements java.util.List<E>, RandomAccess {

        private static final long serialVersionUID = ArrayDeque.serialVersionUID;

        /**
         * Constructs an empty ArrayDeque.List with an initial capacity
         * sufficient to hold 16 elements.
         */
        public List() {

        }

        /**
         * Constructs an empty ArrayDeque.List with an initial capacity
         * sufficient to hold the specified number of elements.
         *
         * @param numElements lower bound on initial capacity of the deque
         */
        public List(int numElements) {
            super(numElements);
        }

        /**
         * Constructs an ArrayDeque.List 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 List(Collection<? extends E> c) {
            super(c);
        }

        public boolean equals(Object o) {
            return o == this || equals(o, head, tail);
        }

        public int hashCode() {
            return hashCode(head, tail);
        }

        public E remove(int index) {
            return removeAt(index);
        }

        public List<E> clone() {
            return (List<E>) super.clone();
        }
    }

    /**
     * A subclass of ArrayDeque.List that accepts null elements (and all other elements).
     */
    public static class WithNulls<E> extends ArrayDeque.List<E> {

        private static final long serialVersionUID = ArrayDeque.serialVersionUID;

        /**
         * Constructs an empty ArrayDeque.WithNulls with an initial capacity
         * sufficient to hold 16 elements.
         */
        public WithNulls() {

        }

        /**
         * Constructs an empty ArrayDeque.WithNulls with an initial capacity
         * sufficient to hold the specified number of elements.
         *
         * @param numElements lower bound on initial capacity of the deque
         */
        public WithNulls(int numElements) {
            super(numElements);
        }

        /**
         * Constructs an ArrayDeque.WithNulls 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 WithNulls(Collection<? extends E> c) {
            super(c);
        }

        void check(E e) {

        }

        void checkArray(Object[] es) {

        }

        boolean isValid(Object e) {
            return true;
        }

        public WithNulls<E> clone() {
            return (WithNulls<E>) super.clone();
        }
    }

    /**
     * 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.
     */
    final E validElementAt(Object[] es, int i) {
        @SuppressWarnings("unchecked") E e = (E) es[i];
        if (!isValid(e))
            throw new ConcurrentModificationException();
        return e;
    }

    // Methods for restricting which elements may be added

    /**
     * Throws one of the specified exceptions if the element can not be
     * added to this ArrayDeque.
     *
     * @param e the element to add
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this deque
     * @throws NullPointerException if the specified element is null and this
     *         deque does not permit null elements
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this deque
     */
    void check(E e) {
        if (e == null)
            throw new NullPointerException();
    }

    /**
     * Throws one of the specified exceptions if any of the elements in the
     * array can not be added to this ArrayDeque.
     *
     * @param a the elements to add
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this deque
     * @throws NullPointerException if the specified element is null and this
     *         deque does not permit null elements
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this deque
     */
    void checkArray(Object[] a) {
        for (Object o : a) {
            if (o == null)
                throw new NullPointerException();
        }
    }

    /**
     * Returns whether or not the element can be in this ArrayDeque.
     *
     * @param e the element to test
     * @return true if the element can be in this ArrayDeque
     */
    boolean isValid(Object e) {
        return e != null;
    }

    // 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) {
        check(e);
        final Object[] es = elements;
        final int h = head = dec(head, es.length);
        if (h == tail) {
            grow()[0] = e;
        } else {
            es[h] = e;
        }
    }

    /**
     * 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) {
        check(e);
        final Object[] es = elements;
        final int t = tail;
        if (head == (tail = inc(t, es.length))) {
            grow()[es.length - 1] = e;
        } else {
            es[t] = e;
        }
    }

    /**
     * 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) {
        Object[] a = c.toArray();
        if (a.length == 0) return false;
        checkArray(a);
        insert(size(), a, 0, a.length);
        return true;
    }

    /**
     * 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() {
        final int h = head;
        final Object[] es = elements;
        E e = elementAt(es, h);
        //optimize for non-empty, non-null case
        if (e == null && h == tail) throw new NoSuchElementException();
        es[h] = null;
        head = inc(h, es.length);
        return e;
    }

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

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

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

    /**
     * @throws NoSuchElementException {@inheritDoc}
     */
    public E getFirst() {
        final int h = head;
        final Object[] es = elements;
        E e = elementAt(es, h);
        if (e == null && h == tail) throw new NoSuchElementException();
        return e;
    }

    /**
     * @throws NoSuchElementException {@inheritDoc}
     */
    public E getLast() {
        final Object[] es = elements;
        final int t = tail;
        E e = elementAt(es, dec(t, es.length));
        if (e == null && t == head) 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) {
        int i = index(o, head, tail);
        if (i == -1) return false;
        delete(i);
        return true;
    }

    /**
     * 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) {
        int i = lastIndex(o, head, tail);
        if (i == -1) return false;
        delete(i);
        return true;
    }

    // *** 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();
    }

    // *** List Methods ***

    /**
     * Returns the element at the specified position in this ArrayDeque.
     *
     * @param index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index >= size()})
     */
    public E get(int index) {
        if (index < 0 || index >= size()) throw new IndexOutOfBoundsException();
        final Object[] es = elements;
        return elementAt(es, inc(head, index, es.length));
    }

    /**
     * Replaces the element at the specified position in this ArrayDeque with
     * the specified element.
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws NullPointerException if the specified element is null
     * @throws IndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index >= size()})
     */
    public E set(int index, E element) {
        if (index < 0 || index >= size()) throw new IndexOutOfBoundsException();
        check(element);
        final Object[] es = elements;
        index = inc(head, index, es.length);
        E old = elementAt(es, index);
        es[index] = element;
        return old;
    }

    /**
     * Inserts the specified element at the specified position in this
     * ArrayDeque. Shifts the element currently at that position
     * (if any) and any subsequent elements to the right (adds one to their
     * indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws NullPointerException if the specified element is null
     * @throws IndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index > size()})
     */
    public void add(int index, E element) {
        if (index < 0 || index > size()) throw new IndexOutOfBoundsException();
        check(element);
        insert(inc(head, index, elements.length), element);
    }

    /**
     * Removes the element at the specified position in this ArrayDeque.
     * Shifts any subsequent elements to the left (subtracts one
     * from their indices).  Returns the element that was removed from the
     * ArrayDeque.
     *
     * @param index the index of the element to be removed
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index >= size()})
     */
    public E removeAt(int index) {
        if (index < 0 || index >= size()) throw new IndexOutOfBoundsException();
        final Object[] es = elements;
        index = inc(head, index, es.length);
        E old = elementAt(es, index);
        delete(index);
        return old;
    }

    /**
     * Inserts all of the elements in the specified collection into this
     * ArrayDeque at the specified position.  Shifts the
     * element currently at that position (if any) and any subsequent
     * elements to the right (increases their indices).  The new elements
     * will appear in this list in the order that they are returned by the
     * specified collection's iterator.  The behavior of this operation is
     * undefined if the specified collection is modified while the
     * operation is in progress.  (Note that this will occur if the specified
     * collection is this ArrayDeque, and it's nonempty.)
     *
     * @param index index at which to insert the first element from the
     *              specified collection
     * @param c collection containing elements to be added to this list
     * @return {@code true} if this list changed as a result of the call
     * @throws NullPointerException if the specified collection contains one
     *         or more null elements or if the specified collection is null
     * @throws IndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index > size()})
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        if (index < 0 || index > size()) throw new IndexOutOfBoundsException();
        Object[] a = c.toArray();
        if (a.length == 0) return false;
        checkArray(a);
        insert(index, a, 0, a.length);
        return true;
    }

    /**
     * Returns the index of the first occurrence of the specified element
     * in this ArrayDeque, or -1 if this ArrayDeque does not contain the element.
     * More formally, returns the lowest index {@code i} such that
     * {@code Objects.equals(o, get(i))},
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the first occurrence of the specified element in
     *         this list, or -1 if this list does not contain the element
     */
    public int indexOf(Object o) {
        int i = index(o, head, tail);
        return i == -1 ? -1 : sub(i, head, elements.length);
    }

    /**
     * Returns the index of the last occurrence of the specified element
     * in this ArrayDeque, or -1 if this ArrayDeque does not contain the element.
     * More formally, returns the highest index {@code i} such that
     * {@code Objects.equals(o, get(i))},
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the last occurrence of the specified element in
     *         this list, or -1 if this list does not contain the element
     */
    public int lastIndexOf(Object o) {
        int i = lastIndex(o, head, tail);
        return i == -1 ? -1 : sub(i, head, elements.length);
    }

    int index(Object o, int from, int to) {
        final Object[] es = elements;
        if (o != null) {
            for (int i = from, e = (i <= to) ? to : es.length; ; i = 0, e = to) {
                for (; i < e; i++)
                    if (o.equals(es[i])) return i;
                if (e == to) break;
            }
        } else {
            for (int i = from, e = (i <= to) ? to : es.length; ; i = 0, e = to) {
                for (; i < e; i++)
                    if (es[i] == null) return i;
                if (e == to) break;
            }
        }
        return -1;
    }

    int lastIndex(Object o, int to, int from) {
        final Object[] es = elements;
        if (o != null) {
            for (int i = from, e = (i >= to) ? to : 0; ; i = es.length, e = to) {
                for ( ; i > e; )
                    if (o.equals(es[--i])) return i;
                if (e == to) break;
            }
        } else {
            for (int i = from, e = (i >= to) ? to : 0; ; i = es.length, e = to) {
                for ( ; i > e; )
                    if (es[--i] == null) return i;
                if (e == to) break;
            }
        }
        return -1;
    }

    /**
     * Replaces each element of this ArrayDeque with the result of applying the
     * operator to that element.  Errors or runtime exceptions thrown by
     * the operator are relayed to the caller.
     *
     * @param operator the operator to apply to each element
     * @throws NullPointerException if the specified operator is null or
     *         if the operator result is a null value
     */
    public void replaceAll(UnaryOperator<E> operator) {
        replaceAll(operator, head, tail);
    }

    void replaceAll(UnaryOperator<E> operator, int from, int to) {
        Objects.requireNonNull(operator);
        final Object[] es = elements;
        for (int i = from, e = (i <= to) ? to : es.length; ; i = 0, e = to) {
            for (; i < e; i++) {
                E v = operator.apply(elementAt(es, i));
                check(v);
                es[i] = v;
            }
            if (e == to) break;
        }
    }

    /**
     * Sorts this ArrayDeque according to the order induced by the specified
     * {@link Comparator}.  The sort is <i>stable</i>: this method must not
     * reorder equal elements.
     *
     * <p>All elements in this ArrayDeque must be <i>mutually comparable</i> using the
     * specified comparator (that is, {@code c.compare(e1, e2)} must not throw
     * a {@code ClassCastException} for any elements {@code e1} and {@code e2}
     * in the ArrayDeque).
     *
     * <p>If the specified comparator is {@code null} then all elements in this
     * ArrayDeque must implement the {@link Comparable} interface and the elements'
     * {@linkplain Comparable natural ordering} should be used.
     *
     * @implNote
     * This implementation is a stable, adaptive, iterative mergesort that
     * requires far fewer than n lg(n) comparisons when the input array is
     * partially sorted, while offering the performance of a traditional
     * mergesort when the input array is randomly ordered.  If the input array
     * is nearly sorted, the implementation requires approximately n
     * comparisons.  Temporary storage requirements vary from a small constant
     * for nearly sorted input arrays to n/2 object references for randomly
     * ordered input arrays.
     *
     * <p>The implementation takes equal advantage of ascending and
     * descending order in its input array, and can take advantage of
     * ascending and descending order in different parts of the same
     * input array.  It is well-suited to merging two or more sorted arrays:
     * simply concatenate the arrays and sort the resulting array.
     *
     * <p>The implementation was adapted from Tim Peters's list sort for Python
     * (<a href="http://svn.python.org/projects/python/trunk/Objects/listsort.txt">
     * TimSort</a>).  It uses techniques from Peter McIlroy's "Optimistic
     * Sorting and Information Theoretic Complexity", in Proceedings of the
     * Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp 467-474,
     * January 1993.
     *
     * @param c the {@code Comparator} used to compare deque elements.
     *          A {@code null} value indicates that the elements'
     *          {@linkplain Comparable natural ordering} should be used
     * @throws ClassCastException if the list contains elements that are not
     *         <i>mutually comparable</i> using the specified comparator
     * @throws IllegalArgumentException
     *         if the comparator is found to violate the {@link Comparator}
     *         contract
     */
    public void sort(Comparator<? super E> c) {
        sort(c, head, tail);
    }

    @SuppressWarnings("unchecked")
    void sort(Comparator<? super E> c, int from, int to) {
        E[] a = (E[]) elements;
        if (from <= to) {
            Arrays.sort(a, from, to, c);
            return;
        }
        if (to == 0) {
            Arrays.sort(a, from, a.length, c);
            return;
        }
        int end = a.length - from;
        int size = end + to;
        int t = tail;
        if (t + (long)size <= head) {
            System.arraycopy(a, from, a, t, end);
            System.arraycopy(a, 0, a, t + end, to);
            try {
                Arrays.sort(a, t, t + size, c);
            } catch(RuntimeException | Error e) {
                Arrays.fill(a, t, t + size, null);
                throw e;
            }
            System.arraycopy(a, t, a, from, end);
            System.arraycopy(a, t + end, a, 0, to);
            Arrays.fill(a, t, t + size, null);
            return;
        }
        E[] e = (E[]) new Object[size];
        System.arraycopy(a, from, e, 0, end);
        System.arraycopy(a, 0, e, end, to);
        Arrays.sort(e, 0, size, c);
        System.arraycopy(e, 0, a, from, end);
        System.arraycopy(e, end, a, 0, to);
    }

    /**
     * Returns a List view of the portion of this ArrayDeque between the specified
     * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.  (If
     * {@code fromIndex} and {@code toIndex} are equal, the returned list is
     * empty.)  The returned list is backed by this ArrayDeque, so
     * changes in the returned list are reflected in this ArrayDeque, and vice-versa.
     * The returned list supports all of the optional list operations supported
     * by this ArrayDeque.<p>
     *
     * This method eliminates the need for explicit range operations (of
     * the sort that commonly exist for arrays).  Any operation that expects
     * a list can be used as a range operation by passing a subList view
     * instead of a whole list.  For example, the following idiom
     * removes a range of elements from a list:
     * <pre>{@code
     *      list.subList(from, to).clear();
     * }</pre>
     * Similar idioms may be constructed for {@code indexOf} and
     * {@code lastIndexOf}, and all of the algorithms in the
     * {@code Collections} class can be applied to a subList.<p>
     *
     * The semantics of the list returned by this method become undefined if
     * the backing ArrayDeque (i.e., this ArrayDeque) is <i>structurally modified</i> in
     * any way other than via the returned list.  (Structural modifications are
     * those that change the size of this ArrayDeque, or otherwise perturb it in such
     * a fashion that iterations in progress may yield incorrect results.)
     *
     * @param fromIndex low endpoint (inclusive) of the subList
     * @param toIndex high endpoint (exclusive) of the subList
     * @return a list view of the specified range within this ArrayDeque
     * @throws IndexOutOfBoundsException for an illegal endpoint index value
     *         ({@code fromIndex < 0 || toIndex > size ||
     *         fromIndex > toIndex})
     */
    public java.util.List<E> subList(int fromIndex, int toIndex) {
        sublistRangeCheck(size(), fromIndex, toIndex);
        return new SubList(null, fromIndex, toIndex - fromIndex);
    }

    static void sublistRangeCheck(int size, int from, int to) {
        if (from < 0) throw new IndexOutOfBoundsException(from + " < 0");
        if (to > size) throw new IndexOutOfBoundsException(to + " > " + size);
        if (from > to) throw new IndexOutOfBoundsException(from + " > " + to);
    }

    private class SubList extends AbstractCollection<E> implements java.util.List<E>, RandomAccess {
        final SubList parent;
        final int offset;
        int size;

        SubList(SubList parent, int offset, int size) {
            this.parent = parent;
            this.size = size;
            this.offset = offset;
        }
        void addToSize(int s) {
            size += s;
            for (SubList p = parent; p != null; p = p.parent) {
                p.size += s;
            }
        }
        public E get(int index) {
            if (index < 0 || index >= size) throw new IndexOutOfBoundsException();
            final Object[] es = elements;
            return elementAt(es, inc(head, offset + index, es.length));
        }
        public E set(int index, E e) {
            if (index < 0 || index >= size) throw new IndexOutOfBoundsException();
            check(e);
            final Object[] es = elements;
            index = inc(head, offset + index, es.length);
            E old = elementAt(es, index);
            es[index] = e;
            return old;
        }
        public void add(int index, E e) {
            if (index < 0 || index > size) throw new IndexOutOfBoundsException();
            check(e);
            insert(inc(head, offset + index, elements.length), e);
            addToSize(1);
        }
        public E remove(int index) {
            if (index < 0 || index >= size) throw new IndexOutOfBoundsException();
            final Object[] es = elements;
            index = inc(head, offset + index, es.length);
            E old = elementAt(es, index);
            delete(index);
            addToSize(-1);
            return old;
        }
        public boolean addAll(int index, Collection<? extends E> c) {
            if (index < 0 || index > size) throw new IndexOutOfBoundsException();
            Object[] a = c.toArray();
            if (a.length == 0) return false;
            checkArray(a);
            insert(index + offset, a, 0, a.length);
            addToSize(a.length);
            return true;
        }
        public java.util.List<E> subList(int from, int to) {
            sublistRangeCheck(size, from, to);
            return new SubList(this, from + offset, to - from);
        }
        public int size() {
            return size;
        }
        public boolean isEmpty() {
            return size == 0;
        }
        public void clear() {
            int s = size;
            if (s == 0) return;
            delete(offset, s);
            addToSize(-s);
        }
        public boolean contains(Object o) {
            int l = elements.length, s = inc(head, offset, l);
            return index(o, s, inc(s, size, l)) != -1;
        }
        public Object[] toArray() {
            int l = elements.length, s = inc(head, offset, l);
            return ArrayDeque.this.toArray(Object[].class, s, inc(s, size, l));
        }
        public <T> T[] toArray(T[] a) {
            int l = elements.length, s = inc(head, offset, l);
            return ArrayDeque.this.toArray(a, s, inc(s, size, l));
        }
        public boolean add(E e) {
            check(e);
            insert(inc(head, offset + size, elements.length), e);
            addToSize(1);
            return true;
        }
        public boolean remove(Object o) {
            int l = elements.length, s = inc(head, offset, l);
            int i = index(o, s, inc(s, size, l));
            if (i == -1) return false;
            delete(i);
            addToSize(-1);
            return true;
        }
        public boolean removeIf(Predicate<? super E> filter) {
            int l = elements.length, s = inc(head, offset, l);
            int r = ArrayDeque.this.bulkRemove(filter, s, inc(s, size, l));
            if (r == 0) return false;
            addToSize(-r);
            return true;
        }
        public boolean addAll(Collection<? extends E> c) {
            return addAll(size, c);
        }
        public boolean containsAll(Collection<?> c) {
            int l = elements.length, s = inc(head, offset, l), t = inc(s, size, l);
            for (Object e : c) {
                if (index(e, s, t) == -1) return false;
            }
            return true;
        }
        public boolean removeAll(Collection<?> c) {
            return removeIf(c::contains);
        }
        public boolean retainAll(Collection<?> c) {
            Objects.requireNonNull(c);
            return removeIf(e -> !c.contains(e));
        }
        public void replaceAll(UnaryOperator<E> operator) {
            int l = elements.length, s = inc(head, offset, l);
            ArrayDeque.this.replaceAll(operator, s, inc(s, size, l));
        }
        public void forEach(Consumer<? super E> action) {
            int l = elements.length, s = inc(head, offset, l);
            ArrayDeque.this.forEach(action, s, inc(s, size, l));
        }
        public void sort(Comparator<? super E> c) {
            int l = elements.length, s = inc(head, offset, l);
            ArrayDeque.this.sort(c, s, inc(s, size, l));
        }
        public boolean equals(Object o) {
            if (o == this) return true;
            int l = elements.length, s = inc(head, offset, l);
            return ArrayDeque.this.equals(o, s, inc(s, size, l));
        }
        public int hashCode() {
            int l = elements.length, s = inc(head, offset, l);
            return ArrayDeque.this.hashCode(s, inc(s, size, l));
        }
        public int indexOf(Object o) {
            int l = elements.length, s = inc(head, offset, l);
            int i = index(o, s, inc(s, size, l));
            return i == -1 ? -1 : sub(i, s, l);
        }
        public int lastIndexOf(Object o) {
            int l = elements.length, s = inc(head, offset, l);
            int i = lastIndex(o, s, inc(s, size, l));
            return i == -1 ? -1 : sub(i, s, l);
        }
        public Spliterator<E> spliterator() {
            int l = elements.length, s = inc(head, offset, l);
            return new DeqSpliterator(s, inc(s, size, l));
        }
        public Iterator<E> iterator() {
            return new SubListIterator(0);
        }
        public ListIterator<E> listIterator() {
            return new SubListListIterator(0);
        }
        public ListIterator<E> listIterator(final int index) {
            if (index < 0 || index > size) throw new IndexOutOfBoundsException();
            return new SubListListIterator(index);
        }
        class SubListIterator implements Iterator<E> {
            final ArrayDeque<E> d = ArrayDeque.this;
            int cursor, lastRet = -1;

            SubListIterator(int index) {
                cursor = index;
            }
            public boolean hasNext() {
                return cursor < size;
            }
            public E next() {
                int c = cursor;
                if (c >= size) throw new NoSuchElementException();
                cursor = c + 1;
                Object[] es = d.elements;
                return elementAt(es, lastRet = inc(d.head, offset + c, es.length));
            }
            public void remove() {
                int l = lastRet;
                if (l < 0) throw new IllegalStateException();
                if (l != inc(d.head, offset + cursor, d.elements.length)) {
                    cursor--;
                }
                d.delete(l);
                lastRet = -1;
                addToSize(-1);
            }
            public void forEachRemaining(Consumer<? super E> action) {
                Objects.requireNonNull(action);
                Object[] a = d.elements;
                int s = offset + d.head;
                int l = a.length, i = inc(s, cursor, l), to = inc(s, size, l);
                if (i == to) return;
                for (int e = (i <= to) ? to : a.length; ; i = 0, e = to) {
                    for (; i < e; i++)
                        action.accept(elementAt(a, i));
                    if (e == to) break;
                }
                lastRet = dec(to, l);
                cursor = size;
            }
        }
        class SubListListIterator extends SubListIterator implements ListIterator<E> {

            SubListListIterator(int index) {
                super(index);
            }
            public boolean hasPrevious() {
                return cursor > 0;
            }
            public E previous() {
                int c = cursor;
                if (c <= 0) throw new NoSuchElementException();
                Object[] es = d.elements;
                return elementAt(es, lastRet = inc(d.head, offset + (cursor = c - 1), es.length));
            }
            public int nextIndex() {
                return cursor;
            }
            public int previousIndex() {
                return cursor - 1;
            }
            public void set(E e) {
                if (lastRet < 0) throw new IllegalStateException();
                d.check(e);
                d.elements[lastRet] = e;
            }
            public void add(E e) {
                d.check(e);
                d.insert(inc(d.head, offset + cursor++, d.elements.length), e);
                addToSize(1);
                lastRet = -1;
            }
        }
    }

    /**
     * Returns a list iterator over the elements in this ArrayDeque (in proper
     * sequence).
     *
     * @return a list iterator over the elements in this ArrayDeque (in proper
     *         sequence)
     */
    public ListIterator<E> listIterator() {
        return new DeqListIterator(head);
    }

    /**
     * Returns a list iterator over the elements in this ArrayDeque (in proper
     * sequence), starting at the specified position in the ArrayDeque.
     * The specified index indicates the first element that would be
     * returned by an initial call to {@link ListIterator#next next}.
     * An initial call to {@link ListIterator#previous previous} would
     * return the element with the specified index minus one.
     *
     * @param index index of the first element to be returned from the
     *        list iterator (by a call to {@link ListIterator#next next})
     * @return a list iterator over the elements in this ArrayDeque (in proper
     *         sequence), starting at the specified position in the ArrayDeque
     * @throws IndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index > size()})
     */
    public ListIterator<E> listIterator(final int index) {
        if (index < 0 || index > size()) throw new IndexOutOfBoundsException();
        return new DeqListIterator(inc(head, index, elements.length));
    }

    private class DeqListIterator implements ListIterator<E> {
        /**
         * Index of element to be returned by subsequent call to next.
         */
        int cursor;
        /**
         * Tail and head recorded at construction (also in remove),
         * to stop iterator and also to check for comodification.
         */
        int end = tail, start = head;
        /**
         * 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;

        DeqListIterator(int index) {
            cursor = index;
        }
        public boolean hasNext() {
            return cursor != end;
        }
        public boolean hasPrevious() {
            return cursor != start;
        }
        public int nextIndex() {
            return sub(cursor, start, elements.length);
        }
        public int previousIndex() {
            return nextIndex() - 1;
        }
        final void checkMod() {
            if (tail != end || head != start) throw new ConcurrentModificationException();
        }
        public E next() {
            int c = cursor;
            if (c == end) throw new NoSuchElementException();
            checkMod();
            Object[] a = elements;
            cursor = inc(c, a.length);
            return elementAt(a, lastRet = c);
        }
        public E previous() {
            int c = cursor;
            if (c == start) throw new NoSuchElementException();
            checkMod();
            Object[] a = elements;
            return elementAt(a, lastRet = cursor = dec(c, a.length));
        }
        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            checkMod();
            int i = cursor, to = end;
            if (i == to) return;
            cursor = to;
            Object[] a = elements;
            for (int e = (i <= to) ? to : a.length; ; i = 0, e = to) {
                for (; i < e; i++)
                    action.accept(elementAt(a, i));
                if (e == to) break;
            }
            lastRet = dec(to, a.length);
        }
        public void remove() {
            int l = lastRet;
            if (l < 0) throw new IllegalStateException();
            checkMod();
            int c = cursor;
            boolean prev = l == c;
            if (delete(l)) {
                if (!prev) cursor = dec(c, elements.length);
                end = tail;
            } else {
                if (prev) cursor = inc(c, elements.length);
                start = head;
            }
            lastRet = -1;
        }
        public void set(E e) {
            int l = lastRet;
            if (l < 0) throw new IllegalStateException();
            checkMod();
            check(e);
            elements[l] = e;
        }
        public void add(E e) {
            checkMod();
            check(e);
            int c = cursor, l = elements.length;
            switch(insert(c, e)) {
            case 0:
                cursor = sub(c, start, l) + 1;
                start = 0;
                end = tail;
                break;
            case 1:
                cursor = inc(c, l);
                end = tail;
                break;
            case -1:
                start = head;
                break;
            }
            lastRet = -1;
        }
    }

    // *** 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 = validElementAt(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 (!isValid(es[cursor]) || 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 = validElementAt(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 (!isValid(es[cursor]) || 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 (!isValid(es[cursor]) || !isValid(es[dec(end, es.length)]))
                    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 = validElementAt(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.ORDERED
                | Spliterator.SIZED
                | Spliterator.SUBSIZED
                | (isValid(null) ? 0 : Spliterator.NONNULL);
        }
    }

    /**
     * @throws NullPointerException {@inheritDoc}
     */
    public void forEach(Consumer<? super E> action) {
        forEach(action, head, tail);
    }

    void forEach(Consumer<? super E> action, int from, int to) {
        Objects.requireNonNull(action);
        final Object[] es = elements;
        for (int i = from, e = (i <= to) ? to : es.length; ; i = 0, e = to) {
            for (; i < e; i++)
                action.accept(elementAt(es, i));
            if (e == to) break;
        }
    }

    /**
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean removeIf(Predicate<? super E> filter) {
        Objects.requireNonNull(filter);
        return bulkRemove(filter, head, tail) != 0;
    }

    /**
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return bulkRemove(e -> c.contains(e), head, tail) != 0;
    }

    /**
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return bulkRemove(e -> !c.contains(e), head, tail) != 0;
    }

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

    // 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 int bulkRemoveModified(
        Predicate<? super E> filter, final int beg, final int end) {
        final Object[] es = elements;
        final int capacity = es.length;
        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;
            }
        }
        int r = sub(end, w, capacity);
        delete(sub(w, head, capacity), r);
        return r;
    }

    /**
     * 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) {
        return index(o, head, tail) != -1;
    }

    public boolean containsAll(Collection<?> c) {
        int h = head, t = tail;
        for (Object e : c) {
            if (index(e, h, t) == -1) return false;
        }
        return true;
    }

    /**
     * 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, head, tail);
    }

    private <T> T[] toArray(Class<T[]> klazz, int head, int tail) {
        final Object[] es = elements;
        final T[] a;
        final int 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
     */
    public <T> T[] toArray(T[] a) {
        return toArray(a, head, tail);
    }

    @SuppressWarnings("unchecked")
    <T> T[] toArray(T[] a, int head, int tail) {
        final Object[] es = elements;
        final int size = sub(tail, head, es.length);
        if (size > a.length)
            return toArray((Class<T[]>) a.getClass(), head, tail);
        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 ***

    boolean equals(Object o, int from, int to) {
        if (!(o instanceof java.util.List)) return false;
        java.util.List<?> l = (java.util.List<?>)o;
        final Object[] es = elements;
        int i = from, s = l.size();
        if (s != sub(to, from, es.length)) return false;
        if (s == 0) return true;
        if (o instanceof RandomAccess) {
            for (int j = 0, e = (i <= to) ? to : es.length; ; i = 0, e = to) {
                for (; i < e; i++) {
                    Object t = es[i];
                    Object c = l.get(j++);
                    if (!(t == null ? c == null : t.equals(c))) return false;
                }
                if (e == to) break;
            }
            return true;
        }
        Iterator<?> it = l.iterator();
        for (int e = (i <= to) ? to : es.length; ; i = 0, e = to) {
            for (; i < e; i++) {
                if (!it.hasNext()) return false;
                Object t = es[i];
                Object c = it.next();
                if (!(t == null ? c == null : t.equals(c))) return false;
            }
            if (e == to) break;
        }
        return !it.hasNext();
    }

    int hashCode(int from, int to) {
        int hashCode = 1;
        final Object[] es = elements;
        for (int i = from, e = (i <= to) ? to : es.length; ; i = 0, e = to) {
            for (; i < e; i++) {
                Object o = es[i];
                hashCode = 31 * hashCode + (o == null ? 0 : o.hashCode());
            }
            if (e == to) break;
        }
        return hashCode;
    }

    /**
     * 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();
            final Object[] es = elements;
            result.elements = es.length == 1 ? es : es.clone();
            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 = size == 0 ? EMPTY : 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;
        }
    }

    // *** Element insertion and deletion  ***

    /**
     * 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 java.util.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;
        }
    }

    /**
     * Inserts the element at the specified position in the elements array.
     * May either shift the element at the specified index and all after forward,
     * of shift all elements before the index backwards and insert the element
     * before the index, or resize the elements array.
     *
     * @return 1 if elements moved forward (and tail was incremented), -1 if
     * elements moved backward, 0 if the elements array was resized
     */
    int insert(int i, E e) {
        Object[] es = elements;
        final int capacity = es.length;
        int h = head;
        final int t = tail;
        final int front = sub(i, h, capacity);
        final int back  = sub(t, i, capacity);

        if (front + back + 1 == capacity) {
            Object[] a = elements = new Object[newCapacity(1)];
            if (h <= t) {
                System.arraycopy(es, h, a, 0, front);
                a[front] = e;
                System.arraycopy(es, h + front, a, front + 1, back);
            } else {
                if (h <= i) {
                    System.arraycopy(es, h, a, 0, front);
                    a[front] = e;
                    System.arraycopy(es, i, a, front + 1, capacity - i);
                    System.arraycopy(es, 0, a, front + 1 + capacity - i, t);
                } else {
                    System.arraycopy(es, h, a, 0, capacity - h);
                    System.arraycopy(es, 0, a, capacity - h, i);
                    a[front] = e;
                    System.arraycopy(es, i, a, front + 1, back);
                }
            }
            head = 0;
            tail = capacity;
            return 0;
        }
        // Optimize for least element motion
        if (front < back) {
            i = dec(i, capacity);
            h = dec(h, capacity);
            if (h <= i) {
                System.arraycopy(es, h + 1, es, h, front);
            } else { // Wrap around
                System.arraycopy(es, h + 1, es, h, capacity - 1 - h);
                es[capacity - 1] = es[0];
                System.arraycopy(es, 1, es, 0, i);
            }
            es[i] = e;
            head = h;
            return -1;
        } else {
            if (i <= t) {
                System.arraycopy(es, i, es, i + 1, back);
            } else { // Wrap around
                System.arraycopy(es, 0, es, 1, t);
                es[0] = es[capacity - 1];
                System.arraycopy(es, i, es, i + 1, capacity - 1 - i);
            }
            es[i] = e;
            tail = inc(t, capacity);
            return 1;
        }
    }

    /**
     * Removes length elements from the elements array starting at index + head (inclusive).
     *
     * @return true if elements moved backward (and tail was decreased)
     */
    boolean delete(int index, int length) {
        int size = size();
        int ahead = size - index - length;
        Object[] a = elements;
        int capacity = a.length;
        int i = inc(head, index, capacity);
        int e = inc(i, length, capacity);
        if (index <= ahead) {
            if (head <= i) {
                if (i <= e) {
                    System.arraycopy(a, head, a, e - index, index);
                    Arrays.fill(a, head, head += length, null);
                } else {
                    if (e >= index) {
                        System.arraycopy(a, head, a, e - index, index);
                        Arrays.fill(a, head, a.length, null);
                        Arrays.fill(a, 0, head = e - index, null);
                    } else {
                        System.arraycopy(a, i - e, a, 0, e);
                        System.arraycopy(a, head, a, head + length, index - e);
                        Arrays.fill(a, head, head += length, null);
                    }
                }
            } else {
                System.arraycopy(a, 0, a, length, i);
                int r = a.length - head;
                if (length >= r) {
                    System.arraycopy(a, head, a, length - r, r);
                    Arrays.fill(a, 0, length - r, null);
                    Arrays.fill(a, head, a.length, null);
                    head = inc(head, length, capacity);
                } else {
                    System.arraycopy(a, a.length - length, a, 0, length);
                    System.arraycopy(a, head, a, head + length, r - length);
                    Arrays.fill(a, head, head += length, null);
                }
            }
            return false;
        } else {
            if (tail >= e) {
                if (i <= e) {
                    System.arraycopy(a, e, a, i, ahead);
                    Arrays.fill(a, tail - length, tail, null);
                    tail -= length;
                } else {
                    if (a.length >= i + ahead) {
                        System.arraycopy(a, e, a, i, ahead);
                        Arrays.fill(a, i + ahead, a.length, null);
                        Arrays.fill(a, 0, tail, null);
                        tail = inc(i, ahead, capacity);
                    } else {
                        int r = a.length - i;
                        System.arraycopy(a, e, a, i, r);
                        System.arraycopy(a, e + r, a, 0, ahead - r);
                        Arrays.fill(a, ahead - r, tail, null);
                        tail = ahead - r;
                    }
                }
            } else {
                int f = a.length - e;
                System.arraycopy(a, e, a, i, f);
                if (length >= tail) {
                    System.arraycopy(a, 0, a, i + f, tail);
                    Arrays.fill(a, 0, tail, null);
                    Arrays.fill(a, i + f + tail, a.length, null);
                    tail = inc(tail, i + f, capacity);
                } else {
                    System.arraycopy(a, 0, a, i + f, length);
                    System.arraycopy(a, length, a, 0, tail - length);
                    Arrays.fill(a, tail - length, tail, null);
                    tail -= length;
                }
            }
            return true;
        }
    }

    /**
     * Inserts length elements from a starting at offset (inclusive) into the
     * elements array at index + head. May shift elements forwards or backwards
     * or resize the elements array.
     */
    void insert(int index, Object[] a, int offset, int length) {
        int size = size();
        int newSize = size + length;
        if (newSize + 1 < 0) {
            throw new IllegalStateException("Sorry, deque too big");
        }
        Object[] es = elements;
        if (newSize >= es.length) {
            Object[] n = elements = new Object[newCapacity(newSize + 1 - es.length)];
            if (tail >= head) {
                System.arraycopy(es, head, n, 0, index);
                System.arraycopy(a, offset, n, index, length);
                System.arraycopy(es, head + index, n, index + length, size - index);
            } else {
                int r = es.length - head;
                if (r < index) {
                    System.arraycopy(es, head, n, 0, r);
                    System.arraycopy(es, 0, n, r, index - r);
                    System.arraycopy(a, offset, n, index, length);
                    System.arraycopy(es, index - r, n, index + length, size - index);
                } else {
                    System.arraycopy(es, head, n, 0, index);
                    System.arraycopy(a, offset, n, index, length);
                    System.arraycopy(es, head + index, n, index + length, r - index);
                    System.arraycopy(es, 0, n, r + length, tail);
                }
            }
            head = 0;
            tail = newSize;
            return;
        }
        int back = size - index;
        int capacity = es.length;
        if (index <= back) {
            int h = head - length;
            if (h < 0) {
                int nh = h + capacity;
                if (index >= -h) {
                    System.arraycopy(es, head, es, nh, -h);
                    System.arraycopy(es, head - h, es, 0, index + h);
                    System.arraycopy(a, offset, es, h + index, length);
                } else {
                    System.arraycopy(es, head, es, nh, index);
                    int f = -h - index;
                    System.arraycopy(a, offset, es, nh + index, f);
                    System.arraycopy(a, offset + f, es, 0, length - f);
                }
                h = nh;
            } else {
                int i = head + index - es.length;
                if (i <= 0) {
                    System.arraycopy(es, head, es, h, index);
                    System.arraycopy(a, offset, es, h + index, length);
                } else {
                    int r = index - i;
                    System.arraycopy(es, head, es, h, r);
                    int ni = i - length;
                    if (ni >= 0) {
                        System.arraycopy(es, 0, es, h + r, length);
                        System.arraycopy(es, length, es, 0, ni);
                        System.arraycopy(a, offset, es, ni, length);
                    } else {
                        System.arraycopy(es, 0, es, h + r, i);
                        System.arraycopy(a, offset, es, h + index, -ni);
                        System.arraycopy(a, offset - ni, es, 0, length + ni);
                    }
                }
            }
            head = h;
        } else {
            int t = tail + length;
            if (t - capacity >= 0) {
                t -= capacity;
                if (t >= back) {
                    int f = t - back;
                    System.arraycopy(es, tail - back, es, f, back);
                    System.arraycopy(a, offset, es, tail - back, length - f);
                    System.arraycopy(a, offset + length - f, es, 0, f);
                } else {
                    System.arraycopy(es, tail - t, es, 0, t);
                    System.arraycopy(es, tail - back, es, es.length - back + t, back - t);
                    System.arraycopy(a, offset, es, tail - back, length);
                }
            } else {
                int i = tail - back;
                if (i >= 0) {
                    System.arraycopy(es, i, es, i + length, back);
                    System.arraycopy(a, offset, es, i, length);
                } else {
                    System.arraycopy(es, 0, es, t - tail, tail);
                    int ni = i + length;
                    int mi = i + capacity;
                    if (ni >= 0) {
                        System.arraycopy(es, mi, es, ni, -i);
                        System.arraycopy(a, offset, es, mi, -i);
                        System.arraycopy(a, offset - i, es, 0, ni);
                    } else {
                        System.arraycopy(es, mi - ni, es, 0, -i + ni);
                        System.arraycopy(es, mi, es, ni + capacity, -ni);
                        System.arraycopy(a, offset, es, mi, length);
                    }
                }
            }
            tail = t;
        }
    }
}