001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import static com.google.common.base.Preconditions.checkArgument;
020import static com.google.common.base.Preconditions.checkNotNull;
021import static com.google.common.collect.CollectPreconditions.checkNonnegative;
022import static java.util.Objects.requireNonNull;
023
024import com.google.common.annotations.GwtCompatible;
025import com.google.common.annotations.GwtIncompatible;
026import com.google.common.annotations.J2ktIncompatible;
027import com.google.common.annotations.VisibleForTesting;
028import com.google.common.math.IntMath;
029import com.google.common.primitives.Ints;
030import com.google.errorprone.annotations.CanIgnoreReturnValue;
031import com.google.errorprone.annotations.concurrent.LazyInit;
032import com.google.j2objc.annotations.RetainedWith;
033import java.io.InvalidObjectException;
034import java.io.ObjectInputStream;
035import java.io.Serializable;
036import java.math.RoundingMode;
037import java.util.Arrays;
038import java.util.Collection;
039import java.util.Collections;
040import java.util.EnumSet;
041import java.util.Iterator;
042import java.util.Set;
043import java.util.SortedSet;
044import java.util.Spliterator;
045import java.util.function.Consumer;
046import java.util.stream.Collector;
047import javax.annotation.CheckForNull;
048import org.checkerframework.checker.nullness.qual.Nullable;
049
050/**
051 * A {@link Set} whose contents will never change, with many other important properties detailed at
052 * {@link ImmutableCollection}.
053 *
054 * @since 2.0
055 */
056@GwtCompatible(serializable = true, emulated = true)
057@SuppressWarnings("serial") // we're overriding default serialization
058@ElementTypesAreNonnullByDefault
059public abstract class ImmutableSet<E> extends ImmutableCollection<E> implements Set<E> {
060  static final int SPLITERATOR_CHARACTERISTICS =
061      ImmutableCollection.SPLITERATOR_CHARACTERISTICS | Spliterator.DISTINCT;
062
063  /**
064   * Returns a {@code Collector} that accumulates the input elements into a new {@code
065   * ImmutableSet}. Elements appear in the resulting set in the encounter order of the stream; if
066   * the stream contains duplicates (according to {@link Object#equals(Object)}), only the first
067   * duplicate in encounter order will appear in the result.
068   *
069   * @since 21.0
070   */
071  public static <E> Collector<E, ?, ImmutableSet<E>> toImmutableSet() {
072    return CollectCollectors.toImmutableSet();
073  }
074
075  /**
076   * Returns the empty immutable set. Preferred over {@link Collections#emptySet} for code
077   * consistency, and because the return type conveys the immutability guarantee.
078   *
079   * <p><b>Performance note:</b> the instance returned is a singleton.
080   */
081  @SuppressWarnings({"unchecked"}) // fully variant implementation (never actually produces any Es)
082  public static <E> ImmutableSet<E> of() {
083    return (ImmutableSet<E>) RegularImmutableSet.EMPTY;
084  }
085
086  /**
087   * Returns an immutable set containing the given element. Preferred over {@link
088   * Collections#singleton} for code consistency, {@code null} rejection, and because the return
089   * type conveys the immutability guarantee.
090   */
091  public static <E> ImmutableSet<E> of(E e1) {
092    return new SingletonImmutableSet<>(e1);
093  }
094
095  /*
096   * TODO: b/315526394 - Skip the Builder entirely for the of(...) methods, since we don't need to
097   * worry that we might trigger the fallback to the JDK-backed implementation? (The varargs one
098   * _could_, so we could keep it as it is. Or we could convince ourselves that hash flooding is
099   * unlikely in practice there, too.)
100   */
101
102  /**
103   * Returns an immutable set containing the given elements, minus duplicates, in the order each was
104   * first specified. That is, if multiple elements are {@linkplain Object#equals equal}, all except
105   * the first are ignored.
106   */
107  public static <E> ImmutableSet<E> of(E e1, E e2) {
108    return new RegularSetBuilderImpl<E>(2).add(e1).add(e2).review().build();
109  }
110
111  /**
112   * Returns an immutable set containing the given elements, minus duplicates, in the order each was
113   * first specified. That is, if multiple elements are {@linkplain Object#equals equal}, all except
114   * the first are ignored.
115   */
116  public static <E> ImmutableSet<E> of(E e1, E e2, E e3) {
117    return new RegularSetBuilderImpl<E>(3).add(e1).add(e2).add(e3).review().build();
118  }
119
120  /**
121   * Returns an immutable set containing the given elements, minus duplicates, in the order each was
122   * first specified. That is, if multiple elements are {@linkplain Object#equals equal}, all except
123   * the first are ignored.
124   */
125  public static <E> ImmutableSet<E> of(E e1, E e2, E e3, E e4) {
126    return new RegularSetBuilderImpl<E>(4).add(e1).add(e2).add(e3).add(e4).review().build();
127  }
128
129  /**
130   * Returns an immutable set containing the given elements, minus duplicates, in the order each was
131   * first specified. That is, if multiple elements are {@linkplain Object#equals equal}, all except
132   * the first are ignored.
133   */
134  public static <E> ImmutableSet<E> of(E e1, E e2, E e3, E e4, E e5) {
135    return new RegularSetBuilderImpl<E>(5).add(e1).add(e2).add(e3).add(e4).add(e5).review().build();
136  }
137
138  /**
139   * Returns an immutable set containing the given elements, minus duplicates, in the order each was
140   * first specified. That is, if multiple elements are {@linkplain Object#equals equal}, all except
141   * the first are ignored.
142   *
143   * <p>The array {@code others} must not be longer than {@code Integer.MAX_VALUE - 6}.
144   *
145   * @since 3.0 (source-compatible since 2.0)
146   */
147  @SafeVarargs // For Eclipse. For internal javac we have disabled this pointless type of warning.
148  public static <E> ImmutableSet<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E... others) {
149    checkArgument(
150        others.length <= Integer.MAX_VALUE - 6, "the total number of elements must fit in an int");
151    SetBuilderImpl<E> builder = new RegularSetBuilderImpl<>(6 + others.length);
152    builder = builder.add(e1).add(e2).add(e3).add(e4).add(e5).add(e6);
153    for (int i = 0; i < others.length; i++) {
154      builder = builder.add(others[i]);
155    }
156    return builder.review().build();
157  }
158
159  /**
160   * Returns an immutable set containing each of {@code elements}, minus duplicates, in the order
161   * each appears first in the source collection.
162   *
163   * <p><b>Performance note:</b> This method will sometimes recognize that the actual copy operation
164   * is unnecessary; for example, {@code copyOf(copyOf(anArrayList))} will copy the data only once.
165   * This reduces the expense of habitually making defensive copies at API boundaries. However, the
166   * precise conditions for skipping the copy operation are undefined.
167   *
168   * @throws NullPointerException if any of {@code elements} is null
169   * @since 7.0 (source-compatible since 2.0)
170   */
171  // This the best we could do to get copyOfEnumSet to compile in the mainline.
172  // The suppression also covers the cast to E[], discussed below.
173  // In the backport, we don't have those cases and thus don't need this suppression.
174  // We keep it to minimize diffs.
175  @SuppressWarnings("unchecked")
176  public static <E> ImmutableSet<E> copyOf(Collection<? extends E> elements) {
177    /*
178     * TODO(lowasser): consider checking for ImmutableAsList here
179     * TODO(lowasser): consider checking for Multiset here
180     */
181    // Don't refer to ImmutableSortedSet by name so it won't pull in all that code
182    if (elements instanceof ImmutableSet && !(elements instanceof SortedSet)) {
183      @SuppressWarnings("unchecked") // all supported methods are covariant
184      ImmutableSet<E> set = (ImmutableSet<E>) elements;
185      if (!set.isPartialView()) {
186        return set;
187      }
188    } else if (elements instanceof EnumSet) {
189      return copyOfEnumSet((EnumSet<?>) elements);
190    }
191
192    if (elements.isEmpty()) {
193      // We avoid allocating anything.
194      return of();
195    }
196    // Collection<E>.toArray() is required to contain only E instances, and all we do is read them.
197    // TODO(cpovirk): Consider using Object[] anyway.
198    E[] array = (E[]) elements.toArray();
199    /*
200     * For a Set, we guess that it contains no duplicates. That's just a guess for purpose of
201     * sizing; if the Set uses different equality semantics, it might contain duplicates according
202     * to equals(), and we will deduplicate those properly, albeit at some cost in allocations.
203     */
204    int expectedSize =
205        elements instanceof Set ? array.length : estimatedSizeForUnknownDuplication(array.length);
206    return fromArrayWithExpectedSize(array, expectedSize);
207  }
208
209  /**
210   * Returns an immutable set containing each of {@code elements}, minus duplicates, in the order
211   * each appears first in the source iterable. This method iterates over {@code elements} only
212   * once.
213   *
214   * <p><b>Performance note:</b> This method will sometimes recognize that the actual copy operation
215   * is unnecessary; for example, {@code copyOf(copyOf(anArrayList))} should copy the data only
216   * once. This reduces the expense of habitually making defensive copies at API boundaries.
217   * However, the precise conditions for skipping the copy operation are undefined.
218   *
219   * @throws NullPointerException if any of {@code elements} is null
220   */
221  public static <E> ImmutableSet<E> copyOf(Iterable<? extends E> elements) {
222    return (elements instanceof Collection)
223        ? copyOf((Collection<? extends E>) elements)
224        : copyOf(elements.iterator());
225  }
226
227  /**
228   * Returns an immutable set containing each of {@code elements}, minus duplicates, in the order
229   * each appears first in the source iterator.
230   *
231   * @throws NullPointerException if any of {@code elements} is null
232   */
233  public static <E> ImmutableSet<E> copyOf(Iterator<? extends E> elements) {
234    // We special-case for 0 or 1 elements, but anything further is madness.
235    if (!elements.hasNext()) {
236      return of();
237    }
238    E first = elements.next();
239    if (!elements.hasNext()) {
240      return of(first);
241    } else {
242      return new ImmutableSet.Builder<E>().add(first).addAll(elements).build();
243    }
244  }
245
246  /**
247   * Returns an immutable set containing each of {@code elements}, minus duplicates, in the order
248   * each appears first in the source array.
249   *
250   * @throws NullPointerException if any of {@code elements} is null
251   * @since 3.0
252   */
253  public static <E> ImmutableSet<E> copyOf(E[] elements) {
254    return fromArrayWithExpectedSize(elements, estimatedSizeForUnknownDuplication(elements.length));
255  }
256
257  private static <E> ImmutableSet<E> fromArrayWithExpectedSize(E[] elements, int expectedSize) {
258    switch (elements.length) {
259      case 0:
260        return of();
261      case 1:
262        return of(elements[0]);
263      default:
264        SetBuilderImpl<E> builder = new RegularSetBuilderImpl<>(expectedSize);
265        for (int i = 0; i < elements.length; i++) {
266          builder = builder.add(elements[i]);
267        }
268        return builder.review().build();
269    }
270  }
271
272  @SuppressWarnings({"rawtypes", "unchecked"}) // necessary to compile against Java 8
273  private static ImmutableSet copyOfEnumSet(EnumSet<?> enumSet) {
274    return ImmutableEnumSet.asImmutable(EnumSet.copyOf((EnumSet) enumSet));
275  }
276
277  ImmutableSet() {}
278
279  /** Returns {@code true} if the {@code hashCode()} method runs quickly. */
280  boolean isHashCodeFast() {
281    return false;
282  }
283
284  @Override
285  public boolean equals(@CheckForNull Object object) {
286    if (object == this) {
287      return true;
288    }
289    if (object instanceof ImmutableSet
290        && isHashCodeFast()
291        && ((ImmutableSet<?>) object).isHashCodeFast()
292        && hashCode() != object.hashCode()) {
293      return false;
294    }
295    return Sets.equalsImpl(this, object);
296  }
297
298  @Override
299  public int hashCode() {
300    return Sets.hashCodeImpl(this);
301  }
302
303  // This declaration is needed to make Set.iterator() and
304  // ImmutableCollection.iterator() consistent.
305  @Override
306  public abstract UnmodifiableIterator<E> iterator();
307
308  @GwtCompatible
309  abstract static class CachingAsList<E> extends ImmutableSet<E> {
310    @LazyInit @RetainedWith @CheckForNull private transient ImmutableList<E> asList;
311
312    @Override
313    public ImmutableList<E> asList() {
314      ImmutableList<E> result = asList;
315      if (result == null) {
316        return asList = createAsList();
317      } else {
318        return result;
319      }
320    }
321
322    ImmutableList<E> createAsList() {
323      return new RegularImmutableAsList<>(this, toArray());
324    }
325
326    // redeclare to help optimizers with b/310253115
327    @SuppressWarnings("RedundantOverride")
328    @Override
329    @J2ktIncompatible // serialization
330    @GwtIncompatible // serialization
331    Object writeReplace() {
332      return super.writeReplace();
333    }
334  }
335
336  abstract static class Indexed<E> extends CachingAsList<E> {
337    abstract E get(int index);
338
339    @Override
340    public UnmodifiableIterator<E> iterator() {
341      return asList().iterator();
342    }
343
344    @Override
345    public Spliterator<E> spliterator() {
346      return CollectSpliterators.indexed(size(), SPLITERATOR_CHARACTERISTICS, this::get);
347    }
348
349    @Override
350    public void forEach(Consumer<? super E> consumer) {
351      checkNotNull(consumer);
352      int n = size();
353      for (int i = 0; i < n; i++) {
354        consumer.accept(get(i));
355      }
356    }
357
358    @Override
359    int copyIntoArray(@Nullable Object[] dst, int offset) {
360      return asList().copyIntoArray(dst, offset);
361    }
362
363    @Override
364    ImmutableList<E> createAsList() {
365      return new ImmutableAsList<E>() {
366        @Override
367        public E get(int index) {
368          return Indexed.this.get(index);
369        }
370
371        @Override
372        Indexed<E> delegateCollection() {
373          return Indexed.this;
374        }
375
376        // redeclare to help optimizers with b/310253115
377        @SuppressWarnings("RedundantOverride")
378        @Override
379        @J2ktIncompatible // serialization
380        @GwtIncompatible // serialization
381        Object writeReplace() {
382          return super.writeReplace();
383        }
384      };
385    }
386
387    // redeclare to help optimizers with b/310253115
388    @SuppressWarnings("RedundantOverride")
389    @Override
390    @J2ktIncompatible // serialization
391    @GwtIncompatible // serialization
392    Object writeReplace() {
393      return super.writeReplace();
394    }
395  }
396
397  /*
398   * This class is used to serialize all ImmutableSet instances, except for
399   * ImmutableEnumSet/ImmutableSortedSet, regardless of implementation type. It
400   * captures their "logical contents" and they are reconstructed using public
401   * static factories. This is necessary to ensure that the existence of a
402   * particular implementation type is an implementation detail.
403   */
404  @J2ktIncompatible // serialization
405  private static class SerializedForm implements Serializable {
406    final Object[] elements;
407
408    SerializedForm(Object[] elements) {
409      this.elements = elements;
410    }
411
412    Object readResolve() {
413      return copyOf(elements);
414    }
415
416    private static final long serialVersionUID = 0;
417  }
418
419  @Override
420  @J2ktIncompatible // serialization
421  Object writeReplace() {
422    return new SerializedForm(toArray());
423  }
424
425  @J2ktIncompatible // serialization
426  private void readObject(ObjectInputStream stream) throws InvalidObjectException {
427    throw new InvalidObjectException("Use SerializedForm");
428  }
429
430  /**
431   * Returns a new builder. The generated builder is equivalent to the builder created by the {@link
432   * Builder} constructor.
433   */
434  public static <E> Builder<E> builder() {
435    return new Builder<>();
436  }
437
438  /**
439   * Returns a new builder, expecting the specified number of distinct elements to be added.
440   *
441   * <p>If {@code expectedSize} is exactly the number of distinct elements added to the builder
442   * before {@link Builder#build} is called, the builder is likely to perform better than an unsized
443   * {@link #builder()} would have.
444   *
445   * <p>It is not specified if any performance benefits apply if {@code expectedSize} is close to,
446   * but not exactly, the number of distinct elements added to the builder.
447   *
448   * @since 23.1
449   */
450  public static <E> Builder<E> builderWithExpectedSize(int expectedSize) {
451    checkNonnegative(expectedSize, "expectedSize");
452    return new Builder<>(expectedSize);
453  }
454
455  /**
456   * A builder for creating {@code ImmutableSet} instances. Example:
457   *
458   * <pre>{@code
459   * static final ImmutableSet<Color> GOOGLE_COLORS =
460   *     ImmutableSet.<Color>builder()
461   *         .addAll(WEBSAFE_COLORS)
462   *         .add(new Color(0, 191, 255))
463   *         .build();
464   * }</pre>
465   *
466   * <p>Elements appear in the resulting set in the same order they were first added to the builder.
467   *
468   * <p>Building does not change the state of the builder, so it is still possible to add more
469   * elements and to build again.
470   *
471   * @since 2.0
472   */
473  public static class Builder<E> extends ImmutableCollection.Builder<E> {
474    /*
475     * `impl` is null only for instances of the subclass, ImmutableSortedSet.Builder. That subclass
476     * overrides all the methods that access it here. Thus, all the methods here can safely assume
477     * that this field is non-null.
478     */
479    @CheckForNull private SetBuilderImpl<E> impl;
480    boolean forceCopy;
481
482    public Builder() {
483      this(0);
484    }
485
486    Builder(int capacity) {
487      if (capacity > 0) {
488        impl = new RegularSetBuilderImpl<>(capacity);
489      } else {
490        impl = EmptySetBuilderImpl.instance();
491      }
492    }
493
494    Builder(@SuppressWarnings("unused") boolean subclass) {
495      this.impl = null; // unused
496    }
497
498    @VisibleForTesting
499    void forceJdk() {
500      requireNonNull(impl); // see the comment on the field
501      this.impl = new JdkBackedSetBuilderImpl<>(impl);
502    }
503
504    final void copyIfNecessary() {
505      if (forceCopy) {
506        copy();
507        forceCopy = false;
508      }
509    }
510
511    void copy() {
512      requireNonNull(impl); // see the comment on the field
513      impl = impl.copy();
514    }
515
516    @Override
517    @CanIgnoreReturnValue
518    public Builder<E> add(E element) {
519      requireNonNull(impl); // see the comment on the field
520      checkNotNull(element);
521      copyIfNecessary();
522      impl = impl.add(element);
523      return this;
524    }
525
526    @Override
527    @CanIgnoreReturnValue
528    public Builder<E> add(E... elements) {
529      super.add(elements);
530      return this;
531    }
532
533    /**
534     * Adds each element of {@code elements} to the {@code ImmutableSet}, ignoring duplicate
535     * elements (only the first duplicate element is added).
536     *
537     * @param elements the elements to add
538     * @return this {@code Builder} object
539     * @throws NullPointerException if {@code elements} is null or contains a null element
540     */
541    @Override
542    @CanIgnoreReturnValue
543    public Builder<E> addAll(Iterable<? extends E> elements) {
544      super.addAll(elements);
545      return this;
546    }
547
548    @Override
549    @CanIgnoreReturnValue
550    public Builder<E> addAll(Iterator<? extends E> elements) {
551      super.addAll(elements);
552      return this;
553    }
554
555    @CanIgnoreReturnValue
556    Builder<E> combine(Builder<E> other) {
557      requireNonNull(impl);
558      requireNonNull(other.impl);
559      /*
560       * For discussion of requireNonNull, see the comment on the field.
561       *
562       * (And I don't believe there's any situation in which we call x.combine(y) when x is a plain
563       * ImmutableSet.Builder but y is an ImmutableSortedSet.Builder (or vice versa). Certainly
564       * ImmutableSortedSet.Builder.combine() is written as if its argument will never be a plain
565       * ImmutableSet.Builder: It casts immediately to ImmutableSortedSet.Builder.)
566       */
567      copyIfNecessary();
568      this.impl = this.impl.combine(other.impl);
569      return this;
570    }
571
572    @Override
573    public ImmutableSet<E> build() {
574      requireNonNull(impl); // see the comment on the field
575      forceCopy = true;
576      impl = impl.review();
577      return impl.build();
578    }
579  }
580
581  /** Swappable internal implementation of an ImmutableSet.Builder. */
582  private abstract static class SetBuilderImpl<E> {
583    // The first `distinct` elements are non-null.
584    // Since we can never access null elements, we don't mark this nullable.
585    E[] dedupedElements;
586    int distinct;
587
588    @SuppressWarnings("unchecked")
589    SetBuilderImpl(int expectedCapacity) {
590      this.dedupedElements = (E[]) new Object[expectedCapacity];
591      this.distinct = 0;
592    }
593
594    /** Initializes this SetBuilderImpl with a copy of the deduped elements array from toCopy. */
595    SetBuilderImpl(SetBuilderImpl<E> toCopy) {
596      this.dedupedElements = Arrays.copyOf(toCopy.dedupedElements, toCopy.dedupedElements.length);
597      this.distinct = toCopy.distinct;
598    }
599
600    /**
601     * Resizes internal data structures if necessary to store the specified number of distinct
602     * elements.
603     */
604    private void ensureCapacity(int minCapacity) {
605      if (minCapacity > dedupedElements.length) {
606        int newCapacity =
607            ImmutableCollection.Builder.expandedCapacity(dedupedElements.length, minCapacity);
608        dedupedElements = Arrays.copyOf(dedupedElements, newCapacity);
609      }
610    }
611
612    /** Adds e to the insertion-order array of deduplicated elements. Calls ensureCapacity. */
613    final void addDedupedElement(E e) {
614      ensureCapacity(distinct + 1);
615      dedupedElements[distinct++] = e;
616    }
617
618    /**
619     * Adds e to this SetBuilderImpl, returning the updated result. Only use the returned
620     * SetBuilderImpl, since we may switch implementations if e.g. hash flooding is detected.
621     */
622    abstract SetBuilderImpl<E> add(E e);
623
624    /** Adds all the elements from the specified SetBuilderImpl to this SetBuilderImpl. */
625    final SetBuilderImpl<E> combine(SetBuilderImpl<E> other) {
626      SetBuilderImpl<E> result = this;
627      for (int i = 0; i < other.distinct; i++) {
628        /*
629         * requireNonNull is safe because we ensure that the first `distinct` elements have been
630         * populated.
631         */
632        result = result.add(requireNonNull(other.dedupedElements[i]));
633      }
634      return result;
635    }
636
637    /**
638     * Creates a new copy of this SetBuilderImpl. Modifications to that SetBuilderImpl will not
639     * affect this SetBuilderImpl or sets constructed from this SetBuilderImpl via build().
640     */
641    abstract SetBuilderImpl<E> copy();
642
643    /**
644     * Call this before build(). Does a final check on the internal data structures, e.g. shrinking
645     * unnecessarily large structures or detecting previously unnoticed hash flooding.
646     */
647    SetBuilderImpl<E> review() {
648      return this;
649    }
650
651    abstract ImmutableSet<E> build();
652  }
653
654  private static final class EmptySetBuilderImpl<E> extends SetBuilderImpl<E> {
655    private static final EmptySetBuilderImpl<Object> INSTANCE = new EmptySetBuilderImpl<>();
656
657    @SuppressWarnings("unchecked")
658    static <E> SetBuilderImpl<E> instance() {
659      return (SetBuilderImpl<E>) INSTANCE;
660    }
661
662    private EmptySetBuilderImpl() {
663      super(0);
664    }
665
666    @Override
667    SetBuilderImpl<E> add(E e) {
668      return new RegularSetBuilderImpl<E>(Builder.DEFAULT_INITIAL_CAPACITY).add(e);
669    }
670
671    @Override
672    SetBuilderImpl<E> copy() {
673      return this;
674    }
675
676    @Override
677    ImmutableSet<E> build() {
678      return ImmutableSet.of();
679    }
680  }
681
682  // We use power-of-2 tables, and this is the highest int that's a power of 2
683  static final int MAX_TABLE_SIZE = Ints.MAX_POWER_OF_TWO;
684
685  // Represents how tightly we can pack things, as a maximum.
686  private static final double DESIRED_LOAD_FACTOR = 0.7;
687
688  // If the set has this many elements, it will "max out" the table size
689  private static final int CUTOFF = (int) (MAX_TABLE_SIZE * DESIRED_LOAD_FACTOR);
690
691  /**
692   * Returns an array size suitable for the backing array of a hash table that uses open addressing
693   * with linear probing in its implementation. The returned size is the smallest power of two that
694   * can hold setSize elements with the desired load factor. Always returns at least setSize + 2.
695   */
696  // TODO(cpovirk): Move to Hashing or something, since it's used elsewhere in the Android version.
697  static int chooseTableSize(int setSize) {
698    setSize = Math.max(setSize, 2);
699    // Correct the size for open addressing to match desired load factor.
700    if (setSize < CUTOFF) {
701      // Round up to the next highest power of 2.
702      int tableSize = Integer.highestOneBit(setSize - 1) << 1;
703      while (tableSize * DESIRED_LOAD_FACTOR < setSize) {
704        tableSize <<= 1;
705      }
706      return tableSize;
707    }
708
709    // The table can't be completely full or we'll get infinite reprobes
710    checkArgument(setSize < MAX_TABLE_SIZE, "collection too large");
711    return MAX_TABLE_SIZE;
712  }
713
714  /**
715   * Default implementation of the guts of ImmutableSet.Builder, creating an open-addressed hash
716   * table and deduplicating elements as they come, so it only allocates O(max(distinct,
717   * expectedCapacity)) rather than O(calls to add).
718   *
719   * <p>This implementation attempts to detect hash flooding, and if it's identified, falls back to
720   * JdkBackedSetBuilderImpl.
721   */
722  private static final class RegularSetBuilderImpl<E> extends SetBuilderImpl<E> {
723    // null until at least two elements are present
724    @CheckForNull private @Nullable Object[] hashTable;
725    private int maxRunBeforeFallback;
726    private int expandTableThreshold;
727    private int hashCode;
728
729    RegularSetBuilderImpl(int expectedCapacity) {
730      super(expectedCapacity);
731      this.hashTable = null;
732      this.maxRunBeforeFallback = 0;
733      this.expandTableThreshold = 0;
734    }
735
736    RegularSetBuilderImpl(RegularSetBuilderImpl<E> toCopy) {
737      super(toCopy);
738      this.hashTable = (toCopy.hashTable == null) ? null : toCopy.hashTable.clone();
739      this.maxRunBeforeFallback = toCopy.maxRunBeforeFallback;
740      this.expandTableThreshold = toCopy.expandTableThreshold;
741      this.hashCode = toCopy.hashCode;
742    }
743
744    @Override
745    SetBuilderImpl<E> add(E e) {
746      checkNotNull(e);
747      if (hashTable == null) {
748        if (distinct == 0) {
749          addDedupedElement(e);
750          return this;
751        } else {
752          ensureTableCapacity(dedupedElements.length);
753          E elem = dedupedElements[0];
754          distinct--;
755          return insertInHashTable(elem).add(e);
756        }
757      }
758      return insertInHashTable(e);
759    }
760
761    private SetBuilderImpl<E> insertInHashTable(E e) {
762      requireNonNull(hashTable);
763      int eHash = e.hashCode();
764      int i0 = Hashing.smear(eHash);
765      int mask = hashTable.length - 1;
766      for (int i = i0; i - i0 < maxRunBeforeFallback; i++) {
767        int index = i & mask;
768        Object tableEntry = hashTable[index];
769        if (tableEntry == null) {
770          addDedupedElement(e);
771          hashTable[index] = e;
772          hashCode += eHash;
773          ensureTableCapacity(distinct); // rebuilds table if necessary
774          return this;
775        } else if (tableEntry.equals(e)) { // not a new element, ignore
776          return this;
777        }
778      }
779      // we fell out of the loop due to a long run; fall back to JDK impl
780      return new JdkBackedSetBuilderImpl<E>(this).add(e);
781    }
782
783    @Override
784    SetBuilderImpl<E> copy() {
785      return new RegularSetBuilderImpl<>(this);
786    }
787
788    @Override
789    SetBuilderImpl<E> review() {
790      if (hashTable == null) {
791        return this;
792      }
793      int targetTableSize = chooseTableSize(distinct);
794      if (targetTableSize * 2 < hashTable.length) {
795        hashTable = rebuildHashTable(targetTableSize, dedupedElements, distinct);
796        maxRunBeforeFallback = maxRunBeforeFallback(targetTableSize);
797        expandTableThreshold = (int) (DESIRED_LOAD_FACTOR * targetTableSize);
798      }
799      return hashFloodingDetected(hashTable) ? new JdkBackedSetBuilderImpl<E>(this) : this;
800    }
801
802    @Override
803    ImmutableSet<E> build() {
804      switch (distinct) {
805        case 0:
806          return of();
807        case 1:
808          /*
809           * requireNonNull is safe because we ensure that the first `distinct` elements have been
810           * populated.
811           */
812          return of(requireNonNull(dedupedElements[0]));
813        default:
814          /*
815           * The suppression is safe because we ensure that the first `distinct` elements have been
816           * populated.
817           */
818          @SuppressWarnings("nullness")
819          Object[] elements =
820              (distinct == dedupedElements.length)
821                  ? dedupedElements
822                  : Arrays.copyOf(dedupedElements, distinct);
823          return new RegularImmutableSet<>(
824              elements, hashCode, requireNonNull(hashTable), hashTable.length - 1);
825      }
826    }
827
828    /** Builds a new open-addressed hash table from the first n objects in elements. */
829    static @Nullable Object[] rebuildHashTable(int newTableSize, Object[] elements, int n) {
830      @Nullable Object[] hashTable = new @Nullable Object[newTableSize];
831      int mask = hashTable.length - 1;
832      for (int i = 0; i < n; i++) {
833        // requireNonNull is safe because we ensure that the first n elements have been populated.
834        Object e = requireNonNull(elements[i]);
835        int j0 = Hashing.smear(e.hashCode());
836        for (int j = j0; ; j++) {
837          int index = j & mask;
838          if (hashTable[index] == null) {
839            hashTable[index] = e;
840            break;
841          }
842        }
843      }
844      return hashTable;
845    }
846
847    void ensureTableCapacity(int minCapacity) {
848      int newTableSize;
849      if (hashTable == null) {
850        newTableSize = chooseTableSize(minCapacity);
851        hashTable = new Object[newTableSize];
852      } else if (minCapacity > expandTableThreshold && hashTable.length < MAX_TABLE_SIZE) {
853        newTableSize = hashTable.length * 2;
854        hashTable = rebuildHashTable(newTableSize, dedupedElements, distinct);
855      } else {
856        return;
857      }
858      maxRunBeforeFallback = maxRunBeforeFallback(newTableSize);
859      expandTableThreshold = (int) (DESIRED_LOAD_FACTOR * newTableSize);
860    }
861
862    /**
863     * We attempt to detect deliberate hash flooding attempts. If one is detected, we fall back to a
864     * wrapper around j.u.HashSet, which has built-in flooding protection. MAX_RUN_MULTIPLIER was
865     * determined experimentally to match our desired probability of false positives.
866     */
867    // NB: yes, this is surprisingly high, but that's what the experiments said was necessary
868    // Raising this number slows the worst-case contains behavior, speeds up hashFloodingDetected,
869    // and reduces the false-positive probability.
870    static final int MAX_RUN_MULTIPLIER = 13;
871
872    /**
873     * Checks the whole hash table for poor hash distribution. Takes O(n) in the worst case, O(n /
874     * log n) on average.
875     *
876     * <p>The online hash flooding detecting in RegularSetBuilderImpl.add can detect e.g. many
877     * exactly matching hash codes, which would cause construction to take O(n^2), but can't detect
878     * e.g. hash codes adversarially designed to go into ascending table locations, which keeps
879     * construction O(n) (as desired) but then can have O(n) queries later.
880     *
881     * <p>If this returns false, then no query can take more than O(log n).
882     *
883     * <p>Note that for a RegularImmutableSet with elements with truly random hash codes, contains
884     * operations take expected O(1) time but with high probability take O(log n) for at least some
885     * element. (https://en.wikipedia.org/wiki/Linear_probing#Analysis)
886     *
887     * <p>This method may return {@code true} even on truly random input, but {@code
888     * ImmutableSetTest} tests that the probability of that is low.
889     */
890    static boolean hashFloodingDetected(@Nullable Object[] hashTable) {
891      int maxRunBeforeFallback = maxRunBeforeFallback(hashTable.length);
892      int mask = hashTable.length - 1;
893
894      // Invariant: all elements at indices in [knownRunStart, knownRunEnd) are nonnull.
895      // If knownRunStart == knownRunEnd, this is vacuously true.
896      // When knownRunEnd exceeds hashTable.length, it "wraps", detecting runs around the end
897      // of the table.
898      int knownRunStart = 0;
899      int knownRunEnd = 0;
900
901      outerLoop:
902      while (knownRunStart < hashTable.length) {
903        if (knownRunStart == knownRunEnd && hashTable[knownRunStart] == null) {
904          if (hashTable[(knownRunStart + maxRunBeforeFallback - 1) & mask] == null) {
905            // There are only maxRunBeforeFallback - 1 elements between here and there,
906            // so even if they were all nonnull, we wouldn't detect a hash flood.  Therefore,
907            // we can skip them all.
908            knownRunStart += maxRunBeforeFallback;
909          } else {
910            knownRunStart++; // the only case in which maxRunEnd doesn't increase by mRBF
911            // happens about f * (1-f) for f = DESIRED_LOAD_FACTOR, so around 21% of the time
912          }
913          knownRunEnd = knownRunStart;
914        } else {
915          for (int j = knownRunStart + maxRunBeforeFallback - 1; j >= knownRunEnd; j--) {
916            if (hashTable[j & mask] == null) {
917              knownRunEnd = knownRunStart + maxRunBeforeFallback;
918              knownRunStart = j + 1;
919              continue outerLoop;
920            }
921          }
922          return true;
923        }
924      }
925      return false;
926    }
927
928    /**
929     * If more than this many consecutive positions are filled in a table of the specified size,
930     * report probable hash flooding. ({@link #hashFloodingDetected} may also report hash flooding
931     * if fewer consecutive positions are filled; see that method for details.)
932     */
933    static int maxRunBeforeFallback(int tableSize) {
934      return MAX_RUN_MULTIPLIER * IntMath.log2(tableSize, RoundingMode.UNNECESSARY);
935    }
936  }
937
938  /**
939   * SetBuilderImpl version that uses a JDK HashSet, which has built in hash flooding protection.
940   */
941  private static final class JdkBackedSetBuilderImpl<E> extends SetBuilderImpl<E> {
942    private final Set<Object> delegate;
943
944    JdkBackedSetBuilderImpl(SetBuilderImpl<E> toCopy) {
945      super(toCopy); // initializes dedupedElements and distinct
946      delegate = Sets.newHashSetWithExpectedSize(distinct);
947      for (int i = 0; i < distinct; i++) {
948        /*
949         * requireNonNull is safe because we ensure that the first `distinct` elements have been
950         * populated.
951         */
952        delegate.add(requireNonNull(dedupedElements[i]));
953      }
954    }
955
956    @Override
957    SetBuilderImpl<E> add(E e) {
958      checkNotNull(e);
959      if (delegate.add(e)) {
960        addDedupedElement(e);
961      }
962      return this;
963    }
964
965    @Override
966    SetBuilderImpl<E> copy() {
967      return new JdkBackedSetBuilderImpl<>(this);
968    }
969
970    @Override
971    ImmutableSet<E> build() {
972      switch (distinct) {
973        case 0:
974          return of();
975        case 1:
976          /*
977           * requireNonNull is safe because we ensure that the first `distinct` elements have been
978           * populated.
979           */
980          return of(requireNonNull(dedupedElements[0]));
981        default:
982          return new JdkBackedImmutableSet<>(
983              delegate, ImmutableList.asImmutableList(dedupedElements, distinct));
984      }
985    }
986  }
987
988  private static int estimatedSizeForUnknownDuplication(int inputElementsIncludingAnyDuplicates) {
989    if (inputElementsIncludingAnyDuplicates
990        < ImmutableCollection.Builder.DEFAULT_INITIAL_CAPACITY) {
991      return inputElementsIncludingAnyDuplicates;
992    }
993    // Guess the size is "halfway between" all duplicates and no duplicates, on a log scale.
994    return Math.max(
995        ImmutableCollection.Builder.DEFAULT_INITIAL_CAPACITY,
996        IntMath.sqrt(inputElementsIncludingAnyDuplicates, RoundingMode.CEILING));
997  }
998
999  private static final long serialVersionUID = 0xcafebabe;
1000}