https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * Copyright (C) 2012 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.collect;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.CollectPreconditions.checkRemove;
import static com.google.common.collect.CompactHashing.UNSET;
import static com.google.common.collect.Hashing.smearedHash;

import com.google.common.annotations.GwtIncompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Objects;
import com.google.common.base.Preconditions;
import com.google.common.primitives.Ints;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.AbstractSet;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.LinkedHashSet;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.Consumer;
import org.checkerframework.checker.nullness.qual.Nullable;


CompactHashSet is an implementation of a Set. All optional operations (adding and removing) are
supported. The elements can be any objects.

contains(x), add(x) and remove(x), are all (expected and amortized) constant time operations. Expected in the hashtable sense (depends on the hash function doing a good job of distributing the elements to the buckets to a distribution not far from uniform), and amortized since some operations can trigger a hash table resize. 
Unlike java.util.HashSet, iteration is only proportional to the actual size(), which is optimal, and not the size of the internal hashtable, which could be much larger than size(). Furthermore, this structure only depends on a fixed number of arrays; 
add(x) operations do not create objects for the garbage collector to deal with, and for every element added, the garbage collector will have to traverse 1.5 references on average, in the marking phase, not 5.0 as in java.util.HashSet. 
If there are no removals, then iteration order is the same as insertion order. Any removal invalidates any ordering guarantees. 
This class should not be assumed to be universally superior to java.util.HashSet. Generally speaking, this class reduces object allocation and memory consumption at the price of moderately increased constant factors of CPU. Only use this class when there is a specific reason to prioritize memory over CPU. 
Author:Dimitris Andreou, Jon Noack
/**
 * CompactHashSet is an implementation of a Set. All optional operations (adding and removing) are
 * supported. The elements can be any objects.
 *
 * <p>{@code contains(x)}, {@code add(x)} and {@code remove(x)}, are all (expected and amortized)
 * constant time operations. Expected in the hashtable sense (depends on the hash function doing a
 * good job of distributing the elements to the buckets to a distribution not far from uniform), and
 * amortized since some operations can trigger a hash table resize.
 *
 * <p>Unlike {@code java.util.HashSet}, iteration is only proportional to the actual {@code size()},
 * which is optimal, and <i>not</i> the size of the internal hashtable, which could be much larger
 * than {@code size()}. Furthermore, this structure only depends on a fixed number of arrays; {@code
 * add(x)} operations <i>do not</i> create objects for the garbage collector to deal with, and for
 * every element added, the garbage collector will have to traverse {@code 1.5} references on
 * average, in the marking phase, not {@code 5.0} as in {@code java.util.HashSet}.
 *
 * <p>If there are no removals, then {@link #iterator iteration} order is the same as insertion
 * order. Any removal invalidates any ordering guarantees.
 *
 * <p>This class should not be assumed to be universally superior to {@code java.util.HashSet}.
 * Generally speaking, this class reduces object allocation and memory consumption at the price of
 * moderately increased constant factors of CPU. Only use this class when there is a specific reason
 * to prioritize memory over CPU.
 *
 * @author Dimitris Andreou
 * @author Jon Noack
 */
@GwtIncompatible // not worth using in GWT for now
class CompactHashSet<E> extends AbstractSet<E> implements Serializable {
  // TODO(user): cache all field accesses in local vars

  
Creates an empty CompactHashSet instance. 
/** Creates an empty {@code CompactHashSet} instance. */
  public static <E> CompactHashSet<E> create() {
    return new CompactHashSet<>();
  }

  
Creates a mutable CompactHashSet instance containing the elements of the given collection in unspecified order. 
Params:
  • collection – the elements that the set should contain
Returns:a new CompactHashSet containing those elements (minus duplicates)
/**
   * Creates a <i>mutable</i> {@code CompactHashSet} instance containing the elements of the given
   * collection in unspecified order.
   *
   * @param collection the elements that the set should contain
   * @return a new {@code CompactHashSet} containing those elements (minus duplicates)
   */
  public static <E> CompactHashSet<E> create(Collection<? extends E> collection) {
    CompactHashSet<E> set = createWithExpectedSize(collection.size());
    set.addAll(collection);
    return set;
  }

  
Creates a mutable CompactHashSet instance containing the given elements in unspecified order. 
Params:
  • elements – the elements that the set should contain
Returns:a new CompactHashSet containing those elements (minus duplicates)
/**
   * Creates a <i>mutable</i> {@code CompactHashSet} instance containing the given elements in
   * unspecified order.
   *
   * @param elements the elements that the set should contain
   * @return a new {@code CompactHashSet} containing those elements (minus duplicates)
   */
  @SafeVarargs
  public static <E> CompactHashSet<E> create(E... elements) {
    CompactHashSet<E> set = createWithExpectedSize(elements.length);
    Collections.addAll(set, elements);
    return set;
  }

  
Creates a CompactHashSet instance, with a high enough "initial capacity" that it should hold expectedSize elements without growth. 
Params:
  • expectedSize – the number of elements you expect to add to the returned set
Throws:
Returns:a new, empty CompactHashSet with enough capacity to hold expectedSize elements without resizing
/**
   * Creates a {@code CompactHashSet} instance, with a high enough "initial capacity" that it
   * <i>should</i> hold {@code expectedSize} elements without growth.
   *
   * @param expectedSize the number of elements you expect to add to the returned set
   * @return a new, empty {@code CompactHashSet} with enough capacity to hold {@code expectedSize}
   *     elements without resizing
   * @throws IllegalArgumentException if {@code expectedSize} is negative
   */
  public static <E> CompactHashSet<E> createWithExpectedSize(int expectedSize) {
    return new CompactHashSet<>(expectedSize);
  }

  
Maximum allowed false positive probability of detecting a hash flooding attack given random
input.

/**
   * Maximum allowed false positive probability of detecting a hash flooding attack given random
   * input.
   */
  @VisibleForTesting(
      )
  static final double HASH_FLOODING_FPP = 0.001;

  
Maximum allowed length of a hash table bucket before falling back to a j.u.LinkedHashSet based
implementation. Experimentally determined.

/**
   * Maximum allowed length of a hash table bucket before falling back to a j.u.LinkedHashSet based
   * implementation. Experimentally determined.
   */
  private static final int MAX_HASH_BUCKET_LENGTH = 9;

  
The hashtable object. This can be either:

    
  
  • a byte[], short[], or int[], with size a power of two, created by
      CompactHashing.createTable, whose values are either
      
      
        
    • UNSET, meaning "null pointer"
        
    • one plus an index into the entries and elements array
      
    
  
  • another java.util.Set delegate implementation. In most modern JDKs, normal java.util hash
      collections intelligently fall back to a binary search tree if hash table collisions are
      detected. Rather than going to all the trouble of reimplementing this ourselves, we
      simply switch over to use the JDK implementation wholesale if probable hash flooding is
      detected, sacrificing the compactness guarantee in very rare cases in exchange for much
      more reliable worst-case behavior.
  
  • null, if no entries have yet been added to the map



/**
   * The hashtable object. This can be either:
   *
   * <ul>
   *   <li>a byte[], short[], or int[], with size a power of two, created by
   *       CompactHashing.createTable, whose values are either
   *       <ul>
   *         <li>UNSET, meaning "null pointer"
   *         <li>one plus an index into the entries and elements array
   *       </ul>
   *   <li>another java.util.Set delegate implementation. In most modern JDKs, normal java.util hash
   *       collections intelligently fall back to a binary search tree if hash table collisions are
   *       detected. Rather than going to all the trouble of reimplementing this ourselves, we
   *       simply switch over to use the JDK implementation wholesale if probable hash flooding is
   *       detected, sacrificing the compactness guarantee in very rare cases in exchange for much
   *       more reliable worst-case behavior.
   *   <li>null, if no entries have yet been added to the map
   * </ul>
   */
  @Nullable private transient Object table;

  
Contains the logical entries, in the range of [0, size()). The high bits of each int are the
part of the smeared hash of the element not covered by the hashtable mask, whereas the low bits
are the "next" pointer (pointing to the next entry in the bucket chain), which will always be
less than or equal to the hashtable mask.

hash  = aaaaaaaa
mask  = 0000ffff
next  = 0000bbbb
entry = aaaabbbb



The pointers in [size(), entries.length) are all "null" (UNSET).

/**
   * Contains the logical entries, in the range of [0, size()). The high bits of each int are the
   * part of the smeared hash of the element not covered by the hashtable mask, whereas the low bits
   * are the "next" pointer (pointing to the next entry in the bucket chain), which will always be
   * less than or equal to the hashtable mask.
   *
   * <pre>
   * hash  = aaaaaaaa
   * mask  = 0000ffff
   * next  = 0000bbbb
   * entry = aaaabbbb
   * </pre>
   *
   * <p>The pointers in [size(), entries.length) are all "null" (UNSET).
   */
  private transient int @Nullable [] entries;

  
The elements contained in the set, in the range of [0, size()). The elements in [size(), elements.length) are all null. 
/**
   * The elements contained in the set, in the range of [0, size()). The elements in [size(),
   * elements.length) are all {@code null}.
   */
  @VisibleForTesting transient Object @Nullable [] elements;

  
Keeps track of metadata like the number of hash table bits and modifications of this data
structure (to make it possible to throw ConcurrentModificationException in the iterator). Note
that we choose not to make this volatile, so we do less of a "best effort" to track such
errors, for better performance.

/**
   * Keeps track of metadata like the number of hash table bits and modifications of this data
   * structure (to make it possible to throw ConcurrentModificationException in the iterator). Note
   * that we choose not to make this volatile, so we do less of a "best effort" to track such
   * errors, for better performance.
   */
  private transient int metadata;

  
The number of elements contained in the set. 
/** The number of elements contained in the set. */
  private transient int size;

  
Constructs a new empty instance of CompactHashSet. 
/** Constructs a new empty instance of {@code CompactHashSet}. */
  CompactHashSet() {
    init(CompactHashing.DEFAULT_SIZE);
  }

  
Constructs a new instance of CompactHashSet with the specified capacity. 
Params:
  • expectedSize – the initial capacity of this CompactHashSet.
/**
   * Constructs a new instance of {@code CompactHashSet} with the specified capacity.
   *
   * @param expectedSize the initial capacity of this {@code CompactHashSet}.
   */
  CompactHashSet(int expectedSize) {
    init(expectedSize);
  }

  
Pseudoconstructor for serialization support. 
/** Pseudoconstructor for serialization support. */
  void init(int expectedSize) {
    Preconditions.checkArgument(expectedSize >= 0, "Expected size must be >= 0");

    // Save expectedSize for use in allocArrays()
    this.metadata = Ints.constrainToRange(expectedSize, 1, CompactHashing.MAX_SIZE);
  }

  
Returns whether arrays need to be allocated. 
/** Returns whether arrays need to be allocated. */
  @VisibleForTesting
  boolean needsAllocArrays() {
    return table == null;
  }

  
Handle lazy allocation of arrays. 
/** Handle lazy allocation of arrays. */
  @CanIgnoreReturnValue
  int allocArrays() {
    Preconditions.checkState(needsAllocArrays(), "Arrays already allocated");

    int expectedSize = metadata;
    int buckets = CompactHashing.tableSize(expectedSize);
    this.table = CompactHashing.createTable(buckets);
    setHashTableMask(buckets - 1);

    this.entries = new int[expectedSize];
    this.elements = new Object[expectedSize];

    return expectedSize;
  }

  @SuppressWarnings("unchecked")
  @VisibleForTesting
  @Nullable
  Set<E> delegateOrNull() {
    if (table instanceof Set) {
      return (Set<E>) table;
    }
    return null;
  }

  private Set<E> createHashFloodingResistantDelegate(int tableSize) {
    return new LinkedHashSet<>(tableSize, 1.0f);
  }

  @SuppressWarnings("unchecked")
  @VisibleForTesting
  @CanIgnoreReturnValue
  Set<E> convertToHashFloodingResistantImplementation() {
    Set<E> newDelegate = createHashFloodingResistantDelegate(hashTableMask() + 1);
    for (int i = firstEntryIndex(); i >= 0; i = getSuccessor(i)) {
      newDelegate.add((E) elements[i]);
    }
    this.table = newDelegate;
    this.entries = null;
    this.elements = null;
    incrementModCount();
    return newDelegate;
  }

  @VisibleForTesting
  boolean isUsingHashFloodingResistance() {
    return delegateOrNull() != null;
  }

  
Stores the hash table mask as the number of bits needed to represent an index. 
/** Stores the hash table mask as the number of bits needed to represent an index. */
  private void setHashTableMask(int mask) {
    int hashTableBits = Integer.SIZE - Integer.numberOfLeadingZeros(mask);
    metadata =
        CompactHashing.maskCombine(metadata, hashTableBits, CompactHashing.HASH_TABLE_BITS_MASK);
  }

  
Gets the hash table mask using the stored number of hash table bits. 
/** Gets the hash table mask using the stored number of hash table bits. */
  private int hashTableMask() {
    return (1 << (metadata & CompactHashing.HASH_TABLE_BITS_MASK)) - 1;
  }

  void incrementModCount() {
    metadata += CompactHashing.MODIFICATION_COUNT_INCREMENT;
  }

  @CanIgnoreReturnValue
  @Override
  public boolean add(@Nullable E object) {
    if (needsAllocArrays()) {
      allocArrays();
    }
    @Nullable Set<E> delegate = delegateOrNull();
    if (delegate != null) {
      return delegate.add(object);
    }
    int[] entries = this.entries;
    Object[] elements = this.elements;

    int newEntryIndex = this.size; // current size, and pointer to the entry to be appended
    int newSize = newEntryIndex + 1;
    int hash = smearedHash(object);
    int mask = hashTableMask();
    int tableIndex = hash & mask;
    int next = CompactHashing.tableGet(table, tableIndex);
    if (next == UNSET) { // uninitialized bucket
      if (newSize > mask) {
        // Resize and add new entry
        mask = resizeTable(mask, CompactHashing.newCapacity(mask), hash, newEntryIndex);
      } else {
        CompactHashing.tableSet(table, tableIndex, newEntryIndex + 1);
      }
    } else {
      int entryIndex;
      int entry;
      int hashPrefix = CompactHashing.getHashPrefix(hash, mask);
      int bucketLength = 0;
      do {
        entryIndex = next - 1;
        entry = entries[entryIndex];
        if (CompactHashing.getHashPrefix(entry, mask) == hashPrefix
            && Objects.equal(object, elements[entryIndex])) {
          return false;
        }
        next = CompactHashing.getNext(entry, mask);
        bucketLength++;
      } while (next != UNSET);

      if (bucketLength >= MAX_HASH_BUCKET_LENGTH) {
        return convertToHashFloodingResistantImplementation().add(object);
      }

      if (newSize > mask) {
        // Resize and add new entry
        mask = resizeTable(mask, CompactHashing.newCapacity(mask), hash, newEntryIndex);
      } else {
        entries[entryIndex] = CompactHashing.maskCombine(entry, newEntryIndex + 1, mask);
      }
    }
    resizeMeMaybe(newSize);
    insertEntry(newEntryIndex, object, hash, mask);
    this.size = newSize;
    incrementModCount();
    return true;
  }

  
Creates a fresh entry with the specified object at the specified position in the entry arrays.

/**
   * Creates a fresh entry with the specified object at the specified position in the entry arrays.
   */
  void insertEntry(int entryIndex, @Nullable E object, int hash, int mask) {
    this.entries[entryIndex] = CompactHashing.maskCombine(hash, UNSET, mask);
    this.elements[entryIndex] = object;
  }

  
Resizes the entries storage if necessary. 
/** Resizes the entries storage if necessary. */
  private void resizeMeMaybe(int newSize) {
    int entriesSize = entries.length;
    if (newSize > entriesSize) {
      // 1.5x but round up to nearest odd (this is optimal for memory consumption on Android)
      int newCapacity =
          Math.min(CompactHashing.MAX_SIZE, (entriesSize + Math.max(1, entriesSize >>> 1)) | 1);
      if (newCapacity != entriesSize) {
        resizeEntries(newCapacity);
      }
    }
  }

  
Resizes the internal entries array to the specified capacity, which may be greater or less than
the current capacity.

/**
   * Resizes the internal entries array to the specified capacity, which may be greater or less than
   * the current capacity.
   */
  void resizeEntries(int newCapacity) {
    this.entries = Arrays.copyOf(entries, newCapacity);
    this.elements = Arrays.copyOf(elements, newCapacity);
  }

  @CanIgnoreReturnValue
  private int resizeTable(int mask, int newCapacity, int targetHash, int targetEntryIndex) {
    Object newTable = CompactHashing.createTable(newCapacity);
    int newMask = newCapacity - 1;

    if (targetEntryIndex != UNSET) {
      // Add target first; it must be last in the chain because its entry hasn't yet been created
      CompactHashing.tableSet(newTable, targetHash & newMask, targetEntryIndex + 1);
    }

    Object table = this.table;
    int[] entries = this.entries;

    // Loop over current hashtable
    for (int tableIndex = 0; tableIndex <= mask; tableIndex++) {
      int next = CompactHashing.tableGet(table, tableIndex);
      while (next != UNSET) {
        int entryIndex = next - 1;
        int entry = entries[entryIndex];

        // Rebuild hash using entry hashPrefix and tableIndex ("hashSuffix")
        int hash = CompactHashing.getHashPrefix(entry, mask) | tableIndex;

        int newTableIndex = hash & newMask;
        int newNext = CompactHashing.tableGet(newTable, newTableIndex);
        CompactHashing.tableSet(newTable, newTableIndex, next);
        entries[entryIndex] = CompactHashing.maskCombine(hash, newNext, newMask);

        next = CompactHashing.getNext(entry, mask);
      }
    }

    this.table = newTable;
    setHashTableMask(newMask);
    return newMask;
  }

  @Override
  public boolean contains(@Nullable Object object) {
    if (needsAllocArrays()) {
      return false;
    }
    @Nullable Set<E> delegate = delegateOrNull();
    if (delegate != null) {
      return delegate.contains(object);
    }
    int hash = smearedHash(object);
    int mask = hashTableMask();
    int next = CompactHashing.tableGet(table, hash & mask);
    if (next == UNSET) {
      return false;
    }
    int hashPrefix = CompactHashing.getHashPrefix(hash, mask);
    do {
      int entryIndex = next - 1;
      int entry = entries[entryIndex];
      if (CompactHashing.getHashPrefix(entry, mask) == hashPrefix
          && Objects.equal(object, elements[entryIndex])) {
        return true;
      }
      next = CompactHashing.getNext(entry, mask);
    } while (next != UNSET);
    return false;
  }

  @CanIgnoreReturnValue
  @Override
  public boolean remove(@Nullable Object object) {
    if (needsAllocArrays()) {
      return false;
    }
    @Nullable Set<E> delegate = delegateOrNull();
    if (delegate != null) {
      return delegate.remove(object);
    }
    int mask = hashTableMask();
    int index =
        CompactHashing.remove(
            object, /* value= */ null, mask, table, entries, elements, /* values= */ null);
    if (index == -1) {
      return false;
    }

    moveLastEntry(index, mask);
    size--;
    incrementModCount();

    return true;
  }

  
Moves the last entry in the entry array into dstIndex, and nulls out its old position. 
/**
   * Moves the last entry in the entry array into {@code dstIndex}, and nulls out its old position.
   */
  void moveLastEntry(int dstIndex, int mask) {
    int srcIndex = size() - 1;
    if (dstIndex < srcIndex) {
      // move last entry to deleted spot
      @Nullable Object object = elements[srcIndex];
      elements[dstIndex] = object;
      elements[srcIndex] = null;

      // move the last entry to the removed spot, just like we moved the element
      entries[dstIndex] = entries[srcIndex];
      entries[srcIndex] = 0;

      // also need to update whoever's "next" pointer was pointing to the last entry place
      int tableIndex = smearedHash(object) & mask;
      int next = CompactHashing.tableGet(table, tableIndex);
      int srcNext = srcIndex + 1;
      if (next == srcNext) {
        // we need to update the root pointer
        CompactHashing.tableSet(table, tableIndex, dstIndex + 1);
      } else {
        // we need to update a pointer in an entry
        int entryIndex;
        int entry;
        do {
          entryIndex = next - 1;
          entry = entries[entryIndex];
          next = CompactHashing.getNext(entry, mask);
        } while (next != srcNext);
        // here, entries[entryIndex] points to the old entry location; update it
        entries[entryIndex] = CompactHashing.maskCombine(entry, dstIndex + 1, mask);
      }
    } else {
      elements[dstIndex] = null;
      entries[dstIndex] = 0;
    }
  }

  int firstEntryIndex() {
    return isEmpty() ? -1 : 0;
  }

  int getSuccessor(int entryIndex) {
    return (entryIndex + 1 < size) ? entryIndex + 1 : -1;
  }

  
Updates the index an iterator is pointing to after a call to remove: returns the index of the
entry that should be looked at after a removal on indexRemoved, with indexBeforeRemove as the
index that *was* the next entry that would be looked at.

/**
   * Updates the index an iterator is pointing to after a call to remove: returns the index of the
   * entry that should be looked at after a removal on indexRemoved, with indexBeforeRemove as the
   * index that *was* the next entry that would be looked at.
   */
  int adjustAfterRemove(int indexBeforeRemove, @SuppressWarnings("unused") int indexRemoved) {
    return indexBeforeRemove - 1;
  }

  @Override
  public Iterator<E> iterator() {
    @Nullable Set<E> delegate = delegateOrNull();
    if (delegate != null) {
      return delegate.iterator();
    }
    return new Iterator<E>() {
      int expectedMetadata = metadata;
      int currentIndex = firstEntryIndex();
      int indexToRemove = -1;

      @Override
      public boolean hasNext() {
        return currentIndex >= 0;
      }

      @SuppressWarnings("unchecked") // known to be Es
      @Override
      public E next() {
        checkForConcurrentModification();
        if (!hasNext()) {
          throw new NoSuchElementException();
        }
        indexToRemove = currentIndex;
        E result = (E) elements[currentIndex];
        currentIndex = getSuccessor(currentIndex);
        return result;
      }

      @Override
      public void remove() {
        checkForConcurrentModification();
        checkRemove(indexToRemove >= 0);
        incrementExpectedModCount();
        CompactHashSet.this.remove(elements[indexToRemove]);
        currentIndex = adjustAfterRemove(currentIndex, indexToRemove);
        indexToRemove = -1;
      }

      void incrementExpectedModCount() {
        expectedMetadata += CompactHashing.MODIFICATION_COUNT_INCREMENT;
      }

      private void checkForConcurrentModification() {
        if (metadata != expectedMetadata) {
          throw new ConcurrentModificationException();
        }
      }
    };
  }

  @Override
  public Spliterator<E> spliterator() {
    if (needsAllocArrays()) {
      return Spliterators.spliterator(new Object[0], Spliterator.DISTINCT | Spliterator.ORDERED);
    }
    @Nullable Set<E> delegate = delegateOrNull();
    return (delegate != null)
        ? delegate.spliterator()
        : Spliterators.spliterator(elements, 0, size, Spliterator.DISTINCT | Spliterator.ORDERED);
  }

  @SuppressWarnings("unchecked") // known to be Es
  @Override
  public void forEach(Consumer<? super E> action) {
    checkNotNull(action);
    @Nullable Set<E> delegate = delegateOrNull();
    if (delegate != null) {
      delegate.forEach(action);
    } else {
      for (int i = firstEntryIndex(); i >= 0; i = getSuccessor(i)) {
        action.accept((E) elements[i]);
      }
    }
  }

  @Override
  public int size() {
    @Nullable Set<E> delegate = delegateOrNull();
    return (delegate != null) ? delegate.size() : size;
  }

  @Override
  public boolean isEmpty() {
    return size() == 0;
  }

  @Override
  public Object[] toArray() {
    if (needsAllocArrays()) {
      return new Object[0];
    }
    @Nullable Set<E> delegate = delegateOrNull();
    return (delegate != null) ? delegate.toArray() : Arrays.copyOf(elements, size);
  }

  @CanIgnoreReturnValue
  @Override
  public <T> T[] toArray(T[] a) {
    if (needsAllocArrays()) {
      if (a.length > 0) {
        a[0] = null;
      }
      return a;
    }
    @Nullable Set<E> delegate = delegateOrNull();
    return (delegate != null)
        ? delegate.toArray(a)
        : ObjectArrays.toArrayImpl(elements, 0, size, a);
  }

  
Ensures that this CompactHashSet has the smallest representation in memory, given its current size. 
/**
   * Ensures that this {@code CompactHashSet} has the smallest representation in memory, given its
   * current size.
   */
  public void trimToSize() {
    if (needsAllocArrays()) {
      return;
    }
    @Nullable Set<E> delegate = delegateOrNull();
    if (delegate != null) {
      Set<E> newDelegate = createHashFloodingResistantDelegate(size());
      newDelegate.addAll(delegate);
      this.table = newDelegate;
      return;
    }
    int size = this.size;
    if (size < entries.length) {
      resizeEntries(size);
    }
    int minimumTableSize = CompactHashing.tableSize(size);
    int mask = hashTableMask();
    if (minimumTableSize < mask) { // smaller table size will always be less than current mask
      resizeTable(mask, minimumTableSize, UNSET, UNSET);
    }
  }

  @Override
  public void clear() {
    if (needsAllocArrays()) {
      return;
    }
    incrementModCount();
    @Nullable Set<E> delegate = delegateOrNull();
    if (delegate != null) {
      metadata =
          Ints.constrainToRange(size(), CompactHashing.DEFAULT_SIZE, CompactHashing.MAX_SIZE);
      delegate.clear(); // invalidate any iterators left over!
      table = null;
      size = 0;
    } else {
      Arrays.fill(elements, 0, size, null);
      CompactHashing.tableClear(table);
      Arrays.fill(entries, 0, size, 0);
      this.size = 0;
    }
  }

  private void writeObject(ObjectOutputStream stream) throws IOException {
    stream.defaultWriteObject();
    stream.writeInt(size());
    for (E e : this) {
      stream.writeObject(e);
    }
  }

  @SuppressWarnings("unchecked")
  private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
    stream.defaultReadObject();
    int elementCount = stream.readInt();
    if (elementCount < 0) {
      throw new InvalidObjectException("Invalid size: " + elementCount);
    }
    init(elementCount);
    for (int i = 0; i < elementCount; i++) {
      E element = (E) stream.readObject();
      add(element);
    }
  }
}