IdentityHashMap.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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package java.util;

import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.reflect.Array;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Consumer;
import jdk.internal.access.SharedSecrets;

/**
 * This class implements the {@code Map} interface with a hash table, using
 * reference-equality in place of object-equality when comparing keys (and
 * values).  In other words, in an {@code IdentityHashMap}, two keys
 * {@code k1} and {@code k2} are considered equal if and only if
 * {@code (k1==k2)}.  (In normal {@code Map} implementations (like
 * {@code HashMap}) two keys {@code k1} and {@code k2} are considered equal
 * if and only if {@code (k1==null ? k2==null : k1.equals(k2))}.)
 *
 * <p><b>This class is <i>not</i> a general-purpose {@code Map}
 * implementation!  While this class implements the {@code Map} interface, it
 * intentionally violates {@code Map's} general contract, which mandates the
 * use of the {@code equals} 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
 * {@code null} values and the {@code null} 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 ({@code get} and {@code put}), 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 {@code iterator} 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 {@code remove} 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 {@code ConcurrentModificationException} 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>This class is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @implNote
 * <p>This is a simple <i>linear-probe</i> hash table,
 * as described for example in texts by Sedgewick and Knuth.  The array
 * contains alternating keys and values, with keys at even indexes and values
 * at odd indexes. (This arrangement has better locality for large
 * tables than does using separate arrays.)  For many Java implementations
 * and operation mixes, this class will yield better performance than
 * {@link HashMap}, which uses <i>chaining</i> rather than linear-probing.
 *
 * @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.
     *
     * In fact, the map can hold no more than MAXIMUM_CAPACITY-1 items
     * because it has to have at least one slot with the key == null
     * in order to avoid infinite loops in get(), put(), remove()
     */
    private static final int MAXIMUM_CAPACITY = 1 << 29;

    /**
     * The table, resized as necessary. Length MUST always be a power of two.
     */
    transient Object[] table; // non-private to simplify nested class access

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

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

    /**
     * Value representing null keys inside tables.
     */
    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.
     */
    static final 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 {@code expectedMaxSize} 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 given 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.
     */
    private static int capacity(int expectedMaxSize) {
        // assert expectedMaxSize >= 0;
        return
            (expectedMaxSize > MAXIMUM_CAPACITY / 3) ? MAXIMUM_CAPACITY :
            (expectedMaxSize <= 2 * MINIMUM_CAPACITY / 3) ? MINIMUM_CAPACITY :
            Integer.highestOneBit(expectedMaxSize + (expectedMaxSize << 1));
    }

    /**
     * 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;

        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 {@code true} if this identity hash map contains no key-value
     * mappings.
     *
     * @return {@code true} 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 -254 to use the hash LSB and to ensure index is even
        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)
     */
    @SuppressWarnings("unchecked")
    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} 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 {@code true} 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} 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 {@code key}, or
     *         {@code null} if there was no mapping for {@code key}.
     *         (A {@code null} return can also indicate that the map
     *         previously associated {@code null} with {@code key}.)
     * @see     Object#equals(Object)
     * @see     #get(Object)
     * @see     #containsKey(Object)
     */
    public V put(K key, V value) {
        final Object k = maskNull(key);

        retryAfterResize: for (;;) {
            final Object[] tab = table;
            final int len = tab.length;
            int i = hash(k, len);

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

            final int s = size + 1;
            // Use optimized form of 3 * s.
            // Next capacity is len, 2 * current capacity.
            if (s + (s << 1) > len && resize(len))
                continue retryAfterResize;

            modCount++;
            tab[i] = k;
            tab[i + 1] = value;
            size = s;
            return null;
        }
    }

    /**
     * Resizes the table if necessary to hold given capacity.
     *
     * @param newCapacity the new capacity, must be a power of two.
     * @return whether a resize did in fact take place
     */
    private boolean 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 (size == MAXIMUM_CAPACITY - 1)
                throw new IllegalStateException("Capacity exhausted.");
            return false;
        }
        if (oldLength >= newLength)
            return false;

        Object[] newTable = new Object[newLength];

        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;
        return true;
    }

    /**
     * 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 > size)
            resize(capacity(n)); // conservatively pre-expand

        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 {@code key}, or
     *         {@code null} if there was no mapping for {@code key}.
     *         (A {@code null} return can also indicate that the map
     *         previously associated {@code null} with {@code key}.)
     */
    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--;
                @SuppressWarnings("unchecked")
                    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} 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
     * {@code true} 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 {@code m} if and only if
     * {@code this.entrySet().equals(m.entrySet())}.
     *
     * <p><b>Owing to the reference-equality-based semantics of this map it is
     * possible that the symmetry and transitivity requirements of the
     * {@code Object.equals} contract may be violated if this map is compared
     * to a normal map.  However, the {@code Object.equals} contract is
     * guaranteed to hold among {@code IdentityHashMap} instances.</b>
     *
     * @param  o object to be compared for equality with this map
     * @return {@code true} 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<?, ?> m) {
            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<?, ?> m) {
            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
     * {@code entrySet()} view.  This ensures that {@code m1.equals(m2)}
     * implies that {@code m1.hashCode()==m2.hashCode()} for any two
     * {@code IdentityHashMap} instances {@code m1} and {@code m2}, as
     * required by the general contract of {@link Object#hashCode}.
     *
     * <p><b>Owing to the reference-equality-based semantics of the
     * {@code Map.Entry} instances in the set returned by this map's
     * {@code entrySet} method, it is possible that the contractual
     * requirement of {@code Object.hashCode} mentioned in the previous
     * paragraph will be violated if one of the two objects being compared is
     * an {@code IdentityHashMap} 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<?,?> m = (IdentityHashMap<?,?>) super.clone();
            m.entrySet = null;
            m.table = table.clone();
            return m;
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e);
        }
    }

    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;
            Object key = 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> {
        @SuppressWarnings("unchecked")
        public K next() {
            return (K) unmaskNull(traversalTable[nextIndex()]);
        }
    }

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

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

        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;
            }

            @SuppressWarnings("unchecked")
            public K getKey() {
                checkIndexForEntryUse();
                return (K) unmaskNull(traversalTable[index]);
            }

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

            @SuppressWarnings("unchecked")
            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);

                return o instanceof Map.Entry<?, ?> e
                        && 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;

    /**
     * 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 {@code Iterator.remove},
     * {@code Set.remove}, {@code removeAll}, {@code retainAll}, and
     * {@code clear} methods.  It does not support the {@code add} or
     * {@code addAll} methods.
     *
     * <p><b>While the object returned by this method implements the
     * {@code Set} interface, it does <i>not</i> obey {@code Set's} 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 {@code contains},
     * {@code remove}, {@code containsAll}, {@code equals}, and
     * {@code hashCode} methods.</b>
     *
     * <p><b>The {@code equals} method of the returned set returns {@code true}
     * 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 {@code Object.equals} contract may be violated if
     * the set returned by this method is compared to a normal set.  However,
     * the {@code Object.equals} contract is guaranteed to hold among sets
     * returned by this method.</b>
     *
     * <p>The {@code hashCode} 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 {@code equals} method, in order to enforce the
     * general contract of the {@code Object.hashCode} 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) {
            ks = new KeySet();
            keySet = ks;
        }
        return ks;
    }

    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) {
            Objects.requireNonNull(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;
        }
        public Object[] toArray() {
            return toArray(new Object[0]);
        }
        @SuppressWarnings("unchecked")
        public <T> T[] toArray(T[] a) {
            int expectedModCount = modCount;
            int size = size();
            if (a.length < size)
                a = (T[]) Array.newInstance(a.getClass().getComponentType(), size);
            Object[] tab = table;
            int ti = 0;
            for (int si = 0; si < tab.length; si += 2) {
                Object key;
                if ((key = tab[si]) != null) { // key present ?
                    // more elements than expected -> concurrent modification from other thread
                    if (ti >= size) {
                        throw new ConcurrentModificationException();
                    }
                    a[ti++] = (T) unmaskNull(key); // unmask key
                }
            }
            // fewer elements than expected or concurrent modification from other thread detected
            if (ti < size || expectedModCount != modCount) {
                throw new ConcurrentModificationException();
            }
            // final null marker as per spec
            if (ti < a.length) {
                a[ti] = null;
            }
            return a;
        }

        public Spliterator<K> spliterator() {
            return new KeySpliterator<>(IdentityHashMap.this, 0, -1, 0, 0);
        }
    }

    /**
     * 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 {@code Iterator.remove},
     * {@code Collection.remove}, {@code removeAll},
     * {@code retainAll} and {@code clear} methods.  It does not
     * support the {@code add} or {@code addAll} methods.
     *
     * <p><b>While the object returned by this method implements the
     * {@code Collection} interface, it does <i>not</i> obey
     * {@code Collection's} 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 {@code contains}, {@code remove} and
     * {@code containsAll} methods.</b>
     */
    public Collection<V> values() {
        Collection<V> vs = values;
        if (vs == null) {
            vs = new Values();
            values = vs;
        }
        return vs;
    }

    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();
        }
        public Object[] toArray() {
            return toArray(new Object[0]);
        }
        @SuppressWarnings("unchecked")
        public <T> T[] toArray(T[] a) {
            int expectedModCount = modCount;
            int size = size();
            if (a.length < size)
                a = (T[]) Array.newInstance(a.getClass().getComponentType(), size);
            Object[] tab = table;
            int ti = 0;
            for (int si = 0; si < tab.length; si += 2) {
                if (tab[si] != null) { // key present ?
                    // more elements than expected -> concurrent modification from other thread
                    if (ti >= size) {
                        throw new ConcurrentModificationException();
                    }
                    a[ti++] = (T) tab[si+1]; // copy value
                }
            }
            // fewer elements than expected or concurrent modification from other thread detected
            if (ti < size || expectedModCount != modCount) {
                throw new ConcurrentModificationException();
            }
            // final null marker as per spec
            if (ti < a.length) {
                a[ti] = null;
            }
            return a;
        }

        public Spliterator<V> spliterator() {
            return new ValueSpliterator<>(IdentityHashMap.this, 0, -1, 0, 0);
        }
    }

    /**
     * Returns a {@link Set} view of the mappings contained in this map.
     * Each element in the returned set is a reference-equality-based
     * {@code Map.Entry}.  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 {@code Iterator.remove}, {@code Set.remove},
     * {@code removeAll}, {@code retainAll} and {@code clear}
     * methods.  It does not support the {@code add} or
     * {@code addAll} methods.
     *
     * <p>Like the backing map, the {@code Map.Entry} 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 {@code equals} and {@code hashCode} methods of these
     * {@code Map.Entry} objects.  A reference-equality based {@code Map.Entry
     * e} is equal to an object {@code o} if and only if {@code o} is a
     * {@code Map.Entry} and {@code e.getKey()==o.getKey() &&
     * e.getValue()==o.getValue()}.  To accommodate these equals
     * semantics, the {@code hashCode} method returns
     * {@code System.identityHashCode(e.getKey()) ^
     * System.identityHashCode(e.getValue())}.
     *
     * <p><b>Owing to the reference-equality-based semantics of the
     * {@code Map.Entry} 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 {@code Object.equals} 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) {
            return o instanceof Entry<?, ?> entry
                    && containsMapping(entry.getKey(), entry.getValue());
        }
        public boolean remove(Object o) {
            return o instanceof Entry<?, ?> entry
                    && 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) {
            Objects.requireNonNull(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() {
            return toArray(new Object[0]);
        }

        @SuppressWarnings("unchecked")
        public <T> T[] toArray(T[] a) {
            int expectedModCount = modCount;
            int size = size();
            if (a.length < size)
                a = (T[]) Array.newInstance(a.getClass().getComponentType(), size);
            Object[] tab = table;
            int ti = 0;
            for (int si = 0; si < tab.length; si += 2) {
                Object key;
                if ((key = tab[si]) != null) { // key present ?
                    // more elements than expected -> concurrent modification from other thread
                    if (ti >= size) {
                        throw new ConcurrentModificationException();
                    }
                    a[ti++] = (T) new AbstractMap.SimpleEntry<>(unmaskNull(key), tab[si + 1]);
                }
            }
            // fewer elements than expected or concurrent modification from other thread detected
            if (ti < size || expectedModCount != modCount) {
                throw new ConcurrentModificationException();
            }
            // final null marker as per spec
            if (ti < a.length) {
                a[ti] = null;
            }
            return a;
        }

        public Spliterator<Map.Entry<K,V>> spliterator() {
            return new EntrySpliterator<>(IdentityHashMap.this, 0, -1, 0, 0);
        }
    }

    @java.io.Serial
    private static final long serialVersionUID = 8188218128353913216L;

    /**
     * Saves the state of the {@code IdentityHashMap} instance to a stream
     * (i.e., serializes it).
     *
     * @serialData The <i>size</i> of the HashMap (the number of key-value
     *          mappings) ({@code int}), 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.
     */
    @java.io.Serial
    private void writeObject(ObjectOutputStream s)
        throws java.io.IOException  {
        // Write out size (number of mappings) and any hidden stuff
        s.defaultWriteObject();

        // Write out size again (maintained for backward compatibility)
        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]);
            }
        }
    }

    /**
     * Reconstitutes the {@code IdentityHashMap} instance from a stream (i.e.,
     * deserializes it).
     */
    @java.io.Serial
    private void readObject(ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException  {
        // Size (number of mappings) is written to the stream twice
        // Read first size value and ignore it
        s.readFields();

        // Read second size value, validate and assign to size field
        int size = s.readInt();
        if (size < 0)
            throw new java.io.StreamCorruptedException
                ("Illegal mappings count: " + size);
        int cap = capacity(size);
        SharedSecrets.getJavaObjectInputStreamAccess().checkArray(s, Object[].class, cap*2);
        this.size = size;
        init(cap);

        // Read the keys and values, and put the mappings in the table
        for (int i=0; i<size; i++) {
            @SuppressWarnings("unchecked")
                K key = (K) s.readObject();
            @SuppressWarnings("unchecked")
                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 java.io.StreamCorruptedException
    {
        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)
                throw new java.io.StreamCorruptedException();
            i = nextKeyIndex(i, len);
        }
        tab[i] = k;
        tab[i + 1] = value;
    }

    @SuppressWarnings("unchecked")
    @Override
    public void forEach(BiConsumer<? super K, ? super V> action) {
        Objects.requireNonNull(action);
        int expectedModCount = modCount;

        Object[] t = table;
        for (int index = 0; index < t.length; index += 2) {
            Object k = t[index];
            if (k != null) {
                action.accept((K) unmaskNull(k), (V) t[index + 1]);
            }

            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
        }
    }

    @SuppressWarnings("unchecked")
    @Override
    public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
        Objects.requireNonNull(function);
        int expectedModCount = modCount;

        Object[] t = table;
        for (int index = 0; index < t.length; index += 2) {
            Object k = t[index];
            if (k != null) {
                t[index + 1] = function.apply((K) unmaskNull(k), (V) t[index + 1]);
            }

            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
        }
    }

    /**
     * Similar form as array-based Spliterators, but skips blank elements,
     * and guestimates size as decreasing by half per split.
     */
    static class IdentityHashMapSpliterator<K,V> {
        final IdentityHashMap<K,V> map;
        int index;             // current index, modified on advance/split
        int fence;             // -1 until first use; then one past last index
        int est;               // size estimate
        int expectedModCount;  // initialized when fence set

        IdentityHashMapSpliterator(IdentityHashMap<K,V> map, int origin,
                                   int fence, int est, int expectedModCount) {
            this.map = map;
            this.index = origin;
            this.fence = fence;
            this.est = est;
            this.expectedModCount = expectedModCount;
        }

        final int getFence() { // initialize fence and size on first use
            int hi;
            if ((hi = fence) < 0) {
                est = map.size;
                expectedModCount = map.modCount;
                hi = fence = map.table.length;
            }
            return hi;
        }

        public final long estimateSize() {
            getFence(); // force init
            return (long) est;
        }
    }

    static final class KeySpliterator<K,V>
        extends IdentityHashMapSpliterator<K,V>
        implements Spliterator<K> {
        KeySpliterator(IdentityHashMap<K,V> map, int origin, int fence, int est,
                       int expectedModCount) {
            super(map, origin, fence, est, expectedModCount);
        }

        public KeySpliterator<K,V> trySplit() {
            int hi = getFence(), lo = index, mid = ((lo + hi) >>> 1) & ~1;
            return (lo >= mid) ? null :
                new KeySpliterator<>(map, lo, index = mid, est >>>= 1,
                                     expectedModCount);
        }

        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super K> action) {
            if (action == null)
                throw new NullPointerException();
            int i, hi, mc; Object key;
            IdentityHashMap<K,V> m; Object[] a;
            if ((m = map) != null && (a = m.table) != null &&
                (i = index) >= 0 && (index = hi = getFence()) <= a.length) {
                for (; i < hi; i += 2) {
                    if ((key = a[i]) != null)
                        action.accept((K)unmaskNull(key));
                }
                if (m.modCount == expectedModCount)
                    return;
            }
            throw new ConcurrentModificationException();
        }

        @SuppressWarnings("unchecked")
        public boolean tryAdvance(Consumer<? super K> action) {
            if (action == null)
                throw new NullPointerException();
            Object[] a = map.table;
            int hi = getFence();
            while (index < hi) {
                Object key = a[index];
                index += 2;
                if (key != null) {
                    action.accept((K)unmaskNull(key));
                    if (map.modCount != expectedModCount)
                        throw new ConcurrentModificationException();
                    return true;
                }
            }
            return false;
        }

        public int characteristics() {
            return (fence < 0 || est == map.size ? SIZED : 0) | Spliterator.DISTINCT;
        }
    }

    static final class ValueSpliterator<K,V>
        extends IdentityHashMapSpliterator<K,V>
        implements Spliterator<V> {
        ValueSpliterator(IdentityHashMap<K,V> m, int origin, int fence, int est,
                         int expectedModCount) {
            super(m, origin, fence, est, expectedModCount);
        }

        public ValueSpliterator<K,V> trySplit() {
            int hi = getFence(), lo = index, mid = ((lo + hi) >>> 1) & ~1;
            return (lo >= mid) ? null :
                new ValueSpliterator<>(map, lo, index = mid, est >>>= 1,
                                       expectedModCount);
        }

        public void forEachRemaining(Consumer<? super V> action) {
            if (action == null)
                throw new NullPointerException();
            int i, hi, mc;
            IdentityHashMap<K,V> m; Object[] a;
            if ((m = map) != null && (a = m.table) != null &&
                (i = index) >= 0 && (index = hi = getFence()) <= a.length) {
                for (; i < hi; i += 2) {
                    if (a[i] != null) {
                        @SuppressWarnings("unchecked") V v = (V)a[i+1];
                        action.accept(v);
                    }
                }
                if (m.modCount == expectedModCount)
                    return;
            }
            throw new ConcurrentModificationException();
        }

        public boolean tryAdvance(Consumer<? super V> action) {
            if (action == null)
                throw new NullPointerException();
            Object[] a = map.table;
            int hi = getFence();
            while (index < hi) {
                Object key = a[index];
                @SuppressWarnings("unchecked") V v = (V)a[index+1];
                index += 2;
                if (key != null) {
                    action.accept(v);
                    if (map.modCount != expectedModCount)
                        throw new ConcurrentModificationException();
                    return true;
                }
            }
            return false;
        }

        public int characteristics() {
            return (fence < 0 || est == map.size ? SIZED : 0);
        }

    }

    static final class EntrySpliterator<K,V>
        extends IdentityHashMapSpliterator<K,V>
        implements Spliterator<Map.Entry<K,V>> {
        EntrySpliterator(IdentityHashMap<K,V> m, int origin, int fence, int est,
                         int expectedModCount) {
            super(m, origin, fence, est, expectedModCount);
        }

        public EntrySpliterator<K,V> trySplit() {
            int hi = getFence(), lo = index, mid = ((lo + hi) >>> 1) & ~1;
            return (lo >= mid) ? null :
                new EntrySpliterator<>(map, lo, index = mid, est >>>= 1,
                                       expectedModCount);
        }

        public void forEachRemaining(Consumer<? super Map.Entry<K, V>> action) {
            if (action == null)
                throw new NullPointerException();
            int i, hi, mc;
            IdentityHashMap<K,V> m; Object[] a;
            if ((m = map) != null && (a = m.table) != null &&
                (i = index) >= 0 && (index = hi = getFence()) <= a.length) {
                for (; i < hi; i += 2) {
                    Object key = a[i];
                    if (key != null) {
                        @SuppressWarnings("unchecked") K k =
                            (K)unmaskNull(key);
                        @SuppressWarnings("unchecked") V v = (V)a[i+1];
                        action.accept
                            (new AbstractMap.SimpleImmutableEntry<>(k, v));

                    }
                }
                if (m.modCount == expectedModCount)
                    return;
            }
            throw new ConcurrentModificationException();
        }

        public boolean tryAdvance(Consumer<? super Map.Entry<K,V>> action) {
            if (action == null)
                throw new NullPointerException();
            Object[] a = map.table;
            int hi = getFence();
            while (index < hi) {
                Object key = a[index];
                @SuppressWarnings("unchecked") V v = (V)a[index+1];
                index += 2;
                if (key != null) {
                    @SuppressWarnings("unchecked") K k =
                        (K)unmaskNull(key);
                    action.accept
                        (new AbstractMap.SimpleImmutableEntry<>(k, v));
                    if (map.modCount != expectedModCount)
                        throw new ConcurrentModificationException();
                    return true;
                }
            }
            return false;
        }

        public int characteristics() {
            return (fence < 0 || est == map.size ? SIZED : 0) | Spliterator.DISTINCT;
        }
    }

}