/*
 * Copyright (C) 2008 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.ObjectArrays.checkElementsNotNull;

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.NavigableSet;
import java.util.SortedSet;

import javax.annotation.Nullable;

import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;

/**
 * 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 the {@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>
 *
 * <p>
 * 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.
 *
 * <p>
 * See the Guava User Guide article on <a href=
 * "http://code.google.com/p/guava-libraries/wiki/ImmutableCollectionsExplained">
 * immutable collections</a>.
 *
 * @see ImmutableSet
 * @author Jared Levy
 * @author Louis Wasserman
 * @since 2.0 (imported from Google Collections Library; implements
 *        {@code NavigableSet} since 12.0)
 */
// TODO(benyu): benchmark and optimize all creation paths, which are a mess now
@GwtCompatible(serializable = true, emulated = true)
@SuppressWarnings("serial") // we're overriding default serialization
public abstract class ImmutableSortedSet<E> extends ImmutableSortedSetFauxverideShim<E>
		implements NavigableSet<E>, SortedIterable<E> {

	private static final Comparator<Comparable> NATURAL_ORDER = Ordering.natural();

	private static final ImmutableSortedSet<Comparable> NATURAL_EMPTY_SET = new EmptyImmutableSortedSet<Comparable>(
			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) {
		return new RegularImmutableSortedSet<E>(ImmutableList.of(element), 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 construct(Ordering.natural(), 2, 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 construct(Ordering.natural(), 3, 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 construct(Ordering.natural(), 4, 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 construct(Ordering.natural(), 5, e1, e2, e3, e4, e5);
	}

	/**
	 * 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
	 * @since 3.0 (source-compatible since 2.0)
	 */
	@SuppressWarnings("unchecked")
	public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E e1, E e2, E e3, E e4, E e5, E e6,
			E... remaining) {
		Comparable[] contents = new Comparable[6 + remaining.length];
		contents[0] = e1;
		contents[1] = e2;
		contents[2] = e3;
		contents[3] = e4;
		contents[4] = e5;
		contents[5] = e6;
		System.arraycopy(remaining, 0, contents, 6, remaining.length);
		return construct(Ordering.natural(), contents.length, (E[]) contents);
	}

	// TODO(kevinb): Consider factory methods that reject duplicates

	/**
	 * 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
	 * @since 3.0
	 */
	public static <E extends Comparable<? super E>> ImmutableSortedSet<E> copyOf(E[] elements) {
		return construct(Ordering.natural(), elements.length, elements.clone());
	}

	/**
	 * 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>
	 * Despite the method name, this method attempts to avoid actually copying the
	 * data when it is safe to do so. The exact circumstances under which a copy
	 * will or will not be performed are undocumented and subject to change.
	 *
	 * <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 E not being a subtype of Comparable.
		// Unsafe, see ImmutableSortedSetFauxverideShim.
		@SuppressWarnings("unchecked")
		Ordering<E> naturalOrder = (Ordering<E>) Ordering.<Comparable>natural();
		return copyOf(naturalOrder, elements);
	}

	/**
	 * 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.
	 *
	 * <p>
	 * This method is safe to use even when {@code elements} is a synchronized or
	 * concurrent collection that is currently being modified by another thread.
	 *
	 * @throws ClassCastException   if the elements are not mutually comparable
	 * @throws NullPointerException if any of {@code elements} is null
	 * @since 7.0 (source-compatible since 2.0)
	 */
	public static <E> ImmutableSortedSet<E> copyOf(Collection<? extends E> elements) {
		// Hack around E not being a subtype of Comparable.
		// Unsafe, see ImmutableSortedSetFauxverideShim.
		@SuppressWarnings("unchecked")
		Ordering<E> naturalOrder = (Ordering<E>) Ordering.<Comparable>natural();
		return copyOf(naturalOrder, elements);
	}

	/**
	 * 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 E not being a subtype of Comparable.
		// Unsafe, see ImmutableSortedSetFauxverideShim.
		@SuppressWarnings("unchecked")
		Ordering<E> naturalOrder = (Ordering<E>) Ordering.<Comparable>natural();
		return copyOf(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 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) {
		return new Builder<E>(comparator).addAll(elements).build();
	}

	/**
	 * 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>
	 * Despite the method name, this method attempts to avoid actually copying the
	 * data when it is safe to do so. The exact circumstances under which a copy
	 * will or will not be performed are undocumented and subject to change.
	 *
	 * @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);
		boolean hasSameComparator = SortedIterables.hasSameComparator(comparator, elements);

		if (hasSameComparator && (elements instanceof ImmutableSortedSet)) {
			@SuppressWarnings("unchecked")
			ImmutableSortedSet<E> original = (ImmutableSortedSet<E>) elements;
			if (!original.isPartialView()) {
				return original;
			}
		}
		@SuppressWarnings("unchecked") // elements only contains E's; it's safe.
		E[] array = (E[]) Iterables.toArray(elements);
		return construct(comparator, array.length, array);
	}

	/**
	 * 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.
	 *
	 * <p>
	 * Despite the method name, this method attempts to avoid actually copying the
	 * data when it is safe to do so. The exact circumstances under which a copy
	 * will or will not be performed are undocumented and subject to change.
	 *
	 * <p>
	 * This method is safe to use even when {@code elements} is a synchronized or
	 * concurrent collection that is currently being modified by another thread.
	 *
	 * @throws NullPointerException if {@code comparator} or any of {@code elements}
	 *                              is null
	 * @since 7.0 (source-compatible since 2.0)
	 */
	public static <E> ImmutableSortedSet<E> copyOf(Comparator<? super E> comparator, Collection<? extends E> elements) {
		return copyOf(comparator, (Iterable<? extends E>) 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>
	 * Despite the method name, this method attempts to avoid actually copying the
	 * data when it is safe to do so. The exact circumstances under which a copy
	 * will or will not be performed are undocumented and subject to change.
	 *
	 * <p>
	 * This method is safe to use even when {@code sortedSet} is a synchronized or
	 * concurrent collection that is currently being modified by another thread.
	 *
	 * @throws NullPointerException if {@code sortedSet} or any of its elements is
	 *                              null
	 */
	public static <E> ImmutableSortedSet<E> copyOfSorted(SortedSet<E> sortedSet) {
		Comparator<? super E> comparator = SortedIterables.comparator(sortedSet);
		ImmutableList<E> list = ImmutableList.copyOf(sortedSet);
		if (list.isEmpty()) {
			return emptySet(comparator);
		} else {
			return new RegularImmutableSortedSet<E>(list, comparator);
		}
	}

	/**
	 * Constructs an {@code ImmutableSortedSet} from the first {@code n} elements of
	 * {@code contents}. If {@code k} is the size of the returned
	 * {@code ImmutableSortedSet}, then the sorted unique elements are in the first
	 * {@code k} positions of {@code contents}, and {@code contents[i] == null} for
	 * {@code k <= i < n}.
	 *
	 * <p>
	 * If {@code k == contents.length}, then {@code contents} may no longer be safe
	 * for modification.
	 *
	 * @throws NullPointerException if any of the first {@code n} elements of
	 *                              {@code contents} is null
	 */
	static <E> ImmutableSortedSet<E> construct(Comparator<? super E> comparator, int n, E... contents) {
		if (n == 0) {
			return emptySet(comparator);
		}
		checkElementsNotNull(contents, n);
		Arrays.sort(contents, 0, n, comparator);
		int uniques = 1;
		for (int i = 1; i < n; i++) {
			E cur = contents[i];
			E prev = contents[uniques - 1];
			if (comparator.compare(cur, prev) != 0) {
				contents[uniques++] = cur;
			}
		}
		Arrays.fill(contents, uniques, n, null);
		return new RegularImmutableSortedSet<E>(ImmutableList.<E>asImmutableList(contents, uniques), comparator);
	}

	/**
	 * 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.
	 */
	public static <E extends Comparable<?>> 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}.
	 */
	public static <E extends Comparable<?>> 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.
	 * Example:
	 * 
	 * <pre>
	 * {
	 * 	&#64;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.
	 *
	 * @since 2.0 (imported from Google Collections Library)
	 */
	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() {
			@SuppressWarnings("unchecked") // we're careful to put only E's in here
			E[] contentsArray = (E[]) contents;
			ImmutableSortedSet<E> result = construct(comparator, size, contentsArray);
			this.size = result.size(); // we eliminated duplicates in-place in contentsArray
			return result;
		}
	}

	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.
	 */
	@Override
	public Comparator<? super E> comparator() {
		return comparator;
	}

	@Override // needed to unify the iterator() methods in Collection and SortedIterable
	public abstract UnmodifiableIterator<E> iterator();

	/**
	 * {@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}.
	 */
	@Override
	public ImmutableSortedSet<E> headSet(E toElement) {
		return headSet(toElement, false);
	}

	/**
	 * @since 12.0
	 */
	@GwtIncompatible("NavigableSet")
	@Override
	public ImmutableSortedSet<E> headSet(E toElement, boolean inclusive) {
		return headSetImpl(checkNotNull(toElement), inclusive);
	}

	/**
	 * {@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}.
	 */
	@Override
	public ImmutableSortedSet<E> subSet(E fromElement, E toElement) {
		return subSet(fromElement, true, toElement, false);
	}

	/**
	 * @since 12.0
	 */
	@GwtIncompatible("NavigableSet")
	@Override
	public ImmutableSortedSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
		checkNotNull(fromElement);
		checkNotNull(toElement);
		checkArgument(comparator.compare(fromElement, toElement) <= 0);
		return subSetImpl(fromElement, fromInclusive, toElement, toInclusive);
	}

	/**
	 * {@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}.
	 */
	@Override
	public ImmutableSortedSet<E> tailSet(E fromElement) {
		return tailSet(fromElement, true);
	}

	/**
	 * @since 12.0
	 */
	@GwtIncompatible("NavigableSet")
	@Override
	public ImmutableSortedSet<E> tailSet(E fromElement, boolean inclusive) {
		return tailSetImpl(checkNotNull(fromElement), inclusive);
	}

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

	abstract ImmutableSortedSet<E> subSetImpl(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive);

	abstract ImmutableSortedSet<E> tailSetImpl(E fromElement, boolean inclusive);

	/**
	 * @since 12.0
	 */
	@GwtIncompatible("NavigableSet")
	@Override
	public E lower(E e) {
		return Iterators.getNext(headSet(e, false).descendingIterator(), null);
	}

	/**
	 * @since 12.0
	 */
	@GwtIncompatible("NavigableSet")
	@Override
	public E floor(E e) {
		return Iterators.getNext(headSet(e, true).descendingIterator(), null);
	}

	/**
	 * @since 12.0
	 */
	@GwtIncompatible("NavigableSet")
	@Override
	public E ceiling(E e) {
		return Iterables.getFirst(tailSet(e, true), null);
	}

	/**
	 * @since 12.0
	 */
	@GwtIncompatible("NavigableSet")
	@Override
	public E higher(E e) {
		return Iterables.getFirst(tailSet(e, false), null);
	}

	@Override
	public E first() {
		return iterator().next();
	}

	@Override
	public E last() {
		return descendingIterator().next();
	}

	/**
	 * Guaranteed to throw an exception and leave the set unmodified.
	 *
	 * @since 12.0
	 * @throws UnsupportedOperationException always
	 * @deprecated Unsupported operation.
	 */
	@Deprecated
	@GwtIncompatible("NavigableSet")
	@Override
	public final E pollFirst() {
		throw new UnsupportedOperationException();
	}

	/**
	 * Guaranteed to throw an exception and leave the set unmodified.
	 *
	 * @since 12.0
	 * @throws UnsupportedOperationException always
	 * @deprecated Unsupported operation.
	 */
	@Deprecated
	@GwtIncompatible("NavigableSet")
	@Override
	public final E pollLast() {
		throw new UnsupportedOperationException();
	}

	@GwtIncompatible("NavigableSet")
	transient ImmutableSortedSet<E> descendingSet;

	/**
	 * @since 12.0
	 */
	@GwtIncompatible("NavigableSet")
	@Override
	public ImmutableSortedSet<E> descendingSet() {
		// racy single-check idiom
		ImmutableSortedSet<E> result = descendingSet;
		if (result == null) {
			result = descendingSet = createDescendingSet();
			result.descendingSet = this;
		}
		return result;
	}

	@GwtIncompatible("NavigableSet")
	ImmutableSortedSet<E> createDescendingSet() {
		return new DescendingImmutableSortedSet<E>(this);
	}

	/**
	 * @since 12.0
	 */
	@GwtIncompatible("NavigableSet")
	@Override
	public abstract UnmodifiableIterator<E> descendingIterator();

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