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 * Copyright (c) 1997, 2019, Oracle and/or its affiliates. All rights reserved.

 * 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

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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

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 * accompanied this code).

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package java.util;

import java.io.IOException;

import java.io.ObjectInputStream;

import java.io.ObjectOutputStream;

import java.io.Serializable;

import java.lang.reflect.Array;

import java.util.function.BiConsumer;

import java.util.function.BiFunction;

import java.util.function.Consumer;

import java.util.function.Function;

import java.util.function.IntFunction;

import java.util.function.Predicate;

import java.util.function.UnaryOperator;

import java.util.stream.IntStream;

import java.util.stream.Stream;

import java.util.stream.StreamSupport;

import jdk.internal.access.SharedSecrets;

/**

 * This class consists exclusively of static methods that operate on or return

 * collections.  It contains polymorphic algorithms that operate on

 * collections, "wrappers", which return a new collection backed by a

 * specified collection, and a few other odds and ends.

 *

 * <p>The methods of this class all throw a {@code NullPointerException}

 * if the collections or class objects provided to them are null.

 *

 * <p>The documentation for the polymorphic algorithms contained in this class

 * generally includes a brief description of the <i>implementation</i>.  Such

 * descriptions should be regarded as <i>implementation notes</i>, rather than

 * parts of the <i>specification</i>.  Implementors should feel free to

 * substitute other algorithms, so long as the specification itself is adhered

 * to.  (For example, the algorithm used by {@code sort} does not have to be

 * a mergesort, but it does have to be <i>stable</i>.)

 *

 * <p>The "destructive" algorithms contained in this class, that is, the

 * algorithms that modify the collection on which they operate, are specified

 * to throw {@code UnsupportedOperationException} if the collection does not

 * support the appropriate mutation primitive(s), such as the {@code set}

 * method.  These algorithms may, but are not required to, throw this

 * exception if an invocation would have no effect on the collection.  For

 * example, invoking the {@code sort} method on an unmodifiable list that is

 * already sorted may or may not throw {@code UnsupportedOperationException}.

 *

 * <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

 * @author  Neal Gafter

 * @see     Collection

 * @see     Set

 * @see     List

 * @see     Map

 * @since   1.2

 */

public class Collections {

    // Suppresses default constructor, ensuring non-instantiability.

    private Collections() {

    }

    // Algorithms

    /*

     * Tuning parameters for algorithms - Many of the List algorithms have

     * two implementations, one of which is appropriate for RandomAccess

     * lists, the other for "sequential."  Often, the random access variant

     * yields better performance on small sequential access lists.  The

     * tuning parameters below determine the cutoff point for what constitutes

     * a "small" sequential access list for each algorithm.  The values below

     * were empirically determined to work well for LinkedList. Hopefully

     * they should be reasonable for other sequential access List

     * implementations.  Those doing performance work on this code would

     * do well to validate the values of these parameters from time to time.

     * (The first word of each tuning parameter name is the algorithm to which

     * it applies.)

     */

    private static final int BINARYSEARCH_THRESHOLD   = 5000;

    private static final int REVERSE_THRESHOLD        =   18;

    private static final int SHUFFLE_THRESHOLD        =    5;

    private static final int FILL_THRESHOLD           =   25;

    private static final int ROTATE_THRESHOLD         =  100;

    private static final int COPY_THRESHOLD           =   10;

    private static final int REPLACEALL_THRESHOLD     =   11;

    private static final int INDEXOFSUBLIST_THRESHOLD =   35;

    /**

     * Sorts the specified list into ascending order, according to the

     * {@linkplain Comparable natural ordering} of its elements.

     * All elements in the list must implement the {@link Comparable}

     * interface.  Furthermore, all elements in the list must be

     * <i>mutually comparable</i> (that is, {@code e1.compareTo(e2)}

     * must not throw a {@code ClassCastException} for any elements

     * {@code e1} and {@code e2} in the list).

     *

     * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will

     * not be reordered as a result of the sort.

     *

     * <p>The specified list must be modifiable, but need not be resizable.

     *

     * @implNote

     * This implementation defers to the {@link List#sort(Comparator)}

     * method using the specified list and a {@code null} comparator.

     *

     * @param  <T> the class of the objects in the list

     * @param  list the list to be sorted.

     * @throws ClassCastException if the list contains elements that are not

     *         <i>mutually comparable</i> (for example, strings and integers).

     * @throws UnsupportedOperationException if the specified list's

     *         list-iterator does not support the {@code set} operation.

     * @throws IllegalArgumentException (optional) if the implementation

     *         detects that the natural ordering of the list elements is

     *         found to violate the {@link Comparable} contract

     * @see List#sort(Comparator)

     */

    @SuppressWarnings("unchecked")

    public static <T extends Comparable<? super T>> void sort(List<T> list) {

        list.sort(null);

    }

    /**

     * Sorts the specified list according to the order induced by the

     * specified comparator.  All elements in the list 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 list).

     *

     * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will

     * not be reordered as a result of the sort.

     *

     * <p>The specified list must be modifiable, but need not be resizable.

     *

     * @implNote

     * This implementation defers to the {@link List#sort(Comparator)}

     * method using the specified list and comparator.

     *

     * @param  <T> the class of the objects in the list

     * @param  list the list to be sorted.

     * @param  c the comparator to determine the order of the list.  A

     *        {@code null} value indicates that the elements' <i>natural

     *        ordering</i> should be used.

     * @throws ClassCastException if the list contains elements that are not

     *         <i>mutually comparable</i> using the specified comparator.

     * @throws UnsupportedOperationException if the specified list's

     *         list-iterator does not support the {@code set} operation.

     * @throws IllegalArgumentException (optional) if the comparator is

     *         found to violate the {@link Comparator} contract

     * @see List#sort(Comparator)

     */

    @SuppressWarnings({"unchecked", "rawtypes"})

    public static <T> void sort(List<T> list, Comparator<? super T> c) {

        list.sort(c);

    }

    /**

     * Searches the specified list for the specified object using the binary

     * search algorithm.  The list must be sorted into ascending order

     * according to the {@linkplain Comparable natural ordering} of its

     * elements (as by the {@link #sort(List)} method) prior to making this

     * call.  If it is not sorted, the results are undefined.  If the list

     * contains multiple elements equal to the specified object, there is no

     * guarantee which one will be found.

     *

     * <p>This method runs in log(n) time for a "random access" list (which

     * provides near-constant-time positional access).  If the specified list

     * does not implement the {@link RandomAccess} interface and is large,

     * this method will do an iterator-based binary search that performs

     * O(n) link traversals and O(log n) element comparisons.

     *

     * @param  <T> the class of the objects in the list

     * @param  list the list to be searched.

     * @param  key the key to be searched for.

     * @return the index of the search key, if it is contained in the list;

     *         otherwise, <code>(-(<i>insertion point</i>) - 1)</code>.  The

     *         <i>insertion point</i> is defined as the point at which the

     *         key would be inserted into the list: the index of the first

     *         element greater than the key, or {@code list.size()} if all

     *         elements in the list are less than the specified key.  Note

     *         that this guarantees that the return value will be >= 0 if

     *         and only if the key is found.

     * @throws ClassCastException if the list contains elements that are not

     *         <i>mutually comparable</i> (for example, strings and

     *         integers), or the search key is not mutually comparable

     *         with the elements of the list.

     */

    public static <T>

    int binarySearch(List<? extends Comparable<? super T>> list, T key) {

        if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)

            return Collections.indexedBinarySearch(list, key);

        else

            return Collections.iteratorBinarySearch(list, key);

    }

    private static <T>

    int indexedBinarySearch(List<? extends Comparable<? super T>> list, T key) {

        int low = 0;

        int high = list.size()-1;

        while (low <= high) {

            int mid = (low + high) >>> 1;

            Comparable<? super T> midVal = list.get(mid);

            int cmp = midVal.compareTo(key);

            if (cmp < 0)

                low = mid + 1;

            else if (cmp > 0)

                high = mid - 1;

            else

                return mid; // key found

        }

        return -(low + 1);  // key not found

    }

    private static <T>

    int iteratorBinarySearch(List<? extends Comparable<? super T>> list, T key)

    {

        int low = 0;

        int high = list.size()-1;

        ListIterator<? extends Comparable<? super T>> i = list.listIterator();

        while (low <= high) {

            int mid = (low + high) >>> 1;

            Comparable<? super T> midVal = get(i, mid);

            int cmp = midVal.compareTo(key);

            if (cmp < 0)

                low = mid + 1;

            else if (cmp > 0)

                high = mid - 1;

            else

                return mid; // key found

        }

        return -(low + 1);  // key not found

    }

    /**

     * Gets the ith element from the given list by repositioning the specified

     * list listIterator.

     */

    private static <T> T get(ListIterator<? extends T> i, int index) {

        T obj = null;

        int pos = i.nextIndex();

        if (pos <= index) {

            do {

                obj = i.next();

            } while (pos++ < index);

        } else {

            do {

                obj = i.previous();

            } while (--pos > index);

        }

        return obj;

    }

    /**

     * Searches the specified list for the specified object using the binary

     * search algorithm.  The list must be sorted into ascending order

     * according to the specified comparator (as by the

     * {@link #sort(List, Comparator) sort(List, Comparator)}

     * method), prior to making this call.  If it is

     * not sorted, the results are undefined.  If the list contains multiple

     * elements equal to the specified object, there is no guarantee which one

     * will be found.

     *

     * <p>This method runs in log(n) time for a "random access" list (which

     * provides near-constant-time positional access).  If the specified list

     * does not implement the {@link RandomAccess} interface and is large,

     * this method will do an iterator-based binary search that performs

     * O(n) link traversals and O(log n) element comparisons.

     *

     * @param  <T> the class of the objects in the list

     * @param  list the list to be searched.

     * @param  key the key to be searched for.

     * @param  c the comparator by which the list is ordered.

     *         A {@code null} value indicates that the elements'

     *         {@linkplain Comparable natural ordering} should be used.

     * @return the index of the search key, if it is contained in the list;

     *         otherwise, <code>(-(<i>insertion point</i>) - 1)</code>.  The

     *         <i>insertion point</i> is defined as the point at which the

     *         key would be inserted into the list: the index of the first

     *         element greater than the key, or {@code list.size()} if all

     *         elements in the list are less than the specified key.  Note

     *         that this guarantees that the return value will be >= 0 if

     *         and only if the key is found.

     * @throws ClassCastException if the list contains elements that are not

     *         <i>mutually comparable</i> using the specified comparator,

     *         or the search key is not mutually comparable with the

     *         elements of the list using this comparator.

     */

    @SuppressWarnings("unchecked")

    public static <T> int binarySearch(List<? extends T> list, T key, Comparator<? super T> c) {

        if (c==null)

            return binarySearch((List<? extends Comparable<? super T>>) list, key);

        if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)

            return Collections.indexedBinarySearch(list, key, c);

        else

            return Collections.iteratorBinarySearch(list, key, c);

    }

    private static <T> int indexedBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {

        int low = 0;

        int high = l.size()-1;

        while (low <= high) {

            int mid = (low + high) >>> 1;

            T midVal = l.get(mid);

            int cmp = c.compare(midVal, key);

            if (cmp < 0)

                low = mid + 1;

            else if (cmp > 0)

                high = mid - 1;

            else

                return mid; // key found

        }

        return -(low + 1);  // key not found

    }

    private static <T> int iteratorBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {

        int low = 0;

        int high = l.size()-1;

        ListIterator<? extends T> i = l.listIterator();

        while (low <= high) {

            int mid = (low + high) >>> 1;

            T midVal = get(i, mid);

            int cmp = c.compare(midVal, key);

            if (cmp < 0)

                low = mid + 1;

            else if (cmp > 0)

                high = mid - 1;

            else

                return mid; // key found

        }

        return -(low + 1);  // key not found

    }

    /**

     * Reverses the order of the elements in the specified list.<p>

     *

     * This method runs in linear time.

     *

     * @param  list the list whose elements are to be reversed.

     * @throws UnsupportedOperationException if the specified list or

     *         its list-iterator does not support the {@code set} operation.

     */

    @SuppressWarnings({"rawtypes", "unchecked"})

    public static void reverse(List<?> list) {

        int size = list.size();

        if (size < REVERSE_THRESHOLD || list instanceof RandomAccess) {

            for (int i=0, mid=size>>1, j=size-1; i<mid; i++, j--)

                swap(list, i, j);

        } else {

            // instead of using a raw type here, it's possible to capture

            // the wildcard but it will require a call to a supplementary

            // private method

            ListIterator fwd = list.listIterator();

            ListIterator rev = list.listIterator(size);

            for (int i=0, mid=list.size()>>1; i<mid; i++) {

                Object tmp = fwd.next();

                fwd.set(rev.previous());

                rev.set(tmp);

            }

        }

    }

    /**

     * Randomly permutes the specified list using a default source of

     * randomness.  All permutations occur with approximately equal

     * likelihood.

     *

     * <p>The hedge "approximately" is used in the foregoing description because

     * default source of randomness is only approximately an unbiased source

     * of independently chosen bits. If it were a perfect source of randomly

     * chosen bits, then the algorithm would choose permutations with perfect

     * uniformity.

     *

     * <p>This implementation traverses the list backwards, from the last

     * element up to the second, repeatedly swapping a randomly selected element

     * into the "current position".  Elements are randomly selected from the

     * portion of the list that runs from the first element to the current

     * position, inclusive.

     *

     * <p>This method runs in linear time.  If the specified list does not

     * implement the {@link RandomAccess} interface and is large, this

     * implementation dumps the specified list into an array before shuffling

     * it, and dumps the shuffled array back into the list.  This avoids the

     * quadratic behavior that would result from shuffling a "sequential

     * access" list in place.

     *

     * @param  list the list to be shuffled.

     * @throws UnsupportedOperationException if the specified list or

     *         its list-iterator does not support the {@code set} operation.

     */

    public static void shuffle(List<?> list) {

        Random rnd = r;

        if (rnd == null)

            r = rnd = new Random(); // harmless race.

        shuffle(list, rnd);

    }

    private static Random r;

    /**

     * Randomly permute the specified list using the specified source of

     * randomness.  All permutations occur with equal likelihood

     * assuming that the source of randomness is fair.<p>

     *

     * This implementation traverses the list backwards, from the last element

     * up to the second, repeatedly swapping a randomly selected element into

     * the "current position".  Elements are randomly selected from the

     * portion of the list that runs from the first element to the current

     * position, inclusive.<p>

     *

     * This method runs in linear time.  If the specified list does not

     * implement the {@link RandomAccess} interface and is large, this

     * implementation dumps the specified list into an array before shuffling

     * it, and dumps the shuffled array back into the list.  This avoids the

     * quadratic behavior that would result from shuffling a "sequential

     * access" list in place.

     *

     * @param  list the list to be shuffled.

     * @param  rnd the source of randomness to use to shuffle the list.

     * @throws UnsupportedOperationException if the specified list or its

     *         list-iterator does not support the {@code set} operation.

     */

    @SuppressWarnings({"rawtypes", "unchecked"})

    public static void shuffle(List<?> list, Random rnd) {

        int size = list.size();

        if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {

            for (int i=size; i>1; i--)

                swap(list, i-1, rnd.nextInt(i));

        } else {

            Object[] arr = list.toArray();

            // Shuffle array

            for (int i=size; i>1; i--)

                swap(arr, i-1, rnd.nextInt(i));

            // Dump array back into list

            // instead of using a raw type here, it's possible to capture

            // the wildcard but it will require a call to a supplementary

            // private method

            ListIterator it = list.listIterator();

            for (Object e : arr) {

                it.next();

                it.set(e);

            }

        }

    }

    /**

     * Swaps the elements at the specified positions in the specified list.

     * (If the specified positions are equal, invoking this method leaves

     * the list unchanged.)

     *

     * @param list The list in which to swap elements.

     * @param i the index of one element to be swapped.

     * @param j the index of the other element to be swapped.

     * @throws IndexOutOfBoundsException if either {@code i} or {@code j}

     *         is out of range (i < 0 || i >= list.size()

     *         || j < 0 || j >= list.size()).

     * @since 1.4

     */

    @SuppressWarnings({"rawtypes", "unchecked"})

    public static void swap(List<?> list, int i, int j) {

        // instead of using a raw type here, it's possible to capture

        // the wildcard but it will require a call to a supplementary

        // private method

        final List l = list;

        l.set(i, l.set(j, l.get(i)));

    }

    /**

     * Swaps the two specified elements in the specified array.

     */

    private static void swap(Object[] arr, int i, int j) {

        Object tmp = arr[i];

        arr[i] = arr[j];

        arr[j] = tmp;

    }

    /**