/*
 * Copyright 2002-2021 the original author or 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
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.avro.util.springframework;

import org.apache.avro.reflect.Nullable;

import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.lang.reflect.Array;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.Collections;
import java.util.EnumSet;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.ReentrantLock;

/**
 * A {@link ConcurrentHashMap} that uses {@link ReferenceType#SOFT soft} or
 * {@linkplain ReferenceType#WEAK weak} references for both {@code keys} and
 * {@code values}.
 *
 * <p>
 * This class can be used as an alternative to
 * {@code Collections.synchronizedMap(new WeakHashMap<K, Reference<V>>())} in
 * order to support better performance when accessed concurrently. This
 * implementation follows the same design constraints as
 * {@link ConcurrentHashMap} with the exception that {@code null} values and
 * {@code null} keys are supported.
 *
 * <p>
 * <b>NOTE:</b> The use of references means that there is no guarantee that
 * items placed into the map will be subsequently available. The garbage
 * collector may discard references at any time, so it may appear that an
 * unknown thread is silently removing entries.
 *
 * <p>
 * If not explicitly specified, this implementation will use
 * {@linkplain SoftReference soft entry references}.
 *
 * @param <K> the key type
 * @param <V> the value type
 * @author Phillip Webb
 * @author Juergen Hoeller
 * @since 3.2
 */
public class ConcurrentReferenceHashMap<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V> {

  private static final int DEFAULT_INITIAL_CAPACITY = 16;

  private static final float DEFAULT_LOAD_FACTOR = 0.75f;

  private static final int DEFAULT_CONCURRENCY_LEVEL = 16;

  private static final ReferenceType DEFAULT_REFERENCE_TYPE = ReferenceType.SOFT;

  private static final int MAXIMUM_CONCURRENCY_LEVEL = 1 << 16;

  private static final int MAXIMUM_SEGMENT_SIZE = 1 << 30;

  /**
   * Array of segments indexed using the high order bits from the hash.
   */
  private final Segment[] segments;

  /**
   * When the average number of references per table exceeds this value resize
   * will be attempted.
   */
  private final float loadFactor;

  /**
   * The reference type: SOFT or WEAK.
   */
  private final ReferenceType referenceType;

  /**
   * The shift value used to calculate the size of the segments array and an index
   * from the hash.
   */
  private final int shift;

  /**
   * Late binding entry set.
   */
  @Nullable
  private volatile Set<Map.Entry<K, V>> entrySet;

  /**
   * Create a new {@code ConcurrentReferenceHashMap} instance.
   */
  public ConcurrentReferenceHashMap() {
    this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL, DEFAULT_REFERENCE_TYPE);
  }

  /**
   * Create a new {@code ConcurrentReferenceHashMap} instance.
   *
   * @param initialCapacity the initial capacity of the map
   */
  public ConcurrentReferenceHashMap(int initialCapacity) {
    this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL, DEFAULT_REFERENCE_TYPE);
  }

  /**
   * Create a new {@code ConcurrentReferenceHashMap} instance.
   *
   * @param initialCapacity the initial capacity of the map
   * @param loadFactor      the load factor. When the average number of references
   *                        per table exceeds this value resize will be attempted
   */
  public ConcurrentReferenceHashMap(int initialCapacity, float loadFactor) {
    this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL, DEFAULT_REFERENCE_TYPE);
  }

  /**
   * Create a new {@code ConcurrentReferenceHashMap} instance.
   *
   * @param initialCapacity  the initial capacity of the map
   * @param concurrencyLevel the expected number of threads that will concurrently
   *                         write to the map
   */
  public ConcurrentReferenceHashMap(int initialCapacity, int concurrencyLevel) {
    this(initialCapacity, DEFAULT_LOAD_FACTOR, concurrencyLevel, DEFAULT_REFERENCE_TYPE);
  }

  /**
   * Create a new {@code ConcurrentReferenceHashMap} instance.
   *
   * @param initialCapacity the initial capacity of the map
   * @param referenceType   the reference type used for entries (soft or weak)
   */
  public ConcurrentReferenceHashMap(int initialCapacity, ReferenceType referenceType) {
    this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL, referenceType);
  }

  /**
   * Create a new {@code ConcurrentReferenceHashMap} instance.
   *
   * @param initialCapacity  the initial capacity of the map
   * @param loadFactor       the load factor. When the average number of
   *                         references per table exceeds this value, resize will
   *                         be attempted.
   * @param concurrencyLevel the expected number of threads that will concurrently
   *                         write to the map
   */
  public ConcurrentReferenceHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) {
    this(initialCapacity, loadFactor, concurrencyLevel, DEFAULT_REFERENCE_TYPE);
  }

  /**
   * Create a new {@code ConcurrentReferenceHashMap} instance.
   *
   * @param initialCapacity  the initial capacity of the map
   * @param loadFactor       the load factor. When the average number of
   *                         references per table exceeds this value, resize will
   *                         be attempted.
   * @param concurrencyLevel the expected number of threads that will concurrently
   *                         write to the map
   * @param referenceType    the reference type used for entries (soft or weak)
   */
  @SuppressWarnings("unchecked")
  public ConcurrentReferenceHashMap(int initialCapacity, float loadFactor, int concurrencyLevel,
      ReferenceType referenceType) {

    Assert.isTrue(initialCapacity >= 0, "Initial capacity must not be negative");
    Assert.isTrue(loadFactor > 0f, "Load factor must be positive");
    Assert.isTrue(concurrencyLevel > 0, "Concurrency level must be positive");
    Assert.notNull(referenceType, "Reference type must not be null");
    this.loadFactor = loadFactor;
    this.shift = calculateShift(concurrencyLevel, MAXIMUM_CONCURRENCY_LEVEL);
    int size = 1 << this.shift;
    this.referenceType = referenceType;
    int roundedUpSegmentCapacity = (int) ((initialCapacity + size - 1L) / size);
    int initialSize = 1 << calculateShift(roundedUpSegmentCapacity, MAXIMUM_SEGMENT_SIZE);
    Segment[] segments = (Segment[]) Array.newInstance(Segment.class, size);
    int resizeThreshold = (int) (initialSize * getLoadFactor());
    for (int i = 0; i < segments.length; i++) {
      segments[i] = new Segment(initialSize, resizeThreshold);
    }
    this.segments = segments;
  }

  protected final float getLoadFactor() {
    return this.loadFactor;
  }

  protected final int getSegmentsSize() {
    return this.segments.length;
  }

  protected final Segment getSegment(int index) {
    return this.segments[index];
  }

  /**
   * Factory method that returns the {@link ReferenceManager}. This method will be
   * called once for each {@link Segment}.
   *
   * @return a new reference manager
   */
  protected ReferenceManager createReferenceManager() {
    return new ReferenceManager();
  }

  /**
   * Get the hash for a given object, apply an additional hash function to reduce
   * collisions. This implementation uses the same Wang/Jenkins algorithm as
   * {@link ConcurrentHashMap}. Subclasses can override to provide alternative
   * hashing.
   *
   * @param o the object to hash (may be null)
   * @return the resulting hash code
   */
  protected int getHash(@Nullable Object o) {
    int hash = (o != null ? o.hashCode() : 0);
    hash += (hash << 15) ^ 0xffffcd7d;
    hash ^= (hash >>> 10);
    hash += (hash << 3);
    hash ^= (hash >>> 6);
    hash += (hash << 2) + (hash << 14);
    hash ^= (hash >>> 16);
    return hash;
  }

  @Override
  @Nullable
  public V get(@Nullable Object key) {
    Reference<K, V> ref = getReference(key, Restructure.WHEN_NECESSARY);
    Entry<K, V> entry = (ref != null ? ref.get() : null);
    return (entry != null ? entry.getValue() : null);
  }

  @Override
  @Nullable
  public V getOrDefault(@Nullable Object key, @Nullable V defaultValue) {
    Reference<K, V> ref = getReference(key, Restructure.WHEN_NECESSARY);
    Entry<K, V> entry = (ref != null ? ref.get() : null);
    return (entry != null ? entry.getValue() : defaultValue);
  }

  @Override
  public boolean containsKey(@Nullable Object key) {
    Reference<K, V> ref = getReference(key, Restructure.WHEN_NECESSARY);
    Entry<K, V> entry = (ref != null ? ref.get() : null);
    return (entry != null && ObjectUtils.nullSafeEquals(entry.getKey(), key));
  }

  /**
   * Return a {@link Reference} to the {@link Entry} for the specified
   * {@code key}, or {@code null} if not found.
   *
   * @param key         the key (can be {@code null})
   * @param restructure types of restructure allowed during this call
   * @return the reference, or {@code null} if not found
   */
  @Nullable
  protected final Reference<K, V> getReference(@Nullable Object key, Restructure restructure) {
    int hash = getHash(key);
    return getSegmentForHash(hash).getReference(key, hash, restructure);
  }

  @Override
  @Nullable
  public V put(@Nullable K key, @Nullable V value) {
    return put(key, value, true);
  }

  @Override
  @Nullable
  public V putIfAbsent(@Nullable K key, @Nullable V value) {
    return put(key, value, false);
  }

  @Nullable
  private V put(@Nullable final K key, @Nullable final V value, final boolean overwriteExisting) {
    return doTask(key, new Task<V>(TaskOption.RESTRUCTURE_BEFORE, TaskOption.RESIZE) {
      @Override
      @Nullable
      protected V execute(@Nullable Reference<K, V> ref, @Nullable Entry<K, V> entry, @Nullable Entries<V> entries) {
        if (entry != null) {
          V oldValue = entry.getValue();
          if (overwriteExisting) {
            entry.setValue(value);
          }
          return oldValue;
        }
        Assert.state(entries != null, "No entries segment");
        entries.add(value);
        return null;
      }
    });
  }

  @Override
  @Nullable
  public V remove(@Nullable Object key) {
    return doTask(key, new Task<V>(TaskOption.RESTRUCTURE_AFTER, TaskOption.SKIP_IF_EMPTY) {
      @Override
      @Nullable
      protected V execute(@Nullable Reference<K, V> ref, @Nullable Entry<K, V> entry) {
        if (entry != null) {
          if (ref != null) {
            ref.release();
          }
          return entry.value;
        }
        return null;
      }
    });
  }

  @Override
  public boolean remove(@Nullable Object key, final @Nullable Object value) {
    Boolean result = doTask(key, new Task<Boolean>(TaskOption.RESTRUCTURE_AFTER, TaskOption.SKIP_IF_EMPTY) {
      @Override
      protected Boolean execute(@Nullable Reference<K, V> ref, @Nullable Entry<K, V> entry) {
        if (entry != null && ObjectUtils.nullSafeEquals(entry.getValue(), value)) {
          if (ref != null) {
            ref.release();
          }
          return true;
        }
        return false;
      }
    });
    return (Boolean.TRUE.equals(result));
  }

  @Override
  public boolean replace(@Nullable K key, final @Nullable V oldValue, final @Nullable V newValue) {
    Boolean result = doTask(key, new Task<Boolean>(TaskOption.RESTRUCTURE_BEFORE, TaskOption.SKIP_IF_EMPTY) {
      @Override
      protected Boolean execute(@Nullable Reference<K, V> ref, @Nullable Entry<K, V> entry) {
        if (entry != null && ObjectUtils.nullSafeEquals(entry.getValue(), oldValue)) {
          entry.setValue(newValue);
          return true;
        }
        return false;
      }
    });
    return (Boolean.TRUE.equals(result));
  }

  @Override
  @Nullable
  public V replace(@Nullable K key, final @Nullable V value) {
    return doTask(key, new Task<V>(TaskOption.RESTRUCTURE_BEFORE, TaskOption.SKIP_IF_EMPTY) {
      @Override
      @Nullable
      protected V execute(@Nullable Reference<K, V> ref, @Nullable Entry<K, V> entry) {
        if (entry != null) {
          V oldValue = entry.getValue();
          entry.setValue(value);
          return oldValue;
        }
        return null;
      }
    });
  }

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

  /**
   * Remove any entries that have been garbage collected and are no longer
   * referenced. Under normal circumstances garbage collected entries are
   * automatically purged as items are added or removed from the Map. This method
   * can be used to force a purge, and is useful when the Map is read frequently
   * but updated less often.
   */
  public void purgeUnreferencedEntries() {
    for (Segment segment : this.segments) {
      segment.restructureIfNecessary(false);
    }
  }

  @Override
  public int size() {
    int size = 0;
    for (Segment segment : this.segments) {
      size += segment.getCount();
    }
    return size;
  }

  @Override
  public boolean isEmpty() {
    for (Segment segment : this.segments) {
      if (segment.getCount() > 0) {
        return false;
      }
    }
    return true;
  }

  @Override
  public Set<Map.Entry<K, V>> entrySet() {
    Set<Map.Entry<K, V>> entrySet = this.entrySet;
    if (entrySet == null) {
      entrySet = new EntrySet();
      this.entrySet = entrySet;
    }
    return entrySet;
  }

  @Nullable
  private <T> T doTask(@Nullable Object key, Task<T> task) {
    int hash = getHash(key);
    return getSegmentForHash(hash).doTask(hash, key, task);
  }

  private Segment getSegmentForHash(int hash) {
    return this.segments[(hash >>> (32 - this.shift)) & (this.segments.length - 1)];
  }

  /**
   * Calculate a shift value that can be used to create a power-of-two value
   * between the specified maximum and minimum values.
   *
   * @param minimumValue the minimum value
   * @param maximumValue the maximum value
   * @return the calculated shift (use {@code 1 << shift} to obtain a value)
   */
  protected static int calculateShift(int minimumValue, int maximumValue) {
    int shift = 0;
    int value = 1;
    while (value < minimumValue && value < maximumValue) {
      value <<= 1;
      shift++;
    }
    return shift;
  }

  /**
   * Various reference types supported by this map.
   */
  public enum ReferenceType {

    /**
     * Use {@link SoftReference SoftReferences}.
     */
    SOFT,

    /**
     * Use {@link WeakReference WeakReferences}.
     */
    WEAK
  }

  /**
   * A single segment used to divide the map to allow better concurrent
   * performance.
   */
  @SuppressWarnings("serial")
  protected final class Segment extends ReentrantLock {

    private final ReferenceManager referenceManager;

    private final int initialSize;

    /**
     * Array of references indexed using the low order bits from the hash. This
     * property should only be set along with {@code resizeThreshold}.
     */
    private volatile Reference<K, V>[] references;

    /**
     * The total number of references contained in this segment. This includes
     * chained references and references that have been garbage collected but not
     * purged.
     */
    private final AtomicInteger count = new AtomicInteger();

    /**
     * The threshold when resizing of the references should occur. When
     * {@code count} exceeds this value references will be resized.
     */
    private int resizeThreshold;

    public Segment(int initialSize, int resizeThreshold) {
      this.referenceManager = createReferenceManager();
      this.initialSize = initialSize;
      this.references = createReferenceArray(initialSize);
      this.resizeThreshold = resizeThreshold;
    }

    @Nullable
    public Reference<K, V> getReference(@Nullable Object key, int hash, Restructure restructure) {
      if (restructure == Restructure.WHEN_NECESSARY) {
        restructureIfNecessary(false);
      }
      if (this.count.get() == 0) {
        return null;
      }
      // Use a local copy to protect against other threads writing
      Reference<K, V>[] references = this.references;
      int index = getIndex(hash, references);
      Reference<K, V> head = references[index];
      return findInChain(head, key, hash);
    }

    /**
     * Apply an update operation to this segment. The segment will be locked during
     * the update.
     *
     * @param hash the hash of the key
     * @param key  the key
     * @param task the update operation
     * @return the result of the operation
     */
    @Nullable
    public <T> T doTask(final int hash, @Nullable final Object key, final Task<T> task) {
      boolean resize = task.hasOption(TaskOption.RESIZE);
      if (task.hasOption(TaskOption.RESTRUCTURE_BEFORE)) {
        restructureIfNecessary(resize);
      }
      if (task.hasOption(TaskOption.SKIP_IF_EMPTY) && this.count.get() == 0) {
        return task.execute(null, null, null);
      }
      lock();
      try {
        final int index = getIndex(hash, this.references);
        final Reference<K, V> head = this.references[index];
        Reference<K, V> ref = findInChain(head, key, hash);
        Entry<K, V> entry = (ref != null ? ref.get() : null);
        Entries<V> entries = value -> {
          @SuppressWarnings("unchecked")
          Entry<K, V> newEntry = new Entry<>((K) key, value);
          Reference<K, V> newReference = Segment.this.referenceManager.createReference(newEntry, hash, head);
          Segment.this.references[index] = newReference;
          Segment.this.count.incrementAndGet();
        };
        return task.execute(ref, entry, entries);
      } finally {
        unlock();
        if (task.hasOption(TaskOption.RESTRUCTURE_AFTER)) {
          restructureIfNecessary(resize);
        }
      }
    }

    /**
     * Clear all items from this segment.
     */
    public void clear() {
      if (this.count.get() == 0) {
        return;
      }
      lock();
      try {
        this.references = createReferenceArray(this.initialSize);
        this.resizeThreshold = (int) (this.references.length * getLoadFactor());
        this.count.set(0);
      } finally {
        unlock();
      }
    }

    /**
     * Restructure the underlying data structure when it becomes necessary. This
     * method can increase the size of the references table as well as purge any
     * references that have been garbage collected.
     *
     * @param allowResize if resizing is permitted
     */
    private void restructureIfNecessary(boolean allowResize) {
      int currCount = this.count.get();
      boolean needsResize = allowResize && (currCount > 0 && currCount >= this.resizeThreshold);
      Reference<K, V> ref = this.referenceManager.pollForPurge();
      if (ref != null || (needsResize)) {
        restructure(allowResize, ref);
      }
    }

    private void restructure(boolean allowResize, @Nullable Reference<K, V> ref) {
      boolean needsResize;
      lock();
      try {
        int countAfterRestructure = this.count.get();
        Set<Reference<K, V>> toPurge = Collections.emptySet();
        if (ref != null) {
          toPurge = new HashSet<>();
          while (ref != null) {
            toPurge.add(ref);
            ref = this.referenceManager.pollForPurge();
          }
        }
        countAfterRestructure -= toPurge.size();

        // Recalculate taking into account count inside lock and items that
        // will be purged
        needsResize = (countAfterRestructure > 0 && countAfterRestructure >= this.resizeThreshold);
        boolean resizing = false;
        int restructureSize = this.references.length;
        if (allowResize && needsResize && restructureSize < MAXIMUM_SEGMENT_SIZE) {
          restructureSize <<= 1;
          resizing = true;
        }

        // Either create a new table or reuse the existing one
        Reference<K, V>[] restructured = (resizing ? createReferenceArray(restructureSize) : this.references);

        // Restructure
        for (int i = 0; i < this.references.length; i++) {
          ref = this.references[i];
          if (!resizing) {
            restructured[i] = null;
          }
          while (ref != null) {
            if (!toPurge.contains(ref)) {
              Entry<K, V> entry = ref.get();
              if (entry != null) {
                int index = getIndex(ref.getHash(), restructured);
                restructured[index] = this.referenceManager.createReference(entry, ref.getHash(), restructured[index]);
              }
            }
            ref = ref.getNext();
          }
        }

        // Replace volatile members
        if (resizing) {
          this.references = restructured;
          this.resizeThreshold = (int) (this.references.length * getLoadFactor());
        }
        this.count.set(Math.max(countAfterRestructure, 0));
      } finally {
        unlock();
      }
    }

    @Nullable
    private Reference<K, V> findInChain(Reference<K, V> ref, @Nullable Object key, int hash) {
      Reference<K, V> currRef = ref;
      while (currRef != null) {
        if (currRef.getHash() == hash) {
          Entry<K, V> entry = currRef.get();
          if (entry != null) {
            K entryKey = entry.getKey();
            if (ObjectUtils.nullSafeEquals(entryKey, key)) {
              return currRef;
            }
          }
        }
        currRef = currRef.getNext();
      }
      return null;
    }

    @SuppressWarnings({ "unchecked" })
    private Reference<K, V>[] createReferenceArray(int size) {
      return new Reference[size];
    }

    private int getIndex(int hash, Reference<K, V>[] references) {
      return (hash & (references.length - 1));
    }

    /**
     * Return the size of the current references array.
     */
    public int getSize() {
      return this.references.length;
    }

    /**
     * Return the total number of references in this segment.
     */
    public int getCount() {
      return this.count.get();
    }
  }

  /**
   * A reference to an {@link Entry} contained in the map. Implementations are
   * usually wrappers around specific Java reference implementations (e.g.,
   * {@link SoftReference}).
   *
   * @param <K> the key type
   * @param <V> the value type
   */
  protected interface Reference<K, V> {

    /**
     * Return the referenced entry, or {@code null} if the entry is no longer
     * available.
     */
    @Nullable
    Entry<K, V> get();

    /**
     * Return the hash for the reference.
     */
    int getHash();

    /**
     * Return the next reference in the chain, or {@code null} if none.
     */
    @Nullable
    Reference<K, V> getNext();

    /**
     * Release this entry and ensure that it will be returned from
     * {@code ReferenceManager#pollForPurge()}.
     */
    void release();
  }

  /**
   * A single map entry.
   *
   * @param <K> the key type
   * @param <V> the value type
   */
  protected static final class Entry<K, V> implements Map.Entry<K, V> {

    @Nullable
    private final K key;

    @Nullable
    private volatile V value;

    public Entry(@Nullable K key, @Nullable V value) {
      this.key = key;
      this.value = value;
    }

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

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

    @Override
    @Nullable
    public V setValue(@Nullable V value) {
      V previous = this.value;
      this.value = value;
      return previous;
    }

    @Override
    public String toString() {
      return (this.key + "=" + this.value);
    }

    @Override
    @SuppressWarnings("rawtypes")
    public boolean equals(@Nullable Object other) {
      if (this == other) {
        return true;
      }
      if (!(other instanceof Map.Entry)) {
        return false;
      }
      Map.Entry otherEntry = (Map.Entry) other;
      return (ObjectUtils.nullSafeEquals(getKey(), otherEntry.getKey())
          && ObjectUtils.nullSafeEquals(getValue(), otherEntry.getValue()));
    }

    @Override
    public int hashCode() {
      return (ObjectUtils.nullSafeHashCode(this.key) ^ ObjectUtils.nullSafeHashCode(this.value));
    }
  }

  /**
   * A task that can be {@link Segment#doTask run} against a {@link Segment}.
   */
  private abstract class Task<T> {

    private final EnumSet<TaskOption> options;

    public Task(TaskOption... options) {
      this.options = (options.length == 0 ? EnumSet.noneOf(TaskOption.class) : EnumSet.of(options[0], options));
    }

    public boolean hasOption(TaskOption option) {
      return this.options.contains(option);
    }

    /**
     * Execute the task.
     *
     * @param ref     the found reference (or {@code null})
     * @param entry   the found entry (or {@code null})
     * @param entries access to the underlying entries
     * @return the result of the task
     * @see #execute(Reference, Entry)
     */
    @Nullable
    protected T execute(@Nullable Reference<K, V> ref, @Nullable Entry<K, V> entry, @Nullable Entries<V> entries) {
      return execute(ref, entry);
    }

    /**
     * Convenience method that can be used for tasks that do not need access to
     * {@link Entries}.
     *
     * @param ref   the found reference (or {@code null})
     * @param entry the found entry (or {@code null})
     * @return the result of the task
     * @see #execute(Reference, Entry, Entries)
     */
    @Nullable
    protected T execute(@Nullable Reference<K, V> ref, @Nullable Entry<K, V> entry) {
      return null;
    }
  }

  /**
   * Various options supported by a {@code Task}.
   */
  private enum TaskOption {

    RESTRUCTURE_BEFORE, RESTRUCTURE_AFTER, SKIP_IF_EMPTY, RESIZE
  }

  /**
   * Allows a task access to {@link Segment} entries.
   */
  private interface Entries<V> {

    /**
     * Add a new entry with the specified value.
     *
     * @param value the value to add
     */
    void add(@Nullable V value);
  }

  /**
   * Internal entry-set implementation.
   */
  private class EntrySet extends AbstractSet<Map.Entry<K, V>> {

    @Override
    public Iterator<Map.Entry<K, V>> iterator() {
      return new EntryIterator();
    }

    @Override
    public boolean contains(@Nullable Object o) {
      if (o instanceof Map.Entry<?, ?>) {
        Map.Entry<?, ?> entry = (Map.Entry<?, ?>) o;
        Reference<K, V> ref = ConcurrentReferenceHashMap.this.getReference(entry.getKey(), Restructure.NEVER);
        Entry<K, V> otherEntry = (ref != null ? ref.get() : null);
        if (otherEntry != null) {
          return ObjectUtils.nullSafeEquals(entry.getValue(), otherEntry.getValue());
        }
      }
      return false;
    }

    @Override
    public boolean remove(Object o) {
      if (o instanceof Map.Entry<?, ?>) {
        Map.Entry<?, ?> entry = (Map.Entry<?, ?>) o;
        return ConcurrentReferenceHashMap.this.remove(entry.getKey(), entry.getValue());
      }
      return false;
    }

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

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

  /**
   * Internal entry iterator implementation.
   */
  private class EntryIterator implements Iterator<Map.Entry<K, V>> {

    private int segmentIndex;

    private int referenceIndex;

    @Nullable
    private Reference<K, V>[] references;

    @Nullable
    private Reference<K, V> reference;

    @Nullable
    private Entry<K, V> next;

    @Nullable
    private Entry<K, V> last;

    public EntryIterator() {
      moveToNextSegment();
    }

    @Override
    public boolean hasNext() {
      getNextIfNecessary();
      return (this.next != null);
    }

    @Override
    public Entry<K, V> next() {
      getNextIfNecessary();
      if (this.next == null) {
        throw new NoSuchElementException();
      }
      this.last = this.next;
      this.next = null;
      return this.last;
    }

    private void getNextIfNecessary() {
      while (this.next == null) {
        moveToNextReference();
        if (this.reference == null) {
          return;
        }
        this.next = this.reference.get();
      }
    }

    private void moveToNextReference() {
      if (this.reference != null) {
        this.reference = this.reference.getNext();
      }
      while (this.reference == null && this.references != null) {
        if (this.referenceIndex >= this.references.length) {
          moveToNextSegment();
          this.referenceIndex = 0;
        } else {
          this.reference = this.references[this.referenceIndex];
          this.referenceIndex++;
        }
      }
    }

    private void moveToNextSegment() {
      this.reference = null;
      this.references = null;
      if (this.segmentIndex < ConcurrentReferenceHashMap.this.segments.length) {
        this.references = ConcurrentReferenceHashMap.this.segments[this.segmentIndex].references;
        this.segmentIndex++;
      }
    }

    @Override
    public void remove() {
      Assert.state(this.last != null, "No element to remove");
      ConcurrentReferenceHashMap.this.remove(this.last.getKey());
      this.last = null;
    }
  }

  /**
   * The types of restructuring that can be performed.
   */
  protected enum Restructure {

    WHEN_NECESSARY, NEVER
  }

  /**
   * Strategy class used to manage {@link Reference References}. This class can be
   * overridden if alternative reference types need to be supported.
   */
  protected class ReferenceManager {

    private final ReferenceQueue<Entry<K, V>> queue = new ReferenceQueue<>();

    /**
     * Factory method used to create a new {@link Reference}.
     *
     * @param entry the entry contained in the reference
     * @param hash  the hash
     * @param next  the next reference in the chain, or {@code null} if none
     * @return a new {@link Reference}
     */
    public Reference<K, V> createReference(Entry<K, V> entry, int hash, @Nullable Reference<K, V> next) {
      if (ConcurrentReferenceHashMap.this.referenceType == ReferenceType.WEAK) {
        return new WeakEntryReference<>(entry, hash, next, this.queue);
      }
      return new SoftEntryReference<>(entry, hash, next, this.queue);
    }

    /**
     * Return any reference that has been garbage collected and can be purged from
     * the underlying structure or {@code null} if no references need purging. This
     * method must be thread safe and ideally should not block when returning
     * {@code null}. References should be returned once and only once.
     *
     * @return a reference to purge or {@code null}
     */
    @SuppressWarnings("unchecked")
    @Nullable
    public Reference<K, V> pollForPurge() {
      return (Reference<K, V>) this.queue.poll();
    }
  }

  /**
   * Internal {@link Reference} implementation for {@link SoftReference
   * SoftReferences}.
   */
  private static final class SoftEntryReference<K, V> extends SoftReference<Entry<K, V>> implements Reference<K, V> {

    private final int hash;

    @Nullable
    private final Reference<K, V> nextReference;

    public SoftEntryReference(Entry<K, V> entry, int hash, @Nullable Reference<K, V> next,
        ReferenceQueue<Entry<K, V>> queue) {

      super(entry, queue);
      this.hash = hash;
      this.nextReference = next;
    }

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

    @Override
    @Nullable
    public Reference<K, V> getNext() {
      return this.nextReference;
    }

    @Override
    public void release() {
      enqueue();
      clear();
    }
  }

  /**
   * Internal {@link Reference} implementation for {@link WeakReference
   * WeakReferences}.
   */
  private static final class WeakEntryReference<K, V> extends WeakReference<Entry<K, V>> implements Reference<K, V> {

    private final int hash;

    @Nullable
    private final Reference<K, V> nextReference;

    public WeakEntryReference(Entry<K, V> entry, int hash, @Nullable Reference<K, V> next,
        ReferenceQueue<Entry<K, V>> queue) {

      super(entry, queue);
      this.hash = hash;
      this.nextReference = next;
    }

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

    @Override
    @Nullable
    public Reference<K, V> getNext() {
      return this.nextReference;
    }

    @Override
    public void release() {
      enqueue();
      clear();
    }
  }

}
