ImmutableSet.java

/*
 * Copyright (C) 2007 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.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.CollectPreconditions.checkNonnegative;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.math.IntMath;
import com.google.common.primitives.Ints;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.concurrent.LazyInit;
import com.google.j2objc.annotations.RetainedWith;
import java.io.Serializable;
import java.math.RoundingMode;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.EnumSet;
import java.util.Iterator;
import java.util.Set;
import java.util.SortedSet;
import java.util.Spliterator;
import java.util.function.Consumer;
import java.util.stream.Collector;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * A {@link Set} whose contents will never change, with many other important properties detailed at
 * {@link ImmutableCollection}.
 *
 * @since 2.0
 */
@GwtCompatible(serializable = true, emulated = true)
@SuppressWarnings("serial") // we're overriding default serialization
public abstract class ImmutableSet<E> extends ImmutableCollection<E> implements Set<E> {
  static final int SPLITERATOR_CHARACTERISTICS =
      ImmutableCollection.SPLITERATOR_CHARACTERISTICS | Spliterator.DISTINCT;

  /**
   * Returns a {@code Collector} that accumulates the input elements into a new {@code
   * ImmutableSet}. Elements appear in the resulting set in the encounter order of the stream; if
   * the stream contains duplicates (according to {@link Object#equals(Object)}), only the first
   * duplicate in encounter order will appear in the result.
   *
   * @since 21.0
   */
  public static <E> Collector<E, ?, ImmutableSet<E>> toImmutableSet() {
    return CollectCollectors.toImmutableSet();
  }

  /**
   * Returns the empty immutable set. Preferred over {@link Collections#emptySet} for code
   * consistency, and because the return type conveys the immutability guarantee.
   */
  @SuppressWarnings({"unchecked"}) // fully variant implementation (never actually produces any Es)
  public static <E> ImmutableSet<E> of() {
    return (ImmutableSet<E>) RegularImmutableSet.EMPTY;
  }

  /**
   * Returns an immutable set containing {@code element}. Preferred over {@link
   * Collections#singleton} for code consistency, {@code null} rejection, and because the return
   * type conveys the immutability guarantee.
   */
  public static <E> ImmutableSet<E> of(E element) {
    return new SingletonImmutableSet<E>(element);
  }

  /**
   * Returns an immutable set containing the given elements, minus duplicates, in the order each was
   * first specified. That is, if multiple elements are {@linkplain Object#equals equal}, all except
   * the first are ignored.
   */
  public static <E> ImmutableSet<E> of(E e1, E e2) {
    return construct(2, 2, e1, e2);
  }

  /**
   * Returns an immutable set containing the given elements, minus duplicates, in the order each was
   * first specified. That is, if multiple elements are {@linkplain Object#equals equal}, all except
   * the first are ignored.
   */
  public static <E> ImmutableSet<E> of(E e1, E e2, E e3) {
    return construct(3, 3, e1, e2, e3);
  }

  /**
   * Returns an immutable set containing the given elements, minus duplicates, in the order each was
   * first specified. That is, if multiple elements are {@linkplain Object#equals equal}, all except
   * the first are ignored.
   */
  public static <E> ImmutableSet<E> of(E e1, E e2, E e3, E e4) {
    return construct(4, 4, e1, e2, e3, e4);
  }

  /**
   * Returns an immutable set containing the given elements, minus duplicates, in the order each was
   * first specified. That is, if multiple elements are {@linkplain Object#equals equal}, all except
   * the first are ignored.
   */
  public static <E> ImmutableSet<E> of(E e1, E e2, E e3, E e4, E e5) {
    return construct(5, 5, e1, e2, e3, e4, e5);
  }

  /**
   * Returns an immutable set containing the given elements, minus duplicates, in the order each was
   * first specified. That is, if multiple elements are {@linkplain Object#equals equal}, all except
   * the first are ignored.
   *
   * <p>The array {@code others} must not be longer than {@code Integer.MAX_VALUE - 6}.
   *
   * @since 3.0 (source-compatible since 2.0)
   */
  @SafeVarargs // For Eclipse. For internal javac we have disabled this pointless type of warning.
  public static <E> ImmutableSet<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E... others) {
    checkArgument(
        others.length <= Integer.MAX_VALUE - 6, "the total number of elements must fit in an int");
    final int paramCount = 6;
    Object[] elements = new Object[paramCount + others.length];
    elements[0] = e1;
    elements[1] = e2;
    elements[2] = e3;
    elements[3] = e4;
    elements[4] = e5;
    elements[5] = e6;
    System.arraycopy(others, 0, elements, paramCount, others.length);
    return construct(elements.length, elements.length, elements);
  }

  /**
   * Constructs an {@code ImmutableSet} from the first {@code n} elements of the specified array,
   * which we have no particular reason to believe does or does not contain duplicates. If {@code k}
   * is the size of the returned {@code ImmutableSet}, then the unique elements of {@code elements}
   * will be in the first {@code k} positions, and {@code elements[i] == null} for {@code k <= i <
   * n}.
   *
   * <p>This may modify {@code elements}. Additionally, if {@code n == elements.length} and {@code
   * elements} contains no duplicates, {@code elements} may be used without copying in the returned
   * {@code ImmutableSet}, in which case the caller must not modify it.
   *
   * <p>{@code elements} may contain only values of type {@code E}.
   *
   * @throws NullPointerException if any of the first {@code n} elements of {@code elements} is null
   */
  private static <E> ImmutableSet<E> constructUnknownDuplication(int n, Object... elements) {
    // Guess the size is "halfway between" all duplicates and no duplicates, on a log scale.
    return construct(
        n,
        Math.max(
            ImmutableCollection.Builder.DEFAULT_INITIAL_CAPACITY,
            IntMath.sqrt(n, RoundingMode.CEILING)),
        elements);
  }

  /**
   * Constructs an {@code ImmutableSet} from the first {@code n} elements of the specified array. If
   * {@code k} is the size of the returned {@code ImmutableSet}, then the unique elements of {@code
   * elements} will be in the first {@code k} positions, and {@code elements[i] == null} for {@code
   * k <= i < n}.
   *
   * <p>This may modify {@code elements}. Additionally, if {@code n == elements.length} and {@code
   * elements} contains no duplicates, {@code elements} may be used without copying in the returned
   * {@code ImmutableSet}, in which case it may no longer be modified.
   *
   * <p>{@code elements} may contain only values of type {@code E}.
   *
   * @throws NullPointerException if any of the first {@code n} elements of {@code elements} is null
   */
  private static <E> ImmutableSet<E> construct(int n, int expectedSize, Object... elements) {
    switch (n) {
      case 0:
        return of();
      case 1:
        @SuppressWarnings("unchecked") // safe; elements contains only E's
        E elem = (E) elements[0];
        return of(elem);
      default:
        SetBuilderImpl<E> builder = new RegularSetBuilderImpl<E>(expectedSize);
        for (int i = 0; i < n; i++) {
          @SuppressWarnings("unchecked")
          E e = (E) checkNotNull(elements[i]);
          builder = builder.add(e);
        }
        return builder.review().build();
    }
  }

  /**
   * Returns an immutable set containing each of {@code elements}, minus duplicates, in the order
   * each appears first in the source collection.
   *
   * <p><b>Performance note:</b> This method will sometimes recognize that the actual copy operation
   * is unnecessary; for example, {@code copyOf(copyOf(anArrayList))} will copy the data only once.
   * This reduces the expense of habitually making defensive copies at API boundaries. However, the
   * precise conditions for skipping the copy operation are undefined.
   *
   * @throws NullPointerException if any of {@code elements} is null
   * @since 7.0 (source-compatible since 2.0)
   */
  public static <E> ImmutableSet<E> copyOf(Collection<? extends E> elements) {
    /*
     * TODO(lowasser): consider checking for ImmutableAsList here
     * TODO(lowasser): consider checking for Multiset here
     */
    // Don't refer to ImmutableSortedSet by name so it won't pull in all that code
    if (elements instanceof ImmutableSet && !(elements instanceof SortedSet)) {
      @SuppressWarnings("unchecked") // all supported methods are covariant
      ImmutableSet<E> set = (ImmutableSet<E>) elements;
      if (!set.isPartialView()) {
        return set;
      }
    } else if (elements instanceof EnumSet) {
      return copyOfEnumSet((EnumSet) elements);
    }
    Object[] array = elements.toArray();
    if (elements instanceof Set) {
      // assume probably no duplicates (though it might be using different equality semantics)
      return construct(array.length, array.length, array);
    } else {
      return constructUnknownDuplication(array.length, array);
    }
  }

  /**
   * Returns an immutable set containing each of {@code elements}, minus duplicates, in the order
   * each appears first in the source iterable. This method iterates over {@code elements} only
   * once.
   *
   * <p><b>Performance note:</b> This method will sometimes recognize that the actual copy operation
   * is unnecessary; for example, {@code copyOf(copyOf(anArrayList))} should copy the data only
   * once. This reduces the expense of habitually making defensive copies at API boundaries.
   * However, the precise conditions for skipping the copy operation are undefined.
   *
   * @throws NullPointerException if any of {@code elements} is null
   */
  public static <E> ImmutableSet<E> copyOf(Iterable<? extends E> elements) {
    return (elements instanceof Collection)
        ? copyOf((Collection<? extends E>) elements)
        : copyOf(elements.iterator());
  }

  /**
   * Returns an immutable set containing each of {@code elements}, minus duplicates, in the order
   * each appears first in the source iterator.
   *
   * @throws NullPointerException if any of {@code elements} is null
   */
  public static <E> ImmutableSet<E> copyOf(Iterator<? extends E> elements) {
    // We special-case for 0 or 1 elements, but anything further is madness.
    if (!elements.hasNext()) {
      return of();
    }
    E first = elements.next();
    if (!elements.hasNext()) {
      return of(first);
    } else {
      return new ImmutableSet.Builder<E>().add(first).addAll(elements).build();
    }
  }

  /**
   * Returns an immutable set containing each of {@code elements}, minus duplicates, in the order
   * each appears first in the source array.
   *
   * @throws NullPointerException if any of {@code elements} is null
   * @since 3.0
   */
  public static <E> ImmutableSet<E> copyOf(E[] elements) {
    switch (elements.length) {
      case 0:
        return of();
      case 1:
        return of(elements[0]);
      default:
        return constructUnknownDuplication(elements.length, elements.clone());
    }
  }

  @SuppressWarnings("rawtypes") // necessary to compile against Java 8
  private static ImmutableSet copyOfEnumSet(EnumSet enumSet) {
    return ImmutableEnumSet.asImmutable(EnumSet.copyOf(enumSet));
  }

  ImmutableSet() {}

  /** Returns {@code true} if the {@code hashCode()} method runs quickly. */
  boolean isHashCodeFast() {
    return false;
  }

  @Override
  public boolean equals(@Nullable Object object) {
    if (object == this) {
      return true;
    } else if (object instanceof ImmutableSet
        && isHashCodeFast()
        && ((ImmutableSet<?>) object).isHashCodeFast()
        && hashCode() != object.hashCode()) {
      return false;
    }
    return Sets.equalsImpl(this, object);
  }

  @Override
  public int hashCode() {
    return Sets.hashCodeImpl(this);
  }

  // This declaration is needed to make Set.iterator() and
  // ImmutableCollection.iterator() consistent.
  @Override
  public abstract UnmodifiableIterator<E> iterator();

  @LazyInit @RetainedWith private transient @Nullable ImmutableList<E> asList;

  @Override
  public ImmutableList<E> asList() {
    ImmutableList<E> result = asList;
    return (result == null) ? asList = createAsList() : result;
  }

  ImmutableList<E> createAsList() {
    return new RegularImmutableAsList<E>(this, toArray());
  }

  abstract static class Indexed<E> extends ImmutableSet<E> {
    abstract E get(int index);

    @Override
    public UnmodifiableIterator<E> iterator() {
      return asList().iterator();
    }

    @Override
    public Spliterator<E> spliterator() {
      return CollectSpliterators.indexed(size(), SPLITERATOR_CHARACTERISTICS, this::get);
    }

    @Override
    public void forEach(Consumer<? super E> consumer) {
      checkNotNull(consumer);
      int n = size();
      for (int i = 0; i < n; i++) {
        consumer.accept(get(i));
      }
    }

    @Override
    int copyIntoArray(Object[] dst, int offset) {
      return asList().copyIntoArray(dst, offset);
    }

    @Override
    ImmutableList<E> createAsList() {
      return new ImmutableAsList<E>() {
        @Override
        public E get(int index) {
          return Indexed.this.get(index);
        }

        @Override
        Indexed<E> delegateCollection() {
          return Indexed.this;
        }
      };
    }
  }

  /*
   * This class is used to serialize all ImmutableSet instances, except for
   * ImmutableEnumSet/ImmutableSortedSet, regardless of implementation type. It
   * captures their "logical contents" and they are reconstructed using public
   * static factories. This is necessary to ensure that the existence of a
   * particular implementation type is an implementation detail.
   */
  private static class SerializedForm implements Serializable {
    final Object[] elements;

    SerializedForm(Object[] elements) {
      this.elements = elements;
    }

    Object readResolve() {
      return copyOf(elements);
    }

    private static final long serialVersionUID = 0;
  }

  @Override
  Object writeReplace() {
    return new SerializedForm(toArray());
  }

  /**
   * Returns a new builder. The generated builder is equivalent to the builder created by the {@link
   * Builder} constructor.
   */
  public static <E> Builder<E> builder() {
    return new Builder<E>();
  }

  /**
   * Returns a new builder, expecting the specified number of distinct elements to be added.
   *
   * <p>If {@code expectedSize} is exactly the number of distinct elements added to the builder
   * before {@link Builder#build} is called, the builder is likely to perform better than an unsized
   * {@link #builder()} would have.
   *
   * <p>It is not specified if any performance benefits apply if {@code expectedSize} is close to,
   * but not exactly, the number of distinct elements added to the builder.
   *
   * @since 23.1
   */
  @Beta
  public static <E> Builder<E> builderWithExpectedSize(int expectedSize) {
    checkNonnegative(expectedSize, "expectedSize");
    return new Builder<E>(expectedSize);
  }

  /** Builds a new open-addressed hash table from the first n objects in elements. */
  static Object[] rebuildHashTable(int newTableSize, Object[] elements, int n) {
    Object[] hashTable = new Object[newTableSize];
    int mask = hashTable.length - 1;
    for (int i = 0; i < n; i++) {
      Object e = elements[i];
      int j0 = Hashing.smear(e.hashCode());
      for (int j = j0; ; j++) {
        int index = j & mask;
        if (hashTable[index] == null) {
          hashTable[index] = e;
          break;
        }
      }
    }
    return hashTable;
  }

  /**
   * A builder for creating {@code ImmutableSet} instances. Example:
   *
   * <pre>{@code
   * static final ImmutableSet<Color> GOOGLE_COLORS =
   *     ImmutableSet.<Color>builder()
   *         .addAll(WEBSAFE_COLORS)
   *         .add(new Color(0, 191, 255))
   *         .build();
   * }</pre>
   *
   * <p>Elements appear in the resulting set in the same order they were first added to the builder.
   *
   * <p>Building does not change the state of the builder, so it is still possible to add more
   * elements and to build again.
   *
   * @since 2.0
   */
  public static class Builder<E> extends ImmutableCollection.Builder<E> {
    private SetBuilderImpl<E> impl;
    boolean forceCopy;

    public Builder() {
      this(DEFAULT_INITIAL_CAPACITY);
    }

    Builder(int capacity) {
      impl = new RegularSetBuilderImpl<E>(capacity);
    }

    Builder(@SuppressWarnings("unused") boolean subclass) {
      this.impl = null; // unused
    }

    @VisibleForTesting
    void forceJdk() {
      this.impl = new JdkBackedSetBuilderImpl<E>(impl);
    }

    final void copyIfNecessary() {
      if (forceCopy) {
        copy();
        forceCopy = false;
      }
    }

    void copy() {
      impl = impl.copy();
    }

    @Override
    @CanIgnoreReturnValue
    public Builder<E> add(E element) {
      checkNotNull(element);
      copyIfNecessary();
      impl = impl.add(element);
      return this;
    }

    @Override
    @CanIgnoreReturnValue
    public Builder<E> add(E... elements) {
      super.add(elements);
      return this;
    }

    @Override
    /**
     * Adds each element of {@code elements} to the {@code ImmutableSet}, ignoring duplicate
     * elements (only the first duplicate element is added).
     *
     * @param elements the elements to add
     * @return this {@code Builder} object
     * @throws NullPointerException if {@code elements} is null or contains a null element
     */
    @CanIgnoreReturnValue
    public Builder<E> addAll(Iterable<? extends E> elements) {
      super.addAll(elements);
      return this;
    }

    @Override
    @CanIgnoreReturnValue
    public Builder<E> addAll(Iterator<? extends E> elements) {
      super.addAll(elements);
      return this;
    }

    Builder<E> combine(Builder<E> other) {
      copyIfNecessary();
      this.impl = this.impl.combine(other.impl);
      return this;
    }

    @Override
    public ImmutableSet<E> build() {
      forceCopy = true;
      impl = impl.review();
      return impl.build();
    }
  }

  /** Swappable internal implementation of an ImmutableSet.Builder. */
  private abstract static class SetBuilderImpl<E> {
    E[] dedupedElements;
    int distinct;

    @SuppressWarnings("unchecked")
    SetBuilderImpl(int expectedCapacity) {
      this.dedupedElements = (E[]) new Object[expectedCapacity];
      this.distinct = 0;
    }

    /** Initializes this SetBuilderImpl with a copy of the deduped elements array from toCopy. */
    SetBuilderImpl(SetBuilderImpl<E> toCopy) {
      this.dedupedElements = Arrays.copyOf(toCopy.dedupedElements, toCopy.dedupedElements.length);
      this.distinct = toCopy.distinct;
    }

    /**
     * Resizes internal data structures if necessary to store the specified number of distinct
     * elements.
     */
    private void ensureCapacity(int minCapacity) {
      if (minCapacity > dedupedElements.length) {
        int newCapacity =
            ImmutableCollection.Builder.expandedCapacity(dedupedElements.length, minCapacity);
        dedupedElements = Arrays.copyOf(dedupedElements, newCapacity);
      }
    }

    /** Adds e to the insertion-order array of deduplicated elements. Calls ensureCapacity. */
    final void addDedupedElement(E e) {
      ensureCapacity(distinct + 1);
      dedupedElements[distinct++] = e;
    }

    /**
     * Adds e to this SetBuilderImpl, returning the updated result. Only use the returned
     * SetBuilderImpl, since we may switch implementations if e.g. hash flooding is detected.
     */
    abstract SetBuilderImpl<E> add(E e);

    /** Adds all the elements from the specified SetBuilderImpl to this SetBuilderImpl. */
    final SetBuilderImpl<E> combine(SetBuilderImpl<E> other) {
      SetBuilderImpl<E> result = this;
      for (int i = 0; i < other.distinct; i++) {
        result = result.add(other.dedupedElements[i]);
      }
      return result;
    }

    /**
     * Creates a new copy of this SetBuilderImpl. Modifications to that SetBuilderImpl will not
     * affect this SetBuilderImpl or sets constructed from this SetBuilderImpl via build().
     */
    abstract SetBuilderImpl<E> copy();

    /**
     * Call this before build(). Does a final check on the internal data structures, e.g. shrinking
     * unnecessarily large structures or detecting previously unnoticed hash flooding.
     */
    SetBuilderImpl<E> review() {
      return this;
    }

    abstract ImmutableSet<E> build();
  }

  // We use power-of-2 tables, and this is the highest int that's a power of 2
  static final int MAX_TABLE_SIZE = Ints.MAX_POWER_OF_TWO;

  // Represents how tightly we can pack things, as a maximum.
  private static final double DESIRED_LOAD_FACTOR = 0.7;

  // If the set has this many elements, it will "max out" the table size
  private static final int CUTOFF = (int) (MAX_TABLE_SIZE * DESIRED_LOAD_FACTOR);

  /**
   * Returns an array size suitable for the backing array of a hash table that uses open addressing
   * with linear probing in its implementation. The returned size is the smallest power of two that
   * can hold setSize elements with the desired load factor. Always returns at least setSize + 2.
   */
  @VisibleForTesting
  static int chooseTableSize(int setSize) {
    setSize = Math.max(setSize, 2);
    // Correct the size for open addressing to match desired load factor.
    if (setSize < CUTOFF) {
      // Round up to the next highest power of 2.
      int tableSize = Integer.highestOneBit(setSize - 1) << 1;
      while (tableSize * DESIRED_LOAD_FACTOR < setSize) {
        tableSize <<= 1;
      }
      return tableSize;
    }

    // The table can't be completely full or we'll get infinite reprobes
    checkArgument(setSize < MAX_TABLE_SIZE, "collection too large");
    return MAX_TABLE_SIZE;
  }

  /**
   * We attempt to detect deliberate hash flooding attempts, and if one is detected, fall back to a
   * wrapper around j.u.HashSet, which has built in flooding protection. HASH_FLOODING_FPP is the
   * maximum allowed probability of falsely detecting a hash flooding attack if the input is
   * randomly generated.
   *
   * <p>MAX_RUN_MULTIPLIER was determined experimentally to match this FPP.
   */
  static final double HASH_FLOODING_FPP = 0.001;

  // NB: yes, this is surprisingly high, but that's what the experiments said was necessary
  // The higher it is, the worse constant factors we are willing to accept.
  static final int MAX_RUN_MULTIPLIER = 13;

  /**
   * Checks the whole hash table for poor hash distribution. Takes O(n) in the worst case, O(n / log
   * n) on average.
   *
   * <p>The online hash flooding detecting in RegularSetBuilderImpl.add can detect e.g. many exactly
   * matching hash codes, which would cause construction to take O(n^2), but can't detect e.g. hash
   * codes adversarially designed to go into ascending table locations, which keeps construction
   * O(n) (as desired) but then can have O(n) queries later.
   *
   * <p>If this returns false, then no query can take more than O(log n).
   *
   * <p>Note that for a RegularImmutableSet with elements with truly random hash codes, contains
   * operations take expected O(1) time but with high probability take O(log n) for at least some
   * element. (https://en.wikipedia.org/wiki/Linear_probing#Analysis)
   *
   * <p>This method may return {@code true} up to {@link #HASH_FLOODING_FPP} of the time even on
   * truly random input.
   *
   * <p>If this method returns false, there are definitely no runs of length at least {@code
   * maxRunBeforeFallback(hashTable.length)} nonnull elements. If there are no runs of length at
   * least {@code maxRunBeforeFallback(hashTable.length) / 2} nonnull elements, this method
   * definitely returns false. In between those constraints, the result of this method is undefined,
   * subject to the above {@link #HASH_FLOODING_FPP} constraint.
   */
  static boolean hashFloodingDetected(Object[] hashTable) {
    int maxRunBeforeFallback = maxRunBeforeFallback(hashTable.length);

    // Test for a run wrapping around the end of the table of length at least maxRunBeforeFallback.
    int endOfStartRun;
    for (endOfStartRun = 0; endOfStartRun < hashTable.length; ) {
      if (hashTable[endOfStartRun] == null) {
        break;
      }
      endOfStartRun++;
      if (endOfStartRun > maxRunBeforeFallback) {
        return true;
      }
    }
    int startOfEndRun;
    for (startOfEndRun = hashTable.length - 1; startOfEndRun > endOfStartRun; startOfEndRun--) {
      if (hashTable[startOfEndRun] == null) {
        break;
      }
      if (endOfStartRun + (hashTable.length - 1 - startOfEndRun) > maxRunBeforeFallback) {
        return true;
      }
    }

    // Now, break the remainder of the table into blocks of maxRunBeforeFallback/2 elements and
    // check that each has at least one null.
    int testBlockSize = maxRunBeforeFallback / 2;
    blockLoop:
    for (int i = endOfStartRun + 1; i + testBlockSize <= startOfEndRun; i += testBlockSize) {
      for (int j = 0; j < testBlockSize; j++) {
        if (hashTable[i + j] == null) {
          continue blockLoop;
        }
      }
      return true;
    }
    return false;
  }

  /**
   * If more than this many consecutive positions are filled in a table of the specified size,
   * report probable hash flooding. ({@link #hashFloodingDetected} may also report hash flooding if
   * fewer consecutive positions are filled; see that method for details.)
   */
  private static int maxRunBeforeFallback(int tableSize) {
    return MAX_RUN_MULTIPLIER * IntMath.log2(tableSize, RoundingMode.UNNECESSARY);
  }

  /**
   * Default implementation of the guts of ImmutableSet.Builder, creating an open-addressed hash
   * table and deduplicating elements as they come, so it only allocates O(max(distinct,
   * expectedCapacity)) rather than O(calls to add).
   *
   * <p>This implementation attempts to detect hash flooding, and if it's identified, falls back to
   * JdkBackedSetBuilderImpl.
   */
  private static final class RegularSetBuilderImpl<E> extends SetBuilderImpl<E> {
    private Object[] hashTable;
    private int maxRunBeforeFallback;
    private int expandTableThreshold;
    private int hashCode;

    RegularSetBuilderImpl(int expectedCapacity) {
      super(expectedCapacity);
      int tableSize = chooseTableSize(expectedCapacity);
      this.hashTable = new Object[tableSize];
      this.maxRunBeforeFallback = maxRunBeforeFallback(tableSize);
      this.expandTableThreshold = (int) (DESIRED_LOAD_FACTOR * tableSize);
    }

    RegularSetBuilderImpl(RegularSetBuilderImpl<E> toCopy) {
      super(toCopy);
      this.hashTable = Arrays.copyOf(toCopy.hashTable, toCopy.hashTable.length);
      this.maxRunBeforeFallback = toCopy.maxRunBeforeFallback;
      this.expandTableThreshold = toCopy.expandTableThreshold;
      this.hashCode = toCopy.hashCode;
    }

    void ensureTableCapacity(int minCapacity) {
      if (minCapacity > expandTableThreshold && hashTable.length < MAX_TABLE_SIZE) {
        int newTableSize = hashTable.length * 2;
        hashTable = rebuildHashTable(newTableSize, dedupedElements, distinct);
        maxRunBeforeFallback = maxRunBeforeFallback(newTableSize);
        expandTableThreshold = (int) (DESIRED_LOAD_FACTOR * newTableSize);
      }
    }

    @Override
    SetBuilderImpl<E> add(E e) {
      checkNotNull(e);
      int eHash = e.hashCode();
      int i0 = Hashing.smear(eHash);
      int mask = hashTable.length - 1;
      for (int i = i0; i - i0 < maxRunBeforeFallback; i++) {
        int index = i & mask;
        Object tableEntry = hashTable[index];
        if (tableEntry == null) {
          addDedupedElement(e);
          hashTable[index] = e;
          hashCode += eHash;
          ensureTableCapacity(distinct); // rebuilds table if necessary
          return this;
        } else if (tableEntry.equals(e)) { // not a new element, ignore
          return this;
        }
      }
      // we fell out of the loop due to a long run; fall back to JDK impl
      return new JdkBackedSetBuilderImpl<E>(this).add(e);
    }

    @Override
    SetBuilderImpl<E> copy() {
      return new RegularSetBuilderImpl<E>(this);
    }

    @Override
    SetBuilderImpl<E> review() {
      int targetTableSize = chooseTableSize(distinct);
      if (targetTableSize * 2 < hashTable.length) {
        hashTable = rebuildHashTable(targetTableSize, dedupedElements, distinct);
        maxRunBeforeFallback = maxRunBeforeFallback(targetTableSize);
        expandTableThreshold = (int) (DESIRED_LOAD_FACTOR * targetTableSize);
      }
      return hashFloodingDetected(hashTable) ? new JdkBackedSetBuilderImpl<E>(this) : this;
    }

    @Override
    ImmutableSet<E> build() {
      switch (distinct) {
        case 0:
          return of();
        case 1:
          return of(dedupedElements[0]);
        default:
          Object[] elements =
              (distinct == dedupedElements.length)
                  ? dedupedElements
                  : Arrays.copyOf(dedupedElements, distinct);
          return new RegularImmutableSet<E>(elements, hashCode, hashTable, hashTable.length - 1);
      }
    }
  }

  /**
   * SetBuilderImpl version that uses a JDK HashSet, which has built in hash flooding protection.
   */
  private static final class JdkBackedSetBuilderImpl<E> extends SetBuilderImpl<E> {
    private final Set<Object> delegate;

    JdkBackedSetBuilderImpl(SetBuilderImpl<E> toCopy) {
      super(toCopy); // initializes dedupedElements and distinct
      delegate = Sets.newHashSetWithExpectedSize(distinct);
      for (int i = 0; i < distinct; i++) {
        delegate.add(dedupedElements[i]);
      }
    }

    @Override
    SetBuilderImpl<E> add(E e) {
      checkNotNull(e);
      if (delegate.add(e)) {
        addDedupedElement(e);
      }
      return this;
    }

    @Override
    SetBuilderImpl<E> copy() {
      return new JdkBackedSetBuilderImpl<>(this);
    }

    @Override
    ImmutableSet<E> build() {
      switch (distinct) {
        case 0:
          return of();
        case 1:
          return of(dedupedElements[0]);
        default:
          return new JdkBackedImmutableSet<E>(
              delegate, ImmutableList.asImmutableList(dedupedElements, distinct));
      }
    }
  }
}