/*
    * Written by Doug Lea with assistance from members of JCP JSR-166
    * Expert Group and released to the public domain, as explained at
    * http://creativecommons.org/licenses/publicdomain
    */
   
   /*
    * Repackaged for use in Ehcache by Chris Dennis. The only changes
    * to this version where to override the AbstractCollection toArray
   * implementation in KeySet.toArray(), Values.toArray(), and
   * EntrySet.toArray() to ensure correct operation when using the 1.5
   * version of AbstractCollection.
   */
  
  package net.sf.ehcache.store.chm;
  
  import java.io.IOException;
  import java.io.Serializable;
  import java.util.AbstractCollection;
  import java.util.AbstractMap;
  import java.util.AbstractSet;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.ConcurrentModificationException;
  import java.util.Enumeration;
  import java.util.HashMap;
  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.ReentrantReadWriteLock;
  
  import net.sf.ehcache.util.FindBugsSuppressWarnings;
  
  /***
   * A hash table supporting full concurrency of retrievals and
   * adjustable expected concurrency for updates. This class obeys the
   * same functional specification as {@link java.util.Hashtable}, and
   * includes versions of methods corresponding to each method of
   * <tt>Hashtable</tt>. However, even though all operations are
   * thread-safe, retrieval operations do <em>not</em> entail locking,
   * and there is <em>not</em> any support for locking the entire table
   * in a way that prevents all access.  This class is fully
   * interoperable with <tt>Hashtable</tt> in programs that rely on its
   * thread safety but not on its synchronization details.
   *
   * <p> Retrieval operations (including <tt>get</tt>) generally do not
   * block, so may overlap with update operations (including
   * <tt>put</tt> and <tt>remove</tt>). Retrievals reflect the results
   * of the most recently <em>completed</em> update operations holding
   * upon their onset.  For aggregate operations such as <tt>putAll</tt>
   * and <tt>clear</tt>, concurrent retrievals may reflect insertion or
   * removal of only some entries.  Similarly, Iterators and
   * Enumerations return elements reflecting the state of the hash table
   * at some point at or since the creation of the iterator/enumeration.
   * They do <em>not</em> throw {@link ConcurrentModificationException}.
   * However, iterators are designed to be used by only one thread at a time.
   *
   * <p> The allowed concurrency among update operations is guided by
   * the optional <tt>concurrencyLevel</tt> constructor argument
   * (default <tt>16</tt>), which is used as a hint for internal sizing.  The
   * table is internally partitioned to try to permit the indicated
   * number of concurrent updates without contention. Because placement
   * in hash tables is essentially random, the actual concurrency will
   * vary.  Ideally, you should choose a value to accommodate as many
   * threads as will ever concurrently modify the table. Using a
   * significantly higher value than you need can waste space and time,
   * and a significantly lower value can lead to thread contention. But
   * overestimates and underestimates within an order of magnitude do
   * not usually have much noticeable impact. A value of one is
   * appropriate when it is known that only one thread will modify and
   * all others will only read. Also, resizing this or any other kind of
   * hash table is a relatively slow operation, so, when possible, it is
   * a good idea to provide estimates of expected table sizes in
   * constructors.
   *
   * <p>This class and its views and iterators implement all of the
   * <em>optional</em> methods of the {@link Map} and {@link Iterator}
   * interfaces.
   *
   * <p> Like {@link Hashtable} but unlike {@link HashMap}, this class
   * does <em>not</em> allow <tt>null</tt> to be used as a key or value.
   *
   * <p>This class is a member of the
   * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   * Java Collections Framework</a>.
   *
   * @since 1.5
   * @author Doug Lea
   * @param <K> the type of keys maintained by this map
   * @param <V> the type of mapped values
   */
  public class ConcurrentHashMap<K, V> extends AbstractMap<K, V>
          implements ConcurrentMap<K, V>, Serializable {
      private static final long serialVersionUID = 7249069246763182397L;
  
      /*
      * The basic strategy is to subdivide the table among Segments,
      * each of which itself is a concurrently readable hash table.
      */
 
     /* ---------------- Constants -------------- */
 
     /***
      * 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 ints.
      */
     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;
 
     transient Set<K> keySet;
     transient Set<Map.Entry<K,V>> entrySet;
     transient 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 ConcurrentHashMap uses power-of-two length hash tables,
      * that otherwise encounter collisions for hashCodes that do not
      * differ in lower or upper bits.
      */
     protected 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];
     }
 
     /* ---------------- Inner Classes -------------- */
 
     /***
      * ConcurrentHashMap 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 K key;
         final int hash;
         volatile V value;
         final HashEntry<K,V> next;
 
         HashEntry(K key, int hash, HashEntry<K,V> next, V value) {
             this.key = key;
             this.hash = hash;
             this.next = next;
             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 ReentrantReadWriteLock implements Serializable {
         /*
          * 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 = 2249069246763182397L;
 
         /***
          * 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.
          */
         transient int modCount;
 
         /***
          * The table is rehashed when its size exceeds this threshold.
          * (The value of this field is always <tt>(int)(capacity *
          * loadFactor)</tt>.)
          */
         transient 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.
          * @serial
          */
         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];
         }
 
         /***
          * 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)];
         }
 
         /* Specialized implementations of map methods */
 
         V get(Object key, int hash) {
             readLock().lock();
             try {
                 if (count != 0) { // read-volatile
                     HashEntry<K,V> e = getFirst(hash);
                     while (e != null) {
                         if (e.hash == hash && key.equals(e.key)) {
                             return e.value;
                         }
                         e = e.next;
                     }
                 }
                 return null;
             } finally {
                 readLock().unlock();
             }
         }
 
         boolean containsKey(Object key, int hash) {
             readLock().lock();
             try {
                 if (count != 0) { // read-volatile
                     HashEntry<K,V> e = getFirst(hash);
                     while (e != null) {
                         if (e.hash == hash && key.equals(e.key))
                             return true;
                         e = e.next;
                     }
                 }
                 return false;
             } finally {
                 readLock().unlock();
             }
         }
 
         boolean containsValue(Object value) {
             readLock().lock();
             try {
                 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 v = e.value;
                             if (value.equals(v))
                                 return true;
                         }
                     }
                 }
                 return false;
             } finally {
                 readLock().unlock();
             }
         }
 
         boolean replace(K key, int hash, V oldValue, V newValue) {
             writeLock().lock();
             try {
                 HashEntry<K,V> e = getFirst(hash);
                 while (e != null && (e.hash != hash || !key.equals(e.key)))
                     e = e.next;
 
                 boolean replaced = false;
                 if (e != null && oldValue.equals(e.value)) {
                     replaced = true;
                     e.value = newValue;
                 }
                 return replaced;
             } finally {
                 writeLock().unlock();
             }
         }
 
         V replace(K key, int hash, V newValue) {
             writeLock().lock();
             try {
                 HashEntry<K,V> e = getFirst(hash);
                 while (e != null && (e.hash != hash || !key.equals(e.key)))
                     e = e.next;
 
                 V oldValue = null;
                 if (e != null) {
                     oldValue = e.value;
                     e.value = newValue;
                 }
                 return oldValue;
             } finally {
                 writeLock().unlock();
             }
         }
 
 
         V put(K key, int hash, V value, boolean onlyIfAbsent) {
             writeLock().lock();
             try {
                 int c = count;
                 if (c++ > threshold) // ensure capacity
                     rehash();
                 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 || !key.equals(e.key)))
                     e = e.next;
 
                 V oldValue;
                 if (e != null) {
                     oldValue = e.value;
                     if (!onlyIfAbsent)
                         e.value = value;
                 }
                 else {
                     oldValue = null;
                     ++modCount;
                     tab[index] = new HashEntry<K,V>(key, hash, first, value);
                     count = c; // write-volatile
                 }
                 return oldValue;
             } finally {
                 writeLock().unlock();
             }
         }
 
         void rehash() {
             HashEntry<K,V>[] oldTable = table;
             int oldCapacity = oldTable.length;
             if (oldCapacity >= MAXIMUM_CAPACITY)
                 return;
 
             /*
              * 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;
             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) {
                             int k = p.hash & sizeMask;
                             HashEntry<K,V> n = newTable[k];
                             newTable[k] = new HashEntry<K,V>(p.key, p.hash,
                                                              n, p.value);
                         }
                     }
                 }
             }
             table = newTable;
         }
 
         /***
          * Remove; match on key only if value null, else match both.
          */
         V remove(Object key, int hash, Object value) {
             writeLock().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;
                 while (e != null && (e.hash != hash || !key.equals(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)
                             newFirst = new HashEntry<K,V>(p.key, p.hash,
                                                           newFirst, p.value);
                         tab[index] = newFirst;
                         count = c; // write-volatile
                     }
                 }
                 return oldValue;
             } finally {
                 writeLock().unlock();
             }
         }
 
         void clear() {
             writeLock().lock();
             try {
                 if (count != 0) {
                     HashEntry<K,V>[] tab = table;
                     for (int i = 0; i < tab.length ; i++)
                         tab[i] = null;
                     ++modCount;
                     count = 0; // write-volatile
                 }
             } finally {
                 writeLock().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 ConcurrentHashMap(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 with default 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 ConcurrentHashMap(int initialCapacity) {
         this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
     }
 
     /***
      * Creates a new, empty map with a default initial capacity (16),
      * load factor (0.75) and concurrencyLevel (16).
      */
     public ConcurrentHashMap() {
         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 ConcurrentHashMap(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
      */
     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
      */
     @FindBugsSuppressWarnings("UL_UNRELEASED_LOCK")
     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].readLock().lock();
             try {
               for (int i = 0; i < segments.length; ++i)
                   sum += segments[i].count;
             } finally {
               for (int i = 0; i < segments.length; ++i)
                   segments[i].readLock().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
      */
     public V get(Object key) {
         int hash = hash(key.hashCode());
         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
      */
     public boolean containsKey(Object key) {
         int hash = hash(key.hashCode());
         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
      */
     @FindBugsSuppressWarnings("UL_UNRELEASED_LOCK")
     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 sum = 0;
             int mcsum = 0;
             for (int i = 0; i < segments.length; ++i) {
                 int c = segments[i].count;
                 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) {
                     int c = segments[i].count;
                     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].readLock().lock();
         try {
             for (int i = 0; i < segments.length; ++i) {
                 if (segments[i].containsValue(value)) {
                     return true;
                 }
             }
         } finally {
             for (int i = 0; i < segments.length; ++i)
                 segments[i].readLock().unlock();
         }
         return false;
     }
 
     /***
      * 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 java.util.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
      */
     public V put(K key, V value) {
         if (value == null)
             throw new NullPointerException();
         int hash = hash(key.hashCode());
         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 = hash(key.hashCode());
         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
      */
     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
      */
     public V remove(Object key) {
         int hash = hash(key.hashCode());
         return segmentFor(hash).remove(key, hash, null);
     }
 
     /***
      * {@inheritDoc}
      *
      * @throws NullPointerException if the specified key is null
      */
     public boolean remove(Object key, Object value) {
         int hash = hash(key.hashCode());
         if (value == null)
             return false;
         return segmentFor(hash).remove(key, hash, value) != 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 = hash(key.hashCode());
         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 = hash(key.hashCode());
         return segmentFor(hash).replace(key, hash, value);
     }
 
     /***
      * Removes all of the mappings from this map.
      */
     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.
      */
     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.
      */
     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.
      */
     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;
 
         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() { return nextEntry != null; }
 
         HashEntry<K,V> nextEntry() {
             if (nextEntry == null)
                 throw new NoSuchElementException();
             lastReturned = nextEntry;
             advance();
             return lastReturned;
         }
 
         public void remove() {
             if (lastReturned == null)
                 throw new IllegalStateException();
             ConcurrentHashMap.this.remove(lastReturned.key);
             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; }
     }
 
     /***
      * 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 cannot guarantee more.
          */
         public V setValue(V value) {
             if (value == null) throw new NullPointerException();
             V v = super.setValue(value);
             ConcurrentHashMap.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> {
         public Iterator<K> iterator() {
             return new KeyIterator();
         }
         public int size() {
             return ConcurrentHashMap.this.size();
         }
         public boolean isEmpty() {
             return ConcurrentHashMap.this.isEmpty();
         }
         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();
         }
         public Object[] toArray() {
             Collection<K> c = new ArrayList<K>();
             for (Iterator<K> i = iterator(); i.hasNext(); )
                 c.add(i.next());
             return c.toArray();
         }
         public <T> T[] toArray(T[] a) {
             Collection<K> c = new ArrayList<K>();
             for (Iterator<K> i = iterator(); i.hasNext(); )
                 c.add(i.next());
             return c.toArray(a);
         }
     }
 
     final class Values extends AbstractCollection<V> {
         public Iterator<V> iterator() {
             return new ValueIterator();
         }
         public int size() {
             return ConcurrentHashMap.this.size();
         }
         public boolean isEmpty() {
             return ConcurrentHashMap.this.isEmpty();
         }
         public boolean contains(Object o) {
             return ConcurrentHashMap.this.containsValue(o);
         }
         public void clear() {
             ConcurrentHashMap.this.clear();
         }
         public Object[] toArray() {
             Collection<V> c = new ArrayList<V>();
             for (Iterator<V> i = iterator(); i.hasNext(); )
                 c.add(i.next());
             return c.toArray();
         }
         public <T> T[] toArray(T[] a) {
             Collection<V> c = new ArrayList<V>();
             for (Iterator<V> i = iterator(); i.hasNext(); )
                 c.add(i.next());
             return c.toArray(a);
         }
     }
 
     final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
         public Iterator<Map.Entry<K,V>> iterator() {
             return new EntryIterator();
         }
         public boolean contains(Object o) {
             if (!(o instanceof Map.Entry))
                 return false;
             Map.Entry<?,?> e = (Map.Entry<?,?>)o;
             V v = ConcurrentHashMap.this.get(e.getKey());
             return v != null && v.equals(e.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());
         }
         public int size() {
             return ConcurrentHashMap.this.size();
         }
         public boolean isEmpty() {
             return ConcurrentHashMap.this.isEmpty();
         }
         public void clear() {
             ConcurrentHashMap.this.clear();
         }
         public Object[] toArray() {
             Collection<Map.Entry<K,V>> c = new ArrayList<Map.Entry<K,V>>();
             for (Iterator<Map.Entry<K,V>> i = iterator(); i.hasNext(); )
                 c.add(i.next());
             return c.toArray();
         }
         public <T> T[] toArray(T[] a) {
             Collection<Map.Entry<K,V>> c = new ArrayList<Map.Entry<K,V>>();
             for (Iterator<Map.Entry<K,V>> i = iterator(); i.hasNext(); )
                 c.add(i.next());
             return c.toArray(a);
         }
     }
 
     /***
      * This duplicates java.util.AbstractMap.SimpleEntry until this class
      * is made accessible.
      */
     static class SimpleEntry<K,V> implements Entry<K,V> {
         K key;
         V value;
 
         public SimpleEntry(K key, V value) {
             this.key   = key;
             this.value = value;
         }
 
         public SimpleEntry(Entry<K,V> e) {
             this.key   = e.getKey();
             this.value = e.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;
         }
 
         public boolean equals(Object o) {
             if (!(o instanceof Map.Entry))
                 return false;
             Map.Entry e = (Map.Entry)o;
             return eq(key, e.getKey()) && eq(value, e.getValue());
         }
 
         public int hashCode() {
             return ((key   == null)   ? 0 :   key.hashCode()) ^
                     ((value == null)   ? 0 : value.hashCode());
         }
 
         public String toString() {
             return key + "=" + value;
         }
 
         static boolean eq(Object o1, Object o2) {
             return (o1 == null ? o2 == null : o1.equals(o2));
         }
     }
 
    /* ---------------- Serialization Support -------------- */
 
     /***
      * Save the state of the <tt>ConcurrentHashMap</tt> instance to a
      * stream (i.e., serialize it).
      * @param s the stream
      * @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 IOException  {
         s.defaultWriteObject();
 
         for (int k = 0; k < segments.length; ++k) {
             Segment<K,V> seg = segments[k];
             seg.readLock().lock();
             try {
                 HashEntry<K,V>[] tab = seg.table;
                 for (int i = 0; i < tab.length; ++i) {
                     for (HashEntry<K,V> e = tab[i]; e != null; e = e.next) {
                         s.writeObject(e.key);
                         s.writeObject(e.value);
                     }
                 }
             } finally {
                 seg.readLock().unlock();
             }
         }
         s.writeObject(null);
         s.writeObject(null);
     }
 
     /***
      * Reconstitute the <tt>ConcurrentHashMap</tt> instance from a
      * stream (i.e., deserialize it).
      * @param s the stream
      */
     private void readObject(java.io.ObjectInputStream s)
         throws IOException, ClassNotFoundException  {
         s.defaultReadObject();
 
         // Initialize each segment to be minimally sized, and let grow.
         for (int i = 0; i < segments.length; ++i) {
             segments[i].setTable(new HashEntry[1]);
         }
 
         // Read the keys and values, and put the mappings in the table
         for (;;) {
             K key = (K) s.readObject();
             V value = (V) s.readObject();
             if (key == null)
                 break;
             put(key, value);
         }
     }
 }