Druid/src/main/java/com/alibaba/druid/util/ConcurrentIdentityHashMap.java

package com.alibaba.druid.util;

import java.util.AbstractCollection;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Enumeration;
import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.locks.ReentrantLock;

/**
 * An alternative identity-comparing {@link ConcurrentMap} which is similar to
 * {@link java.util.concurrent.ConcurrentHashMap}.
 * 
 * @author <a href="http://www.jboss.org/netty/">The Netty Project</a>
 * @author Doug Lea
 * @author Jason T. Greene
 * @author <a href="http://gleamynode.net/">Trustin Lee</a>
 * @version $Rev: 2217 $, $Date: 2010-03-16 18:39:56 +0900 (Tue, 16 Mar 2010) $
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 */
public class ConcurrentIdentityHashMap<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V> {

    /**
     * The default initial capacity for this table, used when not otherwise specified in a constructor.
     */
    static final int      DEFAULT_INITIAL_CAPACITY  = 16;

    /**
     * The default load factor for this table, used when not otherwise specified in a constructor.
     */
    static final float    DEFAULT_LOAD_FACTOR       = 0.75f;

    /**
     * The default concurrency level for this table, used when not otherwise specified in a constructor.
     */
    static final int      DEFAULT_CONCURRENCY_LEVEL = 16;

    /**
     * The maximum capacity, used if a higher value is implicitly specified by either of the constructors with
     * arguments. MUST be a power of two <= 1<<30 to ensure that entries are indexable using integers.
     */
    static final int      MAXIMUM_CAPACITY          = 1 << 30;

    /**
     * The maximum number of segments to allow; used to bound constructor arguments.
     */
    static final int      MAX_SEGMENTS              = 1 << 16; // slightly conservative

    /**
     * Number of unsynchronized retries in size and containsValue methods before resorting to locking. This is used to
     * avoid unbounded retries if tables undergo continuous modification which would make it impossible to obtain an
     * accurate result.
     */
    static final int      RETRIES_BEFORE_LOCK       = 2;

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

    /**
     * Mask value for indexing into segments. The upper bits of a key's hash code are used to choose the segment.
     */
    final int             segmentMask;

    /**
     * Shift value for indexing within segments.
     */
    final int             segmentShift;

    /**
     * The segments, each of which is a specialized hash table
     */
    final Segment<K, V>[] segments;

    Set<K>                keySet;
    Set<Map.Entry<K, V>>  entrySet;
    Collection<V>         values;

    /* ---------------- Small Utilities -------------- */

    /**
     * Applies a supplemental hash function to a given hashCode, which defends against poor quality hash functions. This
     * is critical because ConcurrentReferenceHashMap uses power-of-two length hash tables, that otherwise encounter
     * collisions for hashCodes that do not differ in lower or upper bits.
     */
    private static int hash(int h) {
        // Spread bits to regularize both segment and index locations,
        // using variant of single-word Wang/Jenkins hash.
        h += h << 15 ^ 0xffffcd7d;
        h ^= h >>> 10;
        h += h << 3;
        h ^= h >>> 6;
        h += (h << 2) + (h << 14);
        return h ^ h >>> 16;
    }

    /**
     * Returns the segment that should be used for key with given hash.
     * 
     * @param hash the hash code for the key
     * @return the segment
     */
    final Segment<K, V> segmentFor(int hash) {
        return segments[hash >>> segmentShift & segmentMask];
    }

    private int hashOf(Object key) {
        return hash(System.identityHashCode(key));
    }

    /**
     * ConcurrentReferenceHashMap list entry. Note that this is never exported out as a user-visible Map.Entry. Because
     * the value field is volatile, not final, it is legal wrt the Java Memory Model for an unsynchronized reader to see
     * null instead of initial value when read via a data race. Although a reordering leading to this is not likely to
     * ever actually occur, the Segment.readValueUnderLock method is used as a backup in case a null (pre-initialized)
     * value is ever seen in an unsynchronized access method.
     */
    static final class HashEntry<K, V> {

        final Object          key;
        final int             hash;
        volatile Object       value;
        final HashEntry<K, V> next;

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

        @SuppressWarnings("unchecked")
        final K key() {
            return (K) key;
        }

        @SuppressWarnings("unchecked")
        final V value() {
            return (V) value;
        }

        final void setValue(V value) {
            this.value = value;
        }

        @SuppressWarnings("unchecked")
        static final <K, V> HashEntry<K, V>[] newArray(int i) {
            return new HashEntry[i];
        }
    }

    /**
     * Segments are specialized versions of hash tables. This subclasses from ReentrantLock opportunistically, just to
     * simplify some locking and avoid separate construction.
     */
    static final class Segment<K, V> extends ReentrantLock {

        /*
         * Segments maintain a table of entry lists that are ALWAYS kept in a consistent state, so can be read without
         * locking. Next fields of nodes are immutable (final). All list additions are performed at the front of each
         * bin. This makes it easy to check changes, and also fast to traverse. When nodes would otherwise be changed,
         * new nodes are created to replace them. This works well for hash tables since the bin lists tend to be short.
         * (The average length is less than two for the default load factor threshold.) Read operations can thus proceed
         * without locking, but rely on selected uses of volatiles to ensure that completed write operations performed
         * by other threads are noticed. For most purposes, the "count" field, tracking the number of elements, serves
         * as that volatile variable ensuring visibility. This is convenient because this field needs to be read in many
         * read operations anyway: - All (unsynchronized) read operations must first read the "count" field, and should
         * not look at table entries if it is 0. - All (synchronized) write operations should write to the "count" field
         * after structurally changing any bin. The operations must not take any action that could even momentarily
         * cause a concurrent read operation to see inconsistent data. This is made easier by the nature of the read
         * operations in Map. For example, no operation can reveal that the table has grown but the threshold has not
         * yet been updated, so there are no atomicity requirements for this with respect to reads. As a guide, all
         * critical volatile reads and writes to the count field are marked in code comments.
         */

        private static final long            serialVersionUID = 5207829234977119743L;

        /**
         * The number of elements in this segment's region.
         */
        transient volatile int               count;

        /**
         * Number of updates that alter the size of the table. This is used during bulk-read methods to make sure they
         * see a consistent snapshot: If modCounts change during a traversal of segments computing size or checking
         * containsValue, then we might have an inconsistent view of state so (usually) must retry.
         */
        int                                  modCount;

        /**
         * The table is rehashed when its size exceeds this threshold. (The value of this field is always
         * <tt>(capacity * loadFactor)</tt>.)
         */
        int                                  threshold;

        /**
         * The per-segment table.
         */
        transient volatile HashEntry<K, V>[] table;

        /**
         * The load factor for the hash table. Even though this value is same for all segments, it is replicated to
         * avoid needing links to outer object.
         */
        final float                          loadFactor;

        Segment(int initialCapacity, float lf){
            loadFactor = lf;
            setTable(HashEntry.<K, V> newArray(initialCapacity));
        }

        @SuppressWarnings("unchecked")
        static final <K, V> Segment<K, V>[] newArray(int i) {
            return new Segment[i];
        }

        private boolean keyEq(Object src, Object dest) {
            return src == dest;
        }

        /**
         * Sets table to new HashEntry array. Call only while holding lock or in constructor.
         */
        void setTable(HashEntry<K, V>[] newTable) {
            threshold = (int) (newTable.length * loadFactor);
            table = newTable;
        }

        /**
         * Returns properly casted first entry of bin for given hash.
         */
        HashEntry<K, V> getFirst(int hash) {
            HashEntry<K, V>[] tab = table;
            return tab[hash & tab.length - 1];
        }

        HashEntry<K, V> newHashEntry(K key, int hash, HashEntry<K, V> next, V value) {
            return new HashEntry<K, V>(key, hash, next, value);
        }

        /**
         * Reads value field of an entry under lock. Called if value field ever appears to be null. This is possible
         * only if a compiler happens to reorder a HashEntry initialization with its table assignment, which is legal
         * under memory model but is not known to ever occur.
         */
        V readValueUnderLock(HashEntry<K, V> e) {
            lock();
            try {
                return e.value();
            } finally {
                unlock();
            }
        }

        /* Specialized implementations of map methods */

        V get(Object key, int hash) {
            if (count != 0) { // read-volatile
                HashEntry<K, V> e = getFirst(hash);
                while (e != null) {
                    if (e.hash == hash && keyEq(key, e.key())) {
                        V opaque = e.value();
                        if (opaque != null) {
                            return opaque;
                        }

                        return readValueUnderLock(e); // recheck
                    }
                    e = e.next;
                }
            }
            return null;
        }

        boolean containsKey(Object key, int hash) {
            if (count != 0) { // read-volatile
                HashEntry<K, V> e = getFirst(hash);
                while (e != null) {
                    if (e.hash == hash && keyEq(key, e.key())) {
                        return true;
                    }
                    e = e.next;
                }
            }
            return false;
        }

        boolean containsValue(Object value) {
            if (count != 0) { // read-volatile
                HashEntry<K, V>[] tab = table;
                int len = tab.length;
                for (int i = 0; i < len; i++) {
                    for (HashEntry<K, V> e = tab[i]; e != null; e = e.next) {
                        V opaque = e.value();
                        V v;

                        if (opaque == null) {
                            v = readValueUnderLock(e); // recheck
                        } else {
                            v = opaque;
                        }

                        if (value.equals(v)) {
                            return true;
                        }
                    }
                }
            }
            return false;
        }

        boolean replace(K key, int hash, V oldValue, V newValue) {
            lock();
            try {
                HashEntry<K, V> e = getFirst(hash);
                while (e != null && (e.hash != hash || !keyEq(key, e.key()))) {
                    e = e.next;
                }

                boolean replaced = false;
                if (e != null && oldValue.equals(e.value())) {
                    replaced = true;
                    e.setValue(newValue);
                }
                return replaced;
            } finally {
                unlock();
            }
        }

        V replace(K key, int hash, V newValue) {
            lock();
            try {
                HashEntry<K, V> e = getFirst(hash);
                while (e != null && (e.hash != hash || !keyEq(key, e.key()))) {
                    e = e.next;
                }

                V oldValue = null;
                if (e != null) {
                    oldValue = e.value();
                    e.setValue(newValue);
                }
                return oldValue;
            } finally {
                unlock();
            }
        }

        V put(K key, int hash, V value, boolean onlyIfAbsent) {
            lock();
            try {
                int c = count;
                if (c++ > threshold) { // ensure capacity
                    int reduced = rehash();
                    if (reduced > 0) {
                        count = (c -= reduced) - 1; // write-volatile
                    }
                }

                HashEntry<K, V>[] tab = table;
                int index = hash & tab.length - 1;
                HashEntry<K, V> first = tab[index];
                HashEntry<K, V> e = first;
                while (e != null && (e.hash != hash || !keyEq(key, e.key()))) {
                    e = e.next;
                }

                V oldValue;
                if (e != null) {
                    oldValue = e.value();
                    if (!onlyIfAbsent) {
                        e.setValue(value);
                    }
                } else {
                    oldValue = null;
                    ++modCount;
                    tab[index] = newHashEntry(key, hash, first, value);
                    count = c; // write-volatile
                }
                return oldValue;
            } finally {
                unlock();
            }
        }

        int rehash() {
            HashEntry<K, V>[] oldTable = table;
            int oldCapacity = oldTable.length;
            if (oldCapacity >= MAXIMUM_CAPACITY) {
                return 0;
            }

            /*
             * Reclassify nodes in each list to new Map. Because we are using power-of-two expansion, the elements from
             * each bin must either stay at same index, or move with a power of two offset. We eliminate unnecessary
             * node creation by catching cases where old nodes can be reused because their next fields won't change.
             * Statistically, at the default threshold, only about one-sixth of them need cloning when a table doubles.
             * The nodes they replace will be garbage collectable as soon as they are no longer referenced by any reader
             * thread that may be in the midst of traversing table right now.
             */

            HashEntry<K, V>[] newTable = HashEntry.newArray(oldCapacity << 1);
            threshold = (int) (newTable.length * loadFactor);
            int sizeMask = newTable.length - 1;
            int reduce = 0;
            for (int i = 0; i < oldCapacity; i++) {
                // We need to guarantee that any existing reads of old Map can
                // proceed. So we cannot yet null out each bin.
                HashEntry<K, V> e = oldTable[i];

                if (e != null) {
                    HashEntry<K, V> next = e.next;
                    int idx = e.hash & sizeMask;

                    // Single node on list
                    if (next == null) {
                        newTable[idx] = e;
                    } else {
                        // Reuse trailing consecutive sequence at same slot
                        HashEntry<K, V> lastRun = e;
                        int lastIdx = idx;
                        for (HashEntry<K, V> last = next; last != null; last = last.next) {
                            int k = last.hash & sizeMask;
                            if (k != lastIdx) {
                                lastIdx = k;
                                lastRun = last;
                            }
                        }
                        newTable[lastIdx] = lastRun;
                        // Clone all remaining nodes
                        for (HashEntry<K, V> p = e; p != lastRun; p = p.next) {
                            // Skip GC'd weak references
                            K key = p.key();
                            if (key == null) {
                                reduce++;
                                continue;
                            }
                            int k = p.hash & sizeMask;
                            HashEntry<K, V> n = newTable[k];
                            newTable[k] = newHashEntry(key, p.hash, n, p.value());
                        }
                    }
                }
            }
            table = newTable;
            return reduce;
        }

        /**
         * Remove; match on key only if value null, else match both.
         */
        V remove(Object key, int hash, Object value, boolean refRemove) {
            lock();
            try {
                int c = count - 1;
                HashEntry<K, V>[] tab = table;
                int index = hash & tab.length - 1;
                HashEntry<K, V> first = tab[index];
                HashEntry<K, V> e = first;
                // a reference remove operation compares the Reference instance
                while (e != null && key != e.key && (refRemove || hash != e.hash || !keyEq(key, e.key()))) {
                    e = e.next;
                }

                V oldValue = null;
                if (e != null) {
                    V v = e.value();
                    if (value == null || value.equals(v)) {
                        oldValue = v;
                        // All entries following removed node can stay in list,
                        // but all preceding ones need to be cloned.
                        ++modCount;
                        HashEntry<K, V> newFirst = e.next;
                        for (HashEntry<K, V> p = first; p != e; p = p.next) {
                            K pKey = p.key();
                            if (pKey == null) { // Skip GC'd keys
                                c--;
                                continue;
                            }

                            newFirst = newHashEntry(pKey, p.hash, newFirst, p.value());
                        }
                        tab[index] = newFirst;
                        count = c; // write-volatile
                    }
                }
                return oldValue;
            } finally {
                unlock();
            }
        }

        void clear() {
            if (count != 0) {
                lock();
                try {
                    HashEntry<K, V>[] tab = table;
                    for (int i = 0; i < tab.length; i++) {
                        tab[i] = null;
                    }
                    ++modCount;
                    count = 0; // write-volatile
                } finally {
                    unlock();
                }
            }
        }
    }

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

    /**
     * Creates a new, empty map with the specified initial capacity, load factor and concurrency level.
     * 
     * @param initialCapacity the initial capacity. The implementation performs internal sizing to accommodate this many
     * elements.
     * @param loadFactor the load factor threshold, used to control resizing. Resizing may be performed when the average
     * number of elements per bin exceeds this threshold.
     * @param concurrencyLevel the estimated number of concurrently updating threads. The implementation performs
     * internal sizing to try to accommodate this many threads.
     * @throws IllegalArgumentException if the initial capacity is negative or the load factor or concurrencyLevel are
     * nonpositive.
     */
    public ConcurrentIdentityHashMap(int initialCapacity, float loadFactor, int concurrencyLevel){
        if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0) {
            throw new IllegalArgumentException();
        }

        if (concurrencyLevel > MAX_SEGMENTS) {
            concurrencyLevel = MAX_SEGMENTS;
        }

        // Find power-of-two sizes best matching arguments
        int sshift = 0;
        int ssize = 1;
        while (ssize < concurrencyLevel) {
            ++sshift;
            ssize <<= 1;
        }
        segmentShift = 32 - sshift;
        segmentMask = ssize - 1;
        this.segments = Segment.newArray(ssize);

        if (initialCapacity > MAXIMUM_CAPACITY) {
            initialCapacity = MAXIMUM_CAPACITY;
        }
        int c = initialCapacity / ssize;
        if (c * ssize < initialCapacity) {
            ++c;
        }
        int cap = 1;
        while (cap < c) {
            cap <<= 1;
        }

        for (int i = 0; i < this.segments.length; ++i) {
            this.segments[i] = new Segment<K, V>(cap, loadFactor);
        }
    }

    /**
     * Creates a new, empty map with the specified initial capacity and load factor and with the default reference types
     * (weak keys, strong values), and concurrencyLevel (16).
     * 
     * @param initialCapacity The implementation performs internal sizing to accommodate this many elements.
     * @param loadFactor the load factor threshold, used to control resizing. Resizing may be performed when the average
     * number of elements per bin exceeds this threshold.
     * @throws IllegalArgumentException if the initial capacity of elements is negative or the load factor is
     * nonpositive
     */
    public ConcurrentIdentityHashMap(int initialCapacity, float loadFactor){
        this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new, empty map with the specified initial capacity, and with default reference types (weak keys, strong
     * values), load factor (0.75) and concurrencyLevel (16).
     * 
     * @param initialCapacity the initial capacity. The implementation performs internal sizing to accommodate this many
     * elements.
     * @throws IllegalArgumentException if the initial capacity of elements is negative.
     */
    public ConcurrentIdentityHashMap(int initialCapacity){
        this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new, empty map with a default initial capacity (16), reference types (weak keys, strong values),
     * default load factor (0.75) and concurrencyLevel (16).
     */
    public ConcurrentIdentityHashMap(){
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new map with the same mappings as the given map. The map is created with a capacity of 1.5 times the
     * number of mappings in the given map or 16 (whichever is greater), and a default load factor (0.75) and
     * concurrencyLevel (16).
     * 
     * @param m the map
     */
    public ConcurrentIdentityHashMap(Map<? extends K, ? extends V> m){
        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR,
             DEFAULT_CONCURRENCY_LEVEL);
        putAll(m);
    }

    /**
     * Returns <tt>true</tt> if this map contains no key-value mappings.
     * 
     * @return <tt>true</tt> if this map contains no key-value mappings
     */
    @Override
    public boolean isEmpty() {
        final Segment<K, V>[] segments = this.segments;
        /*
         * We keep track of per-segment modCounts to avoid ABA problems in which an element in one segment was added and
         * in another removed during traversal, in which case the table was never actually empty at any point. Note the
         * similar use of modCounts in the size() and containsValue() methods, which are the only other methods also
         * susceptible to ABA problems.
         */
        int[] mc = new int[segments.length];
        int mcsum = 0;
        for (int i = 0; i < segments.length; ++i) {
            if (segments[i].count != 0) {
                return false;
            } else {
                mcsum += mc[i] = segments[i].modCount;
            }
        }
        // If mcsum happens to be zero, then we know we got a snapshot before
        // any modifications at all were made. This is probably common enough
        // to bother tracking.
        if (mcsum != 0) {
            for (int i = 0; i < segments.length; ++i) {
                if (segments[i].count != 0 || mc[i] != segments[i].modCount) {
                    return false;
                }
            }
        }
        return true;
    }

    /**
     * Returns the number of key-value mappings in this map. If the map contains more than <tt>Integer.MAX_VALUE</tt>
     * elements, returns <tt>Integer.MAX_VALUE</tt>.
     * 
     * @return the number of key-value mappings in this map
     */
    @Override
    public int size() {
        final Segment<K, V>[] segments = this.segments;
        long sum = 0;
        long check = 0;
        int[] mc = new int[segments.length];
        // Try a few times to get accurate count. On failure due to continuous
        // async changes in table, resort to locking.
        for (int k = 0; k < RETRIES_BEFORE_LOCK; ++k) {
            check = 0;
            sum = 0;
            int mcsum = 0;
            for (int i = 0; i < segments.length; ++i) {
                sum += segments[i].count;
                mcsum += mc[i] = segments[i].modCount;
            }
            if (mcsum != 0) {
                for (int i = 0; i < segments.length; ++i) {
                    check += segments[i].count;
                    if (mc[i] != segments[i].modCount) {
                        check = -1; // force retry
                        break;
                    }
                }
            }
            if (check == sum) {
                break;
            }
        }
        if (check != sum) { // Resort to locking all segments
            sum = 0;
            for (int i = 0; i < segments.length; ++i) {
                segments[i].lock();
            }
            for (int i = 0; i < segments.length; ++i) {
                sum += segments[i].count;
            }
            for (int i = 0; i < segments.length; ++i) {
                segments[i].unlock();
            }
        }
        if (sum > Integer.MAX_VALUE) {
            return Integer.MAX_VALUE;
        } else {
            return (int) sum;
        }
    }

    /**
     * Returns the value to which the specified key is mapped, or {@code null} if this map contains no mapping for the
     * key.
     * <p>
     * More formally, if this map contains a mapping from a key {@code k} to a value {@code v} such that
     * {@code key.equals(k)}, then this method returns {@code v}; otherwise it returns {@code null}. (There can be at
     * most one such mapping.)
     * 
     * @throws NullPointerException if the specified key is null
     */
    @Override
    public V get(Object key) {
        int hash = hashOf(key);
        return segmentFor(hash).get(key, hash);
    }

    /**
     * Tests if the specified object is a key in this table.
     * 
     * @param key possible key
     * @return <tt>true</tt> if and only if the specified object is a key in this table, as determined by the
     * <tt>equals</tt> method; <tt>false</tt> otherwise.
     * @throws NullPointerException if the specified key is null
     */
    @Override
    public boolean containsKey(Object key) {
        int hash = hashOf(key);
        return segmentFor(hash).containsKey(key, hash);
    }

    /**
     * Returns <tt>true</tt> if this map maps one or more keys to the specified value. Note: This method requires a full
     * internal traversal of the hash table, and so is much slower than method <tt>containsKey</tt>.
     * 
     * @param value value whose presence in this map is to be tested
     * @return <tt>true</tt> if this map maps one or more keys to the specified value
     * @throws NullPointerException if the specified value is null
     */

    @Override
    public boolean containsValue(Object value) {
        if (value == null) {
            throw new NullPointerException();
        }

        // See explanation of modCount use above

        final Segment<K, V>[] segments = this.segments;
        int[] mc = new int[segments.length];

        // Try a few times without locking
        for (int k = 0; k < RETRIES_BEFORE_LOCK; ++k) {
            int mcsum = 0;
            for (int i = 0; i < segments.length; ++i) {
                mcsum += mc[i] = segments[i].modCount;
                if (segments[i].containsValue(value)) {
                    return true;
                }
            }
            boolean cleanSweep = true;
            if (mcsum != 0) {
                for (int i = 0; i < segments.length; ++i) {
                    if (mc[i] != segments[i].modCount) {
                        cleanSweep = false;
                        break;
                    }
                }
            }
            if (cleanSweep) {
                return false;
            }
        }
        // Resort to locking all segments
        for (int i = 0; i < segments.length; ++i) {
            segments[i].lock();
        }
        boolean found = false;
        try {
            for (int i = 0; i < segments.length; ++i) {
                if (segments[i].containsValue(value)) {
                    found = true;
                    break;
                }
            }
        } finally {
            for (int i = 0; i < segments.length; ++i) {
                segments[i].unlock();
            }
        }
        return found;
    }

    /**
     * Legacy method testing if some key maps into the specified value in this table. This method is identical in
     * functionality to {@link #containsValue}, and exists solely to ensure full compatibility with class
     * {@link Hashtable}, which supported this method prior to introduction of the Java Collections framework.
     * 
     * @param value a value to search for
     * @return <tt>true</tt> if and only if some key maps to the <tt>value</tt> argument in this table as determined by
     * the <tt>equals</tt> method; <tt>false</tt> otherwise
     * @throws NullPointerException if the specified value is null
     */
    public boolean contains(Object value) {
        return containsValue(value);
    }

    /**
     * Maps the specified key to the specified value in this table. Neither the key nor the value can be null.
     * <p>
     * The value can be retrieved by calling the <tt>get</tt> method with a key that is equal to the original key.
     * 
     * @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 <tt>key</tt>, or <tt>null</tt> if there was no mapping for
     * <tt>key</tt>
     * @throws NullPointerException if the specified key or value is null
     */
    @Override
    public V put(K key, V value) {
        if (value == null) {
            throw new NullPointerException();
        }
        int hash = hashOf(key);
        return segmentFor(hash).put(key, hash, value, false);
    }

    /**
     * {@inheritDoc}
     * 
     * @return the previous value associated with the specified key, or <tt>null</tt> 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) {
        if (value == null) {
            throw new NullPointerException();
        }
        int hash = hashOf(key);
        return segmentFor(hash).put(key, hash, value, true);
    }

    /**
     * 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
     */
    @Override
    public void putAll(Map<? extends K, ? extends V> m) {
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
            put(e.getKey(), e.getValue());
        }
    }

    /**
     * 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 <tt>key</tt>, or <tt>null</tt> if there was no mapping for
     * <tt>key</tt>
     * @throws NullPointerException if the specified key is null
     */
    @Override
    public V remove(Object key) {
        int hash = hashOf(key);
        return segmentFor(hash).remove(key, hash, null, false);
    }

    /**
     * {@inheritDoc}
     * 
     * @throws NullPointerException if the specified key is null
     */
    public boolean remove(Object key, Object value) {
        int hash = hashOf(key);
        if (value == null) {
            return false;
        }
        return segmentFor(hash).remove(key, hash, value, false) != null;
    }

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

    /**
     * {@inheritDoc}
     * 
     * @return the previous value associated with the specified key, or <tt>null</tt> 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 (value == null) {
            throw new NullPointerException();
        }
        int hash = hashOf(key);
        return segmentFor(hash).replace(key, hash, value);
    }

    /**
     * Removes all of the mappings from this map.
     */
    @Override
    public void clear() {
        for (int i = 0; i < segments.length; ++i) {
            segments[i].clear();
        }
    }

    /**
     * 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 <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, <tt>removeAll</tt>,
     * <tt>retainAll</tt>, and <tt>clear</tt> operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
     * operations.
     * <p>
     * The view's <tt>iterator</tt> is a "weakly consistent" iterator that will never throw
     * {@link ConcurrentModificationException}, and guarantees to traverse elements as they existed upon construction of
     * the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.
     */
    @Override
    public Set<K> keySet() {
        Set<K> ks = keySet;
        return ks != null ? ks : (keySet = new KeySet());
    }

    /**
     * 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 <tt>Iterator.remove</tt>,
     * <tt>Collection.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> operations. It does not
     * support the <tt>add</tt> or <tt>addAll</tt> operations.
     * <p>
     * The view's <tt>iterator</tt> is a "weakly consistent" iterator that will never throw
     * {@link ConcurrentModificationException}, and guarantees to traverse elements as they existed upon construction of
     * the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.
     */
    @Override
    public Collection<V> values() {
        Collection<V> vs = values;
        return vs != null ? vs : (values = new Values());
    }

    /**
     * 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 <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, <tt>removeAll</tt>,
     * <tt>retainAll</tt>, and <tt>clear</tt> operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
     * operations.
     * <p>
     * The view's <tt>iterator</tt> is a "weakly consistent" iterator that will never throw
     * {@link ConcurrentModificationException}, and guarantees to traverse elements as they existed upon construction of
     * the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.
     */
    @Override
    public Set<Map.Entry<K, V>> entrySet() {
        Set<Map.Entry<K, V>> es = entrySet;
        return es != null ? es : (entrySet = new EntrySet());
    }

    /**
     * Returns an enumeration of the keys in this table.
     * 
     * @return an enumeration of the keys in this table
     * @see #keySet()
     */
    public Enumeration<K> keys() {
        return new KeyIterator();
    }

    /**
     * Returns an enumeration of the values in this table.
     * 
     * @return an enumeration of the values in this table
     * @see #values()
     */
    public Enumeration<V> elements() {
        return new ValueIterator();
    }

    /* ---------------- Iterator Support -------------- */

    abstract class HashIterator {

        int               nextSegmentIndex;
        int               nextTableIndex;
        HashEntry<K, V>[] currentTable;
        HashEntry<K, V>   nextEntry;
        HashEntry<K, V>   lastReturned;
        K                 currentKey;      // Strong reference to weak key (prevents gc)

        HashIterator(){
            nextSegmentIndex = segments.length - 1;
            nextTableIndex = -1;
            advance();
        }

        public boolean hasMoreElements() {
            return hasNext();
        }

        final void advance() {
            if (nextEntry != null && (nextEntry = nextEntry.next) != null) {
                return;
            }

            while (nextTableIndex >= 0) {
                if ((nextEntry = currentTable[nextTableIndex--]) != null) {
                    return;
                }
            }

            while (nextSegmentIndex >= 0) {
                Segment<K, V> seg = segments[nextSegmentIndex--];
                if (seg.count != 0) {
                    currentTable = seg.table;
                    for (int j = currentTable.length - 1; j >= 0; --j) {
                        if ((nextEntry = currentTable[j]) != null) {
                            nextTableIndex = j - 1;
                            return;
                        }
                    }
                }
            }
        }

        public boolean hasNext() {
            while (nextEntry != null) {
                if (nextEntry.key() != null) {
                    return true;
                }
                advance();
            }

            return false;
        }

        HashEntry<K, V> nextEntry() {
            do {
                if (nextEntry == null) {
                    throw new NoSuchElementException();
                }

                lastReturned = nextEntry;
                currentKey = lastReturned.key();
                advance();
            } while (currentKey == null); // Skip GC'd keys

            return lastReturned;
        }

        public void remove() {
            if (lastReturned == null) {
                throw new IllegalStateException();
            }
            ConcurrentIdentityHashMap.this.remove(currentKey);
            lastReturned = null;
        }
    }

    final class KeyIterator extends HashIterator implements Iterator<K>, Enumeration<K> {

        public K next() {
            return super.nextEntry().key();
        }

        public K nextElement() {
            return super.nextEntry().key();
        }
    }

    final class ValueIterator extends HashIterator implements Iterator<V>, Enumeration<V> {

        public V next() {
            return super.nextEntry().value();
        }

        public V nextElement() {
            return super.nextEntry().value();
        }
    }

    /*
     * This class is needed for JDK5 compatibility.
     */
    static class SimpleEntry<K, V> implements Entry<K, V> {

        private final K key;

        private V       value;

        public SimpleEntry(K key, V value){
            this.key = key;
            this.value = value;

        }

        public SimpleEntry(Entry<? extends K, ? extends V> entry){
            this.key = entry.getKey();
            this.value = entry.getValue();

        }

        public K getKey() {
            return key;
        }

        public V getValue() {
            return value;
        }

        public V setValue(V value) {
            V oldValue = this.value;
            this.value = value;
            return oldValue;
        }

        @Override
        public boolean equals(Object o) {
            if (!(o instanceof Map.Entry<?, ?>)) {
                return false;
            }
            @SuppressWarnings("unchecked")
            Map.Entry<K, V> e = (Map.Entry<K, V>) o;
            return eq(key, e.getKey()) && eq(value, e.getValue());
        }

        @Override
        public int hashCode() {
            return (key == null ? 0 : key.hashCode()) ^ (value == null ? 0 : value.hashCode());
        }

        @Override
        public String toString() {
            return key + "=" + value;
        }

        private static boolean eq(Object o1, Object o2) {
            return o1 == null ? o2 == null : o1.equals(o2);
        }
    }

    /**
     * Custom Entry class used by EntryIterator.next(), that relays setValue changes to the underlying map.
     */
    final class WriteThroughEntry extends SimpleEntry<K, V> {

        WriteThroughEntry(K k, V v){
            super(k, v);
        }

        /**
         * Set our entry's value and write through to the map. The value to return is somewhat arbitrary here. Since a
         * WriteThroughEntry does not necessarily track asynchronous changes, the most recent "previous" value could be
         * different from what we return (or could even have been removed in which case the put will re-establish). We
         * do not and can not guarantee more.
         */
        @Override
        public V setValue(V value) {

            if (value == null) {
                throw new NullPointerException();
            }
            V v = super.setValue(value);
            ConcurrentIdentityHashMap.this.put(getKey(), value);
            return v;
        }

    }

    final class EntryIterator extends HashIterator implements Iterator<Entry<K, V>> {

        public Map.Entry<K, V> next() {
            HashEntry<K, V> e = super.nextEntry();
            return new WriteThroughEntry(e.key(), e.value());
        }
    }

    final class KeySet extends AbstractSet<K> {

        @Override
        public Iterator<K> iterator() {

            return new KeyIterator();
        }

        @Override
        public int size() {
            return ConcurrentIdentityHashMap.this.size();
        }

        @Override
        public boolean isEmpty() {
            return ConcurrentIdentityHashMap.this.isEmpty();
        }

        @Override
        public boolean contains(Object o) {
            return ConcurrentIdentityHashMap.this.containsKey(o);
        }

        @Override
        public boolean remove(Object o) {
            return ConcurrentIdentityHashMap.this.remove(o) != null;

        }

        @Override
        public void clear() {
            ConcurrentIdentityHashMap.this.clear();
        }
    }

    final class Values extends AbstractCollection<V> {

        @Override
        public Iterator<V> iterator() {
            return new ValueIterator();
        }

        @Override
        public int size() {
            return ConcurrentIdentityHashMap.this.size();
        }

        @Override
        public boolean isEmpty() {
            return ConcurrentIdentityHashMap.this.isEmpty();
        }

        @Override
        public boolean contains(Object o) {
            return ConcurrentIdentityHashMap.this.containsValue(o);
        }

        @Override
        public void clear() {
            ConcurrentIdentityHashMap.this.clear();
        }
    }

    final class EntrySet extends AbstractSet<Map.Entry<K, V>> {

        @Override
        public Iterator<Map.Entry<K, V>> iterator() {
            return new EntryIterator();
        }

        @Override
        public boolean contains(Object o) {
            if (!(o instanceof Map.Entry<?, ?>)) {
                return false;
            }
            Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
            V v = ConcurrentIdentityHashMap.this.get(e.getKey());
            return v != null && v.equals(e.getValue());
        }

        @Override
        public boolean remove(Object o) {
            if (!(o instanceof Map.Entry<?, ?>)) {
                return false;
            }
            Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
            return ConcurrentIdentityHashMap.this.remove(e.getKey(), e.getValue());
        }

        @Override
        public int size() {
            return ConcurrentIdentityHashMap.this.size();
        }

        @Override
        public boolean isEmpty() {
            return ConcurrentIdentityHashMap.this.isEmpty();
        }

        @Override
        public void clear() {
            ConcurrentIdentityHashMap.this.clear();
        }
    }
}