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	/*
	* Licensed to the Apache Software Foundation (ASF) under one or more
	* contributor license agreements. See the NOTICE file distributed with
	* this work for additional information regarding copyright ownership.
	* The ASF licenses this file to You 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 org.apache.commons.collections4;

	import java.util.AbstractSet;
	import java.util.Arrays;
	import java.util.Collection;
	import java.util.Collections;
	import java.util.HashSet;
	import java.util.IdentityHashMap;
	import java.util.Iterator;
	import java.util.NavigableSet;
	import java.util.Set;
	import java.util.SortedSet;
	import java.util.TreeSet;

	import org.apache.commons.collections4.set.ListOrderedSet;
	import org.apache.commons.collections4.set.PredicatedNavigableSet;
	import org.apache.commons.collections4.set.PredicatedSet;
	import org.apache.commons.collections4.set.PredicatedSortedSet;
	import org.apache.commons.collections4.set.TransformedNavigableSet;
	import org.apache.commons.collections4.set.TransformedSet;
	import org.apache.commons.collections4.set.TransformedSortedSet;
	import org.apache.commons.collections4.set.UnmodifiableNavigableSet;
	import org.apache.commons.collections4.set.UnmodifiableSet;
	import org.apache.commons.collections4.set.UnmodifiableSortedSet;

	/**
	* Provides utility methods and decorators for
	* {@link Set} and {@link SortedSet} instances.
	*
	* @since 2.1
	*/
	public class SetUtils {

	/**
	* An unmodifiable <b>view</b> of a set that may be backed by other sets.
	* <p>
	* If the decorated sets change, this view will change as well. The contents
	* of this view can be transferred to another instance via the {@link #copyInto(Set)}
	* and {@link #toSet()} methods.
	*
	* @param <E> the element type
	* @since 4.1
	*/
	public static abstract class SetView<E> extends AbstractSet<E> {

	/**
	* Copies the contents of this view into the provided set.
	*
	* @param <S> the set type
	* @param set the set for copying the contents
	*/
	public <S extends Set<E>> void copyInto(final S set) {
	CollectionUtils.addAll(set, this);
	}

	/**
	* Return an iterator for this view; the returned iterator is
	* not required to be unmodifiable.
	* @return a new iterator for this view
	*/
	protected abstract Iterator<E> createIterator();

	@Override
	public Iterator<E> iterator() {
	return IteratorUtils.unmodifiableIterator(createIterator());
	}

	@Override
	public int size() {
	return IteratorUtils.size(iterator());
	}

	/**
	* Returns a new set containing the contents of this view.
	*
	* @return a new set containing all elements of this view
	*/
	public Set<E> toSet() {
	final Set<E> set = new HashSet<>(size());
	copyInto(set);
	return set;
	}
	}

	/**
	* An empty unmodifiable sorted set.
	* This is not provided in the JDK.
	*/
	@SuppressWarnings("rawtypes")
	public static final SortedSet EMPTY_SORTED_SET =
	UnmodifiableSortedSet.unmodifiableSortedSet(new TreeSet<>());

	/**
	* Returns a unmodifiable <b>view</b> containing the difference of the given
	* {@link Set}s, denoted by {@code a \ b} (or {@code a - b}).
	* <p>
	* The returned view contains all elements of {@code a} that are not a member
	* of {@code b}.
	*
	* @param <E> the generic type that is able to represent the types contained
	* in both input sets.
	* @param a the set to subtract from, must not be null
	* @param b the set to subtract, must not be null
	* @return a view of the relative complement of of the two sets
	* @since 4.1
	*/
	public static <E> SetView<E> difference(final Set<? extends E> a, final Set<? extends E> b) {
	if (a == null \|\| b == null) {
	throw new NullPointerException("Sets must not be null.");
	}

	final Predicate<E> notContainedInB = new Predicate<E>() {
	@Override
	public boolean evaluate(final E object) {
	return !b.contains(object);
	}
	};

	return new SetView<E>() {
	@Override
	public boolean contains(final Object o) {
	return a.contains(o) && !b.contains(o);
	}

	@Override
	public Iterator<E> createIterator() {
	return IteratorUtils.filteredIterator(a.iterator(), notContainedInB);
	}
	};
	}

	/**
	* Returns a unmodifiable <b>view</b> of the symmetric difference of the given
	* {@link Set}s.
	* <p>
	* The returned view contains all elements of {@code a} and {@code b} that are
	* not a member of the other set.
	* <p>
	* This is equivalent to {@code union(difference(a, b), difference(b, a))}.
	*
	* @param <E> the generic type that is able to represent the types contained
	* in both input sets.
	* @param a the first set, must not be null
	* @param b the second set, must not be null
	* @return a view of the symmetric difference of the two sets
	* @since 4.1
	*/
	public static <E> SetView<E> disjunction(final Set<? extends E> a, final Set<? extends E> b) {
	if (a == null \|\| b == null) {
	throw new NullPointerException("Sets must not be null.");
	}

	final SetView<E> aMinusB = difference(a, b);
	final SetView<E> bMinusA = difference(b, a);

	return new SetView<E>() {
	@Override
	public boolean contains(final Object o) {
	return a.contains(o) ^ b.contains(o);
	}

	@Override
	public Iterator<E> createIterator() {
	return IteratorUtils.chainedIterator(aMinusB.iterator(), bMinusA.iterator());
	}

	@Override
	public boolean isEmpty() {
	return aMinusB.isEmpty() && bMinusA.isEmpty();
	}

	@Override
	public int size() {
	return aMinusB.size() + bMinusA.size();
	}
	};
	}

	/**
	* Returns an immutable empty set if the argument is <code>null</code>,
	* or the argument itself otherwise.
	*
	* @param <T> the element type
	* @param set the set, possibly <code>null</code>
	* @return an empty set if the argument is <code>null</code>
	*/
	public static <T> Set<T> emptyIfNull(final Set<T> set) {
	return set == null ? Collections.<T>emptySet() : set;
	}

	//-----------------------------------------------------------------------

	/**
	* Get a typed empty unmodifiable Set.
	* @param <E> the element type
	* @return an empty Set
	*/
	public static <E> Set<E> emptySet() {
	return Collections.<E>emptySet();
	}

	/**
	* Get a typed empty unmodifiable sorted set.
	* @param <E> the element type
	* @return an empty sorted Set
	*/
	@SuppressWarnings("unchecked") // empty set is OK for any type
	public static <E> SortedSet<E> emptySortedSet() {
	return EMPTY_SORTED_SET;
	}

	/**
	* Generates a hash code using the algorithm specified in
	* {@link java.util.Set#hashCode()}.
	* <p>
	* This method is useful for implementing <code>Set</code> when you cannot
	* extend AbstractSet. The method takes Collection instances to enable other
	* collection types to use the Set implementation algorithm.
	*
	* @param <T> the element type
	* @see java.util.Set#hashCode()
	* @param set the set to calculate the hash code for, may be null
	* @return the hash code
	*/
	public static <T> int hashCodeForSet(final Collection<T> set) {
	if (set == null) {
	return 0;
	}

	int hashCode = 0;
	for (final T obj : set) {
	if (obj != null) {
	hashCode += obj.hashCode();
	}
	}
	return hashCode;
	}

	/**
	* Creates a set from the given items. If the passed var-args argument is {@code
	* null}, then the method returns {@code null}.
	* @param <E> the element type
	* @param items the elements that make up the new set
	* @return a set
	* @since 4.3
	*/
	public static <E> HashSet<E> hashSet(final E... items) {
	if (items == null) {
	return null;
	}
	return new HashSet<>(Arrays.asList(items));
	}

	/**
	* Returns a unmodifiable <b>view</b> of the intersection of the given {@link Set}s.
	* <p>
	* The returned view contains all elements that are members of both input sets
	* ({@code a} and {@code b}).
	*
	* @param <E> the generic type that is able to represent the types contained
	* in both input sets.
	* @param a the first set, must not be null
	* @param b the second set, must not be null
	* @return a view of the intersection of the two sets
	* @since 4.1
	*/
	public static <E> SetView<E> intersection(final Set<? extends E> a, final Set<? extends E> b) {
	if (a == null \|\| b == null) {
	throw new NullPointerException("Sets must not be null.");
	}

	final Predicate<E> containedInB = new Predicate<E>() {
	@Override
	public boolean evaluate(final E object) {
	return b.contains(object);
	}
	};

	return new SetView<E>() {
	@Override
	public boolean contains(final Object o) {
	return a.contains(o) && b.contains(o);
	}

	@Override
	public Iterator<E> createIterator() {
	return IteratorUtils.filteredIterator(a.iterator(), containedInB);
	}
	};
	}

	/**
	* Tests two sets for equality as per the <code>equals()</code> contract
	* in {@link java.util.Set#equals(java.lang.Object)}.
	* <p>
	* This method is useful for implementing <code>Set</code> when you cannot
	* extend AbstractSet. The method takes Collection instances to enable other
	* collection types to use the Set implementation algorithm.
	* <p>
	* The relevant text (slightly paraphrased as this is a static method) is:
	* <blockquote>
	* <p>Two sets are considered equal if they have
	* the same size, and every member of the first set is contained in
	* the second. This ensures that the {@code equals} method works
	* properly across different implementations of the {@code Set}
	* interface.</p>
	*
	* <p>
	* This implementation first checks if the two sets are the same object:
	* if so it returns {@code true}. Then, it checks if the two sets are
	* identical in size; if not, it returns false. If so, it returns
	* {@code a.containsAll((Collection) b)}.</p>
	* </blockquote>
	*
	* @see java.util.Set
	* @param set1 the first set, may be null
	* @param set2 the second set, may be null
	* @return whether the sets are equal by value comparison
	*/
	public static boolean isEqualSet(final Collection<?> set1, final Collection<?> set2) {
	if (set1 == set2) {
	return true;
	}
	if (set1 == null \|\| set2 == null \|\| set1.size() != set2.size()) {
	return false;
	}

	return set1.containsAll(set2);
	}

	/**
	* Returns a new hash set that matches elements based on <code>==</code> not
	* <code>equals()</code>.
	* <p>
	* <strong>This set will violate the detail of various Set contracts.</strong>
	* As a general rule, don't compare this set to other sets. In particular, you can't
	* use decorators like {@link ListOrderedSet} on it, which silently assume that these
	* contracts are fulfilled.
	* <p>
	* <strong>Note that the returned set is not synchronized and is not thread-safe.</strong>
	* If you wish to use this set from multiple threads concurrently, you must use
	* appropriate synchronization. The simplest approach is to wrap this map
	* using {@link java.util.Collections#synchronizedSet(Set)}. This class may throw
	* exceptions when accessed by concurrent threads without synchronization.
	*
	* @param <E> the element type
	* @return a new identity hash set
	* @since 4.1
	*/
	public static <E> Set<E> newIdentityHashSet() {
	return Collections.newSetFromMap(new IdentityHashMap<E, Boolean>());
	}

	/**
	* Returns a set that maintains the order of elements that are added
	* backed by the given set.
	* <p>
	* If an element is added twice, the order is determined by the first add.
	* The order is observed through the iterator or toArray.
	*
	* @param <E> the element type
	* @param set the set to order, must not be null
	* @return an ordered set backed by the given set
	* @throws NullPointerException if the set is null
	*/
	public static <E> Set<E> orderedSet(final Set<E> set) {
	return ListOrderedSet.listOrderedSet(set);
	}

	/**
	* Returns a predicated (validating) navigable set backed by the given navigable set.
	* <p>
	* Only objects that pass the test in the given predicate can be added to the set.
	* Trying to add an invalid object results in an IllegalArgumentException.
	* It is important not to use the original set after invoking this method,
	* as it is a backdoor for adding invalid objects.
	*
	* @param <E> the element type
	* @param set the navigable set to predicate, must not be null
	* @param predicate the predicate for the navigable set, must not be null
	* @return a predicated navigable set backed by the given navigable set
	* @throws NullPointerException if the set or predicate is null
	* @since 4.1
	*/
	public static <E> SortedSet<E> predicatedNavigableSet(final NavigableSet<E> set,
	final Predicate<? super E> predicate) {
	return PredicatedNavigableSet.predicatedNavigableSet(set, predicate);
	}

	/**
	* Returns a predicated (validating) set backed by the given set.
	* <p>
	* Only objects that pass the test in the given predicate can be added to the set.
	* Trying to add an invalid object results in an IllegalArgumentException.
	* It is important not to use the original set after invoking this method,
	* as it is a backdoor for adding invalid objects.
	*
	* @param <E> the element type
	* @param set the set to predicate, must not be null
	* @param predicate the predicate for the set, must not be null
	* @return a predicated set backed by the given set
	* @throws NullPointerException if the set or predicate is null
	*/
	public static <E> Set<E> predicatedSet(final Set<E> set, final Predicate<? super E> predicate) {
	return PredicatedSet.predicatedSet(set, predicate);
	}

	/**
	* Returns a predicated (validating) sorted set backed by the given sorted set.
	* <p>
	* Only objects that pass the test in the given predicate can be added to the set.
	* Trying to add an invalid object results in an IllegalArgumentException.
	* It is important not to use the original set after invoking this method,
	* as it is a backdoor for adding invalid objects.
	*
	* @param <E> the element type
	* @param set the sorted set to predicate, must not be null
	* @param predicate the predicate for the sorted set, must not be null
	* @return a predicated sorted set backed by the given sorted set
	* @throws NullPointerException if the set or predicate is null
	*/
	public static <E> SortedSet<E> predicatedSortedSet(final SortedSet<E> set,
	final Predicate<? super E> predicate) {
	return PredicatedSortedSet.predicatedSortedSet(set, predicate);
	}

	// Set
	//-----------------------------------------------------------------------
	/**
	* Returns a synchronized set backed by the given set.
	* <p>
	* You must manually synchronize on the returned set's iterator to
	* avoid non-deterministic behavior:
	*
	* <pre>
	* Set s = SetUtils.synchronizedSet(mySet);
	* synchronized (s) {
	* Iterator i = s.iterator();
	* while (i.hasNext()) {
	* process (i.next());
	* }
	* }
	* </pre>
	*
	* This method is just a wrapper for {@link Collections#synchronizedSet(Set)}.
	*
	* @param <E> the element type
	* @param set the set to synchronize, must not be null
	* @return a synchronized set backed by the given set
	* @throws NullPointerException if the set is null
	*/
	public static <E> Set<E> synchronizedSet(final Set<E> set) {
	return Collections.synchronizedSet(set);
	}

	// SortedSet
	//-----------------------------------------------------------------------
	/**
	* Returns a synchronized sorted set backed by the given sorted set.
	* <p>
	* You must manually synchronize on the returned set's iterator to
	* avoid non-deterministic behavior:
	*
	* <pre>
	* Set s = SetUtils.synchronizedSortedSet(mySet);
	* synchronized (s) {
	* Iterator i = s.iterator();
	* while (i.hasNext()) {
	* process (i.next());
	* }
	* }
	* </pre>
	*
	* This method is just a wrapper for {@link Collections#synchronizedSortedSet(SortedSet)}.
	*
	* @param <E> the element type
	* @param set the sorted set to synchronize, must not be null
	* @return a synchronized set backed by the given set
	* @throws NullPointerException if the set is null
	*/
	public static <E> SortedSet<E> synchronizedSortedSet(final SortedSet<E> set) {
	return Collections.synchronizedSortedSet(set);
	}

	/**
	* Returns a transformed navigable set backed by the given navigable set.
	* <p>
	* Each object is passed through the transformer as it is added to the
	* Set. It is important not to use the original set after invoking this
	* method, as it is a backdoor for adding untransformed objects.
	* <p>
	* Existing entries in the specified set will not be transformed.
	* If you want that behaviour, see {@link TransformedNavigableSet#transformedNavigableSet}.
	*
	* @param <E> the element type
	* @param set the navigable set to transform, must not be null
	* @param transformer the transformer for the set, must not be null
	* @return a transformed set backed by the given set
	* @throws NullPointerException if the set or transformer is null
	* @since 4.1
	*/
	public static <E> SortedSet<E> transformedNavigableSet(final NavigableSet<E> set,
	final Transformer<? super E, ? extends E> transformer) {
	return TransformedNavigableSet.transformingNavigableSet(set, transformer);
	}

	/**
	* Returns a transformed set backed by the given set.
	* <p>
	* Each object is passed through the transformer as it is added to the
	* Set. It is important not to use the original set after invoking this
	* method, as it is a backdoor for adding untransformed objects.
	* <p>
	* Existing entries in the specified set will not be transformed.
	* If you want that behaviour, see {@link TransformedSet#transformedSet}.
	*
	* @param <E> the element type
	* @param set the set to transform, must not be null
	* @param transformer the transformer for the set, must not be null
	* @return a transformed set backed by the given set
	* @throws NullPointerException if the set or transformer is null
	*/
	public static <E> Set<E> transformedSet(final Set<E> set,
	final Transformer<? super E, ? extends E> transformer) {
	return TransformedSet.transformingSet(set, transformer);
	}

	/**
	* Returns a transformed sorted set backed by the given set.
	* <p>
	* Each object is passed through the transformer as it is added to the
	* Set. It is important not to use the original set after invoking this
	* method, as it is a backdoor for adding untransformed objects.
	* <p>
	* Existing entries in the specified set will not be transformed.
	* If you want that behaviour, see {@link TransformedSortedSet#transformedSortedSet}.
	*
	* @param <E> the element type
	* @param set the set to transform, must not be null
	* @param transformer the transformer for the set, must not be null
	* @return a transformed set backed by the given set
	* @throws NullPointerException if the set or transformer is null
	*/
	public static <E> SortedSet<E> transformedSortedSet(final SortedSet<E> set,
	final Transformer<? super E, ? extends E> transformer) {
	return TransformedSortedSet.transformingSortedSet(set, transformer);
	}

	// Set operations
	//-----------------------------------------------------------------------

	/**
	* Returns a unmodifiable <b>view</b> of the union of the given {@link Set}s.
	* <p>
	* The returned view contains all elements of {@code a} and {@code b}.
	*
	* @param <E> the generic type that is able to represent the types contained
	* in both input sets.
	* @param a the first set, must not be null
	* @param b the second set, must not be null
	* @return a view of the union of the two set
	* @throws NullPointerException if either input set is null
	* @since 4.1
	*/
	public static <E> SetView<E> union(final Set<? extends E> a, final Set<? extends E> b) {
	if (a == null \|\| b == null) {
	throw new NullPointerException("Sets must not be null.");
	}

	final SetView<E> bMinusA = difference(b, a);

	return new SetView<E>() {
	@Override
	public boolean contains(final Object o) {
	return a.contains(o) \|\| b.contains(o);
	}

	@Override
	public Iterator<E> createIterator() {
	return IteratorUtils.chainedIterator(a.iterator(), bMinusA.iterator());
	}

	@Override
	public boolean isEmpty() {
	return a.isEmpty() && b.isEmpty();
	}

	@Override
	public int size() {
	return a.size() + bMinusA.size();
	}
	};
	}

	// NavigableSet
	//-----------------------------------------------------------------------
	/**
	* Returns an unmodifiable navigable set backed by the given navigable set.
	* <p>
	* This method uses the implementation in the decorators subpackage.
	*
	* @param <E> the element type
	* @param set the navigable set to make unmodifiable, must not be null
	* @return an unmodifiable set backed by the given set
	* @throws NullPointerException if the set is null
	* @since 4.1
	*/
	public static <E> SortedSet<E> unmodifiableNavigableSet(final NavigableSet<E> set) {
	return UnmodifiableNavigableSet.unmodifiableNavigableSet(set);
	}

	/**
	* Creates an unmodifiable set from the given items. If the passed var-args argument is {@code
	* null}, then the method returns {@code null}.
	* @param <E> the element type
	* @param items the elements that make up the new set
	* @return a set
	* @since 4.3
	*/
	public static <E> Set<E> unmodifiableSet(final E... items) {
	if (items == null) {
	return null;
	}
	return UnmodifiableSet.unmodifiableSet(hashSet(items));
	}

	/**
	* Returns an unmodifiable set backed by the given set.
	* <p>
	* This method uses the implementation in the decorators subpackage.
	*
	* @param <E> the element type
	* @param set the set to make unmodifiable, must not be null
	* @return an unmodifiable set backed by the given set
	* @throws NullPointerException if the set is null
	*/
	public static <E> Set<E> unmodifiableSet(final Set<? extends E> set) {
	return UnmodifiableSet.unmodifiableSet(set);
	}

	/**
	* Returns an unmodifiable sorted set backed by the given sorted set.
	* <p>
	* This method uses the implementation in the decorators subpackage.
	*
	* @param <E> the element type
	* @param set the sorted set to make unmodifiable, must not be null
	* @return an unmodifiable set backed by the given set
	* @throws NullPointerException if the set is null
	*/
	public static <E> SortedSet<E> unmodifiableSortedSet(final SortedSet<E> set) {
	return UnmodifiableSortedSet.unmodifiableSortedSet(set);
	}

	/**
	* <code>SetUtils</code> should not normally be instantiated.
	*/
	private SetUtils() {}
	}

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