/*
     File: ConcurrentHashMap
   
     Written by Doug Lea. Adapted and released, under explicit
     permission, from JDK1.2 HashMap.java and Hashtable.java which
     carries the following copyright:
   
        * Copyright 1997 by Sun Microsystems, Inc.,
        * 901 San Antonio Road, Palo Alto, California, 94303, U.S.A.
       * All rights reserved.
       *
       * This software is the confidential and proprietary information
       * of Sun Microsystems, Inc. ("Confidential Information").  You
       * shall not disclose such Confidential Information and shall use
       * it only in accordance with the terms of the license agreement
       * you entered into with Sun.
  
    History:
    Date       Who                What
    26nov2000  dl               Created, based on ConcurrentReaderHashMap
    12jan2001  dl               public release
    17nov2001  dl               Minor tunings
    24oct2003  dl               Segment implements Serializable
  */
  
  package org.jdiagnose.concurrent;
  
  import java.io.IOException;
  import java.io.Serializable;
  import java.util.AbstractCollection;
  import java.util.AbstractMap;
  import java.util.AbstractSet;
  import java.util.Collection;
  import java.util.Enumeration;
  import java.util.Iterator;
  import java.util.Map;
  import java.util.NoSuchElementException;
  import java.util.Set;
  
  
  /***
   * A version of Hashtable supporting 
   * concurrency for both retrievals and updates:
   *
   * <dl> 
   * <dt> Retrievals
   *
   * <dd> Retrievals may overlap updates.  (This is the same policy as
   * ConcurrentReaderHashMap.)  Successful retrievals using get(key) and
   * containsKey(key) usually run without locking. Unsuccessful
   * retrievals (i.e., when the key is not present) do involve brief
   * synchronization (locking).  Because retrieval operations can
   * ordinarily overlap with update operations (i.e., put, remove, and
   * their derivatives), retrievals can only be guaranteed to return the
   * results of the most recently <em>completed</em> operations holding
   * upon their onset. Retrieval operations may or may not return
   * results reflecting in-progress writing operations.  However, the
   * retrieval operations do always return consistent results -- either
   * those holding before any single modification or after it, but never
   * a nonsense result.  For aggregate operations such as putAll and
   * clear, concurrent reads may reflect insertion or removal of only
   * some entries.  
   * <p>
   *
   * Iterators and Enumerations (i.e., those returned by
   * keySet().iterator(), entrySet().iterator(), values().iterator(),
   * keys(), and elements()) return elements reflecting the state of the
   * hash table at some point at or since the creation of the
   * iterator/enumeration.  They will return at most one instance of
   * each element (via next()/nextElement()), but might or might not
   * reflect puts and removes that have been processed since they were
   * created.  They do <em>not</em> throw ConcurrentModificationException.
   * However, these iterators are designed to be used by only one
   * thread at a time. Passing an iterator across multiple threads may
   * lead to unpredictable results if the table is being concurrently
   * modified.  <p>
   *
   *
   * <dt> Updates
   *
   * <dd> This class supports a hard-wired preset <em>concurrency
   * level</em> of 32. This allows a maximum of 32 put and/or remove
   * operations to proceed concurrently. This level is an upper bound on
   * concurrency, not a guarantee, since it interacts with how
   * well-strewn elements are across bins of the table. (The preset
   * value in part reflects the fact that even on large multiprocessors,
   * factors other than synchronization tend to be bottlenecks when more
   * than 32 threads concurrently attempt updates.)
   * Additionally, operations triggering internal resizing and clearing
   * do not execute concurrently with any operation.  
   * <p>
   *
   * There is <em>NOT</em> any support for locking the entire table to
   * prevent updates. This makes it imposssible, for example, to
   * add an element only if it is not already present, since another
   * thread may be in the process of doing the same thing.
   * If you need such capabilities, consider instead using the
   * ConcurrentReaderHashMap class. 
   *
  * </dl>
  *
  * Because of how concurrency control is split up, the
  * size() and isEmpty() methods require accumulations across 32
  * control segments, and so might be slightly slower than you expect.
  * <p>
  *
  * This class may be used as a direct replacement for
  * java.util.Hashtable in any application that does not rely
  * on the ability to lock the entire table to prevent updates.  
  * As of this writing, it performs much faster than Hashtable in
  * typical multi-threaded applications with multiple readers and writers.
  * Like Hashtable but unlike java.util.HashMap,
  * this class does NOT allow <tt>null</tt> to be used as a key or
  * value. 
  * <p> 
  *
  * Implementation note: A slightly
  * faster implementation of this class will be possible once planned
  * Java Memory Model revisions are in place.
  *
  * <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>]
 
  **/
 
 
 public class ConcurrentHashMap 
   extends    AbstractMap 
   implements Map, Cloneable, Serializable {
 
   /*
     The basic strategy is an optimistic-style scheme based on
     the guarantee that the hash table and its lists are always
     kept in a consistent enough state to be read without locking:
 
     * Read operations first proceed without locking, by traversing the
        apparently correct list of the apparently correct bin. If an
        entry is found, but not invalidated (value field null), it is
        returned. If not found, operations must recheck (after a memory
        barrier) to make sure they are using both the right list and
        the right table (which can change under resizes). If
        invalidated, reads must acquire main update lock to wait out
        the update, and then re-traverse.
 
     * All list additions are at the front of each bin, making it easy
        to check changes, and also fast to traverse.  Entry next
        pointers are never assigned. Remove() builds new nodes when
        necessary to preserve this.
 
     * Remove() (also clear()) invalidates removed nodes to alert read
        operations that they must wait out the full modifications.
 
     * Locking for puts, removes (and, when necessary gets, etc)
       is controlled by Segments, each covering a portion of the
       table. During operations requiring global exclusivity (mainly
       resize and clear), ALL of these locks are acquired at once.
       Note that these segments are NOT contiguous -- they are based
       on the least 5 bits of hashcodes. This ensures that the same
       segment controls the same slots before and after resizing, which
       is necessary for supporting concurrent retrievals. This
       comes at the price of a mismatch of logical vs physical locality,
       but this seems not to be a performance problem in practice.
  
   */
 
   /***
    * The hash table data.
    */
   protected transient Entry[] table;
 
 
   /*** 
    * The number of concurrency control segments.
    * The value can be at most 32 since ints are used
    * as bitsets over segments. Emprically, it doesn't
    * seem to pay to decrease it either, so the value should be at least 32.
    * In other words, do not redefine this :-)
    **/
 
   protected static final int CONCURRENCY_LEVEL = 32;
 
   /***
    * Mask value for indexing into segments
    **/
 
   protected static final int SEGMENT_MASK = CONCURRENCY_LEVEL - 1;
 
   /***
    * Bookkeeping for each concurrency control segment.
    * Each segment contains a local count of the number of 
    * elements in its region.
    * However, the main use of a Segment is for its lock.
    **/
   protected final static class Segment implements Serializable {
     /***
      * The number of elements in this segment's region.
      * It is always updated within synchronized blocks.
      **/
     protected int count;
 
     /***
      * Get the count under synch. 
      **/
     protected synchronized int getCount() { return count; }
 
     /***
      * Force a synchronization
      **/
     protected synchronized void synch() {}
   }
 
   /***
    * The array of concurrency control segments.
    **/
 
   protected final Segment[] segments = new Segment[CONCURRENCY_LEVEL]; 
 
 
   /***
    * The default initial number of table slots for this table (32).
    * Used when not otherwise specified in constructor.
    **/
   public static int DEFAULT_INITIAL_CAPACITY = 32; 
 
 
   /***
    * The minimum capacity, used if a lower value is implicitly specified
    * by either of the constructors with arguments.  
    * MUST be a power of two.
    */
   private static final int MINIMUM_CAPACITY = 32;
   
   /***
    * 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.
    */
   private static final int MAXIMUM_CAPACITY = 1 << 30;
   
   /***
    * The default load factor for this table (0.75)
    * Used when not otherwise specified in constructor.
    **/
 
   public static final float DEFAULT_LOAD_FACTOR = 0.75f; 
 
   /***
    * The load factor for the hash table.
    *
    * @serial
    */
   protected final float loadFactor;
 
   /***
    * Per-segment resize threshold.  
    *
    * @serial
    */
   protected int threshold;
 
 
   /***
    * Number of segments voting for resize. The table is
    * doubled when 1/4 of the segments reach threshold.
    * Volatile but updated without synch since this is just a heuristic.
    **/
 
   protected transient volatile int votesForResize;
 
 
   /***
    * Return the number of set bits in w.
    * For a derivation of this algorithm, see
    * "Algorithms and data structures with applications to 
    *  graphics and geometry", by Jurg Nievergelt and Klaus Hinrichs,
    *  Prentice Hall, 1993.
    * See also notes by Torsten Sillke at
    * http://www.mathematik.uni-bielefeld.de/~sillke/PROBLEMS/bitcount
    **/
   protected static int bitcount(int w) {
     w -= (0xaaaaaaaa & w) >>> 1;
     w = (w & 0x33333333) + ((w >>> 2) & 0x33333333);
     w = (w + (w >>> 4)) & 0x0f0f0f0f;
     w += w >>> 8;
     w += w >>> 16;
     return w & 0xff;
   }
 
   /***
    * Returns the appropriate capacity (power of two) for the specified 
    * initial capacity argument.
    */
   private int p2capacity(int initialCapacity) {
     int cap = initialCapacity;
     
     // Compute the appropriate capacity
     int result;
     if (cap > MAXIMUM_CAPACITY || cap < 0) {
       result = MAXIMUM_CAPACITY;
     } else {
       result = MINIMUM_CAPACITY;
       while (result < cap)
         result <<= 1;
     }
     return result;
   }
 
   /***
    * Return hash code for Object x. Since we are using power-of-two
    * tables, it is worth the effort to improve hashcode via
    * the same multiplicative scheme as used in IdentityHashMap.
    */
   protected static int hash(Object x) {
     int h = x.hashCode();
     // Multiply by 127 (quickly, via shifts), and mix in some high
     // bits to help guard against bunching of codes that are
     // consecutive or equally spaced.
     return ((h << 7) - h + (h >>> 9) + (h >>> 17));
   }
 
 
   /*** 
    * Check for equality of non-null references x and y. 
    **/
   protected boolean eq(Object x, Object y) {
     return x == y || x.equals(y);
   }
 
   /*** Create table array and set the per-segment threshold **/
   protected Entry[] newTable(int capacity) {
     threshold = (int)(capacity * loadFactor / CONCURRENCY_LEVEL) + 1;
     return new Entry[capacity];
   }
 
   /***
    * Constructs a new, empty map with the specified initial 
    * capacity and the specified load factor.
    *
    * @param initialCapacity the initial capacity.
    *  The actual initial capacity is rounded to the nearest power of two.
    * @param loadFactor  the load factor threshold, used to control resizing.
    *   This value is used in an approximate way: When at least
    *   a quarter of the segments of the table reach per-segment threshold, or
    *   one of the segments itself exceeds overall threshold,
    *   the table is doubled. 
    *   This will on average cause resizing when the table-wide
    *   load factor is slightly less than the threshold. If you'd like
    *   to avoid resizing, you can set this to a ridiculously large
    *   value.
    * @throws IllegalArgumentException  if the load factor is nonpositive.
    */
   public ConcurrentHashMap(int initialCapacity, 
                            float loadFactor) {
     if (!(loadFactor > 0))
       throw new IllegalArgumentException("Illegal Load factor: "+
                                          loadFactor);
     this.loadFactor = loadFactor;
     for (int i = 0; i < segments.length; ++i) 
       segments[i] = new Segment();
     int cap = p2capacity(initialCapacity);
     table = newTable(cap);
   }
 
   /***
    * Constructs a new, empty map with the specified initial 
    * capacity and default load factor.
    *
    * @param   initialCapacity   the initial capacity of the 
    *                            ConcurrentHashMap.
    * @throws    IllegalArgumentException if the initial maximum number 
    *              of elements is less
    *              than zero.
    */
   public ConcurrentHashMap(int initialCapacity) {
     this(initialCapacity, DEFAULT_LOAD_FACTOR);
   }
 
   /***
    * Constructs a new, empty map with a default initial capacity
    * and default load factor.
    */
   public ConcurrentHashMap() {
     this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
   }
 
   /***
    * Constructs a new map with the same mappings as the given map.  The
    * map is created with a capacity of twice the number of mappings in
    * the given map or 32 (whichever is greater), and a default load factor.
    */
   public ConcurrentHashMap(Map t) {
     this(Math.max((int) (t.size() / DEFAULT_LOAD_FACTOR) + 1, 
                   MINIMUM_CAPACITY),
          DEFAULT_LOAD_FACTOR);
     putAll(t);
   }
 
   /***
    * Returns the number of key-value mappings in this map.
    *
    * @return the number of key-value mappings in this map.
    */
   public int size() {
     int c = 0;
     for (int i = 0; i < segments.length; ++i) 
       c += segments[i].getCount();
     return c;
   }
 
   /***
    * Returns <tt>true</tt> if this map contains no key-value mappings.
    *
    * @return <tt>true</tt> if this map contains no key-value mappings.
    */
   public boolean isEmpty() {
     for (int i = 0; i < segments.length; ++i) 
       if (segments[i].getCount() != 0)
         return false;
     return true;
   }
 
 
   /***
    * Returns the value to which the specified key is mapped in this table.
    *
    * @param   key   a key in the table.
    * @return  the value to which the key is mapped in this table;
    *          <code>null</code> if the key is not mapped to any value in
    *          this table.
    * @exception  NullPointerException  if the key is
    *               <code>null</code>.
    * @see     #put(Object, Object)
    */
   public Object get(Object key) {
     int hash = hash(key);     // throws null pointer exception if key null
 
     // Try first without locking...
     Entry[] tab = table;
     int index = hash & (tab.length - 1);
     Entry first = tab[index];
     Entry e;
 
     for (e = first; e != null; e = e.next) {
       if (e.hash == hash && eq(key, e.key)) {
         Object value = e.value;
         if (value != null) 
           return value;
         else
           break;
       }
     }
 
     // Recheck under synch if key apparently not there or interference
     Segment seg = segments[hash & SEGMENT_MASK];
     synchronized(seg) { 
       tab = table;
       index = hash & (tab.length - 1);
       Entry newFirst = tab[index];
       if (e != null || first != newFirst) {
         for (e = newFirst; e != null; e = e.next) {
           if (e.hash == hash && eq(key, e.key)) 
             return e.value;
         }
       }
       return null;
     }
   }
 
   /***
    * Tests if the specified object is a key in this table.
    * 
    * @param   key   possible key.
    * @return  <code>true</code> if and only if the specified object 
    *          is a key in this table, as determined by the 
    *          <tt>equals</tt> method; <code>false</code> otherwise.
    * @exception  NullPointerException  if the key is
    *               <code>null</code>.
    * @see     #contains(Object)
    */
 
   public boolean containsKey(Object key) {
     return get(key) != null;
   }
   
 
   /***
    * Maps the specified <code>key</code> to the specified 
    * <code>value</code> in this table. Neither the key nor the 
    * value can be <code>null</code>. (Note that this policy is
    * the same as for java.util.Hashtable, but unlike java.util.HashMap,
    * which does accept nulls as valid keys and values.)<p>
    *
    * The value can be retrieved by calling the <code>get</code> method 
    * with a key that is equal to the original key. 
    *
    * @param      key     the table key.
    * @param      value   the value.
    * @return     the previous value of the specified key in this table,
    *             or <code>null</code> if it did not have one.
    * @exception  NullPointerException  if the key or value is
    *               <code>null</code>.
    * @see     Object#equals(Object)
    * @see     #get(Object)
    */
   public Object put(Object key, Object value) {
     if (value == null) 
       throw new NullPointerException();
     
     int hash = hash(key);
     Segment seg = segments[hash & SEGMENT_MASK];
     int segcount;
     Entry[] tab;
     int votes;
 
     synchronized(seg) {
       tab = table;
       int index = hash & (tab.length-1);
       Entry first = tab[index];
 
       for (Entry e = first; e != null; e = e.next) {
         if (e.hash == hash && eq(key, e.key)) {
           Object oldValue = e.value; 
           e.value = value;
           return oldValue;
         }
       }
 
       //  Add to front of list
       Entry newEntry = new Entry(hash, key, value, first);
       tab[index] = newEntry;
 
       if ((segcount = ++seg.count) < threshold)
         return null;
 
       int bit = (1 << (hash & SEGMENT_MASK));
       votes = votesForResize;
       if ((votes & bit) == 0)
         votes = votesForResize |= bit;
     }
 
     // Attempt resize if 1/4 segs vote,
     // or if this seg itself reaches the overall threshold.
     // (The latter check is just a safeguard to avoid pathological cases.)
     if (bitcount(votes) >= CONCURRENCY_LEVEL / 4  ||
         segcount > (threshold * CONCURRENCY_LEVEL)) 
       resize(0, tab);
 
     return null;
   }
 
   /***
    * Gather all locks in order to call rehash, by
    * recursing within synch blocks for each segment index.
    * @param index the current segment. initially call value must be 0
    * @param assumedTab the state of table on first call to resize. If
    * this changes on any call, the attempt is aborted because the
    * table has already been resized by another thread.
    */
 
   protected void resize(int index, Entry[] assumedTab) {
     Segment seg = segments[index];
     synchronized(seg) {
       if (assumedTab == table) {
         int next = index+1;
         if (next < segments.length)
           resize(next, assumedTab);
         else
           rehash();
       }
     }
   }
 
   /***
    * Rehashes the contents of this map into a new table
    * with a larger capacity. 
    */
   protected void rehash() {
     votesForResize = 0; // reset
 
     Entry[] oldTable = table;
     int oldCapacity = oldTable.length;
 
     if (oldCapacity >= MAXIMUM_CAPACITY) {
       threshold = Integer.MAX_VALUE; // avoid retriggering
       return;
     }
     
     int newCapacity = oldCapacity << 1;
     Entry[] newTable = newTable(newCapacity);
     int mask = newCapacity - 1;
 
     /*
      * 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 to
      * oldCapacity+index. We also eliminate unnecessary node
      * creation by catching cases where old nodes can be reused
      * because their next fields won't change. Statistically, at
      * the default threshhold, only about one-sixth of them need
      * cloning. (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.)
      */
     
     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.  
       Entry e = oldTable[i];
       
       if (e != null) {
         int idx = e.hash & mask;
         Entry next = e.next;
         
         //  Single node on list
         if (next == null) 
           newTable[idx] = e;
         
         else {    
           // Reuse trailing consecutive sequence of all same bit
           Entry lastRun = e;
           int lastIdx = idx;
           for (Entry last = next; last != null; last = last.next) {
             int k = last.hash & mask;
             if (k != lastIdx) {
               lastIdx = k;
               lastRun = last;
             }
           }
           newTable[lastIdx] = lastRun;
           
           // Clone all remaining nodes
           for (Entry p = e; p != lastRun; p = p.next) {
             int k = p.hash & mask;
             newTable[k] = new Entry(p.hash, p.key, 
                                     p.value, newTable[k]);
           }
         }
       }
     }
     
     table = newTable;
   }
 
 
   /***
    * Removes the key (and its corresponding value) from this 
    * table. This method does nothing if the key is not in the table.
    *
    * @param   key   the key that needs to be removed.
    * @return  the value to which the key had been mapped in this table,
    *          or <code>null</code> if the key did not have a mapping.
    * @exception  NullPointerException  if the key is
    *               <code>null</code>.
    */
   public Object remove(Object key) {
     return remove(key, null); 
   }
 
 
   /***
    * Removes the (key, value) pair from this 
    * table. This method does nothing if the key is not in the table,
    * or if the key is associated with a different value. This method
    * is needed by EntrySet.
    *
    * @param   key   the key that needs to be removed.
    * @param   value   the associated value. If the value is null,
    *                   it means "any value".
    * @return  the value to which the key had been mapped in this table,
    *          or <code>null</code> if the key did not have a mapping.
    * @exception  NullPointerException  if the key is
    *               <code>null</code>.
    */
 
   protected Object remove(Object key, Object value) {
     /*
       Find the entry, then 
         1. Set value field to null, to force get() to retry
         2. Rebuild the list without this entry.
            All entries following removed node can stay in list, but
            all preceeding ones need to be cloned.  Traversals rely
            on this strategy to ensure that elements will not be
           repeated during iteration.
     */
 
     int hash = hash(key);
     Segment seg = segments[hash & SEGMENT_MASK];
 
     synchronized(seg) {
       Entry[] tab = table;
       int index = hash & (tab.length-1);
       Entry first = tab[index];
       Entry e = first;
 
       for (;;) {
         if (e == null)
           return null;
         if (e.hash == hash && eq(key, e.key)) 
           break;
         e = e.next;
       }
 
       Object oldValue = e.value;
       if (value != null && !value.equals(oldValue))
         return null;
      
       e.value = null;
 
       Entry head = e.next;
       for (Entry p = first; p != e; p = p.next) 
         head = new Entry(p.hash, p.key, p.value, head);
       tab[index] = head;
       seg.count--;
       return oldValue;
     }
   }
 
 
   /***
    * 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.  
    * @exception  NullPointerException  if the value is <code>null</code>.
    */
   public boolean containsValue(Object value) {
 
     if (value == null) throw new NullPointerException();
 
     for (int s = 0; s < segments.length; ++s) {
       Segment seg = segments[s];
       Entry[] tab;
       synchronized(seg) { tab = table; }
       for (int i = s; i < tab.length; i+= segments.length) {
         for (Entry e = tab[i]; e != null; e = e.next) 
           if (value.equals(e.value))
             return true;
       }
     }
     return false;
   }
 
   /***
    * Tests if some key maps into the specified value in this table.
    * This operation is more expensive than the <code>containsKey</code>
    * method.<p>
    *
    * Note that this method is identical in functionality to containsValue,
    * (which is part of the Map interface in the collections framework).
    * 
    * @param      value   a value to search for.
    * @return     <code>true</code> if and only if some key maps to the
    *             <code>value</code> argument in this table as 
    *             determined by the <tt>equals</tt> method;
    *             <code>false</code> otherwise.
    * @exception  NullPointerException  if the value is <code>null</code>.
    * @see        #containsKey(Object)
    * @see        #containsValue(Object)
    * @see	   Map
    */
 
   public boolean contains(Object value) {
     return containsValue(value);
   }
 
   /***
    * 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 t Mappings to be stored in this map.
    */
 
   public void putAll(Map t) {
     int n = t.size();
     if (n == 0)
       return;
 
     // Expand enough to hold at least n elements without resizing.
     // We can only resize table by factor of two at a time.
     // It is faster to rehash with fewer elements, so do it now.
     for(;;) {
       Entry[] tab;
       int max;
       synchronized(segments[0]) { // must synch on some segment. pick 0.
         tab = table;
         max = threshold * CONCURRENCY_LEVEL;
       }
       if (n < max)
         break;
       resize(0, tab);
     }
 
     for (Iterator it = t.entrySet().iterator(); it.hasNext();) {
       Map.Entry entry = (Map.Entry) it.next();
       put(entry.getKey(), entry.getValue());
     }
   }
 
   /***
    * Removes all mappings from this map.
    */
 
   public void clear() {
     // We don't need all locks at once so long as locks
     //   are obtained in low to high order
     for (int s = 0; s < segments.length; ++s) {
       Segment seg = segments[s];
       synchronized(seg) {
         Entry[] tab = table;
         for (int i = s; i < tab.length; i+= segments.length) {
           for (Entry e = tab[i]; e != null; e = e.next) 
             e.value = null; 
           tab[i] = null;
           seg.count = 0;
         }
       }
     }
   }
 
   /***
    * Returns a shallow copy of this 
    * <tt>ConcurrentHashMap</tt> instance: the keys and
    * values themselves are not cloned.
    *
    * @return a shallow copy of this map.
    */
 
   public Object clone() {
     // We cannot call super.clone, since it would share final segments array,
     // and there's no way to reassign finals.
     return new ConcurrentHashMap(this);
   }
 
   // Views
 
   protected transient Set keySet = null;
   protected transient Set entrySet = null;
   protected transient Collection values = null;
 
   /***
    * Returns a 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.
    *
    * @return a set view of the keys contained in this map.
    */
   
   public Set keySet() {
     Set ks = keySet;
     return (ks != null)? ks : (keySet = new KeySet());
   }
   
   private class KeySet extends AbstractSet {
     public Iterator iterator() {
       return new KeyIterator();
     }
     public int size() {
       return ConcurrentHashMap.this.size();
     }
     public boolean contains(Object o) {
       return ConcurrentHashMap.this.containsKey(o);
     }
     public boolean remove(Object o) {
       return ConcurrentHashMap.this.remove(o) != null;
     }
     public void clear() {
       ConcurrentHashMap.this.clear();
     }
   }
 
   /***
    * Returns a 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.
    *
    * @return a collection view of the values contained in this map.
    */
   
   public Collection values() {
     Collection vs = values;
     return (vs != null)? vs : (values = new Values());
   }
   
   private class Values extends AbstractCollection {
     public Iterator iterator() {
       return new ValueIterator();
     }
     public int size() {
       return ConcurrentHashMap.this.size();
     }
     public boolean contains(Object o) {
       return ConcurrentHashMap.this.containsValue(o);
     }
     public void clear() {
       ConcurrentHashMap.this.clear();
     }
   }
 
   /***
    * Returns a collection view of the mappings contained in this map.  Each
    * element in the returned collection is a <tt>Map.Entry</tt>.  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 the 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.
    *
    * @return a collection view of the mappings contained in this map.
    */
   
   public Set entrySet() {
     Set es = entrySet;
     return (es != null) ? es : (entrySet = new EntrySet());
   }
 
   private class EntrySet extends AbstractSet {
     public Iterator iterator() {
       return new HashIterator();
     }
     public boolean contains(Object o) {
       if (!(o instanceof Map.Entry))
         return false;
       Map.Entry entry = (Map.Entry)o;
       Object v = ConcurrentHashMap.this.get(entry.getKey());
       return v != null && v.equals(entry.getValue());
     }
     public boolean remove(Object o) {
       if (!(o instanceof Map.Entry))
         return false;
       Map.Entry e = (Map.Entry)o;
       return ConcurrentHashMap.this.remove(e.getKey(), e.getValue()) != null;
     }
     public int size() {
       return ConcurrentHashMap.this.size();
     }
     public void clear() {
       ConcurrentHashMap.this.clear();
     }
   }
 
   /***
    * Returns an enumeration of the keys in this table.
    *
    * @return  an enumeration of the keys in this table.
    * @see     Enumeration
    * @see     #elements()
    * @see	#keySet()
    * @see	Map
    */
   public Enumeration keys() {
     return new KeyIterator();
   }
 
   /***
    * Returns an enumeration of the values in this table.
    * Use the Enumeration methods on the returned object to fetch the elements
    * sequentially.
    *
    * @return  an enumeration of the values in this table.
    * @see     java.util.Enumeration
    * @see     #keys()
    * @see	#values()
    * @see	Map
    */
   
   public Enumeration elements() {
     return new ValueIterator();
   }
 
   /***
    * ConcurrentHashMap collision list entry.
    */
 
   protected static class Entry implements Map.Entry {
     /* 
        The use of volatile for value field ensures that
        we can detect status changes without synchronization.
        The other fields are never changed, and are
        marked as final. 
     */
 
     protected final Object key;
     protected volatile Object value;
     protected final int hash;
     protected final Entry next;
 
     Entry(int hash, Object key, Object value, Entry next) {
       this.value = value;
       this.hash = hash;
       this.key = key;
       this.next = next;
     }
 
     // Map.Entry Ops 
 
     public Object getKey() {
       return key;
     }
 
     /***
      * Get the value.  Note: In an entrySet or entrySet.iterator,
      * unless you can guarantee lack of concurrent modification,
      * <tt>getValue</tt> <em>might</em> return null, reflecting the
      * fact that the entry has been concurrently removed. However,
      * there are no assurances that concurrent removals will be
      * reflected using this method.
      * 
      * @return     the current value, or null if the entry has been 
      * detectably removed.
      **/
     public Object getValue() {
       return value; 
     }
 
     /***
      * Set the value of this entry.  Note: In an entrySet or
      * entrySet.iterator), unless you can guarantee lack of concurrent
      * modification, <tt>setValue</tt> is not strictly guaranteed to
      * actually replace the value field obtained via the <tt>get</tt>
      * operation of the underlying hash table in multithreaded
      * applications.  If iterator-wide synchronization is not used,
      * and any other concurrent <tt>put</tt> or <tt>remove</tt>
      * operations occur, sometimes even to <em>other</em> entries,
      * then this change is not guaranteed to be reflected in the hash
      * table. (It might, or it might not. There are no assurances
      * either way.)
      *
      * @param      value   the new value.
      * @return     the previous value, or null if entry has been detectably
      * removed.
      * @exception  NullPointerException  if the value is <code>null</code>.
      * 
      **/
 
     public Object setValue(Object value) {
       if (value == null)
         throw new NullPointerException();
       Object oldValue = this.value;
       this.value = value;
       return oldValue;
     }
 
     public boolean equals(Object o) {
       if (!(o instanceof Map.Entry))
         return false;
       Map.Entry e = (Map.Entry)o;
       return (key.equals(e.getKey()) && value.equals(e.getValue()));
     }
     
     public int hashCode() {
       return  key.hashCode() ^ value.hashCode();
     }
     
     public String toString() {
       return key + "=" + value;
     }
 
   }
 
   protected class HashIterator implements Iterator, Enumeration {
     protected final Entry[] tab;           // snapshot of table
     protected int index;                   // current slot 
     protected Entry entry = null;          // current node of slot
     protected Object currentKey;           // key for current node
     protected Object currentValue;         // value for current node
     protected Entry lastReturned = null;   // last node returned by next
 
     protected HashIterator() {
       // force all segments to synch
       synchronized(segments[0]) { tab = table; }
       for (int i = 1; i < segments.length; ++i) segments[i].synch();
       index = tab.length - 1;
     }
 
     public boolean hasMoreElements() { return hasNext(); }
     public Object nextElement() { return next(); }
 
     public boolean hasNext() {
       /*
         currentkey and currentValue are set here to ensure that next()
         returns normally if hasNext() returns true. This avoids
         surprises especially when final element is removed during
         traversal -- instead, we just ignore the removal during
         current traversal.  
       */
 
       for (;;) {
         if (entry != null) {
           Object v = entry.value;
           if (v != null) {
             currentKey = entry.key;
             currentValue = v;
             return true;
           }
           else
             entry = entry.next;
         }
 
         while (entry == null && index >= 0)
           entry = tab[index--];
 
         if (entry == null) {
           currentKey = currentValue = null;
           return false;
         }
       }
     }
 
     protected Object returnValueOfNext() { return entry; }
 
     public Object next() {
       if (currentKey == null && !hasNext())
         throw new NoSuchElementException();
 
       Object result = returnValueOfNext();
       lastReturned = entry;
       currentKey = currentValue = null;
       entry = entry.next;
       return result;
     }
 
     public void remove() {
       if (lastReturned == null)
         throw new IllegalStateException();
       ConcurrentHashMap.this.remove(lastReturned.key);
       lastReturned = null;
     }
 
   }
 
   protected class KeyIterator extends HashIterator {
     protected Object returnValueOfNext() { return currentKey; }
   }
   
   protected class ValueIterator extends HashIterator {
     protected Object returnValueOfNext() { return currentValue; }
   }
   
   /***
    * Save the state of the <tt>ConcurrentHashMap</tt> 
    * instance to a stream (i.e.,
    * serialize it).
    *
    * @serialData  
    * An estimate of the table size, followed by
    * 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 IOException  {
     // Write out the loadfactor, and any hidden stuff
     s.defaultWriteObject();
 
     // Write out capacity estimate. It is OK if this
     // changes during the write, since it is only used by
     // readObject to set initial capacity, to avoid needless resizings.
 
     int cap;
     synchronized(segments[0]) { cap = table.length; }
     s.writeInt(cap);
 
     // Write out keys and values (alternating)
     for (int k = 0; k < segments.length; ++k) {
       Segment seg = segments[k];
       Entry[] tab;
       synchronized(seg) { tab = table; }
       for (int i = k; i < tab.length; i+= segments.length) {
         for (Entry e = tab[i]; e != null; e = e.next) {
           s.writeObject(e.key);
           s.writeObject(e.value);
         }
       }
     }
 
     s.writeObject(null);
     s.writeObject(null);
   }
 
   /***
    * Reconstitute the <tt>ConcurrentHashMap</tt> 
    * instance from a stream (i.e.,
    * deserialize it).
    */
   private void readObject(java.io.ObjectInputStream s)
     throws IOException, ClassNotFoundException  {
     // Read in the threshold, loadfactor, and any hidden stuff
     s.defaultReadObject();
 
     int cap = s.readInt();
     table = newTable(cap);
     for (int i = 0; i < segments.length; ++i) 
       segments[i] = new Segment();
 
 
     // Read the keys and values, and put the mappings in the table
     for (;;) {
       Object key = s.readObject();
       Object value = s.readObject();
       if (key == null)
         break;
       put(key, value);
     }
   }
 }