package java.util.concurrent;

import java.io.ObjectStreamField;
import java.io.Serializable;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.*;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.LockSupport;
import java.util.concurrent.locks.ReentrantLock;
import java.util.function.*;
import java.util.stream.Stream;


public class ConcurrentHashMap<K, V> extends AbstractMap<K, V>
        implements ConcurrentMap<K, V>, Serializable {
    private static final long serialVersionUID = 7249069246763182397L;


    /* ---------------- Constants -------------- */

    /**
     * node陣列最大容量
     */
    private static final int MAXIMUM_CAPACITY = 1 << 30;


    /**
     * 預設初始值，必須是2的冪數
     */
    private static final int DEFAULT_CAPACITY = 16;

    /**
     * 陣列可能最大值，需要與toArray（）相關方法關聯
     */
    static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * 併發級別，遺留下來的，為相容以前的版本
     */
    private static final int DEFAULT_CONCURRENCY_LEVEL = 16;

    /**
     * 負載因子
     */
    private static final float LOAD_FACTOR = 0.75f;

    /**
     * 連結串列轉樹的閥值，如果table[i]下面的連結串列長度大於8時就轉化為數
     */
    static final int TREEIFY_THRESHOLD = 8;

    /**
     * 樹轉連結串列的閥值，小於等於6是轉為連結串列，僅在擴容tranfer時才可能樹轉連結串列
     */
    static final int UNTREEIFY_THRESHOLD = 6;

    /**
     * 在轉變成樹之前，還會有一次判斷，只有鍵值對數量大於 64 才會發生轉換。
     * 這是為了避免在雜湊表建立初期，多個鍵值對恰好被放入了同一個連結串列中而導致不必要的轉化。
     */
    static final int MIN_TREEIFY_CAPACITY = 64;

    /**
     * Minimum number of rebinnings per transfer step. Ranges are
     * subdivided to allow multiple resizer threads.  This value
     * serves as a lower bound to avoid resizers encountering
     * excessive memory contention.  The value should be at least
     * DEFAULT_CAPACITY.
     */
    private static final int MIN_TRANSFER_STRIDE = 16;

    /**
     * The number of bits used for generation stamp in sizeCtl.
     * Must be at least 6 for 32bit arrays.
     */
    private static int RESIZE_STAMP_BITS = 16;

    /**
     * 2^15-1，help resize的最大執行緒數
     */
    private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;

    /**
     * 32-16=16，sizeCtl中記錄size大小的偏移量
     */
    private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;

    /*
     * Encodings for Node hash fields. See above for explanation.
     */
    static final int MOVED = -1; // hash for forwarding nodes （forwarding nodes的hash值）、標示位
    static final int TREEBIN = -2; // hash值是-2  表示這是一個TreeBin節點
    static final int RESERVED = -3; // hash for transient reservations
    static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash （ReservationNode的hash值）

    /**
     * 可用處理器數量
     */
    static final int NCPU = Runtime.getRuntime().availableProcessors();

    /**
     * For serialization compatibility.
     */
    private static final ObjectStreamField[] serialPersistentFields = {
            new ObjectStreamField("segments", Segment[].class),
            new ObjectStreamField("segmentMask", Integer.TYPE),
            new ObjectStreamField("segmentShift", Integer.TYPE)
    };

    /* ---------------- Nodes -------------- */

    /**
     * Node是最核心的內部類，它包裝了key-value鍵值對，所有插入ConcurrentHashMap的資料都包裝在這裡面。
     * 它與HashMap中的定義很相似，但是但是有一些差別，它對value和next屬性設定了volatile同步鎖，
     * 它不允許呼叫setValue方法直接改變Node的value域，它增加了find方法輔助map.get()方法。
     */
    static class Node<K, V> implements Map.Entry<K, V> {
        final int hash;
        final K key;
        //val和next都會在擴容時發生變化，所以加上volatile來保持可見性和禁止重排序
        volatile V val;
        volatile Node<K, V> next;

        Node(int hash, K key, V val, Node<K, V> next) {
            this.hash = hash;
            this.key = key;
            this.val = val;
            this.next = next;
        }

        public final K getKey() {
            return key;
        }

        public final V getValue() {
            return val;
        }

        /**
         * HashMap中Node類的hashCode()方法中的程式碼為：Objects.hashCode(key) ^ Objects.hashCode(value)
         * 而Objects.hashCode(key)最終也是呼叫了 key.hashCode()，但是效果一樣
         */
        public final int hashCode() {
            return key.hashCode() ^ val.hashCode();
        }

        public final String toString() {
            return key + "=" + val;
        }

        //不允許直接改變value的值
        public final V setValue(V value) {
            throw new UnsupportedOperationException();
        }

        /**
         * HashMap使用if (o == this)，且巢狀if；ConcurrentHashMap使用&&
         */
        public final boolean equals(Object o) {
            Object k, v, u;
            Map.Entry<?, ?> e;
            return ((o instanceof Map.Entry) &&
                    (k = (e = (Map.Entry<?, ?>) o).getKey()) != null &&
                    (v = e.getValue()) != null &&
                    (k == key || k.equals(key)) &&
                    (v == (u = val) || v.equals(u)));
        }

        /**
         * 增加find方法輔助get方法 ，HashMap中的Node類中沒有此方法
         */
        Node<K, V> find(int h, Object k) {
            Node<K, V> e = this;
            if (k != null) {
                do {
                    K ek;
                    if (e.hash == h &&
                            ((ek = e.key) == k || (ek != null && k.equals(ek))))
                        return e;
                } while ((e = e.next) != null);
            }
            return null;
        }
    }

    /* ---------------- Static utilities -------------- */

    /**
     * 對hashCode進行再雜湊，演算法為(h ^ (h >>> 16)) & HASH_BITS
     */
    static final int spread(int h) {
        return (h ^ (h >>> 16)) & HASH_BITS;
    }

    /**
     * 返回大於等於count的最小的2的冪次方
     */
    private static final int tableSizeFor(int c) {
        int n = c - 1;
        n |= n >>> 1;
        n |= n >>> 2;
        n |= n >>> 4;
        n |= n >>> 8;
        n |= n >>> 16;
        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
    }

    /**
     * Returns x's Class if it is of the form "class C implements
     * Comparable<C>", else null.
     */
    static Class<?> comparableClassFor(Object x) {
        if (x instanceof Comparable) {
            Class<?> c;
            Type[] ts, as;
            Type t;
            ParameterizedType p;
            if ((c = x.getClass()) == String.class) // bypass checks
                return c;
            if ((ts = c.getGenericInterfaces()) != null) {
                for (int i = 0; i < ts.length; ++i) {
                    if (((t = ts[i]) instanceof ParameterizedType) &&
                            ((p = (ParameterizedType) t).getRawType() ==
                                    Comparable.class) &&
                            (as = p.getActualTypeArguments()) != null &&
                            as.length == 1 && as[0] == c) // type arg is c
                        return c;
                }
            }
        }
        return null;
    }

    /**
     * Returns k.compareTo(x) if x matches kc (k's screened comparable
     * class), else 0.
     */
    @SuppressWarnings({"rawtypes", "unchecked"}) // for cast to Comparable
    static int compareComparables(Class<?> kc, Object k, Object x) {
        return (x == null || x.getClass() != kc ? 0 :
                ((Comparable) k).compareTo(x));
    }

    /* ---------------- Table element access -------------- */

    /*
     * Volatile access methods are used for table elements as well as
     * elements of in-progress next table while resizing.  All uses of
     * the tab arguments must be null checked by callers.  All callers
     * also paranoically precheck that tab's length is not zero (or an
     * equivalent check), thus ensuring that any index argument taking
     * the form of a hash value anded with (length - 1) is a valid
     * index.  Note that, to be correct wrt arbitrary concurrency
     * errors by users, these checks must operate on local variables,
     * which accounts for some odd-looking inline assignments below.
     * Note that calls to setTabAt always occur within locked regions,
     * and so in principle require only release ordering, not
     * full volatile semantics, but are currently coded as volatile
     * writes to be conservative.
     */

    /**
     * 獲得在i位置上的Node節點
     */
    @SuppressWarnings("unchecked")
    static final <K, V> Node<K, V> tabAt(Node<K, V>[] tab, int i) {
        return (Node<K, V>) U.getObjectVolatile(tab, ((long) i << ASHIFT) + ABASE);
    }

    /**
     * 利用CAS演算法設定i位置上的Node節點（將c和table[i]比較，相同則插入v）。
     */
    static final <K, V> boolean casTabAt(Node<K, V>[] tab, int i,
                                         Node<K, V> c, Node<K, V> v) {
        return U.compareAndSwapObject(tab, ((long) i << ASHIFT) + ABASE, c, v);
    }

    /**
     * 利用volatile方法設定第i個節點的值，這個操作一定是成功的。
     */
    static final <K, V> void setTabAt(Node<K, V>[] tab, int i, Node<K, V> v) {
        U.putObjectVolatile(tab, ((long) i << ASHIFT) + ABASE, v);
    }

    /* ---------------- Fields -------------- */

    /**
     * 存放node的陣列,大小是2的冪次方
     */
    transient volatile Node<K, V>[] table;

    /**
     * 擴容時用於存放資料的變數，擴容完成後會置為null。
     */
    private transient volatile Node<K, V>[] nextTable;

    /**
     * 記錄容器的容量大小，通過CAS更新
     */
    private transient volatile long baseCount;

    /**
     * 負數代表正在進行初始化或擴容操作 ,其中-1代表正在初始化 ,-N 表示有N-1個執行緒正在進行擴容操作
     * 正數或0代表hash表還沒有被初始化，這個數值表示初始化或下一次進行擴容的大小，類似於擴容閾值。
     * 它的值始終是當前ConcurrentHashMap容量的0.75倍，這與loadfactor是對應的。實際容量>=sizeCtl，則擴容。
     */
    private transient volatile int sizeCtl;//控制識別符號

    /**
     * The next table index (plus one) to split while resizing.
     */
    private transient volatile int transferIndex;

    /**
     * 自旋鎖 （鎖定通過 CAS） 在調整大小和/或建立 CounterCells 時使用。
     * 在CounterCell類更新value中會使用，功能類似顯示鎖和內建鎖，效能更好
     */
    private transient volatile int cellsBusy;

    /**
     * counter cell表，長度總為2的冪次
     */
    private transient volatile CounterCell[] counterCells;

    // views
    private transient KeySetView<K, V> keySet;
    private transient ValuesView<K, V> values;
    private transient EntrySetView<K, V> entrySet;


    /* ---------------- Public operations -------------- */

    /**
     * 預設的建構函式
     */
    public ConcurrentHashMap() {
    }

    /**
     * 指定容量的建構函式
     *
     * @param initialCapacity 初始化容量
     * @throws IllegalArgumentException if the initial capacity of
     *                                  elements is negative
     */
    public ConcurrentHashMap(int initialCapacity) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException();
        int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ?
                MAXIMUM_CAPACITY :
                tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));
        this.sizeCtl = cap;//初始化sizeCtl
    }

    /**
     * 建立與給定map具有相同對映的新map
     *
     * @param m the map
     */
    public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
        this.sizeCtl = DEFAULT_CAPACITY;
        putAll(m);
    }

    /**
     * Creates a new, empty map with an initial table size based on
     * the given number of elements ({@code initialCapacity}) and
     * initial table density ({@code loadFactor}).
     *
     * @param initialCapacity 初始容量
     * @param loadFactor      負載因子,當容量達到initialCapacity*loadFactor時，執行擴容
     * @throws IllegalArgumentException if the initial capacity of
     *                                  elements is negative or the load factor is nonpositive
     * @since 1.6
     */
    public ConcurrentHashMap(int initialCapacity, float loadFactor) {
        this(initialCapacity, loadFactor, 1);
    }

    /**
     * Creates a new, empty map with an initial table size based on
     * the given number of elements ({@code initialCapacity}), table
     * density ({@code loadFactor}), and number of concurrently
     * updating threads ({@code concurrencyLevel}).
     *
     * @param initialCapacity  初始容量
     * @param loadFactor       負載因子,當容量達到initialCapacity*loadFactor時，執行擴容
     * @param concurrencyLevel 預估的併發更新執行緒數
     * @throws IllegalArgumentException if the initial capacity is
     *                                  negative or the load factor or concurrencyLevel are
     *                                  nonpositive
     */
    public ConcurrentHashMap(int initialCapacity,
                             float loadFactor, int concurrencyLevel) {
        if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
            throw new IllegalArgumentException();
        if (initialCapacity < concurrencyLevel)   // Use at least as many bins
            initialCapacity = concurrencyLevel;   // as estimated threads
        long size = (long) (1.0 + (long) initialCapacity / loadFactor);
        int cap = (size >= (long) MAXIMUM_CAPACITY) ?
                MAXIMUM_CAPACITY : tableSizeFor((int) size);
        this.sizeCtl = cap;
    }

    // Original (since JDK1.2) Map methods

    /**
     * {@inheritDoc}
     */
    public int size() {
        long n = sumCount();
        return ((n < 0L) ? 0 :
                (n > (long) Integer.MAX_VALUE) ? Integer.MAX_VALUE :
                        (int) n);
    }

    /**
     * {@inheritDoc}
     */
    public boolean isEmpty() {
        return sumCount() <= 0L; // ignore transient negative values
    }

    /**
     * 根據key在Map中找出其對應的value，如果不存在key，則返回null，
     * 其中key不允許為null，否則拋異常
     * 對於節點可能在連結串列或樹上的情況，需要分別去查詢
     *
     * @throws NullPointerException if the specified key is null
     */
    public V get(Object key) {
        Node<K, V>[] tab;
        Node<K, V> e, p;
        int n, eh;
        K ek;
        int h = spread(key.hashCode());//兩次hash計算出hash值
        //根據hash值確定節點位置
        if ((tab = table) != null && (n = tab.length) > 0 &&
                (e = tabAt(tab, (n - 1) & h)) != null) {
            // 搜尋到的節點key與傳入的key相同且不為null,直接返回這個節點
            if ((eh = e.hash) == h) {
                if ((ek = e.key) == key || (ek != null && key.equals(ek)))
                    return e.val;
            } else if (eh < 0)//如果eh<0 說明這個節點在樹上 直接尋找
                return (p = e.find(h, key)) != null ? p.val : null;
            //否則遍歷連結串列 找到對應的值並返回
            while ((e = e.next) != null) {
                if (e.hash == h &&
                        ((ek = e.key) == key || (ek != null && key.equals(ek))))
                    return e.val;
            }
        }
        return null;
    }

    /**
     * 檢查table中是否含有key
     *
     * @param key possible key
     * @return {@code true} if and only if the specified object
     * is a key in this table, as determined by the
     * {@code equals} method; {@code false} otherwise
     * @throws NullPointerException if the specified key is null
     */
    public boolean containsKey(Object key) {
        //直接呼叫get(int key)方法即可，如果有返回值，則說明是包含key的
        return get(key) != null;
    }

    /**
     * 檢查在所有對映(k,v)中只要出現一次及以上的v==value，返回true
     * 這個方法可能需要完全遍歷Map，因此比containsKey要慢的多
     *
     * @param value value whose presence in this map is to be tested
     * @return {@code true} if this map maps one or more keys to the
     * specified value
     * @throws NullPointerException if the specified value is null
     */
    public boolean containsValue(Object value) {
        if (value == null)
            throw new NullPointerException();
        Node<K, V>[] t;
        if ((t = table) != null) {
            Traverser<K, V> it = new Traverser<K, V>(t, t.length, 0, t.length);
            for (Node<K, V> p; (p = it.advance()) != null; ) {
                V v;
                if ((v = p.val) == value || (v != null && value.equals(v)))
                    return true;
            }
        }
        return false;
    }

    /**
     * 直接呼叫putVal(key, value, false)方法
     *
     * @param key   key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with {@code key}, or
     * {@code null} if there was no mapping for {@code key}
     * @throws NullPointerException if the specified key or value is null
     */
    public V put(K key, V value) {
        return putVal(key, value, false);
    }

    /**
     * putVal方法可以分為以下幾步：
     * 1、檢查key/value是否為空，如果為空，則拋異常，否則進行2
     * 2、進入for死迴圈，進行3
     * 3、檢查table是否初始化了，如果沒有，則呼叫initTable()進行初始化然後進行 2，否則進行4
     * 4、根據key的hash值計算出其應該在table中儲存的位置i，取出table[i]的節點用f表示。
     * 根據f的不同有如下三種情況：
     * 1）如果table[i]==null(即該位置的節點為空，沒有發生碰撞)，則利用CAS操作直接儲存在該位置，如果CAS操作成功則退出死迴圈。
     * 2）如果table[i]!=null(即該位置已經有其它節點，發生碰撞)，碰撞處理也有兩種情況
     * 2.1）檢查table[i]的節點的hash是否等於MOVED，如果等於，則檢測到正在擴容，則幫助其擴容
     * 2.2）說明table[i]的節點的hash值不等於MOVED，如果table[i]為連結串列節點，則將此節點插入連結串列中即可
     * 如果table[i]為樹節點，則將此節點插入樹中即可。插入成功後，進行 5
     * 5、如果table[i]的節點是連結串列節點，則檢查table的第i個位置的連結串列是否需要轉化為數，如果需要則呼叫treeifyBin函式進行轉化
     */
    final V putVal(K key, V value, boolean onlyIfAbsent) {
        if (key == null || value == null) throw new NullPointerException();// key和value不允許null
        int hash = spread(key.hashCode());//兩次hash，減少hash衝突，可以均勻分佈
        int binCount = 0;//i處結點標誌，0: 未加入新結點, 2: TreeBin或連結串列結點數, 其它：連結串列結點數。主要用於每次加入結點後檢視是否要由連結串列轉為紅黑樹
        for (Node<K, V>[] tab = table; ; ) {//CAS經典寫法，不成功無限重試
            Node<K, V> f;
            int n, i, fh;
            //檢查是否初始化了，如果沒有，則初始化
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();
            /**
             * i=(n-1)&hash 等價於i=hash%n(前提是n為2的冪次方).即取出table中位置的節點用f表示。 有如下兩種情況：
             * 1、如果table[i]==null(即該位置的節點為空，沒有發生碰撞)，則利用CAS操作直接儲存在該位置， 如果CAS操作成功則退出死迴圈。
             * 2、如果table[i]!=null(即該位置已經有其它節點，發生碰撞)
             */
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
                if (casTabAt(tab, i, null,
                        new Node<K, V>(hash, key, value, null)))
                    break;                   // no lock when adding to empty bin
            } else if ((fh = f.hash) == MOVED)//檢查table[i]的節點的hash是否等於MOVED，如果等於，則檢測到正在擴容，則幫助其擴容
                tab = helpTransfer(tab, f);
            else {//table[i]的節點的hash值不等於MOVED。
                V oldVal = null;
                // 針對首個節點進行加鎖操作，而不是segment，進一步減少執行緒衝突
                synchronized (f) {
                    if (tabAt(tab, i) == f) {
                        if (fh >= 0) {
                            binCount = 1;
                            for (Node<K, V> e = f; ; ++binCount) {
                                K ek;
                                // 如果在連結串列中找到值為key的節點e，直接設定e.val = value即可
                                if (e.hash == hash &&
                                        ((ek = e.key) == key ||
                                                (ek != null && key.equals(ek)))) {
                                    oldVal = e.val;
                                    if (!onlyIfAbsent)
                                        e.val = value;
                                    break;
                                }
                                // 如果沒有找到值為key的節點，直接新建Node並加入連結串列即可
                                Node<K, V> pred = e;
                                if ((e = e.next) == null) {//插入到連結串列末尾並跳出迴圈
                                    pred.next = new Node<K, V>(hash, key,
                                            value, null);
                                    break;
                                }
                            }
                        } else if (f instanceof TreeBin) {// 如果首節點為TreeBin型別，說明為紅黑樹結構，執行putTreeVal操作
                            Node<K, V> p;
                            binCount = 2;
                            if ((p = ((TreeBin<K, V>) f).putTreeVal(hash, key,
                                    value)) != null) {
                                oldVal = p.val;
                                if (!onlyIfAbsent)
                                    p.val = value;
                            }
                        }
                    }
                }
                if (binCount != 0) {
                    // 如果節點數>＝8，那麼轉換連結串列結構為紅黑樹結構
                    if (binCount >= TREEIFY_THRESHOLD)
                        treeifyBin(tab, i);//若length<64,直接tryPresize,兩倍table.length;不轉紅黑樹
                    if (oldVal != null)
                        return oldVal;
                    break;
                }
            }
        }
        // 計數增加1，有可能觸發transfer操作(擴容)
        addCount(1L, binCount);
        return null;
    }

    /**
     * Copies all of the mappings from the specified map to this one.
     * These mappings replace any mappings that this map had for any of the
     * keys currently in the specified map.
     *
     * @param m mappings to be stored in this map
     */
    public void putAll(Map<? extends K, ? extends V> m) {
        tryPresize(m.size());
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
            putVal(e.getKey(), e.getValue(), false);
    }

    /**
     * Removes the key (and its corresponding value) from this map.
     * This method does nothing if the key is not in the map.
     *
     * @param key the key that needs to be removed
     * @return the previous value associated with {@code key}, or
     * {@code null} if there was no mapping for {@code key}
     * @throws NullPointerException if the specified key is null
     */
    public V remove(Object key) {
        return replaceNode(key, null, null);
    }

    /**
     * Implementation for the four public remove/replace methods:
     * Replaces node value with v, conditional upon match of cv if
     * non-null.  If resulting value is null, delete.
     */
    final V replaceNode(Object key, V value, Object cv) {
        int hash = spread(key.hashCode());
        for (Node<K, V>[] tab = table; ; ) {
            Node<K, V> f;
            int n, i, fh;
            if (tab == null || (n = tab.length) == 0 ||
                    (f = tabAt(tab, i = (n - 1) & hash)) == null)
                break;
            else if ((fh = f.hash) == MOVED)
                tab = helpTransfer(tab, f);
            else {
                V oldVal = null;
                boolean validated = false;
                synchronized (f) {
                    if (tabAt(tab, i) == f) {
                        if (fh >= 0) {
                            validated = true;
                            for (Node<K, V> e = f, pred = null; ; ) {
                                K ek;
                                if (e.hash == hash &&
                                        ((ek = e.key) == key ||
                                                (ek != null && key.equals(ek)))) {
                                    V ev = e.val;
                                    if (cv == null || cv == ev ||
                                            (ev != null && cv.equals(ev))) {
                                        oldVal = ev;
                                        if (value != null)
                                            e.val = value;
                                        else if (pred != null)
                                            pred.next = e.next;
                                        else
                                            setTabAt(tab, i, e.next);
                                    }
                                    break;
                                }
                                pred = e;
                                if ((e = e.next) == null)
                                    break;
                            }
                        } else if (f instanceof TreeBin) {
                            validated = true;
                            TreeBin<K, V> t = (TreeBin<K, V>) f;
                            TreeNode<K, V> r, p;
                            if ((r = t.root) != null &&
                                    (p = r.findTreeNode(hash, key, null)) != null) {
                                V pv = p.val;
                                if (cv == null || cv == pv ||
                                        (pv != null && cv.equals(pv))) {
                                    oldVal = pv;
                                    if (value != null)
                                        p.val = value;
                                    else if (t.removeTreeNode(p))
                                        setTabAt(tab, i, untreeify(t.first));
                                }
                            }
                        }
                    }
                }
                if (validated) {
                    if (oldVal != null) {
                        if (value == null)
                            addCount(-1L, -1);
                        return oldVal;
                    }
                    break;
                }
            }
        }
        return null;
    }

    /**
     * Removes all of the mappings from this map.
     */
    public void clear() {
        long delta = 0L; // negative number of deletions
        int i = 0;
        Node<K, V>[] tab = table;
        while (tab != null && i < tab.length) {
            int fh;
            Node<K, V> f = tabAt(tab, i);
            if (f == null)
                ++i;
            else if ((fh = f.hash) == MOVED) {
                tab = helpTransfer(tab, f);
                i = 0; // restart
            } else {
                synchronized (f) {
                    if (tabAt(tab, i) == f) {
                        Node<K, V> p = (fh >= 0 ? f :
                                (f instanceof TreeBin) ?
                                        ((TreeBin<K, V>) f).first : null);
                        while (p != null) {
                            --delta;
                            p = p.next;
                        }
                        setTabAt(tab, i++, null);
                    }
                }
            }
        }
        if (delta != 0L)
            addCount(delta, -1);
    }

    /**
     * Returns a {@link Set} view of the keys contained in this map.
     * The set is backed by the map, so changes to the map are
     * reflected in the set, and vice-versa. The set supports element
     * removal, which removes the corresponding mapping from this map,
     * via the {@code Iterator.remove}, {@code Set.remove},
     * {@code removeAll}, {@code retainAll}, and {@code clear}
     * operations.  It does not support the {@code add} or
     * {@code addAll} operations.
     * <p>
     * <p>The view's iterators and spliterators are
     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
     * <p>
     * <p>The view's {@code spliterator} reports {@link Spliterator#CONCURRENT},
     * {@link Spliterator#DISTINCT}, and {@link Spliterator#NONNULL}.
     *
     * @return the set view
     */
    public KeySetView<K, V> keySet() {
        KeySetView<K, V> ks;
        return (ks = keySet) != null ? ks : (keySet = new KeySetView<K, V>(this, null));
    }

    /**
     * Returns a {@link Collection} view of the values contained in this map.
     * The collection is backed by the map, so changes to the map are
     * reflected in the collection, and vice-versa.  The collection
     * supports element removal, which removes the corresponding
     * mapping from this map, via the {@code Iterator.remove},
     * {@code Collection.remove}, {@code removeAll},
     * {@code retainAll}, and {@code clear} operations.  It does not
     * support the {@code add} or {@code addAll} operations.
     * <p>
     * <p>The view's iterators and spliterators are
     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
     * <p>
     * <p>The view's {@code spliterator} reports {@link Spliterator#CONCURRENT}
     * and {@link Spliterator#NONNULL}.
     *
     * @return the collection view
     */
    public Collection<V> values() {
        ValuesView<K, V> vs;
        return (vs = values) != null ? vs : (values = new ValuesView<K, V>(this));
    }

    /**
     * Returns a {@link Set} view of the mappings contained in this map.
     * The set is backed by the map, so changes to the map are
     * reflected in the set, and vice-versa.  The set supports element
     * removal, which removes the corresponding mapping from the map,
     * via the {@code Iterator.remove}, {@code Set.remove},
     * {@code removeAll}, {@code retainAll}, and {@code clear}
     * operations.
     * <p>
     * <p>The view's iterators and spliterators are
     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
     * <p>
     * <p>The view's {@code spliterator} reports {@link Spliterator#CONCURRENT},
     * {@link Spliterator#DISTINCT}, and {@link Spliterator#NONNULL}.
     *
     * @return the set view
     */
    public Set<Map.Entry<K, V>> entrySet() {
        EntrySetView<K, V> es;
        return (es = entrySet) != null ? es : (entrySet = new EntrySetView<K, V>(this));
    }

    /**
     * Returns the hash code value for this {@link Map}, i.e.,
     * the sum of, for each key-value pair in the map,
     * {@code key.hashCode() ^ value.hashCode()}.
     *
     * @return the hash code value for this map
     */
    public int hashCode() {
        int h = 0;
        Node<K, V>[] t;
        if ((t = table) != null) {
            Traverser<K, V> it = new Traverser<K, V>(t, t.length, 0, t.length);
            for (Node<K, V> p; (p = it.advance()) != null; )
                h += p.key.hashCode() ^ p.val.hashCode();
        }
        return h;
    }

    /**
     * Returns a string representation of this map.  The string
     * representation consists of a list of key-value mappings (in no
     * particular order) enclosed in braces ("{@code {}}").  Adjacent
     * mappings are separated by the characters {@code ", "} (comma
     * and space).  Each key-value mapping is rendered as the key
     * followed by an equals sign ("{@code =}") followed by the
     * associated value.
     *
     * @return a string representation of this map
     */
    public String toString() {
        Node<K, V>[] t;
        int f = (t = table) == null ? 0 : t.length;
        Traverser<K, V> it = new Traverser<K, V>(t, f, 0, f);
        StringBuilder sb = new StringBuilder();
        sb.append('{');
        Node<K, V> p;
        if ((p = it.advance()) != null) {
            for (; ; ) {
                K k = p.key;
                V v = p.val;
                sb.append(k == this ? "(this Map)" : k);
                sb.append('=');
                sb.append(v == this ? "(this Map)" : v);
                if ((p = it.advance()) == null)
                    break;
                sb.append(',').append(' ');
            }
        }
        return sb.append('}').toString();
    }

    /**
     * Compares the specified object with this map for equality.
     * Returns {@code true} if the given object is a map with the same
     * mappings as this map.  This operation may return misleading
     * results if either map is concurrently modified during execution
     * of this method.
     *
     * @param o object to be compared for equality with this map
     * @return {@code true} if the specified object is equal to this map
     */
    public boolean equals(Object o) {
        if (o != this) {
            if (!(o instanceof Map))
                return false;
            Map<?, ?> m = (Map<?, ?>) o;
            Node<K, V>[] t;
            int f = (t = table) == null ? 0 : t.length;
            Traverser<K, V> it = new Traverser<K, V>(t, f, 0, f);
            for (Node<K, V> p; (p = it.advance()) != null; ) {
                V val = p.val;
                Object v = m.get(p.key);
                if (v == null || (v != val && !v.equals(val)))
                    return false;
            }
            for (Map.Entry<?, ?> e : m.entrySet()) {
                Object mk, mv, v;
                if ((mk = e.getKey()) == null ||
                        (mv = e.getValue()) == null ||
                        (v = get(mk)) == null ||
                        (mv != v && !mv.equals(v)))
                    return false;
            }
        }
        return true;
    }

    /**
     * Stripped-down version of helper class used in previous version,
     * declared for the sake of serialization compatibility
     */
    static class Segment<K, V> extends ReentrantLock implements Serializable {
        private static final long serialVersionUID = 2249069246763182397L;
        final float loadFactor;

        Segment(float lf) {
            this.loadFactor = lf;
        }
    }

    /**
     * Saves the state of the {@code ConcurrentHashMap} instance to a
     * stream (i.e., serializes it).
     *
     * @param s the stream
     * @throws java.io.IOException if an I/O error occurs
     * @serialData the key (Object) and value (Object)
     * for each key-value mapping, followed by a null pair.
     * The key-value mappings are emitted in no particular order.
     */
    private void writeObject(java.io.ObjectOutputStream s)
            throws java.io.IOException {
        // For serialization compatibility
        // Emulate segment calculation from previous version of this class
        int sshift = 0;
        int ssize = 1;
        while (ssize < DEFAULT_CONCURRENCY_LEVEL) {
            ++sshift;
            ssize <<= 1;
        }
        int segmentShift = 32 - sshift;
        int segmentMask = ssize - 1;
        @SuppressWarnings("unchecked")
        Segment<K, V>[] segments = (Segment<K, V>[])
                new Segment<?, ?>[DEFAULT_CONCURRENCY_LEVEL];
        for (int i = 0; i < segments.length; ++i)
            segments[i] = new Segment<K, V>(LOAD_FACTOR);
        s.putFields().put("segments", segments);
        s.putFields().put("segmentShift", segmentShift);
        s.putFields().put("segmentMask", segmentMask);
        s.writeFields();

        Node<K, V>[] t;
        if ((t = table) != null) {
            Traverser<K, V> it = new Traverser<K, V>(t, t.length, 0, t.length);
            for (Node<K, V> p; (p = it.advance()) != null; ) {
                s.writeObject(p.key);
                s.writeObject(p.val);
            }
        }
        s.writeObject(null);
        s.writeObject(null);
        segments = null; // throw away
    }

    /**
     * Reconstitutes the instance 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 {
        /*
         * To improve performance in typical cases, we create nodes
         * while reading, then place in table once size is known.
         * However, we must also validate uniqueness and deal with
         * overpopulated bins while doing so, which requires
         * specialized versions of putVal mechanics.
         */
        sizeCtl = -1; // force exclusion for table construction
        s.defaultReadObject();
        long size = 0L;
        Node<K, V> p = null;
        for (; ; ) {
            @SuppressWarnings("unchecked")
            K k = (K) s.readObject();
            @SuppressWarnings("unchecked")
            V v = (V) s.readObject();
            if (k != null && v != null) {
                p = new Node<K, V>(spread(k.hashCode()), k, v, p);
                ++size;
            } else
                break;
        }
        if (size == 0L)
            sizeCtl = 0;
        else {
            int n;
            if (size >= (long) (MAXIMUM_CAPACITY >>> 1))
                n = MAXIMUM_CAPACITY;
            else {
                int sz = (int) size;
                n = tableSizeFor(sz + (sz >>> 1) + 1);
            }
            @SuppressWarnings("unchecked")
            Node<K, V>[] tab = (Node<K, V>[]) new Node<?, ?>[n];
            int mask = n - 1;
            long added = 0L;
            while (p != null) {
                boolean insertAtFront;
                Node<K, V> next = p.next, first;
                int h = p.hash, j = h & mask;
                if ((first = tabAt(tab, j)) == null)
                    insertAtFront = true;
                else {
                    K k = p.key;
                    if (first.hash < 0) {
                        TreeBin<K, V> t = (TreeBin<K, V>) first;
                        if (t.putTreeVal(h, k, p.val) == null)
                            ++added;
                        insertAtFront = false;
                    } else {
                        int binCount = 0;
                        insertAtFront = true;
                        Node<K, V> q;
                        K qk;
                        for (q = first; q != null; q = q.next) {
                            if (q.hash == h &&
                                    ((qk = q.key) == k ||
                                            (qk != null && k.equals(qk)))) {
                                insertAtFront = false;
                                break;
                            }
                            ++binCount;
                        }
                        if (insertAtFront && binCount >= TREEIFY_THRESHOLD) {
                            insertAtFront = false;
                            ++added;
                            p.next = first;
                            TreeNode<K, V> hd = null, tl = null;
                            for (q = p; q != null; q = q.next) {
                                TreeNode<K, V> t = new TreeNode<K, V>
                                        (q.hash, q.key, q.val, null, null);
                                if ((t.prev = tl) == null)
                                    hd = t;
                                else
                                    tl.next = t;
                                tl = t;
                            }
                            setTabAt(tab, j, new TreeBin<K, V>(hd));
                        }
                    }
                }
                if (insertAtFront) {
                    ++added;
                    p.next = first;
                    setTabAt(tab, j, p);
                }
                p = next;
            }
            table = tab;
            sizeCtl = n - (n >>> 2);
            baseCount = added;
        }
    }

    // ConcurrentMap methods

    /**
     * {@inheritDoc}
     *
     * @return the previous value associated with the specified key,
     * or {@code null} if there was no mapping for the key
     * @throws NullPointerException if the specified key or value is null
     */
    public V putIfAbsent(K key, V value) {
        return putVal(key, value, true);
    }

    /**
     * {@inheritDoc}
     *
     * @throws NullPointerException if the specified key is null
     */
    public boolean remove(Object key, Object value) {
        if (key == null)
            throw new NullPointerException();
        return value != null && replaceNode(key, null, value) != null;
    }

    /**
     * {@inheritDoc}
     *
     * @throws NullPointerException if any of the arguments are null
     */
    public boolean replace(K key, V oldValue, V newValue) {
        if (key == null || oldValue == null || newValue == null)
            throw new NullPointerException();
        return replaceNode(key, newValue, oldValue) != null;
    }

    /**
     * {@inheritDoc}
     *
     * @return the previous value associated with the specified key,
     * or {@code null} if there was no mapping for the key
     * @throws NullPointerException if the specified key or value is null
     */
    public V replace(K key, V value) {
        if (key == null || value == null)
            throw new NullPointerException();
        return replaceNode(key, value, null);
    }

    // Overrides of JDK8+ Map extension method defaults

    /**
     * Returns the value to which the specified key is mapped, or the
     * given default value if this map contains no mapping for the
     * key.
     *
     * @param key          the key whose associated value is to be returned
     * @param defaultValue the value to return if this map contains
     *                     no mapping for the given key
     * @return the mapping for the key, if present; else the default value
     * @throws NullPointerException if the specified key is null
     */
    public V getOrDefault(Object key, V defaultValue) {
        V v;
        return (v = get(key)) == null ? defaultValue : v;
    }

    public void forEach(BiConsumer<? super K, ? super V> action) {
        if (action == null) throw new NullPointerException();
        Node<K, V>[] t;
        if ((t = table) != null) {
            Traverser<K, V> it = new Traverser<K, V>(t, t.length, 0, t.length);
            for (Node<K, V> p; (p = it.advance()) != null; ) {
                action.accept(p.key, p.val);
            }
        }
    }

    public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
        if (function == null) throw new NullPointerException();
        Node<K, V>[] t;
        if ((t = table) != null) {
            Traverser<K, V> it = new Traverser<K, V>(t, t.length, 0, t.length);
            for (Node<K, V> p; (p = it.advance()) != null; ) {
                V oldValue = p.val;
                for (K key = p.key; ; ) {
                    V newValue = function.apply(key, oldValue);
                    if (newValue == null)
                        throw new NullPointerException();
                    if (replaceNode(key, newValue, oldValue) != null ||
                            (oldValue = get(key)) == null)
                        break;
                }
            }
        }
    }

    /**
     * If the specified key is not already associated with a value,
     * attempts to compute its value using the given mapping function
     * and enters it into this map unless {@code null}.  The entire
     * method invocation is performed atomically, so the function is
     * applied at most once per key.  Some attempted update operations
     * on this map by other threads may be blocked while computation
     * is in progress, so the computation should be short and simple,
     * and must not attempt to update any other mappings of this map.
     *
     * @param key             key with which the specified value is to be associated
     * @param mappingFunction the function to compute a value
     * @return the current (existing or computed) value associated with
     * the specified key, or null if the computed value is null
     * @throws NullPointerException  if the specified key or mappingFunction
     *                               is null
     * @throws IllegalStateException if the computation detectably
     *                               attempts a recursive update to this map that would
     *                               otherwise never complete
     * @throws RuntimeException      or Error if the mappingFunction does so,
     *                               in which case the mapping is left unestablished
     */
    public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
        if (key == null || mappingFunction == null)
            throw new NullPointerException();
        int h = spread(key.hashCode());
        V val = null;
        int binCount = 0;
        for (Node<K, V>[] tab = table; ; ) {
            Node<K, V> f;
            int n, i, fh;
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();
            else if ((f = tabAt(tab, i = (n - 1) & h)) == null) {
                Node<K, V> r = new ReservationNode<K, V>();
                synchronized (r) {
                    if (casTabAt(tab, i, null, r)) {
                        binCount = 1;
                        Node<K, V> node = null;
                        try {
                            if ((val = mappingFunction.apply(key)) != null)
                                node = new Node<K, V>(h, key, val, null);
                        } finally {
                            setTabAt(tab, i, node);
                        }
                    }
                }
                if (binCount != 0)
                    break;
            } else if ((fh = f.hash) == MOVED)
                tab = helpTransfer(tab, f);
            else {
                boolean added = false;
                synchronized (f) {
                    if (tabAt(tab, i) == f) {
                        if (fh >= 0) {
                            binCount = 1;
                            for (Node<K, V> e = f; ; ++binCount) {
                                K ek;
                                V ev;
                                if (e.hash == h &&
                                        ((ek = e.key) == key ||
                                                (ek != null && key.equals(ek)))) {
                                    val = e.val;
                                    break;
                                }
                                Node<K, V> pred = e;
                                if ((e = e.next) == null) {
                                    if ((val = mappingFunction.apply(key)) != null) {
                                        added = true;
                                        pred.next = new Node<K, V>(h, key, val, null);
                                    }
                                    break;
                                }
                            }
                        } else if (f instanceof TreeBin) {
                            binCount = 2;
                            TreeBin<K, V> t = (TreeBin<K, V>) f;
                            TreeNode<K, V> r, p;
                            if ((r = t.root) != null &&
                                    (p = r.findTreeNode(h, key, null)) != null)
                                val = p.val;
                            else if ((val = mappingFunction.apply(key)) != null) {
                                added = true;
                                t.putTreeVal(h, key, val);
                            }
                        }
                    }
                }
                if (binCount != 0) {
                    if (binCount >= TREEIFY_THRESHOLD)
                        treeifyBin(tab, i);
                    if (!added)
                        return val;
                    break;
                }
            }
        }
        if (val != null)
            addCount(1L, binCount);
        return val;
    }

    /**
     * If the value for the specified key is present, attempts to
     * compute a new mapping given the key and its current mapped
     * value.  The entire method invocation is performed atomically.
     * Some attempted update operations on this map by other threads
     * may be blocked while computation is in progress, so the
     * computation should be short and simple, and must not attempt to
     * update any other mappings of this map.
     *
     * @param key               key with which a value may be associated
     * @param remappingFunction the function to compute a value
     * @return the new value associated with the specified key, or null if none
     * @throws NullPointerException  if the specified key or remappingFunction
     *                               is null
     * @throws IllegalStateException if the computation detectably
     *                               attempts a recursive update to this map that would
     *                               otherwise never complete
     * @throws RuntimeException      or Error if the remappingFunction does so,
     *                               in which case the mapping is unchanged
     */
    public V computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        if (key == null || remappingFunction == null)
            throw new NullPointerException();
        int h = spread(key.hashCode());
        V val = null;
        int delta = 0;
        int binCount = 0;
        for (Node<K, V>[] tab = table; ; ) {
            Node<K, V> f;
            int n, i, fh;
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();
            else if ((f = tabAt(tab, i = (n - 1) & h)) == null)
                break;
            else if ((fh = f.hash) == MOVED)
                tab = helpTransfer(tab, f);
            else {
                synchronized (f) {
                    if (tabAt(tab, i) == f) {
                        if (fh >= 0) {
                            binCount = 1;
                            for (Node<K, V> e = f, pred = null; ; ++binCount) {
                                K ek;
                                if (e.hash == h &&
                                        ((ek = e.key) == key ||
                                                (ek != null && key.equals(ek)))) {
                                    val = remappingFunction.apply(key, e.val);
                                    if (val != null)
                                        e.val = val;
                                    else {
                                        delta = -1;
                                        Node<K, V> en = e.next;
                                        if (pred != null)
                                            pred.next = en;
                                        else
                                            setTabAt(tab, i, en);
                                    }
                                    break;
                                }
                                pred = e;
                                if ((e = e.next) == null)
                                    break;
                            }
                        } else if (f instanceof TreeBin) {
                            binCount = 2;
                            TreeBin<K, V> t = (TreeBin<K, V>) f;
                            TreeNode<K, V> r, p;
                            if ((r = t.root) != null &&
                                    (p = r.findTreeNode(h, key, null)) != null) {
                                val = remappingFunction.apply(key, p.val);
                                if (val != null)
                                    p.val = val;
                                else {
                                    delta = -1;
                                    if (t.removeTreeNode(p))
                                        setTabAt(tab, i, untreeify(t.first));
                                }
                            }
                        }
                    }
                }
                if (binCount != 0)
                    break;
            }
        }
        if (delta != 0)
            addCount((long) delta, binCount);
        return val;
    }

    /**
     * Attempts to compute a mapping for the specified key and its
     * current mapped value (or {@code null} if there is no current
     * mapping). The entire method invocation is performed atomically.
     * Some attempted update operations on this map by other threads
     * may be blocked while computation is in progress, so the
     * computation should be short and simple, and must not attempt to
     * update any other mappings of this Map.
     *
     * @param key               key with which the specified value is to be associated
     * @param remappingFunction the function to compute a value
     * @return the new value associated with the specified key, or null if none
     * @throws NullPointerException  if the specified key or remappingFunction
     *                               is null
     * @throws IllegalStateException if the computation detectably
     *                               attempts a recursive update to this map that would
     *                               otherwise never complete
     * @throws RuntimeException      or Error if the remappingFunction does so,
     *                               in which case the mapping is unchanged
     */
    public V compute(K key,
                     BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        if (key == null || remappingFunction == null)
            throw new NullPointerException();
        int h = spread(key.hashCode());
        V val = null;
        int delta = 0;
        int binCount = 0;
        for (Node<K, V>[] tab = table; ; ) {
            Node<K, V> f;
            int n, i, fh;
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();
            else if ((f = tabAt(tab, i = (n - 1) & h)) == null) {
                Node<K, V> r = new ReservationNode<K, V>();
                synchronized (r) {
                    if (casTabAt(tab, i, null, r)) {
                        binCount = 1;
                        Node<K, V> node = null;
                        try {
                            if ((val = remappingFunction.apply(key, null)) != null) {
                                delta = 1;
                                node = new Node<K, V>(h, key, val, null);
                            }
                        } finally {
                            setTabAt(tab, i, node);
                        }
                    }
                }
                if (binCount != 0)
                    break;
            } else if ((fh = f.hash) == MOVED)
                tab = helpTransfer(tab, f);
            else {
                synchronized (f) {
                    if (tabAt(tab, i) == f) {
                        if (fh >= 0) {
                            binCount = 1;
                            for (Node<K, V> e = f, pred = null; ; ++binCount) {
                                K ek;
                                if (e.hash == h &&
                                        ((ek = e.key) == key ||
                                                (ek != null && key.equals(ek)))) {
                                    val = remappingFunction.apply(key, e.val);
                                    if (val != null)
                                        e.val = val;
                                    else {
                                        delta = -1;
                                        Node<K, V> en = e.next;
                                        if (pred != null)
                                            pred.next = en;
                                        else
                                            setTabAt(tab, i, en);
                                    }
                                    break;
                                }
                                pred = e;
                                if ((e = e.next) == null) {
                                    val = remappingFunction.apply(key, null);
                                    if (val != null) {
                                        delta = 1;
                                        pred.next =
                                                new Node<K, V>(h, key, val, null);
                                    }
                                    break;
                                }
                            }
                        } else if (f instanceof TreeBin) {
                            binCount = 1;
                            TreeBin<K, V> t = (TreeBin<K, V>) f;
                            TreeNode<K, V> r, p;
                            if ((r = t.root) != null)
                                p = r.findTreeNode(h, key, null);
                            else
                                p = null;
                            V pv = (p == null) ? null : p.val;
                            val = remappingFunction.apply(key, pv);
                            if (val != null) {
                                if (p != null)
                                    p.val = val;
                                else {
                                    delta = 1;
                                    t.putTreeVal(h, key, val);
                                }
                            } else if (p != null) {
                                delta = -1;
                                if (t.removeTreeNode(p))
                                    setTabAt(tab, i, untreeify(t.first));
                            }
                        }
                    }
                }
                if (binCount != 0) {
                    if (binCount >= TREEIFY_THRESHOLD)
                        treeifyBin(tab, i);
                    break;
                }
            }
        }
        if (delta != 0)
            addCount((long) delta, binCount);
        return val;
    }

    /**
     * If the specified key is not already associated with a
     * (non-null) value, associates it with the given value.
     * Otherwise, replaces the value with the results of the given
     * remapping function, or removes if {@code null}. The entire
     * method invocation is performed atomically.  Some attempted
     * update operations on this map by other threads may be blocked
     * while computation is in progress, so the computation should be
     * short and simple, and must not attempt to update any other
     * mappings of this Map.
     *
     * @param key               key with which the specified value is to be associated
     * @param value             the value to use if absent
     * @param remappingFunction the function to recompute a value if present
     * @return the new value associated with the specified key, or null if none
     * @throws NullPointerException if the specified key or the
     *                              remappingFunction is null
     * @throws RuntimeException     or Error if the remappingFunction does so,
     *                              in which case the mapping is unchanged
     */
    public V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
        if (key == null || value == null || remappingFunction == null)
            throw new NullPointerException();
        int h = spread(key.hashCode());
        V val = null;
        int delta = 0;
        int binCount = 0;
        for (Node<K, V>[] tab = table; ; ) {
            Node<K, V> f;
            int n, i, fh;
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();
            else if ((f = tabAt(tab, i = (n - 1) & h)) == null) {
                if (casTabAt(tab, i, null, new Node<K, V>(h, key, value, null))) {
                    delta = 1;
                    val = value;
                    break;
                }
            } else if ((fh = f.hash) == MOVED)
                tab = helpTransfer(tab, f);
            else {
                synchronized (f) {
                    if (tabAt(tab, i) == f) {
                        if (fh >= 0) {
                            binCount = 1;
                            for (Node<K, V> e = f, pred = null; ; ++binCount) {
                                K ek;
                                if (e.hash == h &&
                                        ((ek = e.key) == key ||
                                                (ek != null && key.equals(ek)))) {
                                    val = remappingFunction.apply(e.val, value);
                                    if (val != null)
                                        e.val = val;
                                    else {
                                        delta = -1;
                                        Node<K, V> en = e.next;
                                        if (pred != null)
                                            pred.next = en;
                                        else
                                            setTabAt(tab, i, en);
                                    }
                                    break;
                                }
                                pred = e;
                                if ((e = e.next) == null) {
                                    delta = 1;
                                    val = value;
                                    pred.next =
                                            new Node<K, V>(h, key, val, null);
                                    break;