java/util/IdentityHashMap.java

/*
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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 * accompanied this code).
 *
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 *
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package java.util;
import java.io.*;

/**
 * This class implements the <tt>Map</tt> interface with a hash table, using
 * reference-equality in place of object-equality when comparing keys (and
 * values).  In other words, in an <tt>IdentityHashMap</tt>, two keys
 * <tt>k1</tt> and <tt>k2</tt> are considered equal if and only if
 * <tt>(k1==k2)</tt>.  (In normal <tt>Map</tt> implementations (like
 * <tt>HashMap</tt>) two keys <tt>k1</tt> and <tt>k2</tt> are considered equal
 * if and only if <tt>(k1==null ? k2==null : k1.equals(k2))</tt>.)
 *
 * <p><b>This class is <i>not</i> a general-purpose <tt>Map</tt>
 * implementation!  While this class implements the <tt>Map</tt> interface, it
 * intentionally violates <tt>Map's</tt> general contract, which mandates the
 * use of the <tt>equals</tt> method when comparing objects.  This class is
 * designed for use only in the rare cases wherein reference-equality
 * semantics are required.</b>
 *
 * <p>A typical use of this class is <i>topology-preserving object graph
 * transformations</i>, such as serialization or deep-copying.  To perform such
 * a transformation, a program must maintain a "node table" that keeps track
 * of all the object references that have already been processed.  The node
 * table must not equate distinct objects even if they happen to be equal.
 * Another typical use of this class is to maintain <i>proxy objects</i>.  For
 * example, a debugging facility might wish to maintain a proxy object for
 * each object in the program being debugged.
 *
 * <p>This class provides all of the optional map operations, and permits
 * <tt>null</tt> values and the <tt>null</tt> key.  This class makes no
 * guarantees as to the order of the map; in particular, it does not guarantee
 * that the order will remain constant over time.
 *
 * <p>This class provides constant-time performance for the basic
 * operations (<tt>get</tt> and <tt>put</tt>), assuming the system
 * identity hash function ({@link System#identityHashCode(Object)})
 * disperses elements properly among the buckets.
 *
 * <p>This class has one tuning parameter (which affects performance but not
 * semantics): <i>expected maximum size</i>.  This parameter is the maximum
 * number of key-value mappings that the map is expected to hold.  Internally,
 * this parameter is used to determine the number of buckets initially
 * comprising the hash table.  The precise relationship between the expected
 * maximum size and the number of buckets is unspecified.
 *
 * <p>If the size of the map (the number of key-value mappings) sufficiently
 * exceeds the expected maximum size, the number of buckets is increased
 * Increasing the number of buckets ("rehashing") may be fairly expensive, so
 * it pays to create identity hash maps with a sufficiently large expected
 * maximum size.  On the other hand, iteration over collection views requires
 * time proportional to the number of buckets in the hash table, so it
 * pays not to set the expected maximum size too high if you are especially
 * concerned with iteration performance or memory usage.
 *
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access an identity hash map concurrently, and at
 * least one of the threads modifies the map structurally, it <i>must</i>
 * be synchronized externally.  (A structural modification is any operation
 * that adds or deletes one or more mappings; merely changing the value
 * associated with a key that an instance already contains is not a
 * structural modification.)  This is typically accomplished by
 * synchronizing on some object that naturally encapsulates the map.
 *
 * If no such object exists, the map should be "wrapped" using the
 * {@link Collections#synchronizedMap Collections.synchronizedMap}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the map:<pre>
 *   Map m = Collections.synchronizedMap(new IdentityHashMap(...));</pre>
 *
 * <p>The iterators returned by the <tt>iterator</tt> method of the
 * collections returned by all of this class's "collection view
 * methods" are <i>fail-fast</i>: if the map is structurally modified
 * at any time after the iterator is created, in any way except
 * through the iterator's own <tt>remove</tt> method, the iterator
 * will throw a {@link ConcurrentModificationException}.  Thus, in the
 * face of concurrent modification, the iterator fails quickly and
 * cleanly, rather than risking arbitrary, non-deterministic behavior
 * at an undetermined time in the future.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness: <i>fail-fast iterators should be used only
 * to detect bugs.</i>
 *
 * <p>Implementation note: This is a simple <i>linear-probe</i> hash table,
 * as described for example in texts by Sedgewick and Knuth.  The array
 * alternates holding keys and values.  (This has better locality for large
 * tables than does using separate arrays.)  For many JRE implementations
 * and operation mixes, this class will yield better performance than
 * {@link HashMap} (which uses <i>chaining</i> rather than linear-probing).
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @see     System#identityHashCode(Object)
 * @see     Object#hashCode()
 * @see     Collection
 * @see     Map
 * @see     HashMap
 * @see     TreeMap
 * @author  Doug Lea and Josh Bloch
 * @since   1.4
 */

public class IdentityHashMap<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V>, java.io.Serializable, Cloneable
{
    /**
     * The initial capacity used by the no-args constructor.
     * MUST be a power of two.  The value 32 corresponds to the
     * (specified) expected maximum size of 21, given a load factor
     * of 2/3.
     */
    private static final int DEFAULT_CAPACITY = 32;

    /**
     * The minimum capacity, used if a lower value is implicitly specified
     * by either of the constructors with arguments.  The value 4 corresponds
     * to an expected maximum size of 2, given a load factor of 2/3.
     * MUST be a power of two.
     */
    private static final int MINIMUM_CAPACITY = 4;

    /**
     * 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<<29.
     */
    private static final int MAXIMUM_CAPACITY = 1 << 29;

    /**
     * The table, resized as necessary. Length MUST always be a power of two.
     */
    private transient Object[] table;

    /**
     * The number of key-value mappings contained in this identity hash map.
     *
     * @serial
     */
    private int size;

    /**
     * The number of modifications, to support fast-fail iterators
     */
    private transient int modCount;

    /**
     * The next size value at which to resize (capacity * load factor).
     */
    private transient int threshold;

    /**
     * Value representing null keys inside tables.
     */
    private static final Object NULL_KEY = new Object();

    /**
     * Use NULL_KEY for key if it is null.
     */
    private static Object maskNull(Object key) {
        return (key == null ? NULL_KEY : key);
    }

    /**
     * Returns internal representation of null key back to caller as null.
     */
    private static Object unmaskNull(Object key) {
        return (key == NULL_KEY ? null : key);
    }

    /**
     * Constructs a new, empty identity hash map with a default expected
     * maximum size (21).
     */
    public IdentityHashMap() {
        init(DEFAULT_CAPACITY);
    }

    /**
     * Constructs a new, empty map with the specified expected maximum size.
     * Putting more than the expected number of key-value mappings into
     * the map may cause the internal data structure to grow, which may be
     * somewhat time-consuming.
     *
     * @param expectedMaxSize the expected maximum size of the map
     * @throws IllegalArgumentException if <tt>expectedMaxSize</tt> is negative
     */
    public IdentityHashMap(int expectedMaxSize) {
        if (expectedMaxSize < 0)
            throw new IllegalArgumentException("expectedMaxSize is negative: "
                                               + expectedMaxSize);
        init(capacity(expectedMaxSize));
    }

    /**
     * Returns the appropriate capacity for the specified expected maximum
     * size.  Returns the smallest power of two between MINIMUM_CAPACITY
     * and MAXIMUM_CAPACITY, inclusive, that is greater than
     * (3 * expectedMaxSize)/2, if such a number exists.  Otherwise
     * returns MAXIMUM_CAPACITY.  If (3 * expectedMaxSize)/2 is negative, it
     * is assumed that overflow has occurred, and MAXIMUM_CAPACITY is returned.
     */
    private int capacity(int expectedMaxSize) {
        // Compute min capacity for expectedMaxSize given a load factor of 2/3
        int minCapacity = (3 * expectedMaxSize)/2;

        // Compute the appropriate capacity
        int result;
        if (minCapacity > MAXIMUM_CAPACITY || minCapacity < 0) {
            result = MAXIMUM_CAPACITY;
        } else {
            result = MINIMUM_CAPACITY;
            while (result < minCapacity)
                result <<= 1;
        }
        return result;
    }

    /**
     * Initializes object to be an empty map with the specified initial
     * capacity, which is assumed to be a power of two between
     * MINIMUM_CAPACITY and MAXIMUM_CAPACITY inclusive.
     */
    private void init(int initCapacity) {
        // assert (initCapacity & -initCapacity) == initCapacity; // power of 2
        // assert initCapacity >= MINIMUM_CAPACITY;
        // assert initCapacity <= MAXIMUM_CAPACITY;

        threshold = (initCapacity * 2)/3;
        table = new Object[2 * initCapacity];
    }

    /**
     * Constructs a new identity hash map containing the keys-value mappings
     * in the specified map.
     *
     * @param m the map whose mappings are to be placed into this map
     * @throws NullPointerException if the specified map is null
     */
    public IdentityHashMap(Map<? extends K, ? extends V> m) {
        // Allow for a bit of growth
        this((int) ((1 + m.size()) * 1.1));
        putAll(m);
    }

    /**
     * Returns the number of key-value mappings in this identity hash map.
     *
     * @return the number of key-value mappings in this map
     */
    public int size() {
        return size;
    }

    /**
     * Returns <tt>true</tt> if this identity hash map contains no key-value
     * mappings.
     *
     * @return <tt>true</tt> if this identity hash map contains no key-value
     *         mappings
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * Returns index for Object x.
     */
    private static int hash(Object x, int length) {
        int h = System.identityHashCode(x);
        // Multiply by -127, and left-shift to use least bit as part of hash
        return ((h << 1) - (h << 8)) & (length - 1);
    }

    /**
     * Circularly traverses table of size len.
     */
    private static int nextKeyIndex(int i, int len) {
        return (i + 2 < len ? i + 2 : 0);
    }

    /**
     * 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 == k)},
     * then this method returns {@code v}; otherwise it returns
     * {@code null}.  (There can be at most one such mapping.)
     *
     * <p>A return value of {@code null} does not <i>necessarily</i>
     * indicate that the map contains no mapping for the key; it's also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     *
     * @see #put(Object, Object)
     */
    public V get(Object key) {
        Object k = maskNull(key);
        Object[] tab = table;
        int len = tab.length;
        int i = hash(k, len);
        while (true) {
            Object item = tab[i];
            if (item == k)
                return (V) tab[i + 1];
            if (item == null)
                return null;
            i = nextKeyIndex(i, len);
        }
    }

    /**
     * Tests whether the specified object reference is a key in this identity
     * hash map.
     *
     * @param   key   possible key
     * @return  <code>true</code> if the specified object reference is a key
     *          in this map
     * @see     #containsValue(Object)
     */
    public boolean containsKey(Object key) {
        Object k = maskNull(key);
        Object[] tab = table;
        int len = tab.length;
        int i = hash(k, len);
        while (true) {
            Object item = tab[i];
            if (item == k)
                return true;
            if (item == null)
                return false;
            i = nextKeyIndex(i, len);
        }
    }

    /**
     * Tests whether the specified object reference is a value in this identity
     * hash map.
     *
     * @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 object reference
     * @see     #containsKey(Object)
     */
    public boolean containsValue(Object value) {
        Object[] tab = table;
        for (int i = 1; i < tab.length; i += 2)
            if (tab[i] == value && tab[i - 1] != null)
                return true;

        return false;
    }

    /**
     * Tests if the specified key-value mapping is in the map.
     *
     * @param   key   possible key
     * @param   value possible value
     * @return  <code>true</code> if and only if the specified key-value
     *          mapping is in the map
     */
    private boolean containsMapping(Object key, Object value) {
        Object k = maskNull(key);
        Object[] tab = table;
        int len = tab.length;
        int i = hash(k, len);
        while (true) {
            Object item = tab[i];
            if (item == k)
                return tab[i + 1] == value;
            if (item == null)
                return false;
            i = nextKeyIndex(i, len);
        }
    }

    /**
     * Associates the specified value with the specified key in this identity
     * hash map.  If the map previously contained a mapping for the key, the
     * old value is replaced.
     *
     * @param key the key with which the specified value is to be associated
     * @param value the 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>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     * @see     Object#equals(Object)
     * @see     #get(Object)
     * @see     #containsKey(Object)
     */
    public V put(K key, V value) {
        Object k = maskNull(key);
        Object[] tab = table;
        int len = tab.length;
        int i = hash(k, len);

        Object item;
        while ( (item = tab[i]) != null) {
            if (item == k) {
                V oldValue = (V) tab[i + 1];
                tab[i + 1] = value;
                return oldValue;
            }
            i = nextKeyIndex(i, len);
        }

        modCount++;
        tab[i] = k;
        tab[i + 1] = value;
        if (++size >= threshold)
            resize(len); // len == 2 * current capacity.
        return null;
    }

    /**
     * Resize the table to hold given capacity.
     *
     * @param newCapacity the new capacity, must be a power of two.
     */
    private void resize(int newCapacity) {
        // assert (newCapacity & -newCapacity) == newCapacity; // power of 2
        int newLength = newCapacity * 2;

        Object[] oldTable = table;
        int oldLength = oldTable.length;
        if (oldLength == 2*MAXIMUM_CAPACITY) { // can't expand any further
            if (threshold == MAXIMUM_CAPACITY-1)
                throw new IllegalStateException("Capacity exhausted.");
            threshold = MAXIMUM_CAPACITY-1;  // Gigantic map!
            return;
        }
        if (oldLength >= newLength)
            return;

        Object[] newTable = new Object[newLength];
        threshold = newLength / 3;

        for (int j = 0; j < oldLength; j += 2) {
            Object key = oldTable[j];
            if (key != null) {
                Object value = oldTable[j+1];
                oldTable[j] = null;
                oldTable[j+1] = null;
                int i = hash(key, newLength);
                while (newTable[i] != null)
                    i = nextKeyIndex(i, newLength);
                newTable[i] = key;
                newTable[i + 1] = value;
            }
        }
        table = newTable;
    }

    /**
     * Copies all of the mappings from the specified map to this map.
     * These mappings will 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
     * @throws NullPointerException if the specified map is null
     */
    public void putAll(Map<? extends K, ? extends V> m) {
        int n = m.size();
        if (n == 0)
            return;
        if (n > threshold) // conservatively pre-expand
            resize(capacity(n));

        for (Entry<? extends K, ? extends V> e : m.entrySet())
            put(e.getKey(), e.getValue());
    }

    /**
     * Removes the mapping for this key from this map if present.
     *
     * @param key key whose mapping is to be removed from the map
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
    public V remove(Object key) {
        Object k = maskNull(key);
        Object[] tab = table;
        int len = tab.length;
        int i = hash(k, len);

        while (true) {
            Object item = tab[i];
            if (item == k) {
                modCount++;
                size--;
                V oldValue = (V) tab[i + 1];
                tab[i + 1] = null;
                tab[i] = null;
                closeDeletion(i);
                return oldValue;
            }
            if (item == null)
                return null;
            i = nextKeyIndex(i, len);
        }

    }

    /**
     * Removes the specified key-value mapping from the map if it is present.
     *
     * @param   key   possible key
     * @param   value possible value
     * @return  <code>true</code> if and only if the specified key-value
     *          mapping was in the map
     */
    private boolean removeMapping(Object key, Object value) {
        Object k = maskNull(key);
        Object[] tab = table;
        int len = tab.length;
        int i = hash(k, len);

        while (true) {
            Object item = tab[i];
            if (item == k) {
                if (tab[i + 1] != value)
                    return false;
                modCount++;
                size--;
                tab[i] = null;
                tab[i + 1] = null;
                closeDeletion(i);
                return true;
            }
            if (item == null)
                return false;
            i = nextKeyIndex(i, len);
        }
    }

    /**
     * Rehash all possibly-colliding entries following a
     * deletion. This preserves the linear-probe
     * collision properties required by get, put, etc.
     *
     * @param d the index of a newly empty deleted slot
     */
    private void closeDeletion(int d) {
        // Adapted from Knuth Section 6.4 Algorithm R
        Object[] tab = table;
        int len = tab.length;

        // Look for items to swap into newly vacated slot
        // starting at index immediately following deletion,
        // and continuing until a null slot is seen, indicating
        // the end of a run of possibly-colliding keys.
        Object item;
        for (int i = nextKeyIndex(d, len); (item = tab[i]) != null;
             i = nextKeyIndex(i, len) ) {
            // The following test triggers if the item at slot i (which
            // hashes to be at slot r) should take the spot vacated by d.
            // If so, we swap it in, and then continue with d now at the
            // newly vacated i.  This process will terminate when we hit
            // the null slot at the end of this run.
            // The test is messy because we are using a circular table.
            int r = hash(item, len);
            if ((i < r && (r <= d || d <= i)) || (r <= d && d <= i)) {
                tab[d] = item;
                tab[d + 1] = tab[i + 1];
                tab[i] = null;
                tab[i + 1] = null;
                d = i;
            }
        }
    }

    /**
     * Removes all of the mappings from this map.
     * The map will be empty after this call returns.
     */
    public void clear() {
        modCount++;
        Object[] tab = table;
        for (int i = 0; i < tab.length; i++)
            tab[i] = null;
        size = 0;
    }

    /**
     * Compares the specified object with this map for equality.  Returns
     * <tt>true</tt> if the given object is also a map and the two maps
     * represent identical object-reference mappings.  More formally, this
     * map is equal to another map <tt>m</tt> if and only if
     * <tt>this.entrySet().equals(m.entrySet())</tt>.
     *
     * <p><b>Owing to the reference-equality-based semantics of this map it is
     * possible that the symmetry and transitivity requirements of the
     * <tt>Object.equals</tt> contract may be violated if this map is compared
     * to a normal map.  However, the <tt>Object.equals</tt> contract is
     * guaranteed to hold among <tt>IdentityHashMap</tt> instances.</b>
     *
     * @param  o object to be compared for equality with this map
     * @return <tt>true</tt> if the specified object is equal to this map
     * @see Object#equals(Object)
     */
    public boolean equals(Object o) {
        if (o == this) {
            return true;
        } else if (o instanceof IdentityHashMap) {
            IdentityHashMap m = (IdentityHashMap) o;
            if (m.size() != size)
                return false;

            Object[] tab = m.table;
            for (int i = 0; i < tab.length; i+=2) {
                Object k = tab[i];
                if (k != null && !containsMapping(k, tab[i + 1]))
                    return false;
            }
            return true;
        } else if (o instanceof Map) {
            Map m = (Map)o;
            return entrySet().equals(m.entrySet());
        } else {
            return false;  // o is not a Map
        }
    }

    /**
     * Returns the hash code value for this map.  The hash code of a map is
     * defined to be the sum of the hash codes of each entry in the map's
     * <tt>entrySet()</tt> view.  This ensures that <tt>m1.equals(m2)</tt>
     * implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two
     * <tt>IdentityHashMap</tt> instances <tt>m1</tt> and <tt>m2</tt>, as
     * required by the general contract of {@link Object#hashCode}.
     *
     * <p><b>Owing to the reference-equality-based semantics of the
     * <tt>Map.Entry</tt> instances in the set returned by this map's
     * <tt>entrySet</tt> method, it is possible that the contractual
     * requirement of <tt>Object.hashCode</tt> mentioned in the previous
     * paragraph will be violated if one of the two objects being compared is
     * an <tt>IdentityHashMap</tt> instance and the other is a normal map.</b>
     *
     * @return the hash code value for this map
     * @see Object#equals(Object)
     * @see #equals(Object)
     */
    public int hashCode() {
        int result = 0;
        Object[] tab = table;
        for (int i = 0; i < tab.length; i +=2) {
            Object key = tab[i];
            if (key != null) {
                Object k = unmaskNull(key);
                result += System.identityHashCode(k) ^
                          System.identityHashCode(tab[i + 1]);
            }
        }
        return result;
    }

    /**
     * Returns a shallow copy of this identity hash map: the keys and values
     * themselves are not cloned.
     *
     * @return a shallow copy of this map
     */
    public Object clone() {
        try {
            IdentityHashMap<K,V> m = (IdentityHashMap<K,V>) super.clone();
            m.entrySet = null;
            m.table = table.clone();
            return m;
        } catch (CloneNotSupportedException e) {
            throw new InternalError();
        }
    }

    private abstract class IdentityHashMapIterator<T> implements Iterator<T> {
        int index = (size != 0 ? 0 : table.length); // current slot.
        int expectedModCount = modCount; // to support fast-fail
        int lastReturnedIndex = -1;      // to allow remove()
        boolean indexValid; // To avoid unnecessary next computation
        Object[] traversalTable = table; // reference to main table or copy

        public boolean hasNext() {
            Object[] tab = traversalTable;
            for (int i = index; i < tab.length; i+=2) {
                Object key = tab[i];
                if (key != null) {
                    index = i;
                    return indexValid = true;
                }
            }
            index = tab.length;
            return false;
        }

        protected int nextIndex() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            if (!indexValid && !hasNext())
                throw new NoSuchElementException();

            indexValid = false;
            lastReturnedIndex = index;
            index += 2;
            return lastReturnedIndex;
        }

        public void remove() {
            if (lastReturnedIndex == -1)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();

            expectedModCount = ++modCount;
            int deletedSlot = lastReturnedIndex;
            lastReturnedIndex = -1;
            // back up index to revisit new contents after deletion
            index = deletedSlot;
            indexValid = false;

            // Removal code proceeds as in closeDeletion except that
            // it must catch the rare case where an element already
            // seen is swapped into a vacant slot that will be later
            // traversed by this iterator. We cannot allow future
            // next() calls to return it again.  The likelihood of
            // this occurring under 2/3 load factor is very slim, but
            // when it does happen, we must make a copy of the rest of
            // the table to use for the rest of the traversal. Since
            // this can only happen when we are near the end of the table,
            // even in these rare cases, this is not very expensive in
            // time or space.

            Object[] tab = traversalTable;
            int len = tab.length;

            int d = deletedSlot;
            K key = (K) tab[d];
            tab[d] = null;        // vacate the slot
            tab[d + 1] = null;

            // If traversing a copy, remove in real table.
            // We can skip gap-closure on copy.
            if (tab != IdentityHashMap.this.table) {
                IdentityHashMap.this.remove(key);
                expectedModCount = modCount;
                return;
            }

            size--;

            Object item;
            for (int i = nextKeyIndex(d, len); (item = tab[i]) != null;
                 i = nextKeyIndex(i, len)) {
                int r = hash(item, len);
                // See closeDeletion for explanation of this conditional
                if ((i < r && (r <= d || d <= i)) ||
                    (r <= d && d <= i)) {

                    // If we are about to swap an already-seen element
                    // into a slot that may later be returned by next(),
                    // then clone the rest of table for use in future
                    // next() calls. It is OK that our copy will have
                    // a gap in the "wrong" place, since it will never
                    // be used for searching anyway.

                    if (i < deletedSlot && d >= deletedSlot &&
                        traversalTable == IdentityHashMap.this.table) {
                        int remaining = len - deletedSlot;
                        Object[] newTable = new Object[remaining];
                        System.arraycopy(tab, deletedSlot,
                                         newTable, 0, remaining);
                        traversalTable = newTable;
                        index = 0;
                    }

                    tab[d] = item;
                    tab[d + 1] = tab[i + 1];
                    tab[i] = null;
                    tab[i + 1] = null;
                    d = i;
                }
            }
        }
    }

    private class KeyIterator extends IdentityHashMapIterator<K> {
        public K next() {
            return (K) unmaskNull(traversalTable[nextIndex()]);
        }
    }

    private class ValueIterator extends IdentityHashMapIterator<V> {
        public V next() {
            return (V) traversalTable[nextIndex() + 1];
        }
    }

    private class EntryIterator
        extends IdentityHashMapIterator<Map.Entry<K,V>>
    {
        private Entry lastReturnedEntry = null;

        public Map.Entry<K,V> next() {
            lastReturnedEntry = new Entry(nextIndex());
            return lastReturnedEntry;
        }

        public void remove() {
            lastReturnedIndex =
                ((null == lastReturnedEntry) ? -1 : lastReturnedEntry.index);
            super.remove();
            lastReturnedEntry.index = lastReturnedIndex;
            lastReturnedEntry = null;
        }

        private class Entry implements Map.Entry<K,V> {
            private int index;

            private Entry(int index) {
                this.index = index;
            }

            public K getKey() {
                checkIndexForEntryUse();
                return (K) unmaskNull(traversalTable[index]);
            }

            public V getValue() {
                checkIndexForEntryUse();
                return (V) traversalTable[index+1];
            }

            public V setValue(V value) {
                checkIndexForEntryUse();
                V oldValue = (V) traversalTable[index+1];
                traversalTable[index+1] = value;
                // if shadowing, force into main table
                if (traversalTable != IdentityHashMap.this.table)
                    put((K) traversalTable[index], value);
                return oldValue;
            }

            public boolean equals(Object o) {
                if (index < 0)
                    return super.equals(o);

                if (!(o instanceof Map.Entry))
                    return false;
                Map.Entry e = (Map.Entry)o;
                return (e.getKey() == unmaskNull(traversalTable[index]) &&
                       e.getValue() == traversalTable[index+1]);
            }

            public int hashCode() {
                if (lastReturnedIndex < 0)
                    return super.hashCode();

                return (System.identityHashCode(unmaskNull(traversalTable[index])) ^
                       System.identityHashCode(traversalTable[index+1]));
            }

            public String toString() {
                if (index < 0)
                    return super.toString();

                return (unmaskNull(traversalTable[index]) + "="
                        + traversalTable[index+1]);
            }

            private void checkIndexForEntryUse() {
                if (index < 0)
                    throw new IllegalStateException("Entry was removed");
            }
        }
    }

    // Views

    /**
     * This field is initialized to contain an instance of the entry set
     * view the first time this view is requested.  The view is stateless,
     * so there's no reason to create more than one.
     */
    private transient Set<Map.Entry<K,V>> entrySet = null;

    /**
     * Returns an identity-based 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.  If the map is modified while an iteration
     * over the set is in progress, the results of the iteration are
     * undefined.  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> methods.  It does not support the <tt>add</tt> or
     * <tt>addAll</tt> methods.
     *
     * <p><b>While the object returned by this method implements the
     * <tt>Set</tt> interface, it does <i>not</i> obey <tt>Set's</tt> general
     * contract.  Like its backing map, the set returned by this method
     * defines element equality as reference-equality rather than
     * object-equality.  This affects the behavior of its <tt>contains</tt>,
     * <tt>remove</tt>, <tt>containsAll</tt>, <tt>equals</tt>, and
     * <tt>hashCode</tt> methods.</b>
     *
     * <p><b>The <tt>equals</tt> method of the returned set returns <tt>true</tt>
     * only if the specified object is a set containing exactly the same
     * object references as the returned set.  The symmetry and transitivity
     * requirements of the <tt>Object.equals</tt> contract may be violated if
     * the set returned by this method is compared to a normal set.  However,
     * the <tt>Object.equals</tt> contract is guaranteed to hold among sets
     * returned by this method.</b>
     *
     * <p>The <tt>hashCode</tt> method of the returned set returns the sum of
     * the <i>identity hashcodes</i> of the elements in the set, rather than
     * the sum of their hashcodes.  This is mandated by the change in the
     * semantics of the <tt>equals</tt> method, in order to enforce the
     * general contract of the <tt>Object.hashCode</tt> method among sets
     * returned by this method.
     *
     * @return an identity-based set view of the keys contained in this map
     * @see Object#equals(Object)
     * @see System#identityHashCode(Object)
     */
    public Set<K> keySet() {
        Set<K> ks = keySet;
        if (ks != null)
            return ks;
        else
            return keySet = new KeySet();
    }

    private class KeySet extends AbstractSet<K> {
        public Iterator<K> iterator() {
            return new KeyIterator();
        }
        public int size() {
            return size;
        }
        public boolean contains(Object o) {
            return containsKey(o);
        }
        public boolean remove(Object o) {
            int oldSize = size;
            IdentityHashMap.this.remove(o);
            return size != oldSize;
        }
        /*
         * Must revert from AbstractSet's impl to AbstractCollection's, as
         * the former contains an optimization that results in incorrect
         * behavior when c is a smaller "normal" (non-identity-based) Set.
         */
        public boolean removeAll(Collection<?> c) {
            boolean modified = false;
            for (Iterator<K> i = iterator(); i.hasNext(); ) {
                if (c.contains(i.next())) {
                    i.remove();
                    modified = true;
                }
            }
            return modified;
        }
        public void clear() {
            IdentityHashMap.this.clear();
        }
        public int hashCode() {
            int result = 0;
            for (K key : this)
                result += System.identityHashCode(key);
            return result;
        }
    }

    /**
     * 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.  If the map is
     * modified while an iteration over the collection is in progress,
     * the results of the iteration are undefined.  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> methods.  It does not
     * support the <tt>add</tt> or <tt>addAll</tt> methods.
     *
     * <p><b>While the object returned by this method implements the
     * <tt>Collection</tt> interface, it does <i>not</i> obey
     * <tt>Collection's</tt> general contract.  Like its backing map,
     * the collection returned by this method defines element equality as
     * reference-equality rather than object-equality.  This affects the
     * behavior of its <tt>contains</tt>, <tt>remove</tt> and
     * <tt>containsAll</tt> methods.</b>
     */
    public Collection<V> values() {
        Collection<V> vs = values;
        if (vs != null)
            return vs;
        else
            return values = new Values();
    }

    private class Values extends AbstractCollection<V> {
        public Iterator<V> iterator() {
            return new ValueIterator();
        }
        public int size() {
            return size;
        }
        public boolean contains(Object o) {
            return containsValue(o);
        }
        public boolean remove(Object o) {
            for (Iterator<V> i = iterator(); i.hasNext(); ) {
                if (i.next() == o) {
                    i.remove();
                    return true;
                }
            }
            return false;
        }
        public void clear() {
            IdentityHashMap.this.clear();
        }
    }

    /**
     * Returns a {@link Set} view of the mappings contained in this map.
     * Each element in the returned set is a reference-equality-based
     * <tt>Map.Entry</tt>.  The set is backed by the map, so changes
     * to the map are reflected in the set, and vice-versa.  If the
     * map is modified while an iteration over the set is in progress,
     * the results of the iteration are undefined.  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>
     * methods.  It does not support the <tt>add</tt> or
     * <tt>addAll</tt> methods.
     *
     * <p>Like the backing map, the <tt>Map.Entry</tt> objects in the set
     * returned by this method define key and value equality as
     * reference-equality rather than object-equality.  This affects the
     * behavior of the <tt>equals</tt> and <tt>hashCode</tt> methods of these
     * <tt>Map.Entry</tt> objects.  A reference-equality based <tt>Map.Entry
     * e</tt> is equal to an object <tt>o</tt> if and only if <tt>o</tt> is a
     * <tt>Map.Entry</tt> and <tt>e.getKey()==o.getKey() &amp;&amp;
     * e.getValue()==o.getValue()</tt>.  To accommodate these equals
     * semantics, the <tt>hashCode</tt> method returns
     * <tt>System.identityHashCode(e.getKey()) ^
     * System.identityHashCode(e.getValue())</tt>.
     *
     * <p><b>Owing to the reference-equality-based semantics of the
     * <tt>Map.Entry</tt> instances in the set returned by this method,
     * it is possible that the symmetry and transitivity requirements of
     * the {@link Object#equals(Object)} contract may be violated if any of
     * the entries in the set is compared to a normal map entry, or if
     * the set returned by this method is compared to a set of normal map
     * entries (such as would be returned by a call to this method on a normal
     * map).  However, the <tt>Object.equals</tt> contract is guaranteed to
     * hold among identity-based map entries, and among sets of such entries.
     * </b>
     *
     * @return a set view of the identity-mappings contained in this map
     */
    public Set<Map.Entry<K,V>> entrySet() {
        Set<Map.Entry<K,V>> es = entrySet;
        if (es != null)
            return es;
        else
            return entrySet = new EntrySet();
    }

    private 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 entry = (Map.Entry)o;
            return containsMapping(entry.getKey(), entry.getValue());
        }
        public boolean remove(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry entry = (Map.Entry)o;
            return removeMapping(entry.getKey(), entry.getValue());
        }
        public int size() {
            return size;
        }
        public void clear() {
            IdentityHashMap.this.clear();
        }
        /*
         * Must revert from AbstractSet's impl to AbstractCollection's, as
         * the former contains an optimization that results in incorrect
         * behavior when c is a smaller "normal" (non-identity-based) Set.
         */
        public boolean removeAll(Collection<?> c) {
            boolean modified = false;
            for (Iterator<Map.Entry<K,V>> i = iterator(); i.hasNext(); ) {
                if (c.contains(i.next())) {
                    i.remove();
                    modified = true;
                }
            }
            return modified;
        }

        public Object[] toArray() {
            int size = size();
            Object[] result = new Object[size];
            Iterator<Map.Entry<K,V>> it = iterator();
            for (int i = 0; i < size; i++)
                result[i] = new AbstractMap.SimpleEntry<>(it.next());
            return result;
        }

        @SuppressWarnings("unchecked")
        public <T> T[] toArray(T[] a) {
            int size = size();
            if (a.length < size)
                a = (T[])java.lang.reflect.Array
                    .newInstance(a.getClass().getComponentType(), size);
            Iterator<Map.Entry<K,V>> it = iterator();
            for (int i = 0; i < size; i++)
                a[i] = (T) new AbstractMap.SimpleEntry<>(it.next());
            if (a.length > size)
                a[size] = null;
            return a;
        }
    }


    private static final long serialVersionUID = 8188218128353913216L;

    /**
     * Save the state of the <tt>IdentityHashMap</tt> instance to a stream
     * (i.e., serialize it).
     *
     * @serialData The <i>size</i> of the HashMap (the number of key-value
     *          mappings) (<tt>int</tt>), followed by the key (Object) and
     *          value (Object) for each key-value mapping represented by the
     *          IdentityHashMap.  The key-value mappings are emitted in no
     *          particular order.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException  {
        // Write out and any hidden stuff
        s.defaultWriteObject();

        // Write out size (number of Mappings)
        s.writeInt(size);

        // Write out keys and values (alternating)
        Object[] tab = table;
        for (int i = 0; i < tab.length; i += 2) {
            Object key = tab[i];
            if (key != null) {
                s.writeObject(unmaskNull(key));
                s.writeObject(tab[i + 1]);
            }
        }
    }

    /**
     * Reconstitute the <tt>IdentityHashMap</tt> instance from a stream (i.e.,
     * deserialize it).
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException  {
        // Read in any hidden stuff
        s.defaultReadObject();

        // Read in size (number of Mappings)
        int size = s.readInt();

        // Allow for 33% growth (i.e., capacity is >= 2* size()).
        init(capacity((size*4)/3));

        // Read the keys and values, and put the mappings in the table
        for (int i=0; i<size; i++) {
            K key = (K) s.readObject();
            V value = (V) s.readObject();
            putForCreate(key, value);
        }
    }

    /**
     * The put method for readObject.  It does not resize the table,
     * update modCount, etc.
     */
    private void putForCreate(K key, V value)
        throws IOException
    {
        K k = (K)maskNull(key);
        Object[] tab = table;
        int len = tab.length;
        int i = hash(k, len);

        Object item;
        while ( (item = tab[i]) != null) {
            if (item == k)
                throw new java.io.StreamCorruptedException();
            i = nextKeyIndex(i, len);
        }
        tab[i] = k;
        tab[i + 1] = value;
    }
}