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A bundle project producing JAX-RS RI bundles. The primary artifact is an "all-in-one" OSGi-fied JAX-RS RI bundle (jaxrs-ri.jar). Attached to that are two compressed JAX-RS RI archives. The first archive (jaxrs-ri.zip) consists of binary RI bits and contains the API jar (under "api" directory), RI libraries (under "lib" directory) as well as all external RI dependencies (under "ext" directory). The secondary archive (jaxrs-ri-src.zip) contains buildable JAX-RS RI source bundle and contains the API jar (under "api" directory), RI sources (under "src" directory) as well as all external RI dependencies (under "ext" directory). The second archive also contains "build.xml" ANT script that builds the RI sources. To build the JAX-RS RI simply unzip the archive, cd to the created jaxrs-ri directory and invoke "ant" from the command line.

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/*
 * Copyright (C) 2009 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package org.glassfish.jersey.internal.guava;

import java.io.Serializable;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.AbstractCollection;
import java.util.AbstractMap;
import java.util.AbstractQueue;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Queue;
import java.util.Set;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReferenceArray;
import java.util.concurrent.locks.ReentrantLock;
import java.util.logging.Level;
import java.util.logging.Logger;

import static org.glassfish.jersey.internal.guava.Preconditions.checkNotNull;

/**
 * The concurrent hash map implementation built by {@link MapMaker}.
 * 

 * 
This implementation is heavily derived from revision 1.96 of ConcurrentHashMap.java.
 *
 * @author Bob Lee
 * @author Charles Fry
 * @author Doug Lea ({@code ConcurrentHashMap})
 */
class MapMakerInternalMap
        extends AbstractMap implements ConcurrentMap, Serializable {

  /*
   * The basic strategy is to subdivide the table among Segments, each of which itself is a
   * concurrently readable hash table. The map supports non-blocking reads and concurrent writes
   * across different segments.
   *
   * If a maximum size is specified, a best-effort bounding is performed per segment, using a
   * page-replacement algorithm to determine which entries to evict when the capacity has been
   * exceeded.
   *
   * The page replacement algorithm's data structures are kept casually consistent with the map. The
   * ordering of writes to a segment is sequentially consistent. An update to the map and recording
   * of reads may not be immediately reflected on the algorithm's data structures. These structures
   * are guarded by a lock and operations are applied in batches to avoid lock contention. The
   * penalty of applying the batches is spread across threads so that the amortized cost is slightly
   * higher than performing just the operation without enforcing the capacity constraint.
   *
   * This implementation uses a per-segment queue to record a memento of the additions, removals,
   * and accesses that were performed on the map. The queue is drained on writes and when it exceeds
   * its capacity threshold.
   *
   * The Least Recently Used page replacement algorithm was chosen due to its simplicity, high hit
   * rate, and ability to be implemented with O(1) time complexity. The initial LRU implementation
   * operates per-segment rather than globally for increased implementation simplicity. We expect
   * the cache hit rate to be similar to that of a global LRU algorithm.
   */

    // Constants

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

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

    /**
     * Number of (unsynchronized) retries in the containsValue method.
     */
    private static final int CONTAINS_VALUE_RETRIES = 3;

    /**
     * Number of cache access operations that can be buffered per segment before the cache's recency
     * ordering information is updated. This is used to avoid lock contention by recording a memento
     * of reads and delaying a lock acquisition until the threshold is crossed or a mutation occurs.
     * 

     * 
This must be a (2^n)-1 as it is used as a mask.
     */
    private static final int DRAIN_THRESHOLD = 0x3F;

    /**
     * Maximum number of entries to be drained in a single cleanup run. This applies independently to
     * the cleanup queue and both reference queues.
     */
    // TODO(fry): empirically optimize this
    private static final int DRAIN_MAX = 16;

    // Fields
    /**
     * Placeholder. Indicates that the value hasn't been set yet.
     */
    private static final ValueReference UNSET = new ValueReference() {
        @Override
        public Object get() {
            return null;
        }

        @Override
        public ReferenceEntry getEntry() {
            return null;
        }

        @Override
        public ValueReference copyFor(ReferenceQueue queue,
                                                      Object value, ReferenceEntry entry) {
            return this;
        }

        @Override
        public boolean isComputingReference() {
            return false;
        }

        @Override
        public void clear(ValueReference newValue) {
        }
    };
    private static final Queue DISCARDING_QUEUE = new AbstractQueue() {
        @Override
        public boolean offer(Object o) {
            return true;
        }

        @Override
        public Object peek() {
            return null;
        }

        @Override
        public Object poll() {
            return null;
        }

        @Override
        public int size() {
            return 0;
        }

        @Override
        public Iterator iterator() {
            return Iterators.emptyIterator();
        }
    };
    private static final Logger logger = Logger.getLogger(MapMakerInternalMap.class.getName());
    private static final long serialVersionUID = 5;
    /**
     * Mask value for indexing into segments. The upper bits of a key's hash code are used to choose
     * the segment.
     */
    private final transient int segmentMask;
    /**
     * Shift value for indexing within segments. Helps prevent entries that end up in the same segment
     * from also ending up in the same bucket.
     */
    private final transient int segmentShift;
    /**
     * The segments, each of which is a specialized hash table.
     */
    private final transient Segment[] segments;
    /**
     * The concurrency level.
     */
    private final int concurrencyLevel;
    /**
     * Strategy for comparing keys.
     */
    private final Equivalence keyEquivalence;
    /**
     * Strategy for comparing values.
     */
    private final Equivalence valueEquivalence;
    /**
     * Strategy for referencing keys.
     */
    private final Strength keyStrength;
    /**
     * Strategy for referencing values.
     */
    private final Strength valueStrength;
    /**
     * The maximum size of this map. MapMaker.UNSET_INT if there is no maximum.
     */
    private final int maximumSize;
    /**
     * How long after the last access to an entry the map will retain that entry.
     */
    private final long expireAfterAccessNanos;
    /**
     * How long after the last write to an entry the map will retain that entry.
     */
    private final long expireAfterWriteNanos;
    /**
     * Entries waiting to be consumed by the removal listener.
     */
    // TODO(fry): define a new type which creates event objects and automates the clear logic
    private final Queue> removalNotificationQueue;
    /**
     * A listener that is invoked when an entry is removed due to expiration or garbage collection of
     * soft/weak entries.
     */
    private final MapMaker.RemovalListener removalListener;
    /**
     * Factory used to create new entries.
     */
    private final transient EntryFactory entryFactory;
    /**
     * Measures time in a testable way.
     */
    private final Ticker ticker;
    private transient Set keySet;
    private transient Collection values;
    private transient Set> entrySet;

    /**
     * Creates a new, empty map with the specified strategy, initial capacity and concurrency level.
     */
    private MapMakerInternalMap(MapMaker builder) {
        concurrencyLevel = Math.min(builder.getConcurrencyLevel(), MAX_SEGMENTS);

        keyStrength = builder.getKeyStrength();
        valueStrength = builder.getValueStrength();

        keyEquivalence = builder.getKeyEquivalence();
        valueEquivalence = valueStrength.defaultEquivalence();

        maximumSize = builder.maximumSize;
        expireAfterAccessNanos = builder.getExpireAfterAccessNanos();
        expireAfterWriteNanos = builder.getExpireAfterWriteNanos();

        entryFactory = EntryFactory.getFactory(keyStrength, expires(), evictsBySize());
        ticker = builder.getTicker();

        removalListener = builder.getRemovalListener();
        removalNotificationQueue = (removalListener == GenericMapMaker.NullListener.INSTANCE)
                ? MapMakerInternalMap.discardingQueue()
                : new ConcurrentLinkedQueue>();

        int initialCapacity = Math.min(builder.getInitialCapacity(), MAXIMUM_CAPACITY);
        if (evictsBySize()) {
            initialCapacity = Math.min(initialCapacity, maximumSize);
        }

        // Find power-of-two sizes best matching arguments. Constraints:
        // (segmentCount <= maximumSize)
        // && (concurrencyLevel > maximumSize || segmentCount > concurrencyLevel)
        int segmentShift = 0;
        int segmentCount = 1;
        while (segmentCount < concurrencyLevel
                && (!evictsBySize() || segmentCount * 2 <= maximumSize)) {
            ++segmentShift;
            segmentCount <<= 1;
        }
        this.segmentShift = 32 - segmentShift;
        segmentMask = segmentCount - 1;

        this.segments = newSegmentArray(segmentCount);

        int segmentCapacity = initialCapacity / segmentCount;
        if (segmentCapacity * segmentCount < initialCapacity) {
            ++segmentCapacity;
        }

        int segmentSize = 1;
        while (segmentSize < segmentCapacity) {
            segmentSize <<= 1;
        }

        if (evictsBySize()) {
            // Ensure sum of segment max sizes = overall max size
            int maximumSegmentSize = maximumSize / segmentCount + 1;
            int remainder = maximumSize % segmentCount;
            for (int i = 0; i < this.segments.length; ++i) {
                if (i == remainder) {
                    maximumSegmentSize--;
                }
                this.segments[i] =
                        createSegment(segmentSize, maximumSegmentSize);
            }
        } else {
            for (int i = 0; i < this.segments.length; ++i) {
                this.segments[i] =
                        createSegment(segmentSize, MapMaker.UNSET_INT);
            }
        }
    }

    /**
     * Singleton placeholder that indicates a value is being computed.
     */
    @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
    private static  ValueReference unset() {
        return (ValueReference) UNSET;
    }

    @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
    private static  ReferenceEntry nullEntry() {
        return (ReferenceEntry) NullEntry.INSTANCE;
    }

    /**
     * Queue that discards all elements.
     */
    @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
    private static  Queue discardingQueue() {
        return (Queue) DISCARDING_QUEUE;
    }

    /**
     * Applies a supplemental hash function to a given hash code, which defends against poor quality
     * hash functions. This is critical when the concurrent hash map uses power-of-two length hash
     * tables, that otherwise encounter collisions for hash codes that do not differ in lower or
     * upper bits.
     *
     * @param h hash code
     */
    private static int rehash(int h) {
        // Spread bits to regularize both segment and index locations,
        // using variant of single-word Wang/Jenkins hash.
        // TODO(kevinb): use Hashing/move this to Hashing?
        h += (h << 15) ^ 0xffffcd7d;
        h ^= (h >>> 10);
        h += (h << 3);
        h ^= (h >>> 6);
        h += (h << 2) + (h << 14);
        return h ^ (h >>> 16);
    }

    // Guarded By Segment.this
    private static  void connectExpirables(ReferenceEntry previous, ReferenceEntry next) {
        previous.setNextExpirable(next);
        next.setPreviousExpirable(previous);
    }

    // Guarded By Segment.this
    private static  void nullifyExpirable(ReferenceEntry nulled) {
        ReferenceEntry nullEntry = nullEntry();
        nulled.setNextExpirable(nullEntry);
        nulled.setPreviousExpirable(nullEntry);
    }

    /**
     * Links the evitables together.
     */
    // Guarded By Segment.this
    private static  void connectEvictables(ReferenceEntry previous, ReferenceEntry next) {
        previous.setNextEvictable(next);
        next.setPreviousEvictable(previous);
    }

    // Guarded By Segment.this
    private static  void nullifyEvictable(ReferenceEntry nulled) {
        ReferenceEntry nullEntry = nullEntry();
        nulled.setNextEvictable(nullEntry);
        nulled.setPreviousEvictable(nullEntry);
    }

    boolean evictsBySize() {
        return maximumSize != MapMaker.UNSET_INT;
    }

    boolean expires() {
        return expiresAfterWrite() || expiresAfterAccess();
    }

    private boolean expiresAfterWrite() {
        return expireAfterWriteNanos > 0;
    }

  /*
   * Note: All of this duplicate code sucks, but it saves a lot of memory. If only Java had mixins!
   * To maintain this code, make a change for the strong reference type. Then, cut and paste, and
   * replace "Strong" with "Soft" or "Weak" within the pasted text. The primary difference is that
   * strong entries store the key reference directly while soft and weak entries delegate to their
   * respective superclasses.
   */

    boolean expiresAfterAccess() {
        return expireAfterAccessNanos > 0;
    }

    boolean usesKeyReferences() {
        return keyStrength != Strength.STRONG;
    }

    boolean usesValueReferences() {
        return valueStrength != Strength.STRONG;
    }

    private int hash(Object key) {
        int h = keyEquivalence.hash(key);
        return rehash(h);
    }

    void reclaimValue(ValueReference valueReference) {
        ReferenceEntry entry = valueReference.getEntry();
        int hash = entry.getHash();
        segmentFor(hash).reclaimValue(entry.getKey(), hash, valueReference);
    }

    void reclaimKey(ReferenceEntry entry) {
        int hash = entry.getHash();
        segmentFor(hash).reclaimKey(entry, hash);
    }

    /**
     * Returns the segment that should be used for a key with the given hash.
     *
     * @param hash the hash code for the key
     * @return the segment
     */
    private Segment segmentFor(int hash) {
        // TODO(fry): Lazily create segments?
        return segments[(hash >>> segmentShift) & segmentMask];
    }

    private Segment createSegment(int initialCapacity, int maxSegmentSize) {
        return new Segment(this, initialCapacity, maxSegmentSize);
    }

    /**
     * Gets the value from an entry. Returns {@code null} if the entry is invalid,
     * partially-collected, computing, or expired. Unlike {@link Segment#getLiveValue} this method
     * does not attempt to clean up stale entries.
     */
    private V getLiveValue(ReferenceEntry entry) {
        if (entry.getKey() == null) {
            return null;
        }
        V value = entry.getValueReference().get();
        if (value == null) {
            return null;
        }

        if (expires() && isExpired(entry)) {
            return null;
        }
        return value;
    }

    /**
     * Returns {@code true} if the entry has expired.
     */
    boolean isExpired(ReferenceEntry entry) {
        return isExpired(entry, ticker.read());
    }

    /**
     * Returns {@code true} if the entry has expired.
     */
    boolean isExpired(ReferenceEntry entry, long now) {
        // if the expiration time had overflowed, this "undoes" the overflow
        return now - entry.getExpirationTime() > 0;
    }

    /**
     * Notifies listeners that an entry has been automatically removed due to expiration, eviction,
     * or eligibility for garbage collection. This should be called every time expireEntries or
     * evictEntry is called (once the lock is released).
     */
    void processPendingNotifications() {
        MapMaker.RemovalNotification notification;
        while ((notification = removalNotificationQueue.poll()) != null) {
            try {
                removalListener.onRemoval(notification);
            } catch (Exception e) {
                logger.log(Level.WARNING, "Exception thrown by removal listener", e);
            }
        }
    }

    @SuppressWarnings("unchecked")
    private Segment[] newSegmentArray(int ssize) {
        return new Segment[ssize];
    }

    @Override
    public boolean isEmpty() {
    /*
     * Sum per-segment modCounts to avoid mis-reporting when elements are concurrently added and
     * removed in one segment while checking another, in which case the table was never actually
     * empty at any point. (The sum ensures accuracy up through at least 1<<31 per-segment
     * modifications before recheck.)  Method containsValue() uses similar constructions for
     * stability checks.
     */
        long sum = 0L;
        Segment[] segments = this.segments;
        for (int i = 0; i < segments.length; ++i) {
            if (segments[i].count != 0) {
                return false;
            }
            sum += segments[i].modCount;
        }

        if (sum != 0L) { // recheck unless no modifications
            for (int i = 0; i < segments.length; ++i) {
                if (segments[i].count != 0) {
                    return false;
                }
                sum -= segments[i].modCount;
            }
            if (sum != 0L) {
                return false;
            }
        }
        return true;
    }

    @Override
    public int size() {
        Segment[] segments = this.segments;
        long sum = 0;
        for (int i = 0; i < segments.length; ++i) {
            sum += segments[i].count;
        }
        return Ints.saturatedCast(sum);
    }

    @Override
    public V get(Object key) {
        if (key == null) {
            return null;
        }
        int hash = hash(key);
        return segmentFor(hash).get(key, hash);
    }

    @Override
    public boolean containsKey(Object key) {
        if (key == null) {
            return false;
        }
        int hash = hash(key);
        return segmentFor(hash).containsKey(key, hash);
    }

    @Override
    public boolean containsValue(Object value) {
        if (value == null) {
            return false;
        }

        // This implementation is patterned after ConcurrentHashMap, but without the locking. The only
        // way for it to return a false negative would be for the target value to jump around in the map
        // such that none of the subsequent iterations observed it, despite the fact that at every point
        // in time it was present somewhere int the map. This becomes increasingly unlikely as
        // CONTAINS_VALUE_RETRIES increases, though without locking it is theoretically possible.
        final Segment[] segments = this.segments;
        long last = -1L;
        for (int i = 0; i < CONTAINS_VALUE_RETRIES; i++) {
            long sum = 0L;
            for (Segment segment : segments) {
                // ensure visibility of most recent completed write
                @SuppressWarnings({"UnusedDeclaration", "unused"})
                int c = segment.count; // read-volatile

                AtomicReferenceArray> table = segment.table;
                for (int j = 0; j < table.length(); j++) {
                    for (ReferenceEntry e = table.get(j); e != null; e = e.getNext()) {
                        V v = segment.getLiveValue(e);
                        if (v != null && valueEquivalence.equivalent(value, v)) {
                            return true;
                        }
                    }
                }
                sum += segment.modCount;
            }
            if (sum == last) {
                break;
            }
            last = sum;
        }
        return false;
    }

    // expiration

    @Override
    public V put(K key, V value) {
        checkNotNull(key);
        checkNotNull(value);
        int hash = hash(key);
        return segmentFor(hash).put(key, hash, value, false);
    }

    @Override
    public V putIfAbsent(K key, V value) {
        checkNotNull(key);
        checkNotNull(value);
        int hash = hash(key);
        return segmentFor(hash).put(key, hash, value, true);
    }

    @Override
    public void putAll(Map m) {
        for (Entry e : m.entrySet()) {
            put(e.getKey(), e.getValue());
        }
    }

    @Override
    public V remove(Object key) {
        if (key == null) {
            return null;
        }
        int hash = hash(key);
        return segmentFor(hash).remove(key, hash);
    }

    // eviction

    @Override
    public boolean remove(Object key, Object value) {
        if (key == null || value == null) {
            return false;
        }
        int hash = hash(key);
        return segmentFor(hash).remove(key, hash, value);
    }

    @Override
    public boolean replace(K key, V oldValue, V newValue) {
        checkNotNull(key);
        checkNotNull(newValue);
        if (oldValue == null) {
            return false;
        }
        int hash = hash(key);
        return segmentFor(hash).replace(key, hash, oldValue, newValue);
    }

    @Override
    public V replace(K key, V value) {
        checkNotNull(key);
        checkNotNull(value);
        int hash = hash(key);
        return segmentFor(hash).replace(key, hash, value);
    }

    @Override
    public void clear() {
        for (Segment segment : segments) {
            segment.clear();
        }
    }

    // Inner Classes

    @Override
    public Set keySet() {
        Set ks = keySet;
        return (ks != null) ? ks : (keySet = new KeySet());
    }

    // Queues

    @Override
    public Collection values() {
        Collection vs = values;
        return (vs != null) ? vs : (values = new Values());
    }

    @Override
    public Set> entrySet() {
        Set> es = entrySet;
        return (es != null) ? es : (entrySet = new EntrySet());
    }

    // ConcurrentMap methods

    enum Strength {
    /*
     * TODO(kevinb): If we strongly reference the value and aren't computing, we needn't wrap the
     * value. This could save ~8 bytes per entry.
     */

        STRONG {
            @Override
             ValueReference referenceValue(
                    Segment segment, ReferenceEntry entry, V value) {
                return new StrongValueReference(value);
            }

            @Override
            Equivalence defaultEquivalence() {
                return Equivalence.equals();
            }
        };

        /**
         * Creates a reference for the given value according to this value strength.
         */
        abstract  ValueReference referenceValue(
                Segment segment, ReferenceEntry entry, V value);

        /**
         * Returns the default equivalence strategy used to compare and hash keys or values referenced
         * at this strength. This strategy will be used unless the user explicitly specifies an
         * alternate strategy.
         */
        abstract Equivalence defaultEquivalence();
    }

    /**
     * Creates new entries.
     */
    enum EntryFactory {
        STRONG {
            @Override
             ReferenceEntry newEntry(
                    Segment segment, K key, int hash, ReferenceEntry next) {
                return new StrongEntry(key, hash, next);
            }
        },
        STRONG_EXPIRABLE {
            @Override
             ReferenceEntry newEntry(
                    Segment segment, K key, int hash, ReferenceEntry next) {
                return new StrongExpirableEntry(key, hash, next);
            }

            @Override
             ReferenceEntry copyEntry(
                    Segment segment, ReferenceEntry original, ReferenceEntry newNext) {
                ReferenceEntry newEntry = super.copyEntry(segment, original, newNext);
                copyExpirableEntry(original, newEntry);
                return newEntry;
            }
        },
        STRONG_EVICTABLE {
            @Override
             ReferenceEntry newEntry(
                    Segment segment, K key, int hash, ReferenceEntry next) {
                return new StrongEvictableEntry(key, hash, next);
            }

            @Override
             ReferenceEntry copyEntry(
                    Segment segment, ReferenceEntry original, ReferenceEntry newNext) {
                ReferenceEntry newEntry = super.copyEntry(segment, original, newNext);
                copyEvictableEntry(original, newEntry);
                return newEntry;
            }
        },
        STRONG_EXPIRABLE_EVICTABLE {
            @Override
             ReferenceEntry newEntry(
                    Segment segment, K key, int hash, ReferenceEntry next) {
                return new StrongExpirableEvictableEntry(key, hash, next);
            }

            @Override
             ReferenceEntry copyEntry(
                    Segment segment, ReferenceEntry original, ReferenceEntry newNext) {
                ReferenceEntry newEntry = super.copyEntry(segment, original, newNext);
                copyExpirableEntry(original, newEntry);
                copyEvictableEntry(original, newEntry);
                return newEntry;
            }
        },

        WEAK {
            @Override
             ReferenceEntry newEntry(
                    Segment segment, K key, int hash, ReferenceEntry next) {
                return new WeakEntry(segment.keyReferenceQueue, key, hash, next);
            }
        },
        WEAK_EXPIRABLE {
            @Override
             ReferenceEntry newEntry(
                    Segment segment, K key, int hash, ReferenceEntry next) {
                return new WeakExpirableEntry(segment.keyReferenceQueue, key, hash, next);
            }

            @Override
             ReferenceEntry copyEntry(
                    Segment segment, ReferenceEntry original, ReferenceEntry newNext) {
                ReferenceEntry newEntry = super.copyEntry(segment, original, newNext);
                copyExpirableEntry(original, newEntry);
                return newEntry;
            }
        },
        WEAK_EVICTABLE {
            @Override
             ReferenceEntry newEntry(
                    Segment segment, K key, int hash, ReferenceEntry next) {
                return new WeakEvictableEntry(segment.keyReferenceQueue, key, hash, next);
            }

            @Override
             ReferenceEntry copyEntry(
                    Segment segment, ReferenceEntry original, ReferenceEntry newNext) {
                ReferenceEntry newEntry = super.copyEntry(segment, original, newNext);
                copyEvictableEntry(original, newEntry);
                return newEntry;
            }
        },
        WEAK_EXPIRABLE_EVICTABLE {
            @Override
             ReferenceEntry newEntry(
                    Segment segment, K key, int hash, ReferenceEntry next) {
                return new WeakExpirableEvictableEntry(segment.keyReferenceQueue, key, hash, next);
            }

            @Override
             ReferenceEntry copyEntry(
                    Segment segment, ReferenceEntry original, ReferenceEntry newNext) {
                ReferenceEntry newEntry = super.copyEntry(segment, original, newNext);
                copyExpirableEntry(original, newEntry);
                copyEvictableEntry(original, newEntry);
                return newEntry;
            }
        };

        /**
         * Masks used to compute indices in the following table.
         */
        static final int EXPIRABLE_MASK = 1;
        static final int EVICTABLE_MASK = 2;

        /**
         * Look-up table for factories. First dimension is the reference type. The second dimension is
         * the result of OR-ing the feature masks.
         */
        static final EntryFactory[][] factories = {
                {STRONG, STRONG_EXPIRABLE, STRONG_EVICTABLE, STRONG_EXPIRABLE_EVICTABLE},
                {}, // no support for SOFT keys
                {WEAK, WEAK_EXPIRABLE, WEAK_EVICTABLE, WEAK_EXPIRABLE_EVICTABLE}
        };

        static EntryFactory getFactory(Strength keyStrength, boolean expireAfterWrite,
                                       boolean evictsBySize) {
            int flags = (expireAfterWrite ? EXPIRABLE_MASK : 0) | (evictsBySize ? EVICTABLE_MASK : 0);
            return factories[keyStrength.ordinal()][flags];
        }

        /**
         * Creates a new entry.
         *
         * @param segment to create the entry for
         * @param key     of the entry
         * @param hash    of the key
         * @param next    entry in the same bucket
         */
        abstract  ReferenceEntry newEntry(
                Segment segment, K key, int hash, ReferenceEntry next);

        /**
         * Copies an entry, assigning it a new {@code next} entry.
         *
         * @param original the entry to copy
         * @param newNext  entry in the same bucket
         */
        // Guarded By Segment.this
         ReferenceEntry copyEntry(
                Segment segment, ReferenceEntry original, ReferenceEntry newNext) {
            return newEntry(segment, original.getKey(), original.getHash(), newNext);
        }

        // Guarded By Segment.this
         void copyExpirableEntry(ReferenceEntry original, ReferenceEntry newEntry) {
            // TODO(fry): when we link values instead of entries this method can go
            // away, as can connectExpirables, nullifyExpirable.
            newEntry.setExpirationTime(original.getExpirationTime());

            connectExpirables(original.getPreviousExpirable(), newEntry);
            connectExpirables(newEntry, original.getNextExpirable());

            nullifyExpirable(original);
        }

        // Guarded By Segment.this
         void copyEvictableEntry(ReferenceEntry original, ReferenceEntry newEntry) {
            // TODO(fry): when we link values instead of entries this method can go
            // away, as can connectEvictables, nullifyEvictable.
            connectEvictables(original.getPreviousEvictable(), newEntry);
            connectEvictables(newEntry, original.getNextEvictable());

            nullifyEvictable(original);
        }
    }

    private enum NullEntry implements ReferenceEntry {
        INSTANCE;

        @Override
        public ValueReference getValueReference() {
            return null;
        }

        @Override
        public void setValueReference(ValueReference valueReference) {
        }

        @Override
        public ReferenceEntry getNext() {
            return null;
        }

        @Override
        public int getHash() {
            return 0;
        }

        @Override
        public Object getKey() {
            return null;
        }

        @Override
        public long getExpirationTime() {
            return 0;
        }

        @Override
        public void setExpirationTime(long time) {
        }

        @Override
        public ReferenceEntry getNextExpirable() {
            return this;
        }

        @Override
        public void setNextExpirable(ReferenceEntry next) {
        }

        @Override
        public ReferenceEntry getPreviousExpirable() {
            return this;
        }

        @Override
        public void setPreviousExpirable(ReferenceEntry previous) {
        }

        @Override
        public ReferenceEntry getNextEvictable() {
            return this;
        }

        @Override
        public void setNextEvictable(ReferenceEntry next) {
        }

        @Override
        public ReferenceEntry getPreviousEvictable() {
            return this;
        }

        @Override
        public void setPreviousEvictable(ReferenceEntry previous) {
        }
    }

    /**
     * A reference to a value.
     */
    interface ValueReference {
        /**
         * Gets the value. Does not block or throw exceptions.
         */
        V get();

        /**
         * Returns the entry associated with this value reference, or {@code null} if this value
         * reference is independent of any entry.
         */
        ReferenceEntry getEntry();

        /**
         * Creates a copy of this reference for the given entry.
         * 

         * 
{@code value} may be null only for a loading reference.
         */
        ValueReference copyFor(
                ReferenceQueue queue, V value, ReferenceEntry entry);

        /**
         * Clears this reference object.
         *
         * @param newValue the new value reference which will replace this one; this is only used during
         *                 computation to immediately notify blocked threads of the new value
         */
        void clear(ValueReference newValue);

        /**
         * Returns {@code true} if the value type is a computing reference (regardless of whether or not
         * computation has completed). This is necessary to distiguish between partially-collected
         * entries and computing entries, which need to be cleaned up differently.
         */
        boolean isComputingReference();
    }

    /**
     * An entry in a reference map.
     * 

     * Entries in the map can be in the following states:
     * 

     * Valid:
     * - Live: valid key/value are set
     * - Computing: computation is pending
     * 

     * Invalid:
     * - Expired: time expired (key/value may still be set)
     * - Collected: key/value was partially collected, but not yet cleaned up
     */
    interface ReferenceEntry {
        /**
         * Gets the value reference from this entry.
         */
        ValueReference getValueReference();

        /**
         * Sets the value reference for this entry.
         */
        void setValueReference(ValueReference valueReference);

        /**
         * Gets the next entry in the chain.
         */
        ReferenceEntry getNext();

        /**
         * Gets the entry's hash.
         */
        int getHash();

        /**
         * Gets the key for this entry.
         */
        K getKey();

    /*
     * Used by entries that are expirable. Expirable entries are maintained in a doubly-linked list.
     * New entries are added at the tail of the list at write time; stale entries are expired from
     * the head of the list.
     */

        /**
         * Gets the entry expiration time in ns.
         */
        long getExpirationTime();

        /**
         * Sets the entry expiration time in ns.
         */
        void setExpirationTime(long time);

        /**
         * Gets the next entry in the recency list.
         */
        ReferenceEntry getNextExpirable();

        /**
         * Sets the next entry in the recency list.
         */
        void setNextExpirable(ReferenceEntry next);

        /**
         * Gets the previous entry in the recency list.
         */
        ReferenceEntry getPreviousExpirable();

        /**
         * Sets the previous entry in the recency list.
         */
        void setPreviousExpirable(ReferenceEntry previous);

    /*
     * Implemented by entries that are evictable. Evictable entries are maintained in a
     * doubly-linked list. New entries are added at the tail of the list at write time and stale
     * entries are expired from the head of the list.
     */

        /**
         * Gets the next entry in the recency list.
         */
        ReferenceEntry getNextEvictable();

        /**
         * Sets the next entry in the recency list.
         */
        void setNextEvictable(ReferenceEntry next);

        /**
         * Gets the previous entry in the recency list.
         */
        ReferenceEntry getPreviousEvictable();

        /**
         * Sets the previous entry in the recency list.
         */
        void setPreviousEvictable(ReferenceEntry previous);
    }

    abstract static class AbstractReferenceEntry implements ReferenceEntry {
        @Override
        public ValueReference getValueReference() {
            throw new UnsupportedOperationException();
        }

        @Override
        public void setValueReference(ValueReference valueReference) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ReferenceEntry getNext() {
            throw new UnsupportedOperationException();
        }

        @Override
        public int getHash() {
            throw new UnsupportedOperationException();
        }

        @Override
        public K getKey() {
            throw new UnsupportedOperationException();
        }

        @Override
        public long getExpirationTime() {
            throw new UnsupportedOperationException();
        }

        @Override
        public void setExpirationTime(long time) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ReferenceEntry getNextExpirable() {
            throw new UnsupportedOperationException();
        }

        @Override
        public void setNextExpirable(ReferenceEntry next) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ReferenceEntry getPreviousExpirable() {
            throw new UnsupportedOperationException();
        }

        @Override
        public void setPreviousExpirable(ReferenceEntry previous) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ReferenceEntry getNextEvictable() {
            throw new UnsupportedOperationException();
        }

        @Override
        public void setNextEvictable(ReferenceEntry next) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ReferenceEntry getPreviousEvictable() {
            throw new UnsupportedOperationException();
        }

        @Override
        public void setPreviousEvictable(ReferenceEntry previous) {
            throw new UnsupportedOperationException();
        }
    }

    /**
     * Used for strongly-referenced keys.
     */
    static class StrongEntry implements ReferenceEntry {
        final K key;
        final int hash;
        final ReferenceEntry next;

        // null expiration
        volatile ValueReference valueReference = unset();

        StrongEntry(K key, int hash, ReferenceEntry next) {
            this.key = key;
            this.hash = hash;
            this.next = next;
        }

        @Override
        public K getKey() {
            return this.key;
        }

        @Override
        public long getExpirationTime() {
            throw new UnsupportedOperationException();
        }

        @Override
        public void setExpirationTime(long time) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ReferenceEntry getNextExpirable() {
            throw new UnsupportedOperationException();
        }

        // null eviction

        @Override
        public void setNextExpirable(ReferenceEntry next) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ReferenceEntry getPreviousExpirable() {
            throw new UnsupportedOperationException();
        }

        @Override
        public void setPreviousExpirable(ReferenceEntry previous) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ReferenceEntry getNextEvictable() {
            throw new UnsupportedOperationException();
        }

        // The code below is exactly the same for each entry type.

        @Override
        public void setNextEvictable(ReferenceEntry next) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ReferenceEntry getPreviousEvictable() {
            throw new UnsupportedOperationException();
        }

        @Override
        public void setPreviousEvictable(ReferenceEntry previous) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ValueReference getValueReference() {
            return valueReference;
        }

        @Override
        public void setValueReference(ValueReference valueReference) {
            ValueReference previous = this.valueReference;
            this.valueReference = valueReference;
            previous.clear(valueReference);
        }

        @Override
        public int getHash() {
            return hash;
        }

        @Override
        public ReferenceEntry getNext() {
            return next;
        }
    }

    static final class StrongExpirableEntry extends StrongEntry
            implements ReferenceEntry {
        volatile long time = Long.MAX_VALUE;

        // The code below is exactly the same for each expirable entry type.
        // Guarded By Segment.this
        ReferenceEntry nextExpirable = nullEntry();
        // Guarded By Segment.this
        ReferenceEntry previousExpirable = nullEntry();

        StrongExpirableEntry(K key, int hash, ReferenceEntry next) {
            super(key, hash, next);
        }

        @Override
        public long getExpirationTime() {
            return time;
        }

        @Override
        public void setExpirationTime(long time) {
            this.time = time;
        }

        @Override
        public ReferenceEntry getNextExpirable() {
            return nextExpirable;
        }

        @Override
        public void setNextExpirable(ReferenceEntry next) {
            this.nextExpirable = next;
        }

        @Override
        public ReferenceEntry getPreviousExpirable() {
            return previousExpirable;
        }

        @Override
        public void setPreviousExpirable(ReferenceEntry previous) {
            this.previousExpirable = previous;
        }
    }

    static final class StrongEvictableEntry
            extends StrongEntry implements ReferenceEntry {
        // Guarded By Segment.this
        ReferenceEntry nextEvictable = nullEntry();

        // The code below is exactly the same for each evictable entry type.
        // Guarded By Segment.this
        ReferenceEntry previousEvictable = nullEntry();

        StrongEvictableEntry(K key, int hash, ReferenceEntry next) {
            super(key, hash, next);
        }

        @Override
        public ReferenceEntry getNextEvictable() {
            return nextEvictable;
        }

        @Override
        public void setNextEvictable(ReferenceEntry next) {
            this.nextEvictable = next;
        }

        @Override
        public ReferenceEntry getPreviousEvictable() {
            return previousEvictable;
        }

        @Override
        public void setPreviousEvictable(ReferenceEntry previous) {
            this.previousEvictable = previous;
        }
    }

    static final class StrongExpirableEvictableEntry
            extends StrongEntry implements ReferenceEntry {
        volatile long time = Long.MAX_VALUE;

        // The code below is exactly the same for each expirable entry type.
        // Guarded By Segment.this
        ReferenceEntry nextExpirable = nullEntry();
        // Guarded By Segment.this
        ReferenceEntry previousExpirable = nullEntry();
        // Guarded By Segment.this
        ReferenceEntry nextEvictable = nullEntry();
        // Guarded By Segment.this
        ReferenceEntry previousEvictable = nullEntry();

        StrongExpirableEvictableEntry(K key, int hash, ReferenceEntry next) {
            super(key, hash, next);
        }

        @Override
        public long getExpirationTime() {
            return time;
        }

        @Override
        public void setExpirationTime(long time) {
            this.time = time;
        }

        @Override
        public ReferenceEntry getNextExpirable() {
            return nextExpirable;
        }

        @Override
        public void setNextExpirable(ReferenceEntry next) {
            this.nextExpirable = next;
        }

        // The code below is exactly the same for each evictable entry type.

        @Override
        public ReferenceEntry getPreviousExpirable() {
            return previousExpirable;
        }

        @Override
        public void setPreviousExpirable(ReferenceEntry previous) {
            this.previousExpirable = previous;
        }

        @Override
        public ReferenceEntry getNextEvictable() {
            return nextEvictable;
        }

        @Override
        public void setNextEvictable(ReferenceEntry next) {
            this.nextEvictable = next;
        }

        @Override
        public ReferenceEntry getPreviousEvictable() {
            return previousEvictable;
        }

        @Override
        public void setPreviousEvictable(ReferenceEntry previous) {
            this.previousEvictable = previous;
        }
    }

    /**
     * Used for weakly-referenced keys.
     */
    static class WeakEntry extends WeakReference implements ReferenceEntry {
        final int hash;
        final ReferenceEntry next;

        // null expiration
        volatile ValueReference valueReference = unset();

        WeakEntry(ReferenceQueue queue, K key, int hash, ReferenceEntry next) {
            super(key, queue);
            this.hash = hash;
            this.next = next;
        }

        @Override
        public K getKey() {
            return get();
        }

        @Override
        public long getExpirationTime() {
            throw new UnsupportedOperationException();
        }

        @Override
        public void setExpirationTime(long time) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ReferenceEntry getNextExpirable() {
            throw new UnsupportedOperationException();
        }

        // null eviction

        @Override
        public void setNextExpirable(ReferenceEntry next) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ReferenceEntry getPreviousExpirable() {
            throw new UnsupportedOperationException();
        }

        @Override
        public void setPreviousExpirable(ReferenceEntry previous) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ReferenceEntry getNextEvictable() {
            throw new UnsupportedOperationException();
        }

        // The code below is exactly the same for each entry type.

        @Override
        public void setNextEvictable(ReferenceEntry next) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ReferenceEntry getPreviousEvictable() {
            throw new UnsupportedOperationException();
        }

        @Override
        public void setPreviousEvictable(ReferenceEntry previous) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ValueReference getValueReference() {
            return valueReference;
        }

        @Override
        public void setValueReference(ValueReference valueReference) {
            ValueReference previous = this.valueReference;
            this.valueReference = valueReference;
            previous.clear(valueReference);
        }

        @Override
        public int getHash() {
            return hash;
        }

        @Override
        public ReferenceEntry getNext() {
            return next;
        }
    }

    static final class WeakExpirableEntry
            extends WeakEntry implements ReferenceEntry {
        volatile long time = Long.MAX_VALUE;

        // The code below is exactly the same for each expirable entry type.
        // Guarded By Segment.this
        ReferenceEntry nextExpirable = nullEntry();
        // Guarded By Segment.this
        ReferenceEntry previousExpirable = nullEntry();

        WeakExpirableEntry(
                ReferenceQueue queue, K key, int hash, ReferenceEntry next) {
            super(queue, key, hash, next);
        }

        @Override
        public long getExpirationTime() {
            return time;
        }

        @Override
        public void setExpirationTime(long time) {
            this.time = time;
        }

        @Override
        public ReferenceEntry getNextExpirable() {
            return nextExpirable;
        }

        @Override
        public void setNextExpirable(ReferenceEntry next) {
            this.nextExpirable = next;
        }

        @Override
        public ReferenceEntry getPreviousExpirable() {
            return previousExpirable;
        }

        @Override
        public void setPreviousExpirable(ReferenceEntry previous) {
            this.previousExpirable = previous;
        }
    }

    static final class WeakEvictableEntry
            extends WeakEntry implements ReferenceEntry {
        // Guarded By Segment.this
        ReferenceEntry nextEvictable = nullEntry();

        // The code below is exactly the same for each evictable entry type.
        // Guarded By Segment.this
        ReferenceEntry previousEvictable = nullEntry();

        WeakEvictableEntry(
                ReferenceQueue queue, K key, int hash, ReferenceEntry next) {
            super(queue, key, hash, next);
        }

        @Override
        public ReferenceEntry getNextEvictable() {
            return nextEvictable;
        }

        @Override
        public void setNextEvictable(ReferenceEntry next) {
            this.nextEvictable = next;
        }

        @Override
        public ReferenceEntry getPreviousEvictable() {
            return previousEvictable;
        }

        @Override
        public void setPreviousEvictable(ReferenceEntry previous) {
            this.previousEvictable = previous;
        }
    }

    static final class WeakExpirableEvictableEntry
            extends WeakEntry implements ReferenceEntry {
        volatile long time = Long.MAX_VALUE;

        // The code below is exactly the same for each expirable entry type.
        // Guarded By Segment.this
        ReferenceEntry nextExpirable = nullEntry();
        // Guarded By Segment.this
        ReferenceEntry previousExpirable = nullEntry();
        // Guarded By Segment.this
        ReferenceEntry nextEvictable = nullEntry();
        // Guarded By Segment.this
        ReferenceEntry previousEvictable = nullEntry();

        WeakExpirableEvictableEntry(
                ReferenceQueue queue, K key, int hash, ReferenceEntry next) {
            super(queue, key, hash, next);
        }

        @Override
        public long getExpirationTime() {
            return time;
        }

        @Override
        public void setExpirationTime(long time) {
            this.time = time;
        }

        @Override
        public ReferenceEntry getNextExpirable() {
            return nextExpirable;
        }

        @Override
        public void setNextExpirable(ReferenceEntry next) {
            this.nextExpirable = next;
        }

        // The code below is exactly the same for each evictable entry type.

        @Override
        public ReferenceEntry getPreviousExpirable() {
            return previousExpirable;
        }

        @Override
        public void setPreviousExpirable(ReferenceEntry previous) {
            this.previousExpirable = previous;
        }

        @Override
        public ReferenceEntry getNextEvictable() {
            return nextEvictable;
        }

        @Override
        public void setNextEvictable(ReferenceEntry next) {
            this.nextEvictable = next;
        }

        @Override
        public ReferenceEntry getPreviousEvictable() {
            return previousEvictable;
        }

        @Override
        public void setPreviousEvictable(ReferenceEntry previous) {
            this.previousEvictable = previous;
        }
    }

    /**
     * References a strong value.
     */
    static final class StrongValueReference implements ValueReference {
        final V referent;

        StrongValueReference(V referent) {
            this.referent = referent;
        }

        @Override
        public V get() {
            return referent;
        }

        @Override
        public ReferenceEntry getEntry() {
            return null;
        }

        @Override
        public ValueReference copyFor(
                ReferenceQueue queue, V value, ReferenceEntry entry) {
            return this;
        }

        @Override
        public boolean isComputingReference() {
            return false;
        }

        @Override
        public void clear(ValueReference newValue) {
        }
    }

    /**
     * Segments are specialized versions of hash tables. This subclass inherits from ReentrantLock
     * opportunistically, just to simplify some locking and avoid separate construction.
     */
    @SuppressWarnings("serial") // This class is never serialized.
    static class Segment extends ReentrantLock {

    /*
     * TODO(fry): Consider copying variables (like evictsBySize) from outer class into this class.
     * It will require more memory but will reduce indirection.
     */

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

        final MapMakerInternalMap map;
        /**
         * The maximum size of this map. MapMaker.UNSET_INT if there is no maximum.
         */
        final int maxSegmentSize;
        /**
         * The key reference queue contains entries whose keys have been garbage collected, and which
         * need to be cleaned up internally.
         */
        final ReferenceQueue keyReferenceQueue;
        /**
         * The value reference queue contains value references whose values have been garbage collected,
         * and which need to be cleaned up internally.
         */
        final ReferenceQueue valueReferenceQueue;
        /**
         * The recency queue is used to record which entries were accessed for updating the eviction
         * list's ordering. It is drained as a batch operation when either the DRAIN_THRESHOLD is
         * crossed or a write occurs on the segment.
         */
        final Queue> recencyQueue;
        /**
         * A counter of the number of reads since the last write, used to drain queues on a small
         * fraction of read operations.
         */
        final AtomicInteger readCount = new AtomicInteger();
        /**
         * A queue of elements currently in the map, ordered by access time. Elements are added to the
         * tail of the queue on access/write.
         */
        final Queue> evictionQueue;
        /**
         * A queue of elements currently in the map, ordered by expiration time (either access or write
         * time). Elements are added to the tail of the queue on access/write.
         */
        final Queue> expirationQueue;
        /**
         * The number of live elements in this segment's region. This does not include unset elements
         * which are awaiting cleanup.
         */
        volatile int count;
        /**
         * Number of updates that alter the size of the table. This is used during bulk-read methods to
         * make sure they see a consistent snapshot: If modCounts change during a traversal of segments
         * computing size or checking containsValue, then we might have an inconsistent view of state
         * so (usually) must retry.
         */
        int modCount;
        /**
         * The table is expanded when its size exceeds this threshold. (The value of this field is
         * always {@code (int) (capacity * 0.75)}.)
         */
        int threshold;
        /**
         * The per-segment table.
         */
        volatile AtomicReferenceArray> table;

        Segment(MapMakerInternalMap map, int initialCapacity, int maxSegmentSize) {
            this.map = map;
            this.maxSegmentSize = maxSegmentSize;
            initTable(newEntryArray(initialCapacity));

            keyReferenceQueue = map.usesKeyReferences()
                    ? new ReferenceQueue() : null;

            valueReferenceQueue = map.usesValueReferences()
                    ? new ReferenceQueue() : null;

            recencyQueue = (map.evictsBySize() || map.expiresAfterAccess())
                    ? new ConcurrentLinkedQueue>()
                    : MapMakerInternalMap.discardingQueue();

            evictionQueue = map.evictsBySize()
                    ? new EvictionQueue()
                    : MapMakerInternalMap.discardingQueue();

            expirationQueue = map.expires()
                    ? new ExpirationQueue()
                    : MapMakerInternalMap.discardingQueue();
        }

        AtomicReferenceArray> newEntryArray(int size) {
            return new AtomicReferenceArray>(size);
        }

        void initTable(AtomicReferenceArray> newTable) {
            this.threshold = newTable.length() * 3 / 4; // 0.75
            if (this.threshold == maxSegmentSize) {
                // prevent spurious expansion before eviction
                this.threshold++;
            }
            this.table = newTable;
        }

        ReferenceEntry newEntry(K key, int hash, ReferenceEntry next) {
            return map.entryFactory.newEntry(this, key, hash, next);
        }

        /**
         * Copies {@code original} into a new entry chained to {@code newNext}. Returns the new entry,
         * or {@code null} if {@code original} was already garbage collected.
         */
        ReferenceEntry copyEntry(ReferenceEntry original, ReferenceEntry newNext) {
            if (original.getKey() == null) {
                // key collected
                return null;
            }

            ValueReference valueReference = original.getValueReference();
            V value = valueReference.get();
            if ((value == null) && !valueReference.isComputingReference()) {
                // value collected
                return null;
            }

            ReferenceEntry newEntry = map.entryFactory.copyEntry(this, original, newNext);
            newEntry.setValueReference(valueReference.copyFor(this.valueReferenceQueue, value, newEntry));
            return newEntry;
        }

        /**
         * Sets a new value of an entry. Adds newly created entries at the end of the expiration queue.
         */
        void setValue(ReferenceEntry entry, V value) {
            ValueReference valueReference = map.valueStrength.referenceValue(this, entry, value);
            entry.setValueReference(valueReference);
            recordWrite(entry);
        }

        // reference queues, for garbage collection cleanup

        /**
         * Cleanup collected entries when the lock is available.
         */
        void tryDrainReferenceQueues() {
            if (tryLock()) {
                try {
                    drainReferenceQueues();
                } finally {
                    unlock();
                }
            }
        }

        /**
         * Drain the key and value reference queues, cleaning up internal entries containing garbage
         * collected keys or values.
         */
        void drainReferenceQueues() {
            if (map.usesKeyReferences()) {
                drainKeyReferenceQueue();
            }
            if (map.usesValueReferences()) {
                drainValueReferenceQueue();
            }
        }

        void drainKeyReferenceQueue() {
            Reference ref;
            int i = 0;
            while ((ref = keyReferenceQueue.poll()) != null) {
                @SuppressWarnings("unchecked")
                ReferenceEntry entry = (ReferenceEntry) ref;
                map.reclaimKey(entry);
                if (++i == DRAIN_MAX) {
                    break;
                }
            }
        }

        void drainValueReferenceQueue() {
            Reference ref;
            int i = 0;
            while ((ref = valueReferenceQueue.poll()) != null) {
                @SuppressWarnings("unchecked")
                ValueReference valueReference = (ValueReference) ref;
                map.reclaimValue(valueReference);
                if (++i == DRAIN_MAX) {
                    break;
                }
            }
        }

        /**
         * Clears all entries from the key and value reference queues.
         */
        void clearReferenceQueues() {
            if (map.usesKeyReferences()) {
                clearKeyReferenceQueue();
            }
            if (map.usesValueReferences()) {
                clearValueReferenceQueue();
            }
        }

        void clearKeyReferenceQueue() {
            while (keyReferenceQueue.poll() != null) {
            }
        }

        void clearValueReferenceQueue() {
            while (valueReferenceQueue.poll() != null) {
            }
        }

        // recency queue, shared by expiration and eviction

        /**
         * Records the relative order in which this read was performed by adding {@code entry} to the
         * recency queue. At write-time, or when the queue is full past the threshold, the queue will
         * be drained and the entries therein processed.
         * 

         * 
Note: locked reads should use {@link #recordLockedRead}.
         */
        void recordRead(ReferenceEntry entry) {
            if (map.expiresAfterAccess()) {
                recordExpirationTime(entry, map.expireAfterAccessNanos);
            }
            recencyQueue.add(entry);
        }

        /**
         * Updates the eviction metadata that {@code entry} was just read. This currently amounts to
         * adding {@code entry} to relevant eviction lists.
         * 

         * 
Note: this method should only be called under lock, as it directly manipulates the
         * eviction queues. Unlocked reads should use {@link #recordRead}.
         */
        void recordLockedRead(ReferenceEntry entry) {
            evictionQueue.add(entry);
            if (map.expiresAfterAccess()) {
                recordExpirationTime(entry, map.expireAfterAccessNanos);
                expirationQueue.add(entry);
            }
        }

        /**
         * Updates eviction metadata that {@code entry} was just written. This currently amounts to
         * adding {@code entry} to relevant eviction lists.
         */
        void recordWrite(ReferenceEntry entry) {
            // we are already under lock, so drain the recency queue immediately
            drainRecencyQueue();
            evictionQueue.add(entry);
            if (map.expires()) {
                // currently MapMaker ensures that expireAfterWrite and
                // expireAfterAccess are mutually exclusive
                long expiration = map.expiresAfterAccess()
                        ? map.expireAfterAccessNanos
                        : map.expireAfterWriteNanos;
                recordExpirationTime(entry, expiration);
                expirationQueue.add(entry);
            }
        }

        /**
         * Drains the recency queue, updating eviction metadata that the entries therein were read in
         * the specified relative order. This currently amounts to adding them to relevant eviction
         * lists (accounting for the fact that they could have been removed from the map since being
         * added to the recency queue).
         */
        void drainRecencyQueue() {
            ReferenceEntry e;
            while ((e = recencyQueue.poll()) != null) {
                // An entry may be in the recency queue despite it being removed from
                // the map . This can occur when the entry was concurrently read while a
                // writer is removing it from the segment or after a clear has removed
                // all of the segment's entries.
                if (evictionQueue.contains(e)) {
                    evictionQueue.add(e);
                }
                if (map.expiresAfterAccess() && expirationQueue.contains(e)) {
                    expirationQueue.add(e);
                }
            }
        }

        // expiration

        void recordExpirationTime(ReferenceEntry entry, long expirationNanos) {
            // might overflow, but that's okay (see isExpired())
            entry.setExpirationTime(map.ticker.read() + expirationNanos);
        }

        /**
         * Cleanup expired entries when the lock is available.
         */
        void tryExpireEntries() {
            if (tryLock()) {
                try {
                    expireEntries();
                } finally {
                    unlock();
                    // don't call postWriteCleanup as we're in a read
                }
            }
        }

        void expireEntries() {
            drainRecencyQueue();

            if (expirationQueue.isEmpty()) {
                // There's no point in calling nanoTime() if we have no entries to
                // expire.
                return;
            }
            long now = map.ticker.read();
            ReferenceEntry e;
            while ((e = expirationQueue.peek()) != null && map.isExpired(e, now)) {
                if (!removeEntry(e, e.getHash(), MapMaker.RemovalCause.EXPIRED)) {
                    throw new AssertionError();
                }
            }
        }

        // eviction

        void enqueueNotification(ReferenceEntry entry, MapMaker.RemovalCause cause) {
            enqueueNotification(entry.getKey(), entry.getHash(), entry.getValueReference().get(), cause);
        }

        void enqueueNotification(K key, int hash, V value, MapMaker.RemovalCause cause) {
            if (map.removalNotificationQueue != DISCARDING_QUEUE) {
                MapMaker.RemovalNotification notification = new MapMaker.RemovalNotification(key, value, cause);
                map.removalNotificationQueue.offer(notification);
            }
        }

        /**
         * Performs eviction if the segment is full. This should only be called prior to adding a new
         * entry and increasing {@code count}.
         *
         * @return {@code true} if eviction occurred
         */
        boolean evictEntries() {
            if (map.evictsBySize() && count >= maxSegmentSize) {
                drainRecencyQueue();

                ReferenceEntry e = evictionQueue.remove();
                if (!removeEntry(e, e.getHash(), MapMaker.RemovalCause.SIZE)) {
                    throw new AssertionError();
                }
                return true;
            }
            return false;
        }

        /**
         * Returns first entry of bin for given hash.
         */
        ReferenceEntry getFirst(int hash) {
            // read this volatile field only once
            AtomicReferenceArray> table = this.table;
            return table.get(hash & (table.length() - 1));
        }

        // Specialized implementations of map methods

        ReferenceEntry getEntry(Object key, int hash) {
            if (count != 0) { // read-volatile
                for (ReferenceEntry e = getFirst(hash); e != null; e = e.getNext()) {
                    if (e.getHash() != hash) {
                        continue;
                    }

                    K entryKey = e.getKey();
                    if (entryKey == null) {
                        tryDrainReferenceQueues();
                        continue;
                    }

                    if (map.keyEquivalence.equivalent(key, entryKey)) {
                        return e;
                    }
                }
            }

            return null;
        }

        ReferenceEntry getLiveEntry(Object key, int hash) {
            ReferenceEntry e = getEntry(key, hash);
            if (e == null) {
                return null;
            } else if (map.expires() && map.isExpired(e)) {
                tryExpireEntries();
                return null;
            }
            return e;
        }

        V get(Object key, int hash) {
            try {
                ReferenceEntry e = getLiveEntry(key, hash);
                if (e == null) {
                    return null;
                }

                V value = e.getValueReference().get();
                if (value != null) {
                    recordRead(e);
                } else {
                    tryDrainReferenceQueues();
                }
                return value;
            } finally {
                postReadCleanup();
            }
        }

        boolean containsKey(Object key, int hash) {
            try {
                if (count != 0) { // read-volatile
                    ReferenceEntry e = getLiveEntry(key, hash);
                    if (e == null) {
                        return false;
                    }
                    return e.getValueReference().get() != null;
                }

                return false;
            } finally {
                postReadCleanup();
            }
        }

        V put(K key, int hash, V value, boolean onlyIfAbsent) {
            lock();
            try {
                preWriteCleanup();

                int newCount = this.count + 1;
                if (newCount > this.threshold) { // ensure capacity
                    expand();
                    newCount = this.count + 1;
                }

                AtomicReferenceArray> table = this.table;
                int index = hash & (table.length() - 1);
                ReferenceEntry first = table.get(index);

                // Look for an existing entry.
                for (ReferenceEntry e = first; e != null; e = e.getNext()) {
                    K entryKey = e.getKey();
                    if (e.getHash() == hash && entryKey != null
                            && map.keyEquivalence.equivalent(key, entryKey)) {
                        // We found an existing entry.

                        ValueReference valueReference = e.getValueReference();
                        V entryValue = valueReference.get();

                        if (entryValue == null) {
                            ++modCount;
                            setValue(e, value);
                            if (!valueReference.isComputingReference()) {
                                enqueueNotification(key, hash, entryValue, MapMaker.RemovalCause.COLLECTED);
                                newCount = this.count; // count remains unchanged
                            } else if (evictEntries()) { // evictEntries after setting new value
                                newCount = this.count + 1;
                            }
                            this.count = newCount; // write-volatile
                            return null;
                        } else if (onlyIfAbsent) {
                            // Mimic
                            // "if (!map.containsKey(key)) ...
                            // else return map.get(key);
                            recordLockedRead(e);
                            return entryValue;
                        } else {
                            // clobber existing entry, count remains unchanged
                            ++modCount;
                            enqueueNotification(key, hash, entryValue, MapMaker.RemovalCause.REPLACED);
                            setValue(e, value);
                            return entryValue;
                        }
                    }
                }

                // Create a new entry.
                ++modCount;
                ReferenceEntry newEntry = newEntry(key, hash, first);
                setValue(newEntry, value);
                table.set(index, newEntry);
                if (evictEntries()) { // evictEntries after setting new value
                    newCount = this.count + 1;
                }
                this.count = newCount; // write-volatile
                return null;
            } finally {
                unlock();
                postWriteCleanup();
            }
        }

        /**
         * Expands the table if possible.
         */
        void expand() {
            AtomicReferenceArray> oldTable = table;
            int oldCapacity = oldTable.length();
            if (oldCapacity >= MAXIMUM_CAPACITY) {
                return;
            }

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

            int newCount = count;
            AtomicReferenceArray> newTable = newEntryArray(oldCapacity << 1);
            threshold = newTable.length() * 3 / 4;
            int newMask = newTable.length() - 1;
            for (int oldIndex = 0; oldIndex < oldCapacity; ++oldIndex) {
                // We need to guarantee that any existing reads of old Map can
                // proceed. So we cannot yet null out each bin.
                ReferenceEntry head = oldTable.get(oldIndex);

                if (head != null) {
                    ReferenceEntry next = head.getNext();
                    int headIndex = head.getHash() & newMask;

                    // Single node on list
                    if (next == null) {
                        newTable.set(headIndex, head);
                    } else {
                        // Reuse the consecutive sequence of nodes with the same target
                        // index from the end of the list. tail points to the first
                        // entry in the reusable list.
                        ReferenceEntry tail = head;
                        int tailIndex = headIndex;
                        for (ReferenceEntry e = next; e != null; e = e.getNext()) {
                            int newIndex = e.getHash() & newMask;
                            if (newIndex != tailIndex) {
                                // The index changed. We'll need to copy the previous entry.
                                tailIndex = newIndex;
                                tail = e;
                            }
                        }
                        newTable.set(tailIndex, tail);

                        // Clone nodes leading up to the tail.
                        for (ReferenceEntry e = head; e != tail; e = e.getNext()) {
                            int newIndex = e.getHash() & newMask;
                            ReferenceEntry newNext = newTable.get(newIndex);
                            ReferenceEntry newFirst = copyEntry(e, newNext);
                            if (newFirst != null) {
                                newTable.set(newIndex, newFirst);
                            } else {
                                removeCollectedEntry(e);
                                newCount--;
                            }
                        }
                    }
                }
            }
            table = newTable;
            this.count = newCount;
        }

        boolean replace(K key, int hash, V oldValue, V newValue) {
            lock();
            try {
                preWriteCleanup();

                AtomicReferenceArray> table = this.table;
                int index = hash & (table.length() - 1);
                ReferenceEntry first = table.get(index);

                for (ReferenceEntry e = first; e != null; e = e.getNext()) {
                    K entryKey = e.getKey();
                    if (e.getHash() == hash && entryKey != null
                            && map.keyEquivalence.equivalent(key, entryKey)) {
                        // If the value disappeared, this entry is partially collected,
                        // and we should pretend like it doesn't exist.
                        ValueReference valueReference = e.getValueReference();
                        V entryValue = valueReference.get();
                        if (entryValue == null) {
                            if (isCollected(valueReference)) {
                                int newCount = this.count - 1;
                                ++modCount;
                                enqueueNotification(entryKey, hash, entryValue, MapMaker.RemovalCause.COLLECTED);
                                ReferenceEntry newFirst = removeFromChain(first, e);
                                newCount = this.count - 1;
                                table.set(index, newFirst);
                                this.count = newCount; // write-volatile
                            }
                            return false;
                        }

                        if (map.valueEquivalence.equivalent(oldValue, entryValue)) {
                            ++modCount;
                            enqueueNotification(key, hash, entryValue, MapMaker.RemovalCause.REPLACED);
                            setValue(e, newValue);
                            return true;
                        } else {
                            // Mimic
                            // "if (map.containsKey(key) && map.get(key).equals(oldValue))..."
                            recordLockedRead(e);
                            return false;
                        }
                    }
                }

                return false;
            } finally {
                unlock();
                postWriteCleanup();
            }
        }

        V replace(K key, int hash, V newValue) {
            lock();
            try {
                preWriteCleanup();

                AtomicReferenceArray> table = this.table;
                int index = hash & (table.length() - 1);
                ReferenceEntry first = table.get(index);

                for (ReferenceEntry e = first; e != null; e = e.getNext()) {
                    K entryKey = e.getKey();
                    if (e.getHash() == hash && entryKey != null
                            && map.keyEquivalence.equivalent(key, entryKey)) {
                        // If the value disappeared, this entry is partially collected,
                        // and we should pretend like it doesn't exist.
                        ValueReference valueReference = e.getValueReference();
                        V entryValue = valueReference.get();
                        if (entryValue == null) {
                            if (isCollected(valueReference)) {
                                int newCount = this.count - 1;
                                ++modCount;
                                enqueueNotification(entryKey, hash, entryValue, MapMaker.RemovalCause.COLLECTED);
                                ReferenceEntry newFirst = removeFromChain(first, e);
                                newCount = this.count - 1;
                                table.set(index, newFirst);
                                this.count = newCount; // write-volatile
                            }
                            return null;
                        }

                        ++modCount;
                        enqueueNotification(key, hash, entryValue, MapMaker.RemovalCause.REPLACED);
                        setValue(e, newValue);
                        return entryValue;
                    }
                }

                return null;
            } finally {
                unlock();
                postWriteCleanup();
            }
        }

        V remove(Object key, int hash) {
            lock();
            try {
                preWriteCleanup();

                int newCount = this.count - 1;
                AtomicReferenceArray> table = this.table;
                int index = hash & (table.length() - 1);
                ReferenceEntry first = table.get(index);

                for (ReferenceEntry e = first; e != null; e = e.getNext()) {
                    K entryKey = e.getKey();
                    if (e.getHash() == hash && entryKey != null
                            && map.keyEquivalence.equivalent(key, entryKey)) {
                        ValueReference valueReference = e.getValueReference();
                        V entryValue = valueReference.get();

                        MapMaker.RemovalCause cause;
                        if (entryValue != null) {
                            cause = MapMaker.RemovalCause.EXPLICIT;
                        } else if (isCollected(valueReference)) {
                            cause = MapMaker.RemovalCause.COLLECTED;
                        } else {
                            return null;
                        }

                        ++modCount;
                        enqueueNotification(entryKey, hash, entryValue, cause);
                        ReferenceEntry newFirst = removeFromChain(first, e);
                        newCount = this.count - 1;
                        table.set(index, newFirst);
                        this.count = newCount; // write-volatile
                        return entryValue;
                    }
                }

                return null;
            } finally {
                unlock();
                postWriteCleanup();
            }
        }

        boolean remove(Object key, int hash, Object value) {
            lock();
            try {
                preWriteCleanup();

                int newCount = this.count - 1;
                AtomicReferenceArray> table = this.table;
                int index = hash & (table.length() - 1);
                ReferenceEntry first = table.get(index);

                for (ReferenceEntry e = first; e != null; e = e.getNext()) {
                    K entryKey = e.getKey();
                    if (e.getHash() == hash && entryKey != null
                            && map.keyEquivalence.equivalent(key, entryKey)) {
                        ValueReference valueReference = e.getValueReference();
                        V entryValue = valueReference.get();

                        MapMaker.RemovalCause cause;
                        if (map.valueEquivalence.equivalent(value, entryValue)) {
                            cause = MapMaker.RemovalCause.EXPLICIT;
                        } else if (isCollected(valueReference)) {
                            cause = MapMaker.RemovalCause.COLLECTED;
                        } else {
                            return false;
                        }

                        ++modCount;
                        enqueueNotification(entryKey, hash, entryValue, cause);
                        ReferenceEntry newFirst = removeFromChain(first, e);
                        newCount = this.count - 1;
                        table.set(index, newFirst);
                        this.count = newCount; // write-volatile
                        return (cause == MapMaker.RemovalCause.EXPLICIT);
                    }
                }

                return false;
            } finally {
                unlock();
                postWriteCleanup();
            }
        }

        void clear() {
            if (count != 0) {
                lock();
                try {
                    AtomicReferenceArray> table = this.table;
                    if (map.removalNotificationQueue != DISCARDING_QUEUE) {
                        for (int i = 0; i < table.length(); ++i) {
                            for (ReferenceEntry e = table.get(i); e != null; e = e.getNext()) {
                                // Computing references aren't actually in the map yet.
                                if (!e.getValueReference().isComputingReference()) {
                                    enqueueNotification(e, MapMaker.RemovalCause.EXPLICIT);
                                }
                            }
                        }
                    }
                    for (int i = 0; i < table.length(); ++i) {
                        table.set(i, null);
                    }
                    clearReferenceQueues();
                    evictionQueue.clear();
                    expirationQueue.clear();
                    readCount.set(0);

                    ++modCount;
                    count = 0; // write-volatile
                } finally {
                    unlock();
                    postWriteCleanup();
                }
            }
        }

        /**
         * Removes an entry from within a table. All entries following the removed node can stay, but
         * all preceding ones need to be cloned.
         * 

         * 
This method does not decrement count for the removed entry, but does decrement count for
         * all partially collected entries which are skipped. As such callers which are modifying count
         * must re-read it after calling removeFromChain.
         *
         * @param first the first entry of the table
         * @param entry the entry being removed from the table
         * @return the new first entry for the table
         */
        ReferenceEntry removeFromChain(ReferenceEntry first, ReferenceEntry entry) {
            evictionQueue.remove(entry);
            expirationQueue.remove(entry);

            int newCount = count;
            ReferenceEntry newFirst = entry.getNext();
            for (ReferenceEntry e = first; e != entry; e = e.getNext()) {
                ReferenceEntry next = copyEntry(e, newFirst);
                if (next != null) {
                    newFirst = next;
                } else {
                    removeCollectedEntry(e);
                    newCount--;
                }
            }
            this.count = newCount;
            return newFirst;
        }

        void removeCollectedEntry(ReferenceEntry entry) {
            enqueueNotification(entry, MapMaker.RemovalCause.COLLECTED);
            evictionQueue.remove(entry);
            expirationQueue.remove(entry);
        }

        /**
         * Removes an entry whose key has been garbage collected.
         */
        boolean reclaimKey(ReferenceEntry entry, int hash) {
            lock();
            try {
                int newCount = count - 1;
                AtomicReferenceArray> table = this.table;
                int index = hash & (table.length() - 1);
                ReferenceEntry first = table.get(index);

                for (ReferenceEntry e = first; e != null; e = e.getNext()) {
                    if (e == entry) {
                        ++modCount;
                        enqueueNotification(
                                e.getKey(), hash, e.getValueReference().get(), MapMaker.RemovalCause.COLLECTED);
                        ReferenceEntry newFirst = removeFromChain(first, e);
                        newCount = this.count - 1;
                        table.set(index, newFirst);
                        this.count = newCount; // write-volatile
                        return true;
                    }
                }

                return false;
            } finally {
                unlock();
                postWriteCleanup();
            }
        }

        /**
         * Removes an entry whose value has been garbage collected.
         */
        boolean reclaimValue(K key, int hash, ValueReference valueReference) {
            lock();
            try {
                int newCount = this.count - 1;
                AtomicReferenceArray> table = this.table;
                int index = hash & (table.length() - 1);
                ReferenceEntry first = table.get(index);

                for (ReferenceEntry e = first; e != null; e = e.getNext()) {
                    K entryKey = e.getKey();
                    if (e.getHash() == hash && entryKey != null
                            && map.keyEquivalence.equivalent(key, entryKey)) {
                        ValueReference v = e.getValueReference();
                        if (v == valueReference) {
                            ++modCount;
                            enqueueNotification(key, hash, valueReference.get(), MapMaker.RemovalCause.COLLECTED);
                            ReferenceEntry newFirst = removeFromChain(first, e);
                            newCount = this.count - 1;
                            table.set(index, newFirst);
                            this.count = newCount; // write-volatile
                            return true;
                        }
                        return false;
                    }
                }

                return false;
            } finally {
                unlock();
                if (!isHeldByCurrentThread()) { // don't cleanup inside of put
                    postWriteCleanup();
                }
            }
        }

        boolean removeEntry(ReferenceEntry entry, int hash, MapMaker.RemovalCause cause) {
            int newCount = this.count - 1;
            AtomicReferenceArray> table = this.table;
            int index = hash & (table.length() - 1);
            ReferenceEntry first = table.get(index);

            for (ReferenceEntry e = first; e != null; e = e.getNext()) {
                if (e == entry) {
                    ++modCount;
                    enqueueNotification(e.getKey(), hash, e.getValueReference().get(), cause);
                    ReferenceEntry newFirst = removeFromChain(first, e);
                    newCount = this.count - 1;
                    table.set(index, newFirst);
                    this.count = newCount; // write-volatile
                    return true;
                }
            }

            return false;
        }

        /**
         * Returns {@code true} if the value has been partially collected, meaning that the value is
         * null and it is not computing.
         */
        boolean isCollected(ValueReference valueReference) {
            if (valueReference.isComputingReference()) {
                return false;
            }
            return (valueReference.get() == null);
        }

        /**
         * Gets the value from an entry. Returns {@code null} if the entry is invalid,
         * partially-collected, computing, or expired.
         */
        V getLiveValue(ReferenceEntry entry) {
            if (entry.getKey() == null) {
                tryDrainReferenceQueues();
                return null;
            }
            V value = entry.getValueReference().get();
            if (value == null) {
                tryDrainReferenceQueues();
                return null;
            }

            if (map.expires() && map.isExpired(entry)) {
                tryExpireEntries();
                return null;
            }
            return value;
        }

        /**
         * Performs routine cleanup following a read. Normally cleanup happens during writes, or from
         * the cleanupExecutor. If cleanup is not observed after a sufficient number of reads, try
         * cleaning up from the read thread.
         */
        void postReadCleanup() {
            if ((readCount.incrementAndGet() & DRAIN_THRESHOLD) == 0) {
                runCleanup();
            }
        }

        /**
         * Performs routine cleanup prior to executing a write. This should be called every time a
         * write thread acquires the segment lock, immediately after acquiring the lock.
         * 

         * 
Post-condition: expireEntries has been run.
         */
        void preWriteCleanup() {
            runLockedCleanup();
        }

        /**
         * Performs routine cleanup following a write.
         */
        void postWriteCleanup() {
            runUnlockedCleanup();
        }

        void runCleanup() {
            runLockedCleanup();
            runUnlockedCleanup();
        }

        void runLockedCleanup() {
            if (tryLock()) {
                try {
                    drainReferenceQueues();
                    expireEntries(); // calls drainRecencyQueue
                    readCount.set(0);
                } finally {
                    unlock();
                }
            }
        }

        void runUnlockedCleanup() {
            // locked cleanup may generate notifications we can send unlocked
            if (!isHeldByCurrentThread()) {
                map.processPendingNotifications();
            }
        }

    }

    /**
     * A custom queue for managing eviction order. Note that this is tightly integrated with {@code
     * ReferenceEntry}, upon which it relies to perform its linking.
     * 

     * 
Note that this entire implementation makes the assumption that all elements which are in
     * the map are also in this queue, and that all elements not in the queue are not in the map.
     * 

     * 
The benefits of creating our own queue are that (1) we can replace elements in the middle
     * of the queue as part of copyEvictableEntry, and (2) the contains method is highly optimized
     * for the current model.
     */
    static final class EvictionQueue extends AbstractQueue> {
        final ReferenceEntry head = new AbstractReferenceEntry() {

            ReferenceEntry nextEvictable = this;
            ReferenceEntry previousEvictable = this;

            @Override
            public ReferenceEntry getNextEvictable() {
                return nextEvictable;
            }

            @Override
            public void setNextEvictable(ReferenceEntry next) {
                this.nextEvictable = next;
            }

            @Override
            public ReferenceEntry getPreviousEvictable() {
                return previousEvictable;
            }

            @Override
            public void setPreviousEvictable(ReferenceEntry previous) {
                this.previousEvictable = previous;
            }
        };

        // implements Queue

        @Override
        public boolean offer(ReferenceEntry entry) {
            // unlink
            connectEvictables(entry.getPreviousEvictable(), entry.getNextEvictable());

            // add to tail
            connectEvictables(head.getPreviousEvictable(), entry);
            connectEvictables(entry, head);

            return true;
        }

        @Override
        public ReferenceEntry peek() {
            ReferenceEntry next = head.getNextEvictable();
            return (next == head) ? null : next;
        }

        @Override
        public ReferenceEntry poll() {
            ReferenceEntry next = head.getNextEvictable();
            if (next == head) {
                return null;
            }

            remove(next);
            return next;
        }

        @Override
        @SuppressWarnings("unchecked")
        public boolean remove(Object o) {
            ReferenceEntry e = (ReferenceEntry) o;
            ReferenceEntry previous = e.getPreviousEvictable();
            ReferenceEntry next = e.getNextEvictable();
            connectEvictables(previous, next);
            nullifyEvictable(e);

            return next != NullEntry.INSTANCE;
        }

        @Override
        @SuppressWarnings("unchecked")
        public boolean contains(Object o) {
            ReferenceEntry e = (ReferenceEntry) o;
            return e.getNextEvictable() != NullEntry.INSTANCE;
        }

        @Override
        public boolean isEmpty() {
            return head.getNextEvictable() == head;
        }

        @Override
        public int size() {
            int size = 0;
            for (ReferenceEntry e = head.getNextEvictable(); e != head; e = e.getNextEvictable()) {
                size++;
            }
            return size;
        }

        @Override
        public void clear() {
            ReferenceEntry e = head.getNextEvictable();
            while (e != head) {
                ReferenceEntry next = e.getNextEvictable();
                nullifyEvictable(e);
                e = next;
            }

            head.setNextEvictable(head);
            head.setPreviousEvictable(head);
        }

        @Override
        public Iterator> iterator() {
            return new AbstractSequentialIterator>(peek()) {
                @Override
                protected ReferenceEntry computeNext(ReferenceEntry previous) {
                    ReferenceEntry next = previous.getNextEvictable();
                    return (next == head) ? null : next;
                }
            };
        }
    }

    // Iterator Support

    /**
     * A custom queue for managing expiration order. Note that this is tightly integrated with
     * {@code ReferenceEntry}, upon which it reliese to perform its linking.
     * 

     * 
Note that this entire implementation makes the assumption that all elements which are in
     * the map are also in this queue, and that all elements not in the queue are not in the map.
     * 

     * 
The benefits of creating our own queue are that (1) we can replace elements in the middle
     * of the queue as part of copyEvictableEntry, and (2) the contains method is highly optimized
     * for the current model.
     */
    static final class ExpirationQueue extends AbstractQueue> {
        final ReferenceEntry head = new AbstractReferenceEntry() {

            ReferenceEntry nextExpirable = this;
            ReferenceEntry previousExpirable = this;

            @Override
            public long getExpirationTime() {
                return Long.MAX_VALUE;
            }

            @Override
            public void setExpirationTime(long time) {
            }

            @Override
            public ReferenceEntry getNextExpirable() {
                return nextExpirable;
            }

            @Override
            public void setNextExpirable(ReferenceEntry next) {
                this.nextExpirable = next;
            }

            @Override
            public ReferenceEntry getPreviousExpirable() {
                return previousExpirable;
            }

            @Override
            public void setPreviousExpirable(ReferenceEntry previous) {
                this.previousExpirable = previous;
            }
        };

        // implements Queue

        @Override
        public boolean offer(ReferenceEntry entry) {
            // unlink
            connectExpirables(entry.getPreviousExpirable(), entry.getNextExpirable());

            // add to tail
            connectExpirables(head.getPreviousExpirable(), entry);
            connectExpirables(entry, head);

            return true;
        }

        @Override
        public ReferenceEntry peek() {
            ReferenceEntry next = head.getNextExpirable();
            return (next == head) ? null : next;
        }

        @Override
        public ReferenceEntry poll() {
            ReferenceEntry next = head.getNextExpirable();
            if (next == head) {
                return null;
            }

            remove(next);
            return next;
        }

        @Override
        @SuppressWarnings("unchecked")
        public boolean remove(Object o) {
            ReferenceEntry e = (ReferenceEntry) o;
            ReferenceEntry previous = e.getPreviousExpirable();
            ReferenceEntry next = e.getNextExpirable();
            connectExpirables(previous, next);
            nullifyExpirable(e);

            return next != NullEntry.INSTANCE;
        }

        @Override
        @SuppressWarnings("unchecked")
        public boolean contains(Object o) {
            ReferenceEntry e = (ReferenceEntry) o;
            return e.getNextExpirable() != NullEntry.INSTANCE;
        }

        @Override
        public boolean isEmpty() {
            return head.getNextExpirable() == head;
        }

        @Override
        public int size() {
            int size = 0;
            for (ReferenceEntry e = head.getNextExpirable(); e != head; e = e.getNextExpirable()) {
                size++;
            }
            return size;
        }

        @Override
        public void clear() {
            ReferenceEntry e = head.getNextExpirable();
            while (e != head) {
                ReferenceEntry next = e.getNextExpirable();
                nullifyExpirable(e);
                e = next;
            }

            head.setNextExpirable(head);
            head.setPreviousExpirable(head);
        }

        @Override
        public Iterator> iterator() {
            return new AbstractSequentialIterator>(peek()) {
                @Override
                protected ReferenceEntry computeNext(ReferenceEntry previous) {
                    ReferenceEntry next = previous.getNextExpirable();
                    return (next == head) ? null : next;
                }
            };
        }
    }

    abstract class HashIterator implements Iterator {

        int nextSegmentIndex;
        int nextTableIndex;
        Segment currentSegment;
        AtomicReferenceArray> currentTable;
        ReferenceEntry nextEntry;
        WriteThroughEntry nextExternal;
        WriteThroughEntry lastReturned;

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

        @Override
        public abstract E next();

        final void advance() {
            nextExternal = null;

            if (nextInChain()) {
                return;
            }

            if (nextInTable()) {
                return;
            }

            while (nextSegmentIndex >= 0) {
                currentSegment = segments[nextSegmentIndex--];
                if (currentSegment.count != 0) {
                    currentTable = currentSegment.table;
                    nextTableIndex = currentTable.length() - 1;
                    if (nextInTable()) {
                        return;
                    }
                }
            }
        }

        /**
         * Finds the next entry in the current chain. Returns {@code true} if an entry was found.
         */
        boolean nextInChain() {
            if (nextEntry != null) {
                for (nextEntry = nextEntry.getNext(); nextEntry != null; nextEntry = nextEntry.getNext()) {
                    if (advanceTo(nextEntry)) {
                        return true;
                    }
                }
            }
            return false;
        }

        /**
         * Finds the next entry in the current table. Returns {@code true} if an entry was found.
         */
        boolean nextInTable() {
            while (nextTableIndex >= 0) {
                if ((nextEntry = currentTable.get(nextTableIndex--)) != null) {
                    if (advanceTo(nextEntry) || nextInChain()) {
                        return true;
                    }
                }
            }
            return false;
        }

        /**
         * Advances to the given entry. Returns {@code true} if the entry was valid, {@code false} if it
         * should be skipped.
         */
        boolean advanceTo(ReferenceEntry entry) {
            try {
                K key = entry.getKey();
                V value = getLiveValue(entry);
                if (value != null) {
                    nextExternal = new WriteThroughEntry(key, value);
                    return true;
                } else {
                    // Skip stale entry.
                    return false;
                }
            } finally {
                currentSegment.postReadCleanup();
            }
        }

        @Override
        public boolean hasNext() {
            return nextExternal != null;
        }

        WriteThroughEntry nextEntry() {
            if (nextExternal == null) {
                throw new NoSuchElementException();
            }
            lastReturned = nextExternal;
            advance();
            return lastReturned;
        }

        @Override
        public void remove() {
            CollectPreconditions.checkRemove(lastReturned != null);
            MapMakerInternalMap.this.remove(lastReturned.getKey());
            lastReturned = null;
        }
    }

    final class KeyIterator extends HashIterator {

        @Override
        public K next() {
            return nextEntry().getKey();
        }
    }

    final class ValueIterator extends HashIterator {

        @Override
        public V next() {
            return nextEntry().getValue();
        }
    }

    /**
     * Custom Entry class used by EntryIterator.next(), that relays setValue changes to the
     * underlying map.
     */
    final class WriteThroughEntry extends AbstractMapEntry {
        final K key; // non-null
        V value; // non-null

        WriteThroughEntry(K key, V value) {
            this.key = key;
            this.value = value;
        }

        @Override
        public K getKey() {
            return key;
        }

        @Override
        public V getValue() {
            return value;
        }

        @Override
        public boolean equals(Object object) {
            // Cannot use key and value equivalence
            if (object instanceof Entry) {
                Entry that = (Entry) object;
                return key.equals(that.getKey()) && value.equals(that.getValue());
            }
            return false;
        }

        @Override
        public int hashCode() {
            // Cannot use key and value equivalence
            return key.hashCode() ^ value.hashCode();
        }

        @Override
        public V setValue(V newValue) {
            V oldValue = put(key, newValue);
            value = newValue; // only if put succeeds
            return oldValue;
        }
    }

    final class EntryIterator extends HashIterator> {

        @Override
        public Entry next() {
            return nextEntry();
        }
    }

    private final class KeySet extends AbstractSet {

        @Override
        public Iterator iterator() {
            return new KeyIterator();
        }

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

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

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

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

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

    private final class Values extends AbstractCollection {

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

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

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

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

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

    // Serialization Support

    private final class EntrySet extends AbstractSet> {

        @Override
        public Iterator> iterator() {
            return new EntryIterator();
        }

        @Override
        public boolean contains(Object o) {
            if (!(o instanceof Entry)) {
                return false;
            }
            Entry e = (Entry) o;
            Object key = e.getKey();
            if (key == null) {
                return false;
            }
            V v = MapMakerInternalMap.this.get(key);

            return v != null && valueEquivalence.equivalent(e.getValue(), v);
        }

        @Override
        public boolean remove(Object o) {
            if (!(o instanceof Entry)) {
                return false;
            }
            Entry e = (Entry) o;
            Object key = e.getKey();
            return key != null && MapMakerInternalMap.this.remove(key, e.getValue());
        }

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

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

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

}
 















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