common/collect/ImmutableSortedSet.java

/*
 * Copyright (C) 2008 Google Inc.
 *
 * 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 com.google.common.annotations.GwtCompatible;

import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.SortedSet;

/**
 * An immutable {@code SortedSet} that stores its elements in a sorted array.
 * Some instances are ordered by an explicit comparator, while others follow the
 * natural sort ordering of their elements. Either way, null elements are not
 * supported.
 *
 * <p>Unlike {@link Collections#unmodifiableSortedSet}, which is a <i>view</i>
 * of a separate collection that can still change, an instance of {@code
 * ImmutableSortedSet} contains its own private data and will <i>never</i>
 * change. This class is convenient for {@code public static final} sets
 * ("constant sets") and also lets you easily make a "defensive copy" of a set
 * provided to your class by a caller.
 *
 * <p>The sets returned by {@link #headSet}, {@link #tailSet}, and
 * {@link #subSet} methods share the same array as the original set, preventing
 * that array from being garbage collected. If this is a concern, the data may
 * be copied into a correctly-sized array by calling {@link #copyOfSorted}.
 *
 * <p><b>Note on element equivalence:</b> The {@link #contains(Object)},
 * {@link #containsAll(Collection)}, and {@link #equals(Object)}
 * implementations must check whether a provided object is equivalent to an
 * element in the collection. Unlike most collections, an
 * {@code ImmutableSortedSet} doesn't use {@link Object#equals} to determine if
 * two elements are equivalent. Instead, with an explicit comparator, the
 * following relation determines whether elements {@code x} and {@code y} are
 * equivalent: <pre>   {@code
 *
 *   {(x, y) | comparator.compare(x, y) == 0}}</pre>
 *
 * With natural ordering of elements, the following relation determines whether
 * two elements are equivalent: <pre>   {@code
 *
 *   {(x, y) | x.compareTo(y) == 0}}</pre>
 *
 * <b>Warning:</b> Like most sets, an {@code ImmutableSortedSet} will not
 * function correctly if an element is modified after being placed in the set.
 * For this reason, and to avoid general confusion, it is strongly recommended
 * to place only immutable objects into this collection.
 *
 * <p><b>Note</b>: Although this class is not final, it cannot be subclassed as
 * it has no public or protected constructors. Thus, instances of this type are
 * guaranteed to be immutable.
 *
 * @see ImmutableSet
 * @author Jared Levy
 * @since 2010.01.04 <b>stable</b> (imported from Google Collections Library)
 */
@GwtCompatible(serializable = true)
@SuppressWarnings("serial") // we're overriding default serialization
public abstract class ImmutableSortedSet<E>
    extends ImmutableSortedSetFauxverideShim<E> implements SortedSet<E> {

  // TODO: Can we find a way to remove this @SuppressWarnings even for eclipse?
  @SuppressWarnings("unchecked")
  private static final Comparator NATURAL_ORDER = Ordering.natural();

  @SuppressWarnings("unchecked")
  private static final ImmutableSortedSet<Object> NATURAL_EMPTY_SET =
      new EmptyImmutableSortedSet<Object>(NATURAL_ORDER);

  @SuppressWarnings("unchecked")
  private static <E> ImmutableSortedSet<E> emptySet() {
    return (ImmutableSortedSet<E>) NATURAL_EMPTY_SET;
  }

  static <E> ImmutableSortedSet<E> emptySet(
      Comparator<? super E> comparator) {
    if (NATURAL_ORDER.equals(comparator)) {
      return emptySet();
    } else {
      return new EmptyImmutableSortedSet<E>(comparator);
    }
  }

  /**
   * Returns the empty immutable sorted set.
   */
  public static <E> ImmutableSortedSet<E> of() {
    return emptySet();
  }

  /**
   * Returns an immutable sorted set containing a single element.
   */
  public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(
      E element) {
    Object[] array = { checkNotNull(element) };
    return new RegularImmutableSortedSet<E>(array, Ordering.natural());
  }

  /**
   * Returns an immutable sorted set containing the given elements sorted by
   * their natural ordering. When multiple elements are equivalent according to
   * {@link Comparable#compareTo}, only the first one specified is included.
   *
   * @throws NullPointerException if any element is null
   */
  @SuppressWarnings("unchecked")
  public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(
      E e1, E e2) {
    return ofInternal(Ordering.natural(), e1, e2);
  }

  /**
   * Returns an immutable sorted set containing the given elements sorted by
   * their natural ordering. When multiple elements are equivalent according to
   * {@link Comparable#compareTo}, only the first one specified is included.
   *
   * @throws NullPointerException if any element is null
   */
  @SuppressWarnings("unchecked")
  public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(
      E e1, E e2, E e3) {
    return ofInternal(Ordering.natural(), e1, e2, e3);
  }

  /**
   * Returns an immutable sorted set containing the given elements sorted by
   * their natural ordering. When multiple elements are equivalent according to
   * {@link Comparable#compareTo}, only the first one specified is included.
   *
   * @throws NullPointerException if any element is null
   */
  @SuppressWarnings("unchecked")
  public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(
      E e1, E e2, E e3, E e4) {
    return ofInternal(Ordering.natural(), e1, e2, e3, e4);
  }

  /**
   * Returns an immutable sorted set containing the given elements sorted by
   * their natural ordering. When multiple elements are equivalent according to
   * {@link Comparable#compareTo}, only the first one specified is included.
   *
   * @throws NullPointerException if any element is null
   */
  @SuppressWarnings("unchecked")
  public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(
      E e1, E e2, E e3, E e4, E e5) {
    return ofInternal(Ordering.natural(), e1, e2, e3, e4, e5);
  }

  // TODO: Consider adding factory methods that throw an exception when given
  // duplicate elements.

  /**
   * Returns an immutable sorted set containing the given elements sorted by
   * their natural ordering. When multiple elements are equivalent according to
   * {@link Comparable#compareTo}, only the first one specified is included.
   *
   * @throws NullPointerException if any of {@code elements} is null
   */
  public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(
      E... elements) {
    return ofInternal(Ordering.natural(), elements);
  }

  private static <E> ImmutableSortedSet<E> ofInternal(
      Comparator<? super E> comparator, E... elements) {
    checkNotNull(elements); // for GWT
    switch (elements.length) {
      case 0:
        return emptySet(comparator);
      default:
        /*
         * We can't use Platform.clone() because of GWT bug 3621. See our GWT
         * ImmutableSortedSetTest.testOf_gwtArraycopyBug() for details. We can't
         * use System.arraycopy() here for the same reason.
         */
        Object[] array = new Object[elements.length];
        for (int i = 0; i < elements.length; i++) {
          array[i] = checkNotNull(elements[i]);
        }
        sort(array, comparator);
        array = removeDupes(array, comparator);
        return new RegularImmutableSortedSet<E>(array, comparator);
    }
  }

  /** Sort the array, according to the comparator. */
  @SuppressWarnings("unchecked") // E comparator with Object array
  private static <E> void sort(
      Object[] array, Comparator<? super E> comparator) {
    Arrays.sort(array, (Comparator<Object>) comparator);
  }

  /**
   * Returns an array that removes duplicate consecutive elements, according to
   * the provided comparator. Note that the input array is modified. This method
   * does not support empty arrays.
   */
  private static <E> Object[] removeDupes(Object[] array,
      Comparator<? super E> comparator) {
    int size = 1;
    for (int i = 1; i < array.length; i++) {
      Object element = array[i];
      if (unsafeCompare(comparator, array[size - 1], element) != 0) {
        array[size] = element;
        size++;
      }
    }

    // TODO: Move to ObjectArrays?
    if (size == array.length) {
      return array;
    } else {
      Object[] copy = new Object[size];
      Platform.unsafeArrayCopy(array, 0, copy, 0, size);
      return copy;
    }
  }

  /**
   * Returns an immutable sorted set containing the given elements sorted by
   * their natural ordering. When multiple elements are equivalent according to
   * {@code compareTo()}, only the first one specified is included. To create a
   * copy of a {@code SortedSet} that preserves the comparator, call
   * {@link #copyOfSorted} instead. This method iterates over {@code elements}
   * at most once.
   *
   * <p>Note that if {@code s} is a {@code Set<String>}, then
   * {@code ImmutableSortedSet.copyOf(s)} returns an
   * {@code ImmutableSortedSet<String>} containing each of the strings in
   * {@code s}, while {@code ImmutableSortedSet.of(s)} returns an
   * {@code ImmutableSortedSet<Set<String>>} containing one element (the given
   * set itself).
   *
   * <p><b>Note:</b> Despite what the method name suggests, if {@code elements}
   * is an {@code ImmutableSortedSet}, it may be returned instead of a copy.
   *
   * <p>This method is not type-safe, as it may be called on elements that are
   * not mutually comparable.
   *
   * @throws ClassCastException if the elements are not mutually comparable
   * @throws NullPointerException if any of {@code elements} is null
   */
  public static <E> ImmutableSortedSet<E> copyOf(
      Iterable<? extends E> elements) {
    // Hack around K not being a subtype of Comparable.
    // Unsafe, see ImmutableSortedSetFauxverideShim.
    @SuppressWarnings("unchecked")
    Ordering<E> naturalOrder = (Ordering) Ordering.<Comparable>natural();
    return copyOfInternal(naturalOrder, elements, false);
  }

  /**
   * Returns an immutable sorted set containing the given elements sorted by
   * their natural ordering. When multiple elements are equivalent according to
   * {@code compareTo()}, only the first one specified is included.
   *
   * <p>This method is not type-safe, as it may be called on elements that are
   * not mutually comparable.
   *
   * @throws ClassCastException if the elements are not mutually comparable
   * @throws NullPointerException if any of {@code elements} is null
   */
  public static <E> ImmutableSortedSet<E> copyOf(
      Iterator<? extends E> elements) {
    // Hack around K not being a subtype of Comparable.
    // Unsafe, see ImmutableSortedSetFauxverideShim.
    @SuppressWarnings("unchecked")
    Ordering<E> naturalOrder = (Ordering) Ordering.<Comparable>natural();
    return copyOfInternal(naturalOrder, elements);
  }

  /**
   * Returns an immutable sorted set containing the given elements sorted by
   * the given {@code Comparator}. When multiple elements are equivalent
   * according to {@code compare()}, only the first one specified is
   * included. This method iterates over {@code elements} at most once.
   *
   * <p><b>Note:</b> Despite what the method name suggests, if {@code elements}
   * is an {@code ImmutableSortedSet}, it may be returned instead of a copy.
   *
   * @throws NullPointerException if {@code comparator} or any of
   *     {@code elements} is null
   */
  public static <E> ImmutableSortedSet<E> copyOf(
      Comparator<? super E> comparator, Iterable<? extends E> elements) {
    checkNotNull(comparator);
    return copyOfInternal(comparator, elements, false);
  }

  /**
   * Returns an immutable sorted set containing the given elements sorted by
   * the given {@code Comparator}. When multiple elements are equivalent
   * according to {@code compareTo()}, only the first one specified is
   * included.
   *
   * @throws NullPointerException if {@code comparator} or any of
   *     {@code elements} is null
   */
  public static <E> ImmutableSortedSet<E> copyOf(
      Comparator<? super E> comparator, Iterator<? extends E> elements) {
    checkNotNull(comparator);
    return copyOfInternal(comparator, elements);
  }

  /**
   * Returns an immutable sorted set containing the elements of a sorted set,
   * sorted by the same {@code Comparator}. That behavior differs from
   * {@link #copyOf(Iterable)}, which always uses the natural ordering of the
   * elements.
   *
   * <p><b>Note:</b> Despite what the method name suggests, if {@code sortedSet}
   * is an {@code ImmutableSortedSet}, it may be returned instead of a copy.
   *
   * @throws NullPointerException if any of {@code elements} is null
   */
  @SuppressWarnings("unchecked")
  public static <E> ImmutableSortedSet<E> copyOfSorted(SortedSet<E> sortedSet) {
    Comparator<? super E> comparator = sortedSet.comparator();
    if (comparator == null) {
      comparator = NATURAL_ORDER;
    }
    return copyOfInternal(comparator, sortedSet, true);
  }

  private static <E> ImmutableSortedSet<E> copyOfInternal(
      Comparator<? super E> comparator, Iterable<? extends E> elements,
      boolean fromSortedSet) {
    boolean hasSameComparator
        = fromSortedSet || hasSameComparator(elements, comparator);

    if (hasSameComparator && (elements instanceof ImmutableSortedSet)) {
      @SuppressWarnings("unchecked")
      ImmutableSortedSet<E> result = (ImmutableSortedSet<E>) elements;
      if (!result.hasPartialArray()) {
        return result;
      }
    }

    Object[] array = newObjectArray(elements);
    if (array.length == 0) {
      return emptySet(comparator);
    }

    for (Object e : array) {
      checkNotNull(e);
    }
    if (!hasSameComparator) {
      sort(array, comparator);
      array = removeDupes(array, comparator);
    }
    return new RegularImmutableSortedSet<E>(array, comparator);
  }

  /** Simplified version of {@link Iterables#toArray} that is GWT safe. */
  private static <T> Object[] newObjectArray(Iterable<T> iterable) {
    Collection<T> collection = (iterable instanceof Collection)
        ? (Collection<T>) iterable : Lists.newArrayList(iterable);
    Object[] array = new Object[collection.size()];
    return collection.toArray(array);
  }

  private static <E> ImmutableSortedSet<E> copyOfInternal(
      Comparator<? super E> comparator, Iterator<? extends E> elements) {
    if (!elements.hasNext()) {
      return emptySet(comparator);
    }
    List<E> list = Lists.newArrayList();
    while (elements.hasNext()) {
      list.add(checkNotNull(elements.next()));
    }
    Object[] array = list.toArray();
    sort(array, comparator);
    array = removeDupes(array, comparator);
    return new RegularImmutableSortedSet<E>(array, comparator);
  }

  /**
   * Returns {@code true} if {@code elements} is a {@code SortedSet} that uses
   * {@code comparator} to order its elements. Note that equivalent comparators
   * may still return {@code false}, if {@code equals} doesn't consider them
   * equal. If one comparator is {@code null} and the other is
   * {@link Ordering#natural()}, this method returns {@code true}.
   */
  static boolean hasSameComparator(
      Iterable<?> elements, Comparator<?> comparator) {
    if (elements instanceof SortedSet) {
      SortedSet<?> sortedSet = (SortedSet<?>) elements;
      Comparator<?> comparator2 = sortedSet.comparator();
      return (comparator2 == null)
          ? comparator == Ordering.natural()
          : comparator.equals(comparator2);
    }
    return false;
  }

  /**
   * Returns a builder that creates immutable sorted sets with an explicit
   * comparator. If the comparator has a more general type than the set being
   * generated, such as creating a {@code SortedSet<Integer>} with a
   * {@code Comparator<Number>}, use the {@link Builder} constructor instead.
   *
   * @throws NullPointerException if {@code comparator} is null
   */
  public static <E> Builder<E> orderedBy(Comparator<E> comparator) {
    return new Builder<E>(comparator);
  }

  /**
   * Returns a builder that creates immutable sorted sets whose elements are
   * ordered by the reverse of their natural ordering.
   *
   * <p>Note: the type parameter {@code E} extends {@code Comparable<E>} rather
   * than {@code Comparable<? super E>} as a workaround for javac <a
   * href="http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6468354">bug
   * 6468354</a>.
   */
  public static <E extends Comparable<E>> Builder<E> reverseOrder() {
    return new Builder<E>(Ordering.natural().reverse());
  }

  /**
   * Returns a builder that creates immutable sorted sets whose elements are
   * ordered by their natural ordering. The sorted sets use {@link
   * Ordering#natural()} as the comparator. This method provides more
   * type-safety than {@link #builder}, as it can be called only for classes
   * that implement {@link Comparable}.
   *
   * <p>Note: the type parameter {@code E} extends {@code Comparable<E>} rather
   * than {@code Comparable<? super E>} as a workaround for javac <a
   * href="http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6468354">bug
   * 6468354</a>.
   */
  public static <E extends Comparable<E>> Builder<E> naturalOrder() {
    return new Builder<E>(Ordering.natural());
  }

  /**
   * A builder for creating immutable sorted set instances, especially
   * {@code public static final} sets ("constant sets"), with a given
   * comparator.
   *
   * <p>Example:
   * <pre>{@code
   *   public static final ImmutableSortedSet<Number> LUCKY_NUMBERS
   *       = new ImmutableSortedSet.Builder<Number>(ODDS_FIRST_COMPARATOR)
   *           .addAll(SINGLE_DIGIT_PRIMES)
   *           .add(42)
   *           .build();}</pre>
   *
   * <p>Builder instances can be reused - it is safe to call {@link #build}
   * multiple times to build multiple sets in series. Each set
   * is a superset of the set created before it.
   */
  public static final class Builder<E> extends ImmutableSet.Builder<E> {
    private final Comparator<? super E> comparator;

    /**
     * Creates a new builder. The returned builder is equivalent to the builder
     * generated by {@link ImmutableSortedSet#orderedBy}.
     */
    public Builder(Comparator<? super E> comparator) {
      this.comparator = checkNotNull(comparator);
    }

    /**
     * Adds {@code element} to the {@code ImmutableSortedSet}.  If the
     * {@code ImmutableSortedSet} already contains {@code element}, then
     * {@code add} has no effect. (only the previously added element
     * is retained).
     *
     * @param element the element to add
     * @return this {@code Builder} object
     * @throws NullPointerException if {@code element} is null
     */
    @Override public Builder<E> add(E element) {
      super.add(element);
      return this;
    }

    /**
     * Adds each element of {@code elements} to the {@code ImmutableSortedSet},
     * 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} contains a null element
     */
    @Override public Builder<E> add(E... elements) {
      super.add(elements);
      return this;
    }

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

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

    /**
     * Returns a newly-created {@code ImmutableSortedSet} based on the contents
     * of the {@code Builder} and its comparator.
     */
    @Override public ImmutableSortedSet<E> build() {
      return copyOfInternal(comparator, contents.iterator());
    }
  }

  int unsafeCompare(Object a, Object b) {
    return unsafeCompare(comparator, a, b);
  }

  static int unsafeCompare(
      Comparator<?> comparator, Object a, Object b) {
    // Pretend the comparator can compare anything. If it turns out it can't
    // compare a and b, we should get a CCE on the subsequent line. Only methods
    // that are spec'd to throw CCE should call this.
    @SuppressWarnings("unchecked")
    Comparator<Object> unsafeComparator = (Comparator<Object>) comparator;
    return unsafeComparator.compare(a, b);
  }

  final transient Comparator<? super E> comparator;

  ImmutableSortedSet(Comparator<? super E> comparator) {
    this.comparator = comparator;
  }

  /**
   * Returns the comparator that orders the elements, which is
   * {@link Ordering#natural()} when the natural ordering of the
   * elements is used. Note that its behavior is not consistent with
   * {@link SortedSet#comparator()}, which returns {@code null} to indicate
   * natural ordering.
   */
  public Comparator<? super E> comparator() {
    return comparator;
  }

  /**
   * {@inheritDoc}
   *
   * <p>This method returns a serializable {@code ImmutableSortedSet}.
   *
   * <p>The {@link SortedSet#headSet} documentation states that a subset of a
   * subset throws an {@link IllegalArgumentException} if passed a
   * {@code toElement} greater than an earlier {@code toElement}. However, this
   * method doesn't throw an exception in that situation, but instead keeps the
   * original {@code toElement}.
   */
  public ImmutableSortedSet<E> headSet(E toElement) {
    return headSetImpl(checkNotNull(toElement));
  }

  /**
   * {@inheritDoc}
   *
   * <p>This method returns a serializable {@code ImmutableSortedSet}.
   *
   * <p>The {@link SortedSet#subSet} documentation states that a subset of a
   * subset throws an {@link IllegalArgumentException} if passed a
   * {@code fromElement} smaller than an earlier {@code fromElement}. However,
   * this method doesn't throw an exception in that situation, but instead keeps
   * the original {@code fromElement}. Similarly, this method keeps the
   * original {@code toElement}, instead of throwing an exception, if passed a
   * {@code toElement} greater than an earlier {@code toElement}.
   */
  public ImmutableSortedSet<E> subSet(E fromElement, E toElement) {
    checkNotNull(fromElement);
    checkNotNull(toElement);
    checkArgument(comparator.compare(fromElement, toElement) <= 0);
    return subSetImpl(fromElement, toElement);
  }

  /**
   * {@inheritDoc}
   *
   * <p>This method returns a serializable {@code ImmutableSortedSet}.
   *
   * <p>The {@link SortedSet#tailSet} documentation states that a subset of a
   * subset throws an {@link IllegalArgumentException} if passed a
   * {@code fromElement} smaller than an earlier {@code fromElement}. However,
   * this method doesn't throw an exception in that situation, but instead keeps
   * the original {@code fromElement}.
   */
  public ImmutableSortedSet<E> tailSet(E fromElement) {
    return tailSetImpl(checkNotNull(fromElement));
  }

  /*
   * These methods perform most headSet, subSet, and tailSet logic, besides
   * parameter validation.
   */
  abstract ImmutableSortedSet<E> headSetImpl(E toElement);
  abstract ImmutableSortedSet<E> subSetImpl(E fromElement, E toElement);
  abstract ImmutableSortedSet<E> tailSetImpl(E fromElement);

  /** Returns whether the elements are stored in a subset of a larger array. */
  abstract boolean hasPartialArray();

  /**
   * Returns the position of an element within the set, or -1 if not present.
   */
  abstract int indexOf(Object target);

  /*
   * This class is used to serialize all ImmutableSortedSet instances,
   * regardless of implementation type. It captures their "logical contents"
   * only. This is necessary to ensure that the existence of a particular
   * implementation type is an implementation detail.
   */
  private static class SerializedForm<E> implements Serializable {
    final Comparator<? super E> comparator;
    final Object[] elements;

    public SerializedForm(Comparator<? super E> comparator, Object[] elements) {
      this.comparator = comparator;
      this.elements = elements;
    }

    @SuppressWarnings("unchecked")
    Object readResolve() {
      return new Builder<E>(comparator).add((E[]) elements).build();
    }

    private static final long serialVersionUID = 0;
  }

  private void readObject(ObjectInputStream stream)
      throws InvalidObjectException {
    throw new InvalidObjectException("Use SerializedForm");
  }

  @Override Object writeReplace() {
    return new SerializedForm<E>(comparator, toArray());
  }
}