/*
    * 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 com.google.common.cache;
  
  import static com.google.common.base.Preconditions.checkNotNull;
  import static com.google.common.base.Preconditions.checkState;
  import static com.google.common.cache.CacheBuilder.NULL_TICKER;
  import static com.google.common.cache.CacheBuilder.UNSET_INT;
  import static com.google.common.util.concurrent.Uninterruptibles.getUninterruptibly;
  import static java.util.concurrent.TimeUnit.NANOSECONDS;
  
  import com.google.common.annotations.GwtCompatible;
  import com.google.common.annotations.GwtIncompatible;
  import com.google.common.annotations.VisibleForTesting;
  import com.google.common.base.Equivalence;
  import com.google.common.base.Function;
  import com.google.common.base.Stopwatch;
  import com.google.common.base.Ticker;
  import com.google.common.cache.AbstractCache.SimpleStatsCounter;
  import com.google.common.cache.AbstractCache.StatsCounter;
  import com.google.common.cache.CacheBuilder.NullListener;
  import com.google.common.cache.CacheBuilder.OneWeigher;
  import com.google.common.cache.CacheLoader.InvalidCacheLoadException;
  import com.google.common.cache.CacheLoader.UnsupportedLoadingOperationException;
  import com.google.common.collect.AbstractSequentialIterator;
  import com.google.common.collect.ImmutableMap;
  import com.google.common.collect.Iterators;
  import com.google.common.collect.Maps;
  import com.google.common.collect.Sets;
  import com.google.common.primitives.Ints;
  import com.google.common.util.concurrent.ExecutionError;
  import com.google.common.util.concurrent.Futures;
  import com.google.common.util.concurrent.ListenableFuture;
  import com.google.common.util.concurrent.ListeningExecutorService;
  import com.google.common.util.concurrent.MoreExecutors;
  import com.google.common.util.concurrent.SettableFuture;
  import com.google.common.util.concurrent.UncheckedExecutionException;
  import com.google.common.util.concurrent.Uninterruptibles;
  
  import java.io.IOException;
  import java.io.ObjectInputStream;
  import java.io.Serializable;
  import java.lang.ref.Reference;
  import java.lang.ref.ReferenceQueue;
  import java.lang.ref.SoftReference;
  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.Map.Entry;
  import java.util.NoSuchElementException;
  import java.util.Queue;
  import java.util.Set;
  import java.util.concurrent.Callable;
  import java.util.concurrent.ConcurrentLinkedQueue;
  import java.util.concurrent.ConcurrentMap;
  import java.util.concurrent.ExecutionException;
  import java.util.concurrent.TimeUnit;
  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 javax.annotation.Nullable;
  import javax.annotation.concurrent.GuardedBy;
  
  /**
   * The concurrent hash map implementation built by {@link CacheBuilder}.
   *
   * <p>This implementation is heavily derived from revision 1.96 of <a
   * href="http://tinyurl.com/ConcurrentHashMap">ConcurrentHashMap.java</a>.
   *
   * @author Charles Fry
   * @author Bob Lee ({@code com.google.common.collect.MapMaker})
   * @author Doug Lea ({@code ConcurrentHashMap})
   */
  @GwtCompatible(emulated = true)
  class LocalCache<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V> {
  
    /*
    * 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.
    */
   static final int MAXIMUM_CAPACITY = 1 << 30;
 
   /** The maximum number of segments to allow; used to bound constructor arguments. */
   static final int MAX_SEGMENTS = 1 << 16; // slightly conservative
 
   /** Number of (unsynchronized) retries in the containsValue method. */
   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.
    *
    * <p>This must be a (2^n)-1 as it is used as a mask.
    */
   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
   static final int DRAIN_MAX = 16;
 
   // Fields
 
   static final Logger logger = Logger.getLogger(LocalCache.class.getName());
 
   static final ListeningExecutorService sameThreadExecutor = MoreExecutors.sameThreadExecutor();
 
   /**
    * Mask value for indexing into segments. The upper bits of a key's hash code are used to choose
    * the segment.
    */
   final int segmentMask;
 
   /**
    * Shift value for indexing within segments. Helps prevent entries that end up in the same segment
    * from also ending up in the same bucket.
    */
   final int segmentShift;
 
   /** The segments, each of which is a specialized hash table. */
   final Segment<K, V>[] segments;
 
   /** The concurrency level. */
   final int concurrencyLevel;
 
   /** Strategy for comparing keys. */
   final Equivalence<Object> keyEquivalence;
 
   /** Strategy for comparing values. */
   final Equivalence<Object> valueEquivalence;
 
   /** Strategy for referencing keys. */
   final Strength keyStrength;
 
   /** Strategy for referencing values. */
   final Strength valueStrength;
 
   /** The maximum weight of this map. UNSET_INT if there is no maximum. */
   final long maxWeight;
 
   /** Weigher to weigh cache entries. */
   final Weigher<K, V> weigher;
 
   /** How long after the last access to an entry the map will retain that entry. */
   final long expireAfterAccessNanos;
 
   /** How long after the last write to an entry the map will retain that entry. */
   final long expireAfterWriteNanos;
 
   /** How long after the last write an entry becomes a candidate for refresh. */
   final long refreshNanos;
 
   /** Entries waiting to be consumed by the removal listener. */
   // TODO(fry): define a new type which creates event objects and automates the clear logic
   final Queue<RemovalNotification<K, V>> removalNotificationQueue;
 
   /**
    * A listener that is invoked when an entry is removed due to expiration or garbage collection of
    * soft/weak entries.
    */
   final RemovalListener<K, V> removalListener;
 
   /** Measures time in a testable way. */
   final Ticker ticker;
 
   /** Factory used to create new entries. */
   final EntryFactory entryFactory;
 
   /**
    * Accumulates global cache statistics. Note that there are also per-segments stats counters
    * which must be aggregated to obtain a global stats view.
    */
   final StatsCounter globalStatsCounter;
 
   /**
    * The default cache loader to use on loading operations.
    */
   @Nullable
   final CacheLoader<? super K, V> defaultLoader;
 
   /**
    * Creates a new, empty map with the specified strategy, initial capacity and concurrency level.
    */
   LocalCache(
       CacheBuilder<? super K, ? super V> builder, @Nullable CacheLoader<? super K, V> loader) {
     concurrencyLevel = Math.min(builder.getConcurrencyLevel(), MAX_SEGMENTS);
 
     keyStrength = builder.getKeyStrength();
     valueStrength = builder.getValueStrength();
 
     keyEquivalence = builder.getKeyEquivalence();
     valueEquivalence = builder.getValueEquivalence();
 
     maxWeight = builder.getMaximumWeight();
     weigher = builder.getWeigher();
     expireAfterAccessNanos = builder.getExpireAfterAccessNanos();
     expireAfterWriteNanos = builder.getExpireAfterWriteNanos();
     refreshNanos = builder.getRefreshNanos();
 
     removalListener = builder.getRemovalListener();
     removalNotificationQueue = (removalListener == NullListener.INSTANCE)
         ? LocalCache.<RemovalNotification<K, V>>discardingQueue()
         : new ConcurrentLinkedQueue<RemovalNotification<K, V>>();
 
     ticker = builder.getTicker(recordsTime());
     entryFactory = EntryFactory.getFactory(keyStrength, usesAccessEntries(), usesWriteEntries());
     globalStatsCounter = builder.getStatsCounterSupplier().get();
     defaultLoader = loader;
 
     int initialCapacity = Math.min(builder.getInitialCapacity(), MAXIMUM_CAPACITY);
     if (evictsBySize() && !customWeigher()) {
       initialCapacity = Math.min(initialCapacity, (int) maxWeight);
     }
 
     // Find the lowest power-of-two segmentCount that exceeds concurrencyLevel, unless
     // maximumSize/Weight is specified in which case ensure that each segment gets at least 10
     // entries. The special casing for size-based eviction is only necessary because that eviction
     // happens per segment instead of globally, so too many segments compared to the maximum size
     // will result in random eviction behavior.
     int segmentShift = 0;
     int segmentCount = 1;
     while (segmentCount < concurrencyLevel
            && (!evictsBySize() || segmentCount * 20 <= maxWeight)) {
       ++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 weights = overall max weights
       long maxSegmentWeight = maxWeight / segmentCount + 1;
       long remainder = maxWeight % segmentCount;
       for (int i = 0; i < this.segments.length; ++i) {
         if (i == remainder) {
           maxSegmentWeight--;
         }
         this.segments[i] =
             createSegment(segmentSize, maxSegmentWeight, builder.getStatsCounterSupplier().get());
       }
     } else {
       for (int i = 0; i < this.segments.length; ++i) {
         this.segments[i] =
             createSegment(segmentSize, UNSET_INT, builder.getStatsCounterSupplier().get());
       }
     }
   }
 
   boolean evictsBySize() {
     return maxWeight >= 0;
   }
 
   boolean customWeigher() {
     return weigher != OneWeigher.INSTANCE;
   }
 
   boolean expires() {
     return expiresAfterWrite() || expiresAfterAccess();
   }
 
   boolean expiresAfterWrite() {
     return expireAfterWriteNanos > 0;
   }
 
   boolean expiresAfterAccess() {
     return expireAfterAccessNanos > 0;
   }
 
   boolean refreshes() {
     return refreshNanos > 0;
   }
 
   boolean usesAccessQueue() {
     return expiresAfterAccess() || evictsBySize();
   }
 
   boolean usesWriteQueue() {
     return expiresAfterWrite();
   }
 
   boolean recordsWrite() {
     return expiresAfterWrite() || refreshes();
   }
 
   boolean recordsAccess() {
     return expiresAfterAccess();
   }
 
   boolean recordsTime() {
     return recordsWrite() || recordsAccess();
   }
 
   boolean usesWriteEntries() {
     return usesWriteQueue() || recordsWrite();
   }
 
   boolean usesAccessEntries() {
     return usesAccessQueue() || recordsAccess();
   }
 
   boolean usesKeyReferences() {
     return keyStrength != Strength.STRONG;
   }
 
   boolean usesValueReferences() {
     return valueStrength != Strength.STRONG;
   }
 
   enum Strength {
     /*
      * TODO(kevinb): If we strongly reference the value and aren't loading, we needn't wrap the
      * value. This could save ~8 bytes per entry.
      */
 
     STRONG {
       @Override
       <K, V> ValueReference<K, V> referenceValue(
           Segment<K, V> segment, ReferenceEntry<K, V> entry, V value, int weight) {
         return (weight == 1)
             ? new StrongValueReference<K, V>(value)
             : new WeightedStrongValueReference<K, V>(value, weight);
       }
 
       @Override
       Equivalence<Object> defaultEquivalence() {
         return Equivalence.equals();
       }
     },
 
     SOFT {
       @Override
       <K, V> ValueReference<K, V> referenceValue(
           Segment<K, V> segment, ReferenceEntry<K, V> entry, V value, int weight) {
         return (weight == 1)
             ? new SoftValueReference<K, V>(segment.valueReferenceQueue, value, entry)
             : new WeightedSoftValueReference<K, V>(
                 segment.valueReferenceQueue, value, entry, weight);
       }
 
       @Override
       Equivalence<Object> defaultEquivalence() {
         return Equivalence.identity();
       }
     },
 
     WEAK {
       @Override
       <K, V> ValueReference<K, V> referenceValue(
           Segment<K, V> segment, ReferenceEntry<K, V> entry, V value, int weight) {
         return (weight == 1)
             ? new WeakValueReference<K, V>(segment.valueReferenceQueue, value, entry)
             : new WeightedWeakValueReference<K, V>(
                 segment.valueReferenceQueue, value, entry, weight);
       }
 
       @Override
       Equivalence<Object> defaultEquivalence() {
         return Equivalence.identity();
       }
     };
 
     /**
      * Creates a reference for the given value according to this value strength.
      */
     abstract <K, V> ValueReference<K, V> referenceValue(
         Segment<K, V> segment, ReferenceEntry<K, V> entry, V value, int weight);
 
     /**
      * 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<Object> defaultEquivalence();
   }
 
   /**
    * Creates new entries.
    */
   enum EntryFactory {
     STRONG {
       @Override
       <K, V> ReferenceEntry<K, V> newEntry(
           Segment<K, V> segment, K key, int hash, @Nullable ReferenceEntry<K, V> next) {
         return new StrongEntry<K, V>(key, hash, next);
       }
     },
     STRONG_ACCESS {
       @Override
       <K, V> ReferenceEntry<K, V> newEntry(
           Segment<K, V> segment, K key, int hash, @Nullable ReferenceEntry<K, V> next) {
         return new StrongAccessEntry<K, V>(key, hash, next);
       }
 
       @Override
       <K, V> ReferenceEntry<K, V> copyEntry(
           Segment<K, V> segment, ReferenceEntry<K, V> original, ReferenceEntry<K, V> newNext) {
         ReferenceEntry<K, V> newEntry = super.copyEntry(segment, original, newNext);
         copyAccessEntry(original, newEntry);
         return newEntry;
       }
     },
     STRONG_WRITE {
       @Override
       <K, V> ReferenceEntry<K, V> newEntry(
           Segment<K, V> segment, K key, int hash, @Nullable ReferenceEntry<K, V> next) {
         return new StrongWriteEntry<K, V>(key, hash, next);
       }
 
       @Override
       <K, V> ReferenceEntry<K, V> copyEntry(
           Segment<K, V> segment, ReferenceEntry<K, V> original, ReferenceEntry<K, V> newNext) {
         ReferenceEntry<K, V> newEntry = super.copyEntry(segment, original, newNext);
         copyWriteEntry(original, newEntry);
         return newEntry;
       }
     },
     STRONG_ACCESS_WRITE {
       @Override
       <K, V> ReferenceEntry<K, V> newEntry(
           Segment<K, V> segment, K key, int hash, @Nullable ReferenceEntry<K, V> next) {
         return new StrongAccessWriteEntry<K, V>(key, hash, next);
       }
 
       @Override
       <K, V> ReferenceEntry<K, V> copyEntry(
           Segment<K, V> segment, ReferenceEntry<K, V> original, ReferenceEntry<K, V> newNext) {
         ReferenceEntry<K, V> newEntry = super.copyEntry(segment, original, newNext);
         copyAccessEntry(original, newEntry);
         copyWriteEntry(original, newEntry);
         return newEntry;
       }
     },
 
     WEAK {
       @Override
       <K, V> ReferenceEntry<K, V> newEntry(
           Segment<K, V> segment, K key, int hash, @Nullable ReferenceEntry<K, V> next) {
         return new WeakEntry<K, V>(segment.keyReferenceQueue, key, hash, next);
       }
     },
     WEAK_ACCESS {
       @Override
       <K, V> ReferenceEntry<K, V> newEntry(
           Segment<K, V> segment, K key, int hash, @Nullable ReferenceEntry<K, V> next) {
         return new WeakAccessEntry<K, V>(segment.keyReferenceQueue, key, hash, next);
       }
 
       @Override
       <K, V> ReferenceEntry<K, V> copyEntry(
           Segment<K, V> segment, ReferenceEntry<K, V> original, ReferenceEntry<K, V> newNext) {
         ReferenceEntry<K, V> newEntry = super.copyEntry(segment, original, newNext);
         copyAccessEntry(original, newEntry);
         return newEntry;
       }
     },
     WEAK_WRITE {
       @Override
       <K, V> ReferenceEntry<K, V> newEntry(
           Segment<K, V> segment, K key, int hash, @Nullable ReferenceEntry<K, V> next) {
         return new WeakWriteEntry<K, V>(segment.keyReferenceQueue, key, hash, next);
       }
 
       @Override
       <K, V> ReferenceEntry<K, V> copyEntry(
           Segment<K, V> segment, ReferenceEntry<K, V> original, ReferenceEntry<K, V> newNext) {
         ReferenceEntry<K, V> newEntry = super.copyEntry(segment, original, newNext);
         copyWriteEntry(original, newEntry);
         return newEntry;
       }
     },
     WEAK_ACCESS_WRITE {
       @Override
       <K, V> ReferenceEntry<K, V> newEntry(
           Segment<K, V> segment, K key, int hash, @Nullable ReferenceEntry<K, V> next) {
         return new WeakAccessWriteEntry<K, V>(segment.keyReferenceQueue, key, hash, next);
       }
 
       @Override
       <K, V> ReferenceEntry<K, V> copyEntry(
           Segment<K, V> segment, ReferenceEntry<K, V> original, ReferenceEntry<K, V> newNext) {
         ReferenceEntry<K, V> newEntry = super.copyEntry(segment, original, newNext);
         copyAccessEntry(original, newEntry);
         copyWriteEntry(original, newEntry);
         return newEntry;
       }
     };
 
     /**
      * Masks used to compute indices in the following table.
      */
     static final int ACCESS_MASK = 1;
     static final int WRITE_MASK = 2;
     static final int WEAK_MASK = 4;
 
     /**
      * Look-up table for factories.
      */
     static final EntryFactory[] factories = {
       STRONG, STRONG_ACCESS, STRONG_WRITE, STRONG_ACCESS_WRITE,
       WEAK, WEAK_ACCESS, WEAK_WRITE, WEAK_ACCESS_WRITE,
     };
 
     static EntryFactory getFactory(Strength keyStrength, boolean usesAccessQueue,
         boolean usesWriteQueue) {
       int flags = ((keyStrength == Strength.WEAK) ? WEAK_MASK : 0)
           | (usesAccessQueue ? ACCESS_MASK : 0)
           | (usesWriteQueue ? WRITE_MASK : 0);
       return factories[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 <K, V> ReferenceEntry<K, V> newEntry(
         Segment<K, V> segment, K key, int hash, @Nullable ReferenceEntry<K, V> next);
 
     /**
      * Copies an entry, assigning it a new {@code next} entry.
      *
      * @param original the entry to copy
      * @param newNext entry in the same bucket
      */
     @GuardedBy("Segment.this")
     <K, V> ReferenceEntry<K, V> copyEntry(
         Segment<K, V> segment, ReferenceEntry<K, V> original, ReferenceEntry<K, V> newNext) {
       return newEntry(segment, original.getKey(), original.getHash(), newNext);
     }
 
     @GuardedBy("Segment.this")
     <K, V> void copyAccessEntry(ReferenceEntry<K, V> original, ReferenceEntry<K, V> newEntry) {
       // TODO(fry): when we link values instead of entries this method can go
       // away, as can connectAccessOrder, nullifyAccessOrder.
       newEntry.setAccessTime(original.getAccessTime());
 
       connectAccessOrder(original.getPreviousInAccessQueue(), newEntry);
       connectAccessOrder(newEntry, original.getNextInAccessQueue());
 
       nullifyAccessOrder(original);
     }
 
     @GuardedBy("Segment.this")
     <K, V> void copyWriteEntry(ReferenceEntry<K, V> original, ReferenceEntry<K, V> newEntry) {
       // TODO(fry): when we link values instead of entries this method can go
       // away, as can connectWriteOrder, nullifyWriteOrder.
       newEntry.setWriteTime(original.getWriteTime());
 
       connectWriteOrder(original.getPreviousInWriteQueue(), newEntry);
       connectWriteOrder(newEntry, original.getNextInWriteQueue());
 
       nullifyWriteOrder(original);
     }
   }
 
   /**
    * A reference to a value.
    */
   interface ValueReference<K, V> {
     /**
      * Returns the value. Does not block or throw exceptions.
      */
     @Nullable
     V get();
 
     /**
      * Waits for a value that may still be loading. Unlike get(), this method can block (in the
      * case of FutureValueReference).
      *
      * @throws ExecutionException if the loading thread throws an exception
      * @throws ExecutionError if the loading thread throws an error
      */
     V waitForValue() throws ExecutionException;
 
     /**
      * Returns the weight of this entry. This is assumed to be static between calls to setValue.
      */
     int getWeight();
 
     /**
      * Returns the entry associated with this value reference, or {@code null} if this value
      * reference is independent of any entry.
      */
     @Nullable
     ReferenceEntry<K, V> getEntry();
 
     /**
      * Creates a copy of this reference for the given entry.
      *
      * <p>{@code value} may be null only for a loading reference.
      */
     ValueReference<K, V> copyFor(
         ReferenceQueue<V> queue, @Nullable V value, ReferenceEntry<K, V> entry);
 
     /**
      * Notifify pending loads that a new value was set. This is only relevant to loading
      * value references.
      */
     void notifyNewValue(@Nullable V newValue);
 
     /**
      * Returns true if a new value is currently loading, regardless of whether or not there is an
      * existing value. It is assumed that the return value of this method is constant for any given
      * ValueReference instance.
      */
     boolean isLoading();
 
     /**
      * Returns true if this reference contains an active value, meaning one that is still considered
      * present in the cache. Active values consist of live values, which are returned by cache
      * lookups, and dead values, which have been evicted but awaiting removal. Non-active values
      * consist strictly of loading values, though during refresh a value may be both active and
      * loading.
      */
     boolean isActive();
   }
 
   /**
    * Placeholder. Indicates that the value hasn't been set yet.
    */
   static final ValueReference<Object, Object> UNSET = new ValueReference<Object, Object>() {
     @Override
     public Object get() {
       return null;
     }
 
     @Override
     public int getWeight() {
       return 0;
     }
 
     @Override
     public ReferenceEntry<Object, Object> getEntry() {
       return null;
     }
 
     @Override
     public ValueReference<Object, Object> copyFor(ReferenceQueue<Object> queue,
         @Nullable Object value, ReferenceEntry<Object, Object> entry) {
       return this;
     }
 
     @Override
     public boolean isLoading() {
       return false;
     }
 
     @Override
     public boolean isActive() {
       return false;
     }
 
     @Override
     public Object waitForValue() {
       return null;
     }
 
     @Override
     public void notifyNewValue(Object newValue) {}
   };
 
   /**
    * Singleton placeholder that indicates a value is being loaded.
    */
   @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
   static <K, V> ValueReference<K, V> unset() {
     return (ValueReference<K, V>) UNSET;
   }
 
   /**
    * An entry in a reference map.
    *
    * Entries in the map can be in the following states:
    *
    * Valid:
    * - Live: valid key/value are set
    * - Loading: loading is pending
    *
    * Invalid:
    * - Expired: time expired (key/value may still be set)
    * - Collected: key/value was partially collected, but not yet cleaned up
    * - Unset: marked as unset, awaiting cleanup or reuse
    */
   interface ReferenceEntry<K, V> {
     /**
      * Returns the value reference from this entry.
      */
     ValueReference<K, V> getValueReference();
 
     /**
      * Sets the value reference for this entry.
      */
     void setValueReference(ValueReference<K, V> valueReference);
 
     /**
      * Returns the next entry in the chain.
      */
     @Nullable
     ReferenceEntry<K, V> getNext();
 
     /**
      * Returns the entry's hash.
      */
     int getHash();
 
     /**
      * Returns the key for this entry.
      */
     @Nullable
     K getKey();
 
     /*
      * Used by entries that use access order. Access 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.
      */
 
     /**
      * Returns the time that this entry was last accessed, in ns.
      */
     long getAccessTime();
 
     /**
      * Sets the entry access time in ns.
      */
     void setAccessTime(long time);
 
     /**
      * Returns the next entry in the access queue.
      */
     ReferenceEntry<K, V> getNextInAccessQueue();
 
     /**
      * Sets the next entry in the access queue.
      */
     void setNextInAccessQueue(ReferenceEntry<K, V> next);
 
     /**
      * Returns the previous entry in the access queue.
      */
     ReferenceEntry<K, V> getPreviousInAccessQueue();
 
     /**
      * Sets the previous entry in the access queue.
      */
     void setPreviousInAccessQueue(ReferenceEntry<K, V> previous);
 
     /*
      * Implemented by entries that use write order. Write 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.
      */
 
     /**
      * Returns the time that this entry was last written, in ns.
      */
     long getWriteTime();
 
     /**
      * Sets the entry write time in ns.
      */
     void setWriteTime(long time);
 
     /**
      * Returns the next entry in the write queue.
      */
     ReferenceEntry<K, V> getNextInWriteQueue();
 
     /**
      * Sets the next entry in the write queue.
      */
     void setNextInWriteQueue(ReferenceEntry<K, V> next);
 
     /**
      * Returns the previous entry in the write queue.
      */
     ReferenceEntry<K, V> getPreviousInWriteQueue();
 
     /**
      * Sets the previous entry in the write queue.
      */
     void setPreviousInWriteQueue(ReferenceEntry<K, V> previous);
   }
 
   private enum NullEntry implements ReferenceEntry<Object, Object> {
     INSTANCE;
 
     @Override
     public ValueReference<Object, Object> getValueReference() {
       return null;
     }
 
     @Override
     public void setValueReference(ValueReference<Object, Object> valueReference) {}
 
     @Override
     public ReferenceEntry<Object, Object> getNext() {
       return null;
     }
 
     @Override
     public int getHash() {
       return 0;
     }
 
     @Override
     public Object getKey() {
       return null;
     }
 
     @Override
     public long getAccessTime() {
       return 0;
     }
 
     @Override
     public void setAccessTime(long time) {}
 
     @Override
     public ReferenceEntry<Object, Object> getNextInAccessQueue() {
       return this;
     }
 
     @Override
     public void setNextInAccessQueue(ReferenceEntry<Object, Object> next) {}
 
     @Override
     public ReferenceEntry<Object, Object> getPreviousInAccessQueue() {
       return this;
     }
 
     @Override
     public void setPreviousInAccessQueue(ReferenceEntry<Object, Object> previous) {}
 
     @Override
     public long getWriteTime() {
       return 0;
     }
 
     @Override
     public void setWriteTime(long time) {}
 
     @Override
     public ReferenceEntry<Object, Object> getNextInWriteQueue() {
       return this;
     }
 
     @Override
     public void setNextInWriteQueue(ReferenceEntry<Object, Object> next) {}
 
     @Override
     public ReferenceEntry<Object, Object> getPreviousInWriteQueue() {
       return this;
     }
 
     @Override
     public void setPreviousInWriteQueue(ReferenceEntry<Object, Object> previous) {}
   }
 
   static abstract class AbstractReferenceEntry<K, V> implements ReferenceEntry<K, V> {
     @Override
     public ValueReference<K, V> getValueReference() {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public void setValueReference(ValueReference<K, V> valueReference) {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public ReferenceEntry<K, V> getNext() {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public int getHash() {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public K getKey() {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public long getAccessTime() {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public void setAccessTime(long time) {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public ReferenceEntry<K, V> getNextInAccessQueue() {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public void setNextInAccessQueue(ReferenceEntry<K, V> next) {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public ReferenceEntry<K, V> getPreviousInAccessQueue() {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public void setPreviousInAccessQueue(ReferenceEntry<K, V> previous) {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public long getWriteTime() {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public void setWriteTime(long time) {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public ReferenceEntry<K, V> getNextInWriteQueue() {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public void setNextInWriteQueue(ReferenceEntry<K, V> next) {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public ReferenceEntry<K, V> getPreviousInWriteQueue() {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public void setPreviousInWriteQueue(ReferenceEntry<K, V> previous) {
       throw new UnsupportedOperationException();
     }
   }
 
   @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
   static <K, V> ReferenceEntry<K, V> nullEntry() {
     return (ReferenceEntry<K, V>) NullEntry.INSTANCE;
   }
 
   static final Queue<? extends Object> DISCARDING_QUEUE = new AbstractQueue<Object>() {
     @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<Object> iterator() {
       return Iterators.emptyIterator();
     }
   };
 
   /**
    * Queue that discards all elements.
    */
   @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
   static <E> Queue<E> discardingQueue() {
     return (Queue) DISCARDING_QUEUE;
   }
 
   /*
    * 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.
    */
 
   /**
    * Used for strongly-referenced keys.
    */
   static class StrongEntry<K, V> extends AbstractReferenceEntry<K, V> {
     final K key;
 
     StrongEntry(K key, int hash, @Nullable ReferenceEntry<K, V> next) {
       this.key = key;
       this.hash = hash;
       this.next = next;
     }
 
     @Override
     public K getKey() {
       return this.key;
     }
 
     // The code below is exactly the same for each entry type.
 
     final int hash;
     final ReferenceEntry<K, V> next;
     volatile ValueReference<K, V> valueReference = unset();
 
     @Override
     public ValueReference<K, V> getValueReference() {
       return valueReference;
     }
 
     @Override
     public void setValueReference(ValueReference<K, V> valueReference) {
       this.valueReference = valueReference;
     }
 
     @Override
     public int getHash() {
       return hash;
     }
 
     @Override
     public ReferenceEntry<K, V> getNext() {
       return next;
     }
   }
 
   static final class StrongAccessEntry<K, V> extends StrongEntry<K, V> {
     StrongAccessEntry(K key, int hash, @Nullable ReferenceEntry<K, V> next) {
       super(key, hash, next);
     }
 
     // The code below is exactly the same for each access entry type.
 
     volatile long accessTime = Long.MAX_VALUE;
 
     @Override
     public long getAccessTime() {
       return accessTime;
     }
 
     @Override
     public void setAccessTime(long time) {
       this.accessTime = time;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> nextAccess = nullEntry();
 
     @Override
     public ReferenceEntry<K, V> getNextInAccessQueue() {
       return nextAccess;
     }
 
     @Override
     public void setNextInAccessQueue(ReferenceEntry<K, V> next) {
       this.nextAccess = next;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> previousAccess = nullEntry();
 
     @Override
     public ReferenceEntry<K, V> getPreviousInAccessQueue() {
       return previousAccess;
     }
 
     @Override
     public void setPreviousInAccessQueue(ReferenceEntry<K, V> previous) {
       this.previousAccess = previous;
     }
   }
 
   static final class StrongWriteEntry<K, V> extends StrongEntry<K, V> {
     StrongWriteEntry(K key, int hash, @Nullable ReferenceEntry<K, V> next) {
       super(key, hash, next);
     }
 
     // The code below is exactly the same for each write entry type.
 
     volatile long writeTime = Long.MAX_VALUE;
 
     @Override
     public long getWriteTime() {
       return writeTime;
     }
 
     @Override
     public void setWriteTime(long time) {
       this.writeTime = time;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> nextWrite = nullEntry();
 
     @Override
     public ReferenceEntry<K, V> getNextInWriteQueue() {
       return nextWrite;
     }
 
     @Override
     public void setNextInWriteQueue(ReferenceEntry<K, V> next) {
       this.nextWrite = next;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> previousWrite = nullEntry();
 
     @Override
     public ReferenceEntry<K, V> getPreviousInWriteQueue() {
       return previousWrite;
     }
 
     @Override
     public void setPreviousInWriteQueue(ReferenceEntry<K, V> previous) {
       this.previousWrite = previous;
     }
   }
 
   static final class StrongAccessWriteEntry<K, V> extends StrongEntry<K, V> {
     StrongAccessWriteEntry(K key, int hash, @Nullable ReferenceEntry<K, V> next) {
       super(key, hash, next);
     }
 
     // The code below is exactly the same for each access entry type.
 
     volatile long accessTime = Long.MAX_VALUE;
 
     @Override
     public long getAccessTime() {
       return accessTime;
     }
 
     @Override
     public void setAccessTime(long time) {
       this.accessTime = time;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> nextAccess = nullEntry();
 
     @Override
     public ReferenceEntry<K, V> getNextInAccessQueue() {
       return nextAccess;
     }
 
     @Override
     public void setNextInAccessQueue(ReferenceEntry<K, V> next) {
       this.nextAccess = next;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> previousAccess = nullEntry();
 
     @Override
     public ReferenceEntry<K, V> getPreviousInAccessQueue() {
       return previousAccess;
     }
 
     @Override
     public void setPreviousInAccessQueue(ReferenceEntry<K, V> previous) {
       this.previousAccess = previous;
     }
 
     // The code below is exactly the same for each write entry type.
 
     volatile long writeTime = Long.MAX_VALUE;
 
     @Override
     public long getWriteTime() {
       return writeTime;
     }
 
     @Override
     public void setWriteTime(long time) {
       this.writeTime = time;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> nextWrite = nullEntry();
 
     @Override
     public ReferenceEntry<K, V> getNextInWriteQueue() {
       return nextWrite;
     }
 
     @Override
     public void setNextInWriteQueue(ReferenceEntry<K, V> next) {
       this.nextWrite = next;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> previousWrite = nullEntry();
 
     @Override
     public ReferenceEntry<K, V> getPreviousInWriteQueue() {
       return previousWrite;
     }
 
     @Override
     public void setPreviousInWriteQueue(ReferenceEntry<K, V> previous) {
       this.previousWrite = previous;
     }
   }
 
   /**
    * Used for weakly-referenced keys.
    */
   static class WeakEntry<K, V> extends WeakReference<K> implements ReferenceEntry<K, V> {
     WeakEntry(ReferenceQueue<K> queue, K key, int hash, @Nullable ReferenceEntry<K, V> next) {
       super(key, queue);
       this.hash = hash;
       this.next = next;
     }
 
     @Override
     public K getKey() {
       return get();
     }
 
     /*
      * It'd be nice to get these for free from AbstractReferenceEntry, but we're already extending
      * WeakReference<K>.
      */
 
     // null access
 
     @Override
     public long getAccessTime() {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public void setAccessTime(long time) {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public ReferenceEntry<K, V> getNextInAccessQueue() {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public void setNextInAccessQueue(ReferenceEntry<K, V> next) {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public ReferenceEntry<K, V> getPreviousInAccessQueue() {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public void setPreviousInAccessQueue(ReferenceEntry<K, V> previous) {
       throw new UnsupportedOperationException();
     }
 
     // null write
 
     @Override
     public long getWriteTime() {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public void setWriteTime(long time) {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public ReferenceEntry<K, V> getNextInWriteQueue() {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public void setNextInWriteQueue(ReferenceEntry<K, V> next) {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public ReferenceEntry<K, V> getPreviousInWriteQueue() {
       throw new UnsupportedOperationException();
     }
 
     @Override
     public void setPreviousInWriteQueue(ReferenceEntry<K, V> previous) {
       throw new UnsupportedOperationException();
     }
 
     // The code below is exactly the same for each entry type.
 
     final int hash;
     final ReferenceEntry<K, V> next;
     volatile ValueReference<K, V> valueReference = unset();
 
     @Override
     public ValueReference<K, V> getValueReference() {
       return valueReference;
     }
 
     @Override
     public void setValueReference(ValueReference<K, V> valueReference) {
       this.valueReference = valueReference;
     }
 
     @Override
     public int getHash() {
       return hash;
     }
 
     @Override
     public ReferenceEntry<K, V> getNext() {
       return next;
     }
   }
 
   static final class WeakAccessEntry<K, V> extends WeakEntry<K, V> {
     WeakAccessEntry(
         ReferenceQueue<K> queue, K key, int hash, @Nullable ReferenceEntry<K, V> next) {
       super(queue, key, hash, next);
     }
 
     // The code below is exactly the same for each access entry type.
 
     volatile long accessTime = Long.MAX_VALUE;
 
     @Override
     public long getAccessTime() {
       return accessTime;
     }
 
     @Override
     public void setAccessTime(long time) {
       this.accessTime = time;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> nextAccess = nullEntry();
 
     @Override
     public ReferenceEntry<K, V> getNextInAccessQueue() {
       return nextAccess;
     }
 
     @Override
     public void setNextInAccessQueue(ReferenceEntry<K, V> next) {
       this.nextAccess = next;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> previousAccess = nullEntry();
 
     @Override
     public ReferenceEntry<K, V> getPreviousInAccessQueue() {
       return previousAccess;
     }
 
     @Override
     public void setPreviousInAccessQueue(ReferenceEntry<K, V> previous) {
       this.previousAccess = previous;
     }
   }
 
   static final class WeakWriteEntry<K, V> extends WeakEntry<K, V> {
     WeakWriteEntry(
         ReferenceQueue<K> queue, K key, int hash, @Nullable ReferenceEntry<K, V> next) {
       super(queue, key, hash, next);
     }
 
     // The code below is exactly the same for each write entry type.
 
     volatile long writeTime = Long.MAX_VALUE;
 
     @Override
     public long getWriteTime() {
       return writeTime;
     }
 
     @Override
     public void setWriteTime(long time) {
       this.writeTime = time;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> nextWrite = nullEntry();
 
     @Override
     public ReferenceEntry<K, V> getNextInWriteQueue() {
       return nextWrite;
     }
 
     @Override
     public void setNextInWriteQueue(ReferenceEntry<K, V> next) {
       this.nextWrite = next;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> previousWrite = nullEntry();
 
     @Override
     public ReferenceEntry<K, V> getPreviousInWriteQueue() {
       return previousWrite;
     }
 
     @Override
     public void setPreviousInWriteQueue(ReferenceEntry<K, V> previous) {
       this.previousWrite = previous;
     }
   }
 
   static final class WeakAccessWriteEntry<K, V> extends WeakEntry<K, V> {
     WeakAccessWriteEntry(
         ReferenceQueue<K> queue, K key, int hash, @Nullable ReferenceEntry<K, V> next) {
       super(queue, key, hash, next);
     }
 
     // The code below is exactly the same for each access entry type.
 
     volatile long accessTime = Long.MAX_VALUE;
 
     @Override
     public long getAccessTime() {
       return accessTime;
     }
 
     @Override
     public void setAccessTime(long time) {
       this.accessTime = time;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> nextAccess = nullEntry();
 
     @Override
     public ReferenceEntry<K, V> getNextInAccessQueue() {
       return nextAccess;
     }
 
     @Override
     public void setNextInAccessQueue(ReferenceEntry<K, V> next) {
       this.nextAccess = next;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> previousAccess = nullEntry();
 
     @Override
     public ReferenceEntry<K, V> getPreviousInAccessQueue() {
       return previousAccess;
     }
 
     @Override
     public void setPreviousInAccessQueue(ReferenceEntry<K, V> previous) {
       this.previousAccess = previous;
     }
 
     // The code below is exactly the same for each write entry type.
 
     volatile long writeTime = Long.MAX_VALUE;
 
     @Override
     public long getWriteTime() {
       return writeTime;
     }
 
     @Override
     public void setWriteTime(long time) {
       this.writeTime = time;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> nextWrite = nullEntry();
 
     @Override
     public ReferenceEntry<K, V> getNextInWriteQueue() {
       return nextWrite;
     }
 
     @Override
     public void setNextInWriteQueue(ReferenceEntry<K, V> next) {
       this.nextWrite = next;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> previousWrite = nullEntry();
 
     @Override
     public ReferenceEntry<K, V> getPreviousInWriteQueue() {
       return previousWrite;
     }
 
     @Override
     public void setPreviousInWriteQueue(ReferenceEntry<K, V> previous) {
       this.previousWrite = previous;
     }
   }
 
   /**
    * References a weak value.
    */
   static class WeakValueReference<K, V>
       extends WeakReference<V> implements ValueReference<K, V> {
     final ReferenceEntry<K, V> entry;
 
     WeakValueReference(ReferenceQueue<V> queue, V referent, ReferenceEntry<K, V> entry) {
       super(referent, queue);
       this.entry = entry;
     }
 
     @Override
     public int getWeight() {
       return 1;
     }
 
     @Override
     public ReferenceEntry<K, V> getEntry() {
       return entry;
     }
 
     @Override
     public void notifyNewValue(V newValue) {}
 
     @Override
     public ValueReference<K, V> copyFor(
         ReferenceQueue<V> queue, V value, ReferenceEntry<K, V> entry) {
       return new WeakValueReference<K, V>(queue, value, entry);
     }
 
     @Override
     public boolean isLoading() {
       return false;
     }
 
     @Override
     public boolean isActive() {
       return true;
     }
 
     @Override
     public V waitForValue() {
       return get();
     }
   }
 
   /**
    * References a soft value.
    */
   static class SoftValueReference<K, V>
       extends SoftReference<V> implements ValueReference<K, V> {
     final ReferenceEntry<K, V> entry;
 
     SoftValueReference(ReferenceQueue<V> queue, V referent, ReferenceEntry<K, V> entry) {
       super(referent, queue);
       this.entry = entry;
     }
 
     @Override
     public int getWeight() {
       return 1;
     }
 
     @Override
     public ReferenceEntry<K, V> getEntry() {
       return entry;
     }
 
     @Override
     public void notifyNewValue(V newValue) {}
 
     @Override
     public ValueReference<K, V> copyFor(
         ReferenceQueue<V> queue, V value, ReferenceEntry<K, V> entry) {
       return new SoftValueReference<K, V>(queue, value, entry);
     }
 
     @Override
     public boolean isLoading() {
       return false;
     }
 
     @Override
     public boolean isActive() {
       return true;
     }
 
     @Override
     public V waitForValue() {
       return get();
     }
   }
 
   /**
    * References a strong value.
    */
   static class StrongValueReference<K, V> implements ValueReference<K, V> {
     final V referent;
 
     StrongValueReference(V referent) {
       this.referent = referent;
     }
 
     @Override
     public V get() {
       return referent;
     }
 
     @Override
     public int getWeight() {
       return 1;
     }
 
     @Override
     public ReferenceEntry<K, V> getEntry() {
       return null;
     }
 
     @Override
     public ValueReference<K, V> copyFor(
         ReferenceQueue<V> queue, V value, ReferenceEntry<K, V> entry) {
       return this;
     }
 
     @Override
     public boolean isLoading() {
       return false;
     }
 
     @Override
     public boolean isActive() {
       return true;
     }
 
     @Override
     public V waitForValue() {
       return get();
     }
 
     @Override
     public void notifyNewValue(V newValue) {}
   }
 
   /**
    * References a weak value.
    */
   static final class WeightedWeakValueReference<K, V> extends WeakValueReference<K, V> {
     final int weight;
 
     WeightedWeakValueReference(ReferenceQueue<V> queue, V referent, ReferenceEntry<K, V> entry,
         int weight) {
       super(queue, referent, entry);
       this.weight = weight;
     }
 
     @Override
     public int getWeight() {
       return weight;
     }
 
     @Override
     public ValueReference<K, V> copyFor(
         ReferenceQueue<V> queue, V value, ReferenceEntry<K, V> entry) {
       return new WeightedWeakValueReference<K, V>(queue, value, entry, weight);
     }
   }
 
   /**
    * References a soft value.
    */
   static final class WeightedSoftValueReference<K, V> extends SoftValueReference<K, V> {
     final int weight;
 
     WeightedSoftValueReference(ReferenceQueue<V> queue, V referent, ReferenceEntry<K, V> entry,
         int weight) {
       super(queue, referent, entry);
       this.weight = weight;
     }
 
     @Override
     public int getWeight() {
       return weight;
     }
     @Override
     public ValueReference<K, V> copyFor(
         ReferenceQueue<V> queue, V value, ReferenceEntry<K, V> entry) {
       return new WeightedSoftValueReference<K, V>(queue, value, entry, weight);
     }
 
   }
 
   /**
    * References a strong value.
    */
   static final class WeightedStrongValueReference<K, V> extends StrongValueReference<K, V> {
     final int weight;
 
     WeightedStrongValueReference(V referent, int weight) {
       super(referent);
       this.weight = weight;
     }
 
     @Override
     public int getWeight() {
       return weight;
     }
   }
 
   /**
    * 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
    */
   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);
   }
 
   /**
    * This method is a convenience for testing. Code should call {@link Segment#newEntry} directly.
    */
   @GuardedBy("Segment.this")
   @VisibleForTesting
   ReferenceEntry<K, V> newEntry(K key, int hash, @Nullable ReferenceEntry<K, V> next) {
     return segmentFor(hash).newEntry(key, hash, next);
   }
 
   /**
    * This method is a convenience for testing. Code should call {@link Segment#copyEntry} directly.
    */
   @GuardedBy("Segment.this")
   @VisibleForTesting
   ReferenceEntry<K, V> copyEntry(ReferenceEntry<K, V> original, ReferenceEntry<K, V> newNext) {
     int hash = original.getHash();
     return segmentFor(hash).copyEntry(original, newNext);
   }
 
   /**
    * This method is a convenience for testing. Code should call {@link Segment#setValue} instead.
    */
   @GuardedBy("Segment.this")
   @VisibleForTesting
   ValueReference<K, V> newValueReference(ReferenceEntry<K, V> entry, V value, int weight) {
     int hash = entry.getHash();
     return valueStrength.referenceValue(segmentFor(hash), entry, checkNotNull(value), weight);
   }
 
   int hash(@Nullable Object key) {
     int h = keyEquivalence.hash(key);
     return rehash(h);
   }
 
   void reclaimValue(ValueReference<K, V> valueReference) {
     ReferenceEntry<K, V> entry = valueReference.getEntry();
     int hash = entry.getHash();
     segmentFor(hash).reclaimValue(entry.getKey(), hash, valueReference);
   }
 
   void reclaimKey(ReferenceEntry<K, V> entry) {
     int hash = entry.getHash();
     segmentFor(hash).reclaimKey(entry, hash);
   }
 
   /**
    * This method is a convenience for testing. Code should call {@link Segment#getLiveValue}
    * instead.
    */
   @VisibleForTesting
   boolean isLive(ReferenceEntry<K, V> entry, long now) {
     return segmentFor(entry.getHash()).getLiveValue(entry, now) != null;
   }
 
   /**
    * 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
    */
   Segment<K, V> segmentFor(int hash) {
     // TODO(fry): Lazily create segments?
     return segments[(hash >>> segmentShift) & segmentMask];
   }
 
   Segment<K, V> createSegment(
       int initialCapacity, long maxSegmentWeight, StatsCounter statsCounter) {
     return new Segment<K, V>(this, initialCapacity, maxSegmentWeight, statsCounter);
   }
 
   /**
    * Gets the value from an entry. Returns null if the entry is invalid, partially-collected,
    * loading, or expired. Unlike {@link Segment#getLiveValue} this method does not attempt to
    * cleanup stale entries. As such it should only be called outside of a segment context, such as
    * during iteration.
    */
   @Nullable
   V getLiveValue(ReferenceEntry<K, V> entry, long now) {
     if (entry.getKey() == null) {
       return null;
     }
     V value = entry.getValueReference().get();
     if (value == null) {
       return null;
     }
 
     if (isExpired(entry, now)) {
       return null;
     }
     return value;
   }
 
   // expiration
 
   /**
    * Returns true if the entry has expired.
    */
   boolean isExpired(ReferenceEntry<K, V> entry, long now) {
     checkNotNull(entry);
     if (expiresAfterAccess()
         && (now - entry.getAccessTime() >= expireAfterAccessNanos)) {
       return true;
     }
     if (expiresAfterWrite()
         && (now - entry.getWriteTime() >= expireAfterWriteNanos)) {
       return true;
     }
     return false;
   }
 
   // queues
 
   @GuardedBy("Segment.this")
   static <K, V> void connectAccessOrder(ReferenceEntry<K, V> previous, ReferenceEntry<K, V> next) {
     previous.setNextInAccessQueue(next);
     next.setPreviousInAccessQueue(previous);
   }
 
   @GuardedBy("Segment.this")
   static <K, V> void nullifyAccessOrder(ReferenceEntry<K, V> nulled) {
     ReferenceEntry<K, V> nullEntry = nullEntry();
     nulled.setNextInAccessQueue(nullEntry);
     nulled.setPreviousInAccessQueue(nullEntry);
   }
 
   @GuardedBy("Segment.this")
   static <K, V> void connectWriteOrder(ReferenceEntry<K, V> previous, ReferenceEntry<K, V> next) {
     previous.setNextInWriteQueue(next);
     next.setPreviousInWriteQueue(previous);
   }
 
   @GuardedBy("Segment.this")
   static <K, V> void nullifyWriteOrder(ReferenceEntry<K, V> nulled) {
     ReferenceEntry<K, V> nullEntry = nullEntry();
     nulled.setNextInWriteQueue(nullEntry);
     nulled.setPreviousInWriteQueue(nullEntry);
   }
 
   /**
    * 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() {
     RemovalNotification<K, V> notification;
     while ((notification = removalNotificationQueue.poll()) != null) {
       try {
         removalListener.onRemoval(notification);
       } catch (Throwable e) {
         logger.log(Level.WARNING, "Exception thrown by removal listener", e);
       }
     }
   }
 
   @SuppressWarnings("unchecked")
   final Segment<K, V>[] newSegmentArray(int ssize) {
     return new Segment[ssize];
   }
 
   // Inner Classes
 
   /**
    * 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<K, V> 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 LocalCache<K, V> map;
 
     /**
      * The number of live elements in this segment's region.
      */
     volatile int count;
 
     /**
      * The weight of the live elements in this segment's region.
      */
     @GuardedBy("Segment.this")
     int totalWeight;
 
     /**
      * 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
      * loading 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<ReferenceEntry<K, V>> table;
 
     /**
      * The maximum weight of this segment. UNSET_INT if there is no maximum.
      */
     final long maxSegmentWeight;
 
     /**
      * The key reference queue contains entries whose keys have been garbage collected, and which
      * need to be cleaned up internally.
      */
     final ReferenceQueue<K> keyReferenceQueue;
 
     /**
      * The value reference queue contains value references whose values have been garbage collected,
      * and which need to be cleaned up internally.
      */
     final ReferenceQueue<V> valueReferenceQueue;
 
     /**
      * The recency queue is used to record which entries were accessed for updating the access
      * 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<ReferenceEntry<K, V>> 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 write time. Elements are added to the
      * tail of the queue on write.
      */
     @GuardedBy("Segment.this")
     final Queue<ReferenceEntry<K, V>> writeQueue;
 
     /**
      * A queue of elements currently in the map, ordered by access time. Elements are added to the
      * tail of the queue on access (note that writes count as accesses).
      */
     @GuardedBy("Segment.this")
     final Queue<ReferenceEntry<K, V>> accessQueue;
 
     /** Accumulates cache statistics. */
     final StatsCounter statsCounter;
 
     Segment(LocalCache<K, V> map, int initialCapacity, long maxSegmentWeight,
         StatsCounter statsCounter) {
       this.map = map;
       this.maxSegmentWeight = maxSegmentWeight;
       this.statsCounter = checkNotNull(statsCounter);
       initTable(newEntryArray(initialCapacity));
 
       keyReferenceQueue = map.usesKeyReferences()
            ? new ReferenceQueue<K>() : null;
 
       valueReferenceQueue = map.usesValueReferences()
            ? new ReferenceQueue<V>() : null;
 
       recencyQueue = map.usesAccessQueue()
           ? new ConcurrentLinkedQueue<ReferenceEntry<K, V>>()
           : LocalCache.<ReferenceEntry<K, V>>discardingQueue();
 
       writeQueue = map.usesWriteQueue()
           ? new WriteQueue<K, V>()
           : LocalCache.<ReferenceEntry<K, V>>discardingQueue();
 
       accessQueue = map.usesAccessQueue()
           ? new AccessQueue<K, V>()
           : LocalCache.<ReferenceEntry<K, V>>discardingQueue();
     }
 
     AtomicReferenceArray<ReferenceEntry<K, V>> newEntryArray(int size) {
       return new AtomicReferenceArray<ReferenceEntry<K, V>>(size);
     }
 
     void initTable(AtomicReferenceArray<ReferenceEntry<K, V>> newTable) {
       this.threshold = newTable.length() * 3 / 4; // 0.75
       if (!map.customWeigher() && this.threshold == maxSegmentWeight) {
         // prevent spurious expansion before eviction
         this.threshold++;
       }
       this.table = newTable;
     }
 
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> newEntry(K key, int hash, @Nullable ReferenceEntry<K, V> next) {
       return map.entryFactory.newEntry(this, checkNotNull(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.
      */
     @GuardedBy("Segment.this")
     ReferenceEntry<K, V> copyEntry(ReferenceEntry<K, V> original, ReferenceEntry<K, V> newNext) {
       if (original.getKey() == null) {
         // key collected
         return null;
       }
 
       ValueReference<K, V> valueReference = original.getValueReference();
       V value = valueReference.get();
       if ((value == null) && valueReference.isActive()) {
         // value collected
         return null;
       }
 
       ReferenceEntry<K, V> 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 access queue.
      */
     @GuardedBy("Segment.this")
     void setValue(ReferenceEntry<K, V> entry, K key, V value, long now) {
       ValueReference<K, V> previous = entry.getValueReference();
       int weight = map.weigher.weigh(key, value);
       checkState(weight >= 0, "Weights must be non-negative");
 
       ValueReference<K, V> valueReference =
           map.valueStrength.referenceValue(this, entry, value, weight);
       entry.setValueReference(valueReference);
       recordWrite(entry, weight, now);
       previous.notifyNewValue(value);
     }
 
     // loading
 
     V get(K key, int hash, CacheLoader<? super K, V> loader) throws ExecutionException {
       checkNotNull(key);
       checkNotNull(loader);
       try {
         if (count != 0) { // read-volatile
           // don't call getLiveEntry, which would ignore loading values
           ReferenceEntry<K, V> e = getEntry(key, hash);
           if (e != null) {
             long now = map.ticker.read();
             V value = getLiveValue(e, now);
             if (value != null) {
               recordRead(e, now);
               statsCounter.recordHits(1);
               return scheduleRefresh(e, key, hash, value, now, loader);
             }
             ValueReference<K, V> valueReference = e.getValueReference();
             if (valueReference.isLoading()) {
               return waitForLoadingValue(e, key, valueReference);
             }
           }
         }
 
         // at this point e is either null or expired;
         return lockedGetOrLoad(key, hash, loader);
       } catch (ExecutionException ee) {
         Throwable cause = ee.getCause();
         if (cause instanceof Error) {
           throw new ExecutionError((Error) cause);
         } else if (cause instanceof RuntimeException) {
           throw new UncheckedExecutionException(cause);
         }
         throw ee;
       } finally {
         postReadCleanup();
       }
     }
 
     V lockedGetOrLoad(K key, int hash, CacheLoader<? super K, V> loader)
         throws ExecutionException {
       ReferenceEntry<K, V> e;
       ValueReference<K, V> valueReference = null;
       LoadingValueReference<K, V> loadingValueReference = null;
       boolean createNewEntry = true;
 
       lock();
       try {
         // re-read ticker once inside the lock
         long now = map.ticker.read();
         preWriteCleanup(now);
 
         int newCount = this.count - 1;
         AtomicReferenceArray<ReferenceEntry<K, V>> table = this.table;
         int index = hash & (table.length() - 1);
         ReferenceEntry<K, V> first = table.get(index);
 
         for (e = first; e != null; e = e.getNext()) {
           K entryKey = e.getKey();
           if (e.getHash() == hash && entryKey != null
               && map.keyEquivalence.equivalent(key, entryKey)) {
             valueReference = e.getValueReference();
             if (valueReference.isLoading()) {
               createNewEntry = false;
             } else {
               V value = valueReference.get();
               if (value == null) {
                 enqueueNotification(entryKey, hash, valueReference, RemovalCause.COLLECTED);
               } else if (map.isExpired(e, now)) {
                 // This is a duplicate check, as preWriteCleanup already purged expired
                 // entries, but let's accomodate an incorrect expiration queue.
                 enqueueNotification(entryKey, hash, valueReference, RemovalCause.EXPIRED);
               } else {
                 recordLockedRead(e, now);
                 statsCounter.recordHits(1);
                 // we were concurrent with loading; don't consider refresh
                 return value;
               }
 
               // immediately reuse invalid entries
               writeQueue.remove(e);
               accessQueue.remove(e);
               this.count = newCount; // write-volatile
             }
             break;
           }
         }
 
         if (createNewEntry) {
           loadingValueReference = new LoadingValueReference<K, V>();
 
           if (e == null) {
             e = newEntry(key, hash, first);
             e.setValueReference(loadingValueReference);
             table.set(index, e);
           } else {
             e.setValueReference(loadingValueReference);
           }
         }
       } finally {
         unlock();
         postWriteCleanup();
       }
 
       if (createNewEntry) {
         try {
           // Synchronizes on the entry to allow failing fast when a recursive load is
           // detected. This may be circumvented when an entry is copied, but will fail fast most
           // of the time.
           synchronized (e) {
             return loadSync(key, hash, loadingValueReference, loader);
           }
         } finally {
           statsCounter.recordMisses(1);
         }
       } else {
         // The entry already exists. Wait for loading.
         return waitForLoadingValue(e, key, valueReference);
       }
     }
 
     V waitForLoadingValue(ReferenceEntry<K, V> e, K key, ValueReference<K, V> valueReference)
         throws ExecutionException {
       if (!valueReference.isLoading()) {
         throw new AssertionError();
       }
 
       checkState(!Thread.holdsLock(e), "Recursive load of: %s", key);
       // don't consider expiration as we're concurrent with loading
       try {
         V value = valueReference.waitForValue();
         if (value == null) {
           throw new InvalidCacheLoadException("CacheLoader returned null for key " + key + ".");
         }
         // re-read ticker now that loading has completed
         long now = map.ticker.read();
         recordRead(e, now);
         return value;
       } finally {
         statsCounter.recordMisses(1);
       }
     }
 
     // at most one of loadSync/loadAsync may be called for any given LoadingValueReference
 
     V loadSync(K key, int hash, LoadingValueReference<K, V> loadingValueReference,
         CacheLoader<? super K, V> loader) throws ExecutionException {
       ListenableFuture<V> loadingFuture = loadingValueReference.loadFuture(key, loader);
       return getAndRecordStats(key, hash, loadingValueReference, loadingFuture);
     }
 
     ListenableFuture<V> loadAsync(final K key, final int hash,
         final LoadingValueReference<K, V> loadingValueReference, CacheLoader<? super K, V> loader) {
       final ListenableFuture<V> loadingFuture = loadingValueReference.loadFuture(key, loader);
       loadingFuture.addListener(
           new Runnable() {
             @Override
             public void run() {
               try {
                 V newValue = getAndRecordStats(key, hash, loadingValueReference, loadingFuture);
               } catch (Throwable t) {
                 logger.log(Level.WARNING, "Exception thrown during refresh", t);
                 loadingValueReference.setException(t);
               }
             }
           }, sameThreadExecutor);
       return loadingFuture;
     }
 
     /**
      * Waits uninterruptibly for {@code newValue} to be loaded, and then records loading stats.
      */
     V getAndRecordStats(K key, int hash, LoadingValueReference<K, V> loadingValueReference,
         ListenableFuture<V> newValue) throws ExecutionException {
       V value = null;
       try {
         value = getUninterruptibly(newValue);
         if (value == null) {
           throw new InvalidCacheLoadException("CacheLoader returned null for key " + key + ".");
         }
         statsCounter.recordLoadSuccess(loadingValueReference.elapsedNanos());
         storeLoadedValue(key, hash, loadingValueReference, value);
         return value;
       } finally {
         if (value == null) {
           statsCounter.recordLoadException(loadingValueReference.elapsedNanos());
           removeLoadingValue(key, hash, loadingValueReference);
         }
       }
     }
 
     V scheduleRefresh(ReferenceEntry<K, V> entry, K key, int hash, V oldValue, long now,
         CacheLoader<? super K, V> loader) {
       if (map.refreshes() && (now - entry.getWriteTime() > map.refreshNanos)
           && !entry.getValueReference().isLoading()) {
         V newValue = refresh(key, hash, loader, true);
         if (newValue != null) {
           return newValue;
         }
       }
       return oldValue;
     }
 
     /**
      * Refreshes the value associated with {@code key}, unless another thread is already doing so.
      * Returns the newly refreshed value associated with {@code key} if it was refreshed inline, or
      * {@code null} if another thread is performing the refresh or if an error occurs during
      * refresh.
      */
     @Nullable
     V refresh(K key, int hash, CacheLoader<? super K, V> loader, boolean checkTime) {
       final LoadingValueReference<K, V> loadingValueReference =
           insertLoadingValueReference(key, hash, checkTime);
       if (loadingValueReference == null) {
         return null;
       }
 
       ListenableFuture<V> result = loadAsync(key, hash, loadingValueReference, loader);
       if (result.isDone()) {
         try {
           return Uninterruptibles.getUninterruptibly(result);
         } catch (Throwable t) {
           // don't let refresh exceptions propagate; error was already logged
         }
       }
       return null;
     }
 
     /**
      * Returns a newly inserted {@code LoadingValueReference}, or null if the live value reference
      * is already loading.
      */
     @Nullable
     LoadingValueReference<K, V> insertLoadingValueReference(final K key, final int hash,
         boolean checkTime) {
       ReferenceEntry<K, V> e = null;
       lock();
       try {
         long now = map.ticker.read();
         preWriteCleanup(now);
 
         AtomicReferenceArray<ReferenceEntry<K, V>> table = this.table;
         int index = hash & (table.length() - 1);
         ReferenceEntry<K, V> first = table.get(index);
 
         // Look for an existing entry.
         for (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<K, V> valueReference = e.getValueReference();
             if (valueReference.isLoading()
                 || (checkTime && (now - e.getWriteTime() < map.refreshNanos))) {
               // refresh is a no-op if loading is pending
               // if checkTime, we want to check *after* acquiring the lock if refresh still needs
               // to be scheduled
               return null;
             }
 
             // continue returning old value while loading
             ++modCount;
             LoadingValueReference<K, V> loadingValueReference =
                 new LoadingValueReference<K, V>(valueReference);
             e.setValueReference(loadingValueReference);
             return loadingValueReference;
           }
         }
 
         ++modCount;
         LoadingValueReference<K, V> loadingValueReference = new LoadingValueReference<K, V>();
         e = newEntry(key, hash, first);
         e.setValueReference(loadingValueReference);
         table.set(index, e);
         return loadingValueReference;
       } finally {
         unlock();
         postWriteCleanup();
       }
     }
 
     // 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.
      */
     @GuardedBy("Segment.this")
     void drainReferenceQueues() {
       if (map.usesKeyReferences()) {
         drainKeyReferenceQueue();
       }
       if (map.usesValueReferences()) {
         drainValueReferenceQueue();
       }
     }
 
     @GuardedBy("Segment.this")
     void drainKeyReferenceQueue() {
       Reference<? extends K> ref;
       int i = 0;
       while ((ref = keyReferenceQueue.poll()) != null) {
         @SuppressWarnings("unchecked")
         ReferenceEntry<K, V> entry = (ReferenceEntry<K, V>) ref;
         map.reclaimKey(entry);
         if (++i == DRAIN_MAX) {
           break;
         }
       }
     }
 
     @GuardedBy("Segment.this")
     void drainValueReferenceQueue() {
       Reference<? extends V> ref;
       int i = 0;
       while ((ref = valueReferenceQueue.poll()) != null) {
         @SuppressWarnings("unchecked")
         ValueReference<K, V> valueReference = (ValueReference<K, V>) 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.
      *
      * <p>Note: locked reads should use {@link #recordLockedRead}.
      */
     void recordRead(ReferenceEntry<K, V> entry, long now) {
       if (map.recordsAccess()) {
         entry.setAccessTime(now);
       }
       recencyQueue.add(entry);
     }
 
     /**
      * Updates the eviction metadata that {@code entry} was just read. This currently amounts to
      * adding {@code entry} to relevant eviction lists.
      *
      * <p>Note: this method should only be called under lock, as it directly manipulates the
      * eviction queues. Unlocked reads should use {@link #recordRead}.
      */
     @GuardedBy("Segment.this")
     void recordLockedRead(ReferenceEntry<K, V> entry, long now) {
       if (map.recordsAccess()) {
         entry.setAccessTime(now);
       }
       accessQueue.add(entry);
     }
 
     /**
      * Updates eviction metadata that {@code entry} was just written. This currently amounts to
      * adding {@code entry} to relevant eviction lists.
      */
     @GuardedBy("Segment.this")
     void recordWrite(ReferenceEntry<K, V> entry, int weight, long now) {
       // we are already under lock, so drain the recency queue immediately
       drainRecencyQueue();
       totalWeight += weight;
 
       if (map.recordsAccess()) {
         entry.setAccessTime(now);
       }
       if (map.recordsWrite()) {
         entry.setWriteTime(now);
       }
       accessQueue.add(entry);
       writeQueue.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).
      */
     @GuardedBy("Segment.this")
     void drainRecencyQueue() {
       ReferenceEntry<K, V> 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 (accessQueue.contains(e)) {
           accessQueue.add(e);
         }
       }
     }
 
     // expiration
 
     /**
      * Cleanup expired entries when the lock is available.
      */
     void tryExpireEntries(long now) {
       if (tryLock()) {
         try {
           expireEntries(now);
         } finally {
           unlock();
           // don't call postWriteCleanup as we're in a read
         }
       }
     }
 
     @GuardedBy("Segment.this")
     void expireEntries(long now) {
       drainRecencyQueue();
 
       ReferenceEntry<K, V> e;
       while ((e = writeQueue.peek()) != null && map.isExpired(e, now)) {
         if (!removeEntry(e, e.getHash(), RemovalCause.EXPIRED)) {
           throw new AssertionError();
         }
       }
       while ((e = accessQueue.peek()) != null && map.isExpired(e, now)) {
         if (!removeEntry(e, e.getHash(), RemovalCause.EXPIRED)) {
           throw new AssertionError();
         }
       }
     }
 
     // eviction
 
     @GuardedBy("Segment.this")
     void enqueueNotification(ReferenceEntry<K, V> entry, RemovalCause cause) {
       enqueueNotification(entry.getKey(), entry.getHash(), entry.getValueReference(), cause);
     }
 
     @GuardedBy("Segment.this")
     void enqueueNotification(@Nullable K key, int hash, ValueReference<K, V> valueReference,
         RemovalCause cause) {
       totalWeight -= valueReference.getWeight();
       if (cause.wasEvicted()) {
         statsCounter.recordEviction();
       }
       if (map.removalNotificationQueue != DISCARDING_QUEUE) {
         V value = valueReference.get();
         RemovalNotification<K, V> notification = new RemovalNotification<K, V>(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}.
      */
     @GuardedBy("Segment.this")
     void evictEntries() {
       if (!map.evictsBySize()) {
         return;
       }
 
       drainRecencyQueue();
       while (totalWeight > maxSegmentWeight) {
         ReferenceEntry<K, V> e = getNextEvictable();
         if (!removeEntry(e, e.getHash(), RemovalCause.SIZE)) {
           throw new AssertionError();
         }
       }
     }
 
     // TODO(fry): instead implement this with an eviction head
     ReferenceEntry<K, V> getNextEvictable() {
       for (ReferenceEntry<K, V> e : accessQueue) {
         int weight = e.getValueReference().getWeight();
         if (weight > 0) {
           return e;
         }
       }
       throw new AssertionError();
     }
 
     /**
      * Returns first entry of bin for given hash.
      */
     ReferenceEntry<K, V> getFirst(int hash) {
       // read this volatile field only once
       AtomicReferenceArray<ReferenceEntry<K, V>> table = this.table;
       return table.get(hash & (table.length() - 1));
     }
 
     // Specialized implementations of map methods
 
     @Nullable
     ReferenceEntry<K, V> getEntry(Object key, int hash) {
       for (ReferenceEntry<K, V> 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;
     }
 
     @Nullable
     ReferenceEntry<K, V> getLiveEntry(Object key, int hash, long now) {
       ReferenceEntry<K, V> e = getEntry(key, hash);
       if (e == null) {
         return null;
       } else if (map.isExpired(e, now)) {
         tryExpireEntries(now);
         return null;
       }
       return e;
     }
 
     /**
      * Gets the value from an entry. Returns null if the entry is invalid, partially-collected,
      * loading, or expired.
      */
     V getLiveValue(ReferenceEntry<K, V> entry, long now) {
       if (entry.getKey() == null) {
         tryDrainReferenceQueues();
         return null;
       }
       V value = entry.getValueReference().get();
       if (value == null) {
         tryDrainReferenceQueues();
         return null;
       }
 
       if (map.isExpired(entry, now)) {
         tryExpireEntries(now);
         return null;
       }
       return value;
     }
 
     @Nullable
     V get(Object key, int hash) {
       try {
         if (count != 0) { // read-volatile
           long now = map.ticker.read();
           ReferenceEntry<K, V> e = getLiveEntry(key, hash, now);
           if (e == null) {
             return null;
           }
 
           V value = e.getValueReference().get();
           if (value != null) {
             recordRead(e, now);
             return scheduleRefresh(e, e.getKey(), hash, value, now, map.defaultLoader);
           }
           tryDrainReferenceQueues();
         }
         return null;
       } finally {
         postReadCleanup();
       }
     }
 
     boolean containsKey(Object key, int hash) {
       try {
         if (count != 0) { // read-volatile
           long now = map.ticker.read();
           ReferenceEntry<K, V> e = getLiveEntry(key, hash, now);
           if (e == null) {
             return false;
           }
           return e.getValueReference().get() != null;
         }
 
         return false;
       } finally {
         postReadCleanup();
       }
     }
 
     /**
      * This method is a convenience for testing. Code should call {@link
      * LocalCache#containsValue} directly.
      */
     @VisibleForTesting
     boolean containsValue(Object value) {
       try {
         if (count != 0) { // read-volatile
           long now = map.ticker.read();
           AtomicReferenceArray<ReferenceEntry<K, V>> table = this.table;
           int length = table.length();
           for (int i = 0; i < length; ++i) {
             for (ReferenceEntry<K, V> e = table.get(i); e != null; e = e.getNext()) {
               V entryValue = getLiveValue(e, now);
               if (entryValue == null) {
                 continue;
               }
               if (map.valueEquivalence.equivalent(value, entryValue)) {
                 return true;
               }
             }
           }
         }
 
         return false;
       } finally {
         postReadCleanup();
       }
     }
 
     @Nullable
     V put(K key, int hash, V value, boolean onlyIfAbsent) {
       lock();
       try {
         long now = map.ticker.read();
         preWriteCleanup(now);
 
         int newCount = this.count + 1;
         if (newCount > this.threshold) { // ensure capacity
           expand();
           newCount = this.count + 1;
         }
 
         AtomicReferenceArray<ReferenceEntry<K, V>> table = this.table;
         int index = hash & (table.length() - 1);
         ReferenceEntry<K, V> first = table.get(index);
 
         // Look for an existing entry.
         for (ReferenceEntry<K, V> 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<K, V> valueReference = e.getValueReference();
             V entryValue = valueReference.get();
 
             if (entryValue == null) {
               ++modCount;
               if (valueReference.isActive()) {
                 enqueueNotification(key, hash, valueReference, RemovalCause.COLLECTED);
                 setValue(e, key, value, now);
                 newCount = this.count; // count remains unchanged
               } else {
                 setValue(e, key, value, now);
                 newCount = this.count + 1;
               }
               this.count = newCount; // write-volatile
               evictEntries();
               return null;
             } else if (onlyIfAbsent) {
               // Mimic
               // "if (!map.containsKey(key)) ...
               // else return map.get(key);
               recordLockedRead(e, now);
               return entryValue;
             } else {
               // clobber existing entry, count remains unchanged
               ++modCount;
               enqueueNotification(key, hash, valueReference, RemovalCause.REPLACED);
               setValue(e, key, value, now);
               evictEntries();
               return entryValue;
             }
           }
         }
 
         // Create a new entry.
         ++modCount;
         ReferenceEntry<K, V> newEntry = newEntry(key, hash, first);
         setValue(newEntry, key, value, now);
         table.set(index, newEntry);
         newCount = this.count + 1;
         this.count = newCount; // write-volatile
         evictEntries();
         return null;
       } finally {
         unlock();
         postWriteCleanup();
       }
     }
 
     /**
      * Expands the table if possible.
      */
     @GuardedBy("Segment.this")
     void expand() {
       AtomicReferenceArray<ReferenceEntry<K, V>> oldTable = table;
       int oldCapacity = oldTable.length();
       if (oldCapacity >= MAXIMUM_CAPACITY) {
         return;
       }
 
       /*
        * Reclassify nodes in each list to new Map. Because we are using power-of-two expansion, the
        * elements from each bin must either stay at same index, or move with a power of two offset.
        * We eliminate unnecessary node creation by catching cases where old nodes can be reused
        * because their next fields won't change. Statistically, at the default threshold, only
        * about one-sixth of them need cloning when a table doubles. The nodes they replace will be
        * garbage collectable as soon as they are no longer referenced by any reader thread that may
        * be in the midst of traversing table right now.
        */
 
       int newCount = count;
       AtomicReferenceArray<ReferenceEntry<K, V>> 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<K, V> head = oldTable.get(oldIndex);
 
         if (head != null) {
           ReferenceEntry<K, V> 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<K, V> tail = head;
             int tailIndex = headIndex;
             for (ReferenceEntry<K, V> 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<K, V> e = head; e != tail; e = e.getNext()) {
               int newIndex = e.getHash() & newMask;
               ReferenceEntry<K, V> newNext = newTable.get(newIndex);
               ReferenceEntry<K, V> 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 {
         long now = map.ticker.read();
         preWriteCleanup(now);
 
         AtomicReferenceArray<ReferenceEntry<K, V>> table = this.table;
         int index = hash & (table.length() - 1);
         ReferenceEntry<K, V> first = table.get(index);
 
         for (ReferenceEntry<K, V> e = first; e != null; e = e.getNext()) {
           K entryKey = e.getKey();
           if (e.getHash() == hash && entryKey != null
               && map.keyEquivalence.equivalent(key, entryKey)) {
             ValueReference<K, V> valueReference = e.getValueReference();
             V entryValue = valueReference.get();
             if (entryValue == null) {
               if (valueReference.isActive()) {
                 // If the value disappeared, this entry is partially collected.
                 int newCount = this.count - 1;
                 ++modCount;
                 ReferenceEntry<K, V> newFirst = removeValueFromChain(
                     first, e, entryKey, hash, valueReference, RemovalCause.COLLECTED);
                 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, valueReference, RemovalCause.REPLACED);
               setValue(e, key, newValue, now);
               evictEntries();
               return true;
             } else {
               // Mimic
               // "if (map.containsKey(key) && map.get(key).equals(oldValue))..."
               recordLockedRead(e, now);
               return false;
             }
           }
         }
 
         return false;
       } finally {
         unlock();
         postWriteCleanup();
       }
     }
 
     @Nullable
     V replace(K key, int hash, V newValue) {
       lock();
       try {
         long now = map.ticker.read();
         preWriteCleanup(now);
 
         AtomicReferenceArray<ReferenceEntry<K, V>> table = this.table;
         int index = hash & (table.length() - 1);
         ReferenceEntry<K, V> first = table.get(index);
 
         for (ReferenceEntry<K, V> e = first; e != null; e = e.getNext()) {
           K entryKey = e.getKey();
           if (e.getHash() == hash && entryKey != null
               && map.keyEquivalence.equivalent(key, entryKey)) {
             ValueReference<K, V> valueReference = e.getValueReference();
             V entryValue = valueReference.get();
             if (entryValue == null) {
               if (valueReference.isActive()) {
                 // If the value disappeared, this entry is partially collected.
                 int newCount = this.count - 1;
                 ++modCount;
                 ReferenceEntry<K, V> newFirst = removeValueFromChain(
                     first, e, entryKey, hash, valueReference, RemovalCause.COLLECTED);
                 newCount = this.count - 1;
                 table.set(index, newFirst);
                 this.count = newCount; // write-volatile
               }
               return null;
             }
 
             ++modCount;
             enqueueNotification(key, hash, valueReference, RemovalCause.REPLACED);
             setValue(e, key, newValue, now);
             evictEntries();
             return entryValue;
           }
         }
 
         return null;
       } finally {
         unlock();
         postWriteCleanup();
       }
     }
 
     @Nullable
     V remove(Object key, int hash) {
       lock();
       try {
         long now = map.ticker.read();
         preWriteCleanup(now);
 
         int newCount = this.count - 1;
         AtomicReferenceArray<ReferenceEntry<K, V>> table = this.table;
         int index = hash & (table.length() - 1);
         ReferenceEntry<K, V> first = table.get(index);
 
         for (ReferenceEntry<K, V> e = first; e != null; e = e.getNext()) {
           K entryKey = e.getKey();
           if (e.getHash() == hash && entryKey != null
               && map.keyEquivalence.equivalent(key, entryKey)) {
             ValueReference<K, V> valueReference = e.getValueReference();
             V entryValue = valueReference.get();
 
             RemovalCause cause;
             if (entryValue != null) {
               cause = RemovalCause.EXPLICIT;
             } else if (valueReference.isActive()) {
               cause = RemovalCause.COLLECTED;
             } else {
               // currently loading
               return null;
             }
 
             ++modCount;
             ReferenceEntry<K, V> newFirst = removeValueFromChain(
                 first, e, entryKey, hash, valueReference, cause);
             newCount = this.count - 1;
             table.set(index, newFirst);
             this.count = newCount; // write-volatile
             return entryValue;
           }
         }
 
         return null;
       } finally {
         unlock();
         postWriteCleanup();
       }
     }
 
     boolean storeLoadedValue(K key, int hash, LoadingValueReference<K, V> oldValueReference,
         V newValue) {
       lock();
       try {
         long now = map.ticker.read();
         preWriteCleanup(now);
 
         int newCount = this.count + 1;
         if (newCount > this.threshold) { // ensure capacity
           expand();
           newCount = this.count + 1;
         }
 
         AtomicReferenceArray<ReferenceEntry<K, V>> table = this.table;
         int index = hash & (table.length() - 1);
         ReferenceEntry<K, V> first = table.get(index);
 
         for (ReferenceEntry<K, V> e = first; e != null; e = e.getNext()) {
           K entryKey = e.getKey();
           if (e.getHash() == hash && entryKey != null
               && map.keyEquivalence.equivalent(key, entryKey)) {
             ValueReference<K, V> valueReference = e.getValueReference();
             V entryValue = valueReference.get();
             // replace the old LoadingValueReference if it's live, otherwise
             // perform a putIfAbsent
             if (oldValueReference == valueReference
                 || (entryValue == null && valueReference != UNSET)) {
               ++modCount;
               if (oldValueReference.isActive()) {
                 RemovalCause cause =
                     (entryValue == null) ? RemovalCause.COLLECTED : RemovalCause.REPLACED;
                 enqueueNotification(key, hash, oldValueReference, cause);
                 newCount--;
               }
               setValue(e, key, newValue, now);
               this.count = newCount; // write-volatile
               evictEntries();
               return true;
             }
 
             // the loaded value was already clobbered
             valueReference = new WeightedStrongValueReference<K, V>(newValue, 0);
             enqueueNotification(key, hash, valueReference, RemovalCause.REPLACED);
             return false;
           }
         }
 
         ++modCount;
         ReferenceEntry<K, V> newEntry = newEntry(key, hash, first);
         setValue(newEntry, key, newValue, now);
         table.set(index, newEntry);
         this.count = newCount; // write-volatile
         evictEntries();
         return true;
       } finally {
         unlock();
         postWriteCleanup();
       }
     }
 
     boolean remove(Object key, int hash, Object value) {
       lock();
       try {
         long now = map.ticker.read();
         preWriteCleanup(now);
 
         int newCount = this.count - 1;
         AtomicReferenceArray<ReferenceEntry<K, V>> table = this.table;
         int index = hash & (table.length() - 1);
         ReferenceEntry<K, V> first = table.get(index);
 
         for (ReferenceEntry<K, V> e = first; e != null; e = e.getNext()) {
           K entryKey = e.getKey();
           if (e.getHash() == hash && entryKey != null
               && map.keyEquivalence.equivalent(key, entryKey)) {
             ValueReference<K, V> valueReference = e.getValueReference();
             V entryValue = valueReference.get();
 
             RemovalCause cause;
             if (map.valueEquivalence.equivalent(value, entryValue)) {
               cause = RemovalCause.EXPLICIT;
             } else if (entryValue == null && valueReference.isActive()) {
               cause = RemovalCause.COLLECTED;
             } else {
               // currently loading
               return false;
             }
 
             ++modCount;
             ReferenceEntry<K, V> newFirst = removeValueFromChain(
                 first, e, entryKey, hash, valueReference, cause);
             newCount = this.count - 1;
             table.set(index, newFirst);
             this.count = newCount; // write-volatile
             return (cause == RemovalCause.EXPLICIT);
           }
         }
 
         return false;
       } finally {
         unlock();
         postWriteCleanup();
       }
     }
 
     void clear() {
       if (count != 0) { // read-volatile
         lock();
         try {
           AtomicReferenceArray<ReferenceEntry<K, V>> table = this.table;
           for (int i = 0; i < table.length(); ++i) {
             for (ReferenceEntry<K, V> e = table.get(i); e != null; e = e.getNext()) {
               // Loading references aren't actually in the map yet.
               if (e.getValueReference().isActive()) {
                 enqueueNotification(e, RemovalCause.EXPLICIT);
               }
             }
           }
           for (int i = 0; i < table.length(); ++i) {
             table.set(i, null);
           }
           clearReferenceQueues();
           writeQueue.clear();
           accessQueue.clear();
           readCount.set(0);
 
           ++modCount;
           count = 0; // write-volatile
         } finally {
           unlock();
           postWriteCleanup();
         }
       }
     }
 
     @GuardedBy("Segment.this")
     @Nullable
     ReferenceEntry<K, V> removeValueFromChain(ReferenceEntry<K, V> first,
         ReferenceEntry<K, V> entry, @Nullable K key, int hash, ValueReference<K, V> valueReference,
         RemovalCause cause) {
       enqueueNotification(key, hash, valueReference, cause);
       writeQueue.remove(entry);
       accessQueue.remove(entry);
 
       if (valueReference.isLoading()) {
         valueReference.notifyNewValue(null);
         return first;
       } else {
         return removeEntryFromChain(first, entry);
       }
     }
 
     @GuardedBy("Segment.this")
     @Nullable
     ReferenceEntry<K, V> removeEntryFromChain(ReferenceEntry<K, V> first,
         ReferenceEntry<K, V> entry) {
       int newCount = count;
       ReferenceEntry<K, V> newFirst = entry.getNext();
       for (ReferenceEntry<K, V> e = first; e != entry; e = e.getNext()) {
         ReferenceEntry<K, V> next = copyEntry(e, newFirst);
         if (next != null) {
           newFirst = next;
         } else {
           removeCollectedEntry(e);
           newCount--;
         }
       }
       this.count = newCount;
       return newFirst;
     }
 
     @GuardedBy("Segment.this")
     void removeCollectedEntry(ReferenceEntry<K, V> entry) {
       enqueueNotification(entry, RemovalCause.COLLECTED);
       writeQueue.remove(entry);
       accessQueue.remove(entry);
     }
 
     /**
      * Removes an entry whose key has been garbage collected.
      */
     boolean reclaimKey(ReferenceEntry<K, V> entry, int hash) {
       lock();
       try {
         int newCount = count - 1;
         AtomicReferenceArray<ReferenceEntry<K, V>> table = this.table;
         int index = hash & (table.length() - 1);
         ReferenceEntry<K, V> first = table.get(index);
 
         for (ReferenceEntry<K, V> e = first; e != null; e = e.getNext()) {
           if (e == entry) {
             ++modCount;
             ReferenceEntry<K, V> newFirst = removeValueFromChain(
                 first, e, e.getKey(), hash, e.getValueReference(), RemovalCause.COLLECTED);
             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<K, V> valueReference) {
       lock();
       try {
         int newCount = this.count - 1;
         AtomicReferenceArray<ReferenceEntry<K, V>> table = this.table;
         int index = hash & (table.length() - 1);
         ReferenceEntry<K, V> first = table.get(index);
 
         for (ReferenceEntry<K, V> e = first; e != null; e = e.getNext()) {
           K entryKey = e.getKey();
           if (e.getHash() == hash && entryKey != null
               && map.keyEquivalence.equivalent(key, entryKey)) {
             ValueReference<K, V> v = e.getValueReference();
             if (v == valueReference) {
               ++modCount;
               ReferenceEntry<K, V> newFirst = removeValueFromChain(
                   first, e, entryKey, hash, valueReference, RemovalCause.COLLECTED);
               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 removeLoadingValue(K key, int hash, LoadingValueReference<K, V> valueReference) {
       lock();
       try {
         AtomicReferenceArray<ReferenceEntry<K, V>> table = this.table;
         int index = hash & (table.length() - 1);
         ReferenceEntry<K, V> first = table.get(index);
 
         for (ReferenceEntry<K, V> e = first; e != null; e = e.getNext()) {
           K entryKey = e.getKey();
           if (e.getHash() == hash && entryKey != null
               && map.keyEquivalence.equivalent(key, entryKey)) {
             ValueReference<K, V> v = e.getValueReference();
             if (v == valueReference) {
               if (valueReference.isActive()) {
                 e.setValueReference(valueReference.getOldValue());
               } else {
                 ReferenceEntry<K, V> newFirst = removeEntryFromChain(first, e);
                 table.set(index, newFirst);
               }
               return true;
             }
             return false;
           }
         }
 
         return false;
       } finally {
         unlock();
         postWriteCleanup();
       }
     }
 
     @GuardedBy("Segment.this")
     boolean removeEntry(ReferenceEntry<K, V> entry, int hash, RemovalCause cause) {
       int newCount = this.count - 1;
       AtomicReferenceArray<ReferenceEntry<K, V>> table = this.table;
       int index = hash & (table.length() - 1);
       ReferenceEntry<K, V> first = table.get(index);
 
       for (ReferenceEntry<K, V> e = first; e != null; e = e.getNext()) {
         if (e == entry) {
           ++modCount;
           ReferenceEntry<K, V> newFirst = removeValueFromChain(
               first, e, e.getKey(), hash, e.getValueReference(), cause);
           newCount = this.count - 1;
           table.set(index, newFirst);
           this.count = newCount; // write-volatile
           return true;
         }
       }
 
       return false;
     }
 
     /**
      * Performs routine cleanup following a read. Normally cleanup happens during writes. 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) {
         cleanUp();
       }
     }
 
     /**
      * 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.
      *
      * <p>Post-condition: expireEntries has been run.
      */
     @GuardedBy("Segment.this")
     void preWriteCleanup(long now) {
       runLockedCleanup(now);
     }
 
     /**
      * Performs routine cleanup following a write.
      */
     void postWriteCleanup() {
       runUnlockedCleanup();
     }
 
     void cleanUp() {
       long now = map.ticker.read();
       runLockedCleanup(now);
       runUnlockedCleanup();
     }
 
     void runLockedCleanup(long now) {
       if (tryLock()) {
         try {
           drainReferenceQueues();
           expireEntries(now); // calls drainRecencyQueue
           readCount.set(0);
         } finally {
           unlock();
         }
       }
     }
 
     void runUnlockedCleanup() {
       // locked cleanup may generate notifications we can send unlocked
       if (!isHeldByCurrentThread()) {
         map.processPendingNotifications();
       }
     }
 
   }
 
   static class LoadingValueReference<K, V> implements ValueReference<K, V> {
     volatile ValueReference<K, V> oldValue;
 
     // TODO(fry): rename get, then extend AbstractFuture instead of containing SettableFuture
     final SettableFuture<V> futureValue = SettableFuture.create();
     final Stopwatch stopwatch = Stopwatch.createUnstarted();
 
     public LoadingValueReference() {
       this(LocalCache.<K, V>unset());
     }
 
     public LoadingValueReference(ValueReference<K, V> oldValue) {
       this.oldValue = oldValue;
     }
 
     @Override
     public boolean isLoading() {
       return true;
     }
 
     @Override
     public boolean isActive() {
       return oldValue.isActive();
     }
 
     @Override
     public int getWeight() {
       return oldValue.getWeight();
     }
 
     public boolean set(@Nullable V newValue) {
       return futureValue.set(newValue);
     }
 
     public boolean setException(Throwable t) {
       return futureValue.setException(t);
     }
 
     private ListenableFuture<V> fullyFailedFuture(Throwable t) {
       return Futures.immediateFailedFuture(t);
     }
 
     @Override
     public void notifyNewValue(@Nullable V newValue) {
       if (newValue != null) {
         // The pending load was clobbered by a manual write.
         // Unblock all pending gets, and have them return the new value.
         set(newValue);
       } else {
         // The pending load was removed. Delay notifications until loading completes.
         oldValue = unset();
       }
 
       // TODO(fry): could also cancel loading if we had a handle on its future
     }
 
     public ListenableFuture<V> loadFuture(K key, CacheLoader<? super K, V> loader) {
       stopwatch.start();
       V previousValue = oldValue.get();
       try {
         if (previousValue == null) {
           V newValue = loader.load(key);
           return set(newValue) ? futureValue : Futures.immediateFuture(newValue);
         }
         ListenableFuture<V> newValue = loader.reload(key, previousValue);
         if (newValue == null) {
           return Futures.immediateFuture(null);
         }
         // To avoid a race, make sure the refreshed value is set into loadingValueReference
         // *before* returning newValue from the cache query.
         return Futures.transform(newValue, new Function<V, V>() {
           @Override
           public V apply(V newValue) {
             LoadingValueReference.this.set(newValue);
             return newValue;
           }
         });
       } catch (Throwable t) {
         if (t instanceof InterruptedException) {
           Thread.currentThread().interrupt();
         }
         return setException(t) ? futureValue : fullyFailedFuture(t);
       }
     }
 
     public long elapsedNanos() {
       return stopwatch.elapsed(NANOSECONDS);
     }
 
     @Override
     public V waitForValue() throws ExecutionException {
       return getUninterruptibly(futureValue);
     }
 
     @Override
     public V get() {
       return oldValue.get();
     }
 
     public ValueReference<K, V> getOldValue() {
       return oldValue;
     }
 
     @Override
     public ReferenceEntry<K, V> getEntry() {
       return null;
     }
 
     @Override
     public ValueReference<K, V> copyFor(
         ReferenceQueue<V> queue, @Nullable V value, ReferenceEntry<K, V> entry) {
       return this;
     }
   }
 
   // Queues
 
   /**
    * A custom queue for managing eviction order. Note that this is tightly integrated with {@code
    * ReferenceEntry}, upon which it relies to perform its linking.
    *
    * <p>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.
    *
    * <p>The benefits of creating our own queue are that (1) we can replace elements in the middle
    * of the queue as part of copyWriteEntry, and (2) the contains method is highly optimized
    * for the current model.
    */
   static final class WriteQueue<K, V> extends AbstractQueue<ReferenceEntry<K, V>> {
     final ReferenceEntry<K, V> head = new AbstractReferenceEntry<K, V>() {
 
       @Override
       public long getWriteTime() {
         return Long.MAX_VALUE;
       }
 
       @Override
       public void setWriteTime(long time) {}
 
       ReferenceEntry<K, V> nextWrite = this;
 
       @Override
       public ReferenceEntry<K, V> getNextInWriteQueue() {
         return nextWrite;
       }
 
       @Override
       public void setNextInWriteQueue(ReferenceEntry<K, V> next) {
         this.nextWrite = next;
       }
 
       ReferenceEntry<K, V> previousWrite = this;
 
       @Override
       public ReferenceEntry<K, V> getPreviousInWriteQueue() {
         return previousWrite;
       }
 
       @Override
       public void setPreviousInWriteQueue(ReferenceEntry<K, V> previous) {
         this.previousWrite = previous;
       }
     };
 
     // implements Queue
 
     @Override
     public boolean offer(ReferenceEntry<K, V> entry) {
       // unlink
       connectWriteOrder(entry.getPreviousInWriteQueue(), entry.getNextInWriteQueue());
 
       // add to tail
       connectWriteOrder(head.getPreviousInWriteQueue(), entry);
       connectWriteOrder(entry, head);
 
       return true;
     }
 
     @Override
     public ReferenceEntry<K, V> peek() {
       ReferenceEntry<K, V> next = head.getNextInWriteQueue();
       return (next == head) ? null : next;
     }
 
     @Override
     public ReferenceEntry<K, V> poll() {
       ReferenceEntry<K, V> next = head.getNextInWriteQueue();
       if (next == head) {
         return null;
       }
 
       remove(next);
       return next;
     }
 
     @Override
     @SuppressWarnings("unchecked")
     public boolean remove(Object o) {
       ReferenceEntry<K, V> e = (ReferenceEntry) o;
       ReferenceEntry<K, V> previous = e.getPreviousInWriteQueue();
       ReferenceEntry<K, V> next = e.getNextInWriteQueue();
       connectWriteOrder(previous, next);
       nullifyWriteOrder(e);
 
       return next != NullEntry.INSTANCE;
     }
 
     @Override
     @SuppressWarnings("unchecked")
     public boolean contains(Object o) {
       ReferenceEntry<K, V> e = (ReferenceEntry) o;
       return e.getNextInWriteQueue() != NullEntry.INSTANCE;
     }
 
     @Override
     public boolean isEmpty() {
       return head.getNextInWriteQueue() == head;
     }
 
     @Override
     public int size() {
       int size = 0;
       for (ReferenceEntry<K, V> e = head.getNextInWriteQueue(); e != head;
           e = e.getNextInWriteQueue()) {
         size++;
       }
       return size;
     }
 
     @Override
     public void clear() {
       ReferenceEntry<K, V> e = head.getNextInWriteQueue();
       while (e != head) {
         ReferenceEntry<K, V> next = e.getNextInWriteQueue();
         nullifyWriteOrder(e);
         e = next;
       }
 
       head.setNextInWriteQueue(head);
       head.setPreviousInWriteQueue(head);
     }
 
     @Override
     public Iterator<ReferenceEntry<K, V>> iterator() {
       return new AbstractSequentialIterator<ReferenceEntry<K, V>>(peek()) {
         @Override
         protected ReferenceEntry<K, V> computeNext(ReferenceEntry<K, V> previous) {
           ReferenceEntry<K, V> next = previous.getNextInWriteQueue();
           return (next == head) ? null : next;
         }
       };
     }
   }
 
   /**
    * A custom queue for managing access order. Note that this is tightly integrated with
    * {@code ReferenceEntry}, upon which it reliese to perform its linking.
    *
    * <p>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.
    *
    * <p>The benefits of creating our own queue are that (1) we can replace elements in the middle
    * of the queue as part of copyWriteEntry, and (2) the contains method is highly optimized
    * for the current model.
    */
   static final class AccessQueue<K, V> extends AbstractQueue<ReferenceEntry<K, V>> {
     final ReferenceEntry<K, V> head = new AbstractReferenceEntry<K, V>() {
 
       @Override
       public long getAccessTime() {
         return Long.MAX_VALUE;
       }
 
       @Override
       public void setAccessTime(long time) {}
 
       ReferenceEntry<K, V> nextAccess = this;
 
       @Override
       public ReferenceEntry<K, V> getNextInAccessQueue() {
         return nextAccess;
       }
 
       @Override
       public void setNextInAccessQueue(ReferenceEntry<K, V> next) {
         this.nextAccess = next;
       }
 
       ReferenceEntry<K, V> previousAccess = this;
 
       @Override
       public ReferenceEntry<K, V> getPreviousInAccessQueue() {
         return previousAccess;
       }
 
       @Override
       public void setPreviousInAccessQueue(ReferenceEntry<K, V> previous) {
         this.previousAccess = previous;
       }
     };
 
     // implements Queue
 
     @Override
     public boolean offer(ReferenceEntry<K, V> entry) {
       // unlink
       connectAccessOrder(entry.getPreviousInAccessQueue(), entry.getNextInAccessQueue());
 
       // add to tail
       connectAccessOrder(head.getPreviousInAccessQueue(), entry);
       connectAccessOrder(entry, head);
 
       return true;
     }
 
     @Override
     public ReferenceEntry<K, V> peek() {
       ReferenceEntry<K, V> next = head.getNextInAccessQueue();
       return (next == head) ? null : next;
     }
 
     @Override
     public ReferenceEntry<K, V> poll() {
       ReferenceEntry<K, V> next = head.getNextInAccessQueue();
       if (next == head) {
         return null;
       }
 
       remove(next);
       return next;
     }
 
     @Override
     @SuppressWarnings("unchecked")
     public boolean remove(Object o) {
       ReferenceEntry<K, V> e = (ReferenceEntry) o;
       ReferenceEntry<K, V> previous = e.getPreviousInAccessQueue();
       ReferenceEntry<K, V> next = e.getNextInAccessQueue();
       connectAccessOrder(previous, next);
       nullifyAccessOrder(e);
 
       return next != NullEntry.INSTANCE;
     }
 
     @Override
     @SuppressWarnings("unchecked")
     public boolean contains(Object o) {
       ReferenceEntry<K, V> e = (ReferenceEntry) o;
       return e.getNextInAccessQueue() != NullEntry.INSTANCE;
     }
 
     @Override
     public boolean isEmpty() {
       return head.getNextInAccessQueue() == head;
     }
 
     @Override
     public int size() {
       int size = 0;
       for (ReferenceEntry<K, V> e = head.getNextInAccessQueue(); e != head;
           e = e.getNextInAccessQueue()) {
         size++;
       }
       return size;
     }
 
     @Override
     public void clear() {
       ReferenceEntry<K, V> e = head.getNextInAccessQueue();
       while (e != head) {
         ReferenceEntry<K, V> next = e.getNextInAccessQueue();
         nullifyAccessOrder(e);
         e = next;
       }
 
       head.setNextInAccessQueue(head);
       head.setPreviousInAccessQueue(head);
     }
 
     @Override
     public Iterator<ReferenceEntry<K, V>> iterator() {
       return new AbstractSequentialIterator<ReferenceEntry<K, V>>(peek()) {
         @Override
         protected ReferenceEntry<K, V> computeNext(ReferenceEntry<K, V> previous) {
           ReferenceEntry<K, V> next = previous.getNextInAccessQueue();
           return (next == head) ? null : next;
         }
       };
     }
   }
 
   // Cache support
 
   public void cleanUp() {
     for (Segment<?, ?> segment : segments) {
       segment.cleanUp();
     }
   }
 
   // ConcurrentMap methods
 
   @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<K, V>[] 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;
   }
 
   long longSize() {
     Segment<K, V>[] segments = this.segments;
     long sum = 0;
     for (int i = 0; i < segments.length; ++i) {
       sum += segments[i].count;
     }
     return sum;
   }
 
   @Override
   public int size() {
     return Ints.saturatedCast(longSize());
   }
 
   @Override
   @Nullable
   public V get(@Nullable Object key) {
     if (key == null) {
       return null;
     }
     int hash = hash(key);
     return segmentFor(hash).get(key, hash);
   }
 
   @Nullable
   public V getIfPresent(Object key) {
     int hash = hash(checkNotNull(key));
     V value = segmentFor(hash).get(key, hash);
     if (value == null) {
       globalStatsCounter.recordMisses(1);
     } else {
       globalStatsCounter.recordHits(1);
     }
     return value;
   }
 
   V get(K key, CacheLoader<? super K, V> loader) throws ExecutionException {
     int hash = hash(checkNotNull(key));
     return segmentFor(hash).get(key, hash, loader);
   }
 
   V getOrLoad(K key) throws ExecutionException {
     return get(key, defaultLoader);
   }
 
   ImmutableMap<K, V> getAllPresent(Iterable<?> keys) {
     int hits = 0;
     int misses = 0;
 
     Map<K, V> result = Maps.newLinkedHashMap();
     for (Object key : keys) {
       V value = get(key);
       if (value == null) {
         misses++;
       } else {
         // TODO(fry): store entry key instead of query key
         @SuppressWarnings("unchecked")
         K castKey = (K) key;
         result.put(castKey, value);
         hits++;
       }
     }
     globalStatsCounter.recordHits(hits);
     globalStatsCounter.recordMisses(misses);
     return ImmutableMap.copyOf(result);
   }
 
   ImmutableMap<K, V> getAll(Iterable<? extends K> keys) throws ExecutionException {
     int hits = 0;
     int misses = 0;
 
     Map<K, V> result = Maps.newLinkedHashMap();
     Set<K> keysToLoad = Sets.newLinkedHashSet();
     for (K key : keys) {
       V value = get(key);
       if (!result.containsKey(key)) {
         result.put(key, value);
         if (value == null) {
           misses++;
           keysToLoad.add(key);
         } else {
           hits++;
         }
       }
     }
 
     try {
       if (!keysToLoad.isEmpty()) {
         try {
           Map<K, V> newEntries = loadAll(keysToLoad, defaultLoader);
           for (K key : keysToLoad) {
             V value = newEntries.get(key);
             if (value == null) {
               throw new InvalidCacheLoadException("loadAll failed to return a value for " + key);
             }
             result.put(key, value);
           }
         } catch (UnsupportedLoadingOperationException e) {
           // loadAll not implemented, fallback to load
           for (K key : keysToLoad) {
             misses--; // get will count this miss
             result.put(key, get(key, defaultLoader));
           }
         }
       }
       return ImmutableMap.copyOf(result);
     } finally {
       globalStatsCounter.recordHits(hits);
       globalStatsCounter.recordMisses(misses);
     }
   }
 
   /**
    * Returns the result of calling {@link CacheLoader#loadAll}, or null if {@code loader} doesn't
    * implement {@code loadAll}.
    */
   @Nullable
   Map<K, V> loadAll(Set<? extends K> keys, CacheLoader<? super K, V> loader)
       throws ExecutionException {
     checkNotNull(loader);
     checkNotNull(keys);
     Stopwatch stopwatch = Stopwatch.createStarted();
     Map<K, V> result;
     boolean success = false;
     try {
       @SuppressWarnings("unchecked") // safe since all keys extend K
       Map<K, V> map = (Map<K, V>) loader.loadAll(keys);
       result = map;
       success = true;
     } catch (UnsupportedLoadingOperationException e) {
       success = true;
       throw e;
     } catch (InterruptedException e) {
       Thread.currentThread().interrupt();
       throw new ExecutionException(e);
     } catch (RuntimeException e) {
       throw new UncheckedExecutionException(e);
     } catch (Exception e) {
       throw new ExecutionException(e);
     } catch (Error e) {
       throw new ExecutionError(e);
     } finally {
       if (!success) {
         globalStatsCounter.recordLoadException(stopwatch.elapsed(NANOSECONDS));
       }
     }
 
     if (result == null) {
       globalStatsCounter.recordLoadException(stopwatch.elapsed(NANOSECONDS));
       throw new InvalidCacheLoadException(loader + " returned null map from loadAll");
     }
 
     stopwatch.stop();
     // TODO(fry): batch by segment
     boolean nullsPresent = false;
     for (Map.Entry<K, V> entry : result.entrySet()) {
       K key = entry.getKey();
       V value = entry.getValue();
       if (key == null || value == null) {
         // delay failure until non-null entries are stored
         nullsPresent = true;
       } else {
         put(key, value);
       }
     }
 
     if (nullsPresent) {
       globalStatsCounter.recordLoadException(stopwatch.elapsed(NANOSECONDS));
       throw new InvalidCacheLoadException(loader + " returned null keys or values from loadAll");
     }
 
     // TODO(fry): record count of loaded entries
     globalStatsCounter.recordLoadSuccess(stopwatch.elapsed(NANOSECONDS));
     return result;
   }
 
   /**
    * Returns the internal entry for the specified key. The entry may be loading, expired, or
    * partially collected.
    */
   ReferenceEntry<K, V> getEntry(@Nullable Object key) {
     // does not impact recency ordering
     if (key == null) {
       return null;
     }
     int hash = hash(key);
     return segmentFor(hash).getEntry(key, hash);
   }
 
   void refresh(K key) {
     int hash = hash(checkNotNull(key));
     segmentFor(hash).refresh(key, hash, defaultLoader, false);
   }
 
   @Override
   public boolean containsKey(@Nullable Object key) {
     // does not impact recency ordering
     if (key == null) {
       return false;
     }
     int hash = hash(key);
     return segmentFor(hash).containsKey(key, hash);
   }
 
   @Override
   public boolean containsValue(@Nullable Object value) {
     // does not impact recency ordering
     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.
     long now = ticker.read();
     final Segment<K,V>[] segments = this.segments;
     long last = -1L;
     for (int i = 0; i < CONTAINS_VALUE_RETRIES; i++) {
       long sum = 0L;
       for (Segment<K, V> segment : segments) {
         // ensure visibility of most recent completed write
         @SuppressWarnings({"UnusedDeclaration", "unused"})
         int c = segment.count; // read-volatile
 
         AtomicReferenceArray<ReferenceEntry<K, V>> table = segment.table;
         for (int j = 0 ; j < table.length(); j++) {
           for (ReferenceEntry<K, V> e = table.get(j); e != null; e = e.getNext()) {
             V v = segment.getLiveValue(e, now);
             if (v != null && valueEquivalence.equivalent(value, v)) {
               return true;
             }
           }
         }
         sum += segment.modCount;
       }
       if (sum == last) {
         break;
       }
       last = sum;
     }
     return false;
   }
 
   @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<? extends K, ? extends V> m) {
     for (Entry<? extends K, ? extends V> e : m.entrySet()) {
       put(e.getKey(), e.getValue());
     }
   }
 
   @Override
   public V remove(@Nullable Object key) {
     if (key == null) {
       return null;
     }
     int hash = hash(key);
     return segmentFor(hash).remove(key, hash);
   }
 
   @Override
   public boolean remove(@Nullable Object key, @Nullable 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, @Nullable 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<K, V> segment : segments) {
       segment.clear();
     }
   }
 
   void invalidateAll(Iterable<?> keys) {
     // TODO(fry): batch by segment
     for (Object key : keys) {
       remove(key);
     }
   }
 
   Set<K> keySet;
 
   @Override
   public Set<K> keySet() {
     // does not impact recency ordering
     Set<K> ks = keySet;
     return (ks != null) ? ks : (keySet = new KeySet(this));
   }
 
   Collection<V> values;
 
   @Override
   public Collection<V> values() {
     // does not impact recency ordering
     Collection<V> vs = values;
     return (vs != null) ? vs : (values = new Values(this));
   }
 
   Set<Entry<K, V>> entrySet;
 
   @Override
   @GwtIncompatible("Not supported.")
   public Set<Entry<K, V>> entrySet() {
     // does not impact recency ordering
     Set<Entry<K, V>> es = entrySet;
     return (es != null) ? es : (entrySet = new EntrySet(this));
   }
 
   // Iterator Support
 
   abstract class HashIterator<T> implements Iterator<T> {
 
     int nextSegmentIndex;
     int nextTableIndex;
     Segment<K, V> currentSegment;
     AtomicReferenceArray<ReferenceEntry<K, V>> currentTable;
     ReferenceEntry<K, V> nextEntry;
     WriteThroughEntry nextExternal;
     WriteThroughEntry lastReturned;
 
     HashIterator() {
       nextSegmentIndex = segments.length - 1;
       nextTableIndex = -1;
       advance();
     }
 
     @Override
     public abstract T 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 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 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 true if the entry was valid, false if it should be
      * skipped.
      */
     boolean advanceTo(ReferenceEntry<K, V> entry) {
       try {
         long now = ticker.read();
         K key = entry.getKey();
         V value = getLiveValue(entry, now);
         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() {
       checkState(lastReturned != null);
       LocalCache.this.remove(lastReturned.getKey());
       lastReturned = null;
     }
   }
 
   final class KeyIterator extends HashIterator<K> {
 
     @Override
     public K next() {
       return nextEntry().getKey();
     }
   }
 
   final class ValueIterator extends HashIterator<V> {
 
     @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 implements Entry<K, V> {
     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(@Nullable 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) {
       throw new UnsupportedOperationException();
     }
 
     /**
      * Returns a string representation of the form <code>{key}={value}</code>.
      */
     @Override public String toString() {
       return getKey() + "=" + getValue();
     }
   }
 
   final class EntryIterator extends HashIterator<Entry<K, V>> {
 
     @Override
     public Entry<K, V> next() {
       return nextEntry();
     }
   }
 
   abstract class AbstractCacheSet<T> extends AbstractSet<T> {
     final ConcurrentMap<?, ?> map;
 
     AbstractCacheSet(ConcurrentMap<?, ?> map) {
       this.map = map;
     }
 
     @Override
     public int size() {
       return map.size();
     }
 
     @Override
     public boolean isEmpty() {
       return map.isEmpty();
     }
 
     @Override
     public void clear() {
       map.clear();
     }
   }
 
   final class KeySet extends AbstractCacheSet<K> {
 
     KeySet(ConcurrentMap<?, ?> map) {
       super(map);
     }
 
     @Override
     public Iterator<K> iterator() {
       return new KeyIterator();
     }
 
     @Override
     public boolean contains(Object o) {
       return map.containsKey(o);
     }
 
     @Override
     public boolean remove(Object o) {
       return map.remove(o) != null;
     }
   }
 
   final class Values extends AbstractCollection<V> {
     private final ConcurrentMap<?, ?> map;
 
     Values(ConcurrentMap<?, ?> map) {
       this.map = map;
     }
 
     @Override public int size() {
       return map.size();
     }
 
     @Override public boolean isEmpty() {
       return map.isEmpty();
     }
 
     @Override public void clear() {
       map.clear();
     }
 
     @Override
     public Iterator<V> iterator() {
       return new ValueIterator();
     }
 
     @Override
     public boolean contains(Object o) {
       return map.containsValue(o);
     }
   }
 
   final class EntrySet extends AbstractCacheSet<Entry<K, V>> {
 
     EntrySet(ConcurrentMap<?, ?> map) {
       super(map);
     }
 
     @Override
     public Iterator<Entry<K, V>> 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 = LocalCache.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 && LocalCache.this.remove(key, e.getValue());
     }
   }
 
   // Serialization Support
 
   /**
    * Serializes the configuration of a LocalCache, reconsitituting it as a Cache using
    * CacheBuilder upon deserialization. An instance of this class is fit for use by the writeReplace
    * of LocalManualCache.
    *
    * Unfortunately, readResolve() doesn't get called when a circular dependency is present, so the
    * proxy must be able to behave as the cache itself.
    */
   static class ManualSerializationProxy<K, V>
       extends ForwardingCache<K, V> implements Serializable {
     private static final long serialVersionUID = 1;
 
     final Strength keyStrength;
     final Strength valueStrength;
     final Equivalence<Object> keyEquivalence;
     final Equivalence<Object> valueEquivalence;
     final long expireAfterWriteNanos;
     final long expireAfterAccessNanos;
     final long maxWeight;
     final Weigher<K, V> weigher;
     final int concurrencyLevel;
     final RemovalListener<? super K, ? super V> removalListener;
     final Ticker ticker;
     final CacheLoader<? super K, V> loader;
 
     transient Cache<K, V> delegate;
 
     ManualSerializationProxy(LocalCache<K, V> cache) {
       this(
           cache.keyStrength,
           cache.valueStrength,
           cache.keyEquivalence,
           cache.valueEquivalence,
           cache.expireAfterWriteNanos,
           cache.expireAfterAccessNanos,
           cache.maxWeight,
           cache.weigher,
           cache.concurrencyLevel,
           cache.removalListener,
           cache.ticker,
           cache.defaultLoader);
     }
 
     private ManualSerializationProxy(
         Strength keyStrength, Strength valueStrength,
         Equivalence<Object> keyEquivalence, Equivalence<Object> valueEquivalence,
         long expireAfterWriteNanos, long expireAfterAccessNanos, long maxWeight,
         Weigher<K, V> weigher, int concurrencyLevel,
         RemovalListener<? super K, ? super V> removalListener,
         Ticker ticker, CacheLoader<? super K, V> loader) {
       this.keyStrength = keyStrength;
       this.valueStrength = valueStrength;
       this.keyEquivalence = keyEquivalence;
       this.valueEquivalence = valueEquivalence;
       this.expireAfterWriteNanos = expireAfterWriteNanos;
       this.expireAfterAccessNanos = expireAfterAccessNanos;
       this.maxWeight = maxWeight;
       this.weigher = weigher;
       this.concurrencyLevel = concurrencyLevel;
       this.removalListener = removalListener;
       this.ticker = (ticker == Ticker.systemTicker() || ticker == NULL_TICKER)
           ? null : ticker;
       this.loader = loader;
     }
 
    CacheBuilder<K, V> recreateCacheBuilder() {
       CacheBuilder<K, V> builder = CacheBuilder.newBuilder()
           .setKeyStrength(keyStrength)
           .setValueStrength(valueStrength)
           .keyEquivalence(keyEquivalence)
           .valueEquivalence(valueEquivalence)
           .concurrencyLevel(concurrencyLevel)
           .removalListener(removalListener);
       builder.strictParsing = false;
       if (expireAfterWriteNanos > 0) {
         builder.expireAfterWrite(expireAfterWriteNanos, TimeUnit.NANOSECONDS);
       }
       if (expireAfterAccessNanos > 0) {
         builder.expireAfterAccess(expireAfterAccessNanos, TimeUnit.NANOSECONDS);
       }
       if (weigher != OneWeigher.INSTANCE) {
         builder.weigher(weigher);
         if (maxWeight != UNSET_INT) {
           builder.maximumWeight(maxWeight);
         }
       } else {
         if (maxWeight != UNSET_INT) {
           builder.maximumSize(maxWeight);
         }
       }
       if (ticker != null) {
         builder.ticker(ticker);
       }
       return builder;
     }
 
     private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
       in.defaultReadObject();
       CacheBuilder<K, V> builder = recreateCacheBuilder();
       this.delegate = builder.build();
     }
 
     private Object readResolve() {
       return delegate;
     }
 
     @Override
     protected Cache<K, V> delegate() {
       return delegate;
     }
   }
 
   /**
    * Serializes the configuration of a LocalCache, reconsitituting it as an LoadingCache using
    * CacheBuilder upon deserialization. An instance of this class is fit for use by the writeReplace
    * of LocalLoadingCache.
    *
    * Unfortunately, readResolve() doesn't get called when a circular dependency is present, so the
    * proxy must be able to behave as the cache itself.
    */
   static final class LoadingSerializationProxy<K, V>
       extends ManualSerializationProxy<K, V> implements LoadingCache<K, V>, Serializable {
     private static final long serialVersionUID = 1;
 
     transient LoadingCache<K, V> autoDelegate;
 
     LoadingSerializationProxy(LocalCache<K, V> cache) {
       super(cache);
     }
 
     private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
       in.defaultReadObject();
       CacheBuilder<K, V> builder = recreateCacheBuilder();
       this.autoDelegate = builder.build(loader);
     }
 
     @Override
     public V get(K key) throws ExecutionException {
       return autoDelegate.get(key);
     }
 
     @Override
     public V getUnchecked(K key) {
       return autoDelegate.getUnchecked(key);
     }
 
     @Override
     public ImmutableMap<K, V> getAll(Iterable<? extends K> keys) throws ExecutionException {
       return autoDelegate.getAll(keys);
     }
 
     @Override
     public final V apply(K key) {
       return autoDelegate.apply(key);
     }
 
     @Override
     public void refresh(K key) {
       autoDelegate.refresh(key);
     }
 
     private Object readResolve() {
       return autoDelegate;
     }
   }
 
   static class LocalManualCache<K, V> implements Cache<K, V>, Serializable {
     final LocalCache<K, V> localCache;
 
     LocalManualCache(CacheBuilder<? super K, ? super V> builder) {
       this(new LocalCache<K, V>(builder, null));
     }
 
     private LocalManualCache(LocalCache<K, V> localCache) {
       this.localCache = localCache;
     }
 
     // Cache methods
 
     @Override
     @Nullable
     public V getIfPresent(Object key) {
       return localCache.getIfPresent(key);
     }
 
     @Override
     public V get(K key, final Callable<? extends V> valueLoader) throws ExecutionException {
       checkNotNull(valueLoader);
       return localCache.get(key, new CacheLoader<Object, V>() {
         @Override
         public V load(Object key) throws Exception {
           return valueLoader.call();
         }
       });
     }
 
     @Override
     public ImmutableMap<K, V> getAllPresent(Iterable<?> keys) {
       return localCache.getAllPresent(keys);
     }
 
     @Override
     public void put(K key, V value) {
       localCache.put(key, value);
     }
 
     @Override
     public void putAll(Map<? extends K, ? extends V> m) {
       localCache.putAll(m);
     }
 
     @Override
     public void invalidate(Object key) {
       checkNotNull(key);
       localCache.remove(key);
     }
 
     @Override
     public void invalidateAll(Iterable<?> keys) {
       localCache.invalidateAll(keys);
     }
 
     @Override
     public void invalidateAll() {
       localCache.clear();
     }
 
     @Override
     public long size() {
       return localCache.longSize();
     }
 
     @Override
     public ConcurrentMap<K, V> asMap() {
       return localCache;
     }
 
     @Override
     public CacheStats stats() {
       SimpleStatsCounter aggregator = new SimpleStatsCounter();
       aggregator.incrementBy(localCache.globalStatsCounter);
       for (Segment<K, V> segment : localCache.segments) {
         aggregator.incrementBy(segment.statsCounter);
       }
       return aggregator.snapshot();
     }
 
     @Override
     public void cleanUp() {
       localCache.cleanUp();
     }
 
     // Serialization Support
 
     private static final long serialVersionUID = 1;
 
     Object writeReplace() {
       return new ManualSerializationProxy<K, V>(localCache);
     }
   }
 
   static class LocalLoadingCache<K, V>
       extends LocalManualCache<K, V> implements LoadingCache<K, V> {
 
     LocalLoadingCache(CacheBuilder<? super K, ? super V> builder,
         CacheLoader<? super K, V> loader) {
       super(new LocalCache<K, V>(builder, checkNotNull(loader)));
     }
 
     // LoadingCache methods
 
     @Override
     public V get(K key) throws ExecutionException {
       return localCache.getOrLoad(key);
     }
 
     @Override
     public V getUnchecked(K key) {
       try {
         return get(key);
       } catch (ExecutionException e) {
         throw new UncheckedExecutionException(e.getCause());
       }
     }
 
     @Override
     public ImmutableMap<K, V> getAll(Iterable<? extends K> keys) throws ExecutionException {
       return localCache.getAll(keys);
     }
 
     @Override
     public void refresh(K key) {
       localCache.refresh(key);
     }
 
     @Override
     public final V apply(K key) {
       return getUnchecked(key);
     }
 
     // Serialization Support
 
     private static final long serialVersionUID = 1;
 
     @Override
     Object writeReplace() {
       return new LoadingSerializationProxy<K, V>(localCache);
     }
   }
 }