xref: /external/guava/guava/src/com/google/common/cache/LocalCache.java

/*
 * 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.VisibleForTesting;
import com.google.common.base.Equivalence;
import com.google.common.base.Equivalences;
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.AbstractLinkedIterator;
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 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.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})
 */
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, 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 power-of-two sizes best matching arguments. Constraints:
    // (segmentCount <= maxWeight)
    // && (concurrencyLevel > maxWeight || segmentCount > concurrencyLevel)
    int segmentShift = 0;
    int segmentCount = 1;
    while (segmentCount < concurrencyLevel
        && (!evictsBySize() || customWeigher() || segmentCount * 2 <= 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 Equivalences.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 Equivalences.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 Equivalences.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.
     */
    ValueReference<K, V> copyFor(ReferenceQueue<V> queue, 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, 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> implements ReferenceEntry<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;
    }

    // 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 StrongAccessEntry<K, V> extends StrongEntry<K, V>
      implements ReferenceEntry<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> implements ReferenceEntry<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> implements ReferenceEntry<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();
    }

    // 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> implements ReferenceEntry<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> implements ReferenceEntry<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> implements ReferenceEntry<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, ReferenceEntry<K, V> entry) {
      return new WeakValueReference<K, V>(queue, get(), 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, ReferenceEntry<K, V> entry) {
      return new SoftValueReference<K, V>(queue, get(), 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, 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, ReferenceEntry<K, V> entry) {
      return new WeightedWeakValueReference<K, V>(queue, get(), 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, ReferenceEntry<K, V> entry) {
      return new WeightedSoftValueReference<K, V>(queue, get(), 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, value, weight);
  }

  int hash(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) {
    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 = 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, key, hash, next);
    }

    @GuardedBy("Segment.this")
    ReferenceEntry<K, V> copyEntry(ReferenceEntry<K, V> original, ReferenceEntry<K, V> newNext) {
      ValueReference<K, V> valueReference = original.getValueReference();
      ReferenceEntry<K, V> newEntry = map.entryFactory.copyEntry(this, original, newNext);
      newEntry.setValueReference(valueReference.copyFor(this.valueReferenceQueue, 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 {
      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");
      // 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);
                // update loadingFuture for the sake of other pending requests
                loadingValueReference.set(newValue);
              } 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)) {
        V newValue = refresh(key, hash, loader);
        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) {
      final LoadingValueReference<K, V> loadingValueReference =
          insertLoadingValueReference(key, hash);
      if (loadingValueReference == null) {
        return null;
      }

      ListenableFuture<V> result = loadAsync(key, hash, loadingValueReference, loader);
      if (result.isDone()) {
        try {
          return result.get();
        } 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) {
      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()) {
              // refresh is a no-op if loading is pending
              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()) {
              if (isCollected(e)) {
                removeCollectedEntry(e);
                newCount--;
              } else {
                int newIndex = e.getHash() & newMask;
                ReferenceEntry<K, V> newNext = newTable.get(newIndex);
                ReferenceEntry<K, V> newFirst = copyEntry(e, newNext);
                newTable.set(newIndex, newFirst);
              }
            }
          }
        }
      }
      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;
        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 || oldValueReference == valueReference) {
              ++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()) {
        if (isCollected(e)) {
          removeCollectedEntry(e);
          newCount--;
        } else {
          newFirst = copyEntry(e, newFirst);
        }
      }
      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;
    }

    /**
     * Returns true if the entry has been partially collected, meaning that either the key is null,
     * or the value is active but null.
     */
    boolean isCollected(ReferenceEntry<K, V> entry) {
      if (entry.getKey() == null) {
        return true;
      }
      ValueReference<K, V> valueReference = entry.getValueReference();
      return (valueReference.get() == null) && valueReference.isActive();
    }

    /**
     * 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 = new Stopwatch();

    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 setException(futureValue, t);
    }

    private static boolean setException(SettableFuture<?> future, Throwable t) {
      try {
        return future.setException(t);
      } catch (Error e) {
        // the error will already be propagated by the loading thread
        return false;
      }
    }

    private ListenableFuture<V> fullyFailedFuture(Throwable t) {
      SettableFuture<V> future = SettableFuture.create();
      setException(future, t);
      return future;
    }

    @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);
        } else {
          ListenableFuture<V> newValue = loader.reload(key, previousValue);
          // rely on loadAsync to call set in order to avoid adding a second listener here
          return newValue != null ? newValue : Futures.<V>immediateFuture(null);
        }
      } catch (Throwable t) {
        return setException(t) ? futureValue : fullyFailedFuture(t);
      }
    }

    public long elapsedNanos() {
      return stopwatch.elapsedTime(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, 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 AbstractLinkedIterator<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 AbstractLinkedIterator<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<? extends K> keys) {
    int hits = 0;
    int misses = 0;

    Map<K, V> result = Maps.newLinkedHashMap();
    for (K key : keys) {
      V value = get(key);
      if (value == null) {
        misses++;
      } else {
        result.put(key, 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 {
    Stopwatch stopwatch = new Stopwatch().start();
    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 (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.elapsedTime(NANOSECONDS));
      }
    }

    if (result == null) {
      globalStatsCounter.recordLoadException(stopwatch.elapsedTime(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.elapsedTime(NANOSECONDS));
      throw new InvalidCacheLoadException(loader + " returned null keys or values from loadAll");
    }

    // TODO(fry): record count of loaded entries
    globalStatsCounter.recordLoadSuccess(stopwatch.elapsedTime(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);
  }

  /**
   * Returns the live internal entry for the specified key.
   */
  ReferenceEntry<K, V> getLiveEntry(@Nullable Object key) {
    // does not impact recency ordering
    if (key == null) {
      return null;
    }
    int hash = hash(key);
    return segmentFor(hash).getLiveEntry(key, hash, ticker.read());
  }

  void refresh(K key) {
    int hash = hash(checkNotNull(key));
    segmentFor(hash).refresh(key, hash, defaultLoader);
  }

  @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());
  }

  Collection<V> values;

  @Override
  public Collection<V> values() {
    // does not impact recency ordering
    Collection<V> vs = values;
    return (vs != null) ? vs : (values = new Values());
  }

  Set<Entry<K, V>> entrySet;

  @Override
  public Set<Entry<K, V>> entrySet() {
    // does not impact recency ordering
    Set<Entry<K, V>> es = entrySet;
    return (es != null) ? es : (entrySet = new EntrySet());
  }

  // Iterator Support

  abstract class HashIterator {

    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();
    }

    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();
      }
    }

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

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

    public void remove() {
      checkState(lastReturned != null);
      LocalCache.this.remove(lastReturned.getKey());
      lastReturned = null;
    }
  }

  final class KeyIterator extends HashIterator implements Iterator<K> {

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

  final class ValueIterator extends HashIterator implements Iterator<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 implements Iterator<Entry<K, V>> {

    @Override
    public Entry<K, V> next() {
      return nextEntry();
    }
  }

  final class KeySet extends AbstractSet<K> {

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

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

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

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

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

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

  final class Values extends AbstractCollection<V> {

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

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

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

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

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

  final class EntrySet extends AbstractSet<Entry<K, V>> {

    @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());
    }

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

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

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

  // 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<Object, Object> recreateCacheBuilder() {
      CacheBuilder<Object, Object> builder = CacheBuilder.newBuilder()
          .setKeyStrength(keyStrength)
          .setValueStrength(valueStrength)
          .keyEquivalence(keyEquivalence)
          .valueEquivalence(valueEquivalence)
          .concurrencyLevel(concurrencyLevel);
      builder.strictParsing = false;
      builder.removalListener(removalListener);
      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<Object, Object> 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<Object, Object> 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(builder, null);
    }

    protected LocalManualCache(CacheBuilder<? super K, ? super V> builder,
        CacheLoader<? super K, V> loader) {
      this.localCache = new LocalCache<K, V>(builder, loader);
    }

    // Cache methods

    @Override
    @Nullable
    public V getIfPresent(K 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<? extends K> keys) {
      return localCache.getAllPresent(keys);
    }

    @Override
    public void put(K key, V value) {
      localCache.put(key, value);
    }

    @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();
    }

    /*
     * These methods have been moved to LoadingCache, but they temporarily
     * remain in Cache in Guava.
     */

    public V get(K key) throws ExecutionException {
      return localCache.getOrLoad(key);
    }

    public V getUnchecked(K key) {
      try {
        return get(key);
      } catch (ExecutionException e) {
        throw new UncheckedExecutionException(e.getCause());
      }
    }

    public final V apply(K key) {
      return getUnchecked(key);
    }

    // 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(builder, checkNotNull(loader));
    }

    // Cache methods

    @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);
    }

    // Serialization Support

    private static final long serialVersionUID = 1;

    Object writeReplace() {
      return new LoadingSerializationProxy<K, V>(localCache);
    }
  }
}