/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.collect.ObjectArrays.checkElementNotNull;

import java.io.Serializable;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.EnumSet;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

import javax.annotation.Nullable;

import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.primitives.Ints;

/**
 * A high-performance, immutable {@code Set} with reliable, user-specified
 * iteration order. Does not permit null elements.
 *
 * <p>
 * Unlike {@link Collections#unmodifiableSet}, which is a <i>view</i> of a
 * separate collection that can still change, an instance of this class 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>
 * <b>Warning:</b> Like most sets, an {@code ImmutableSet} 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>
 * This class has been observed to perform significantly better than
 * {@link HashSet} for objects with very fast {@link Object#hashCode}
 * implementations (as a well-behaved immutable object should). While this
 * class's factory methods create hash-based instances, the
 * {@link ImmutableSortedSet} subclass performs binary searches instead.
 *
 * <p>
 * <b>Note:</b> Although this class is not final, it cannot be subclassed
 * outside its package 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 ImmutableList
 * @see ImmutableMap
 * @author Kevin Bourrillion
 * @author Nick Kralevich
 * @since 2.0 (imported from Google Collections Library)
 */
@GwtCompatible(serializable = true, emulated = true)
@SuppressWarnings("serial") // we're overriding default serialization
public abstract class ImmutableSet<E> extends ImmutableCollection<E> implements Set<E> {
	/**
	 * Returns the empty immutable set. This set behaves and performs comparably to
	 * {@link Collections#emptySet}, and is preferable mainly for consistency and
	 * maintainability of your code.
	 */
	// Casting to any type is safe because the set will never hold any elements.
	@SuppressWarnings({ "unchecked" })
	public static <E> ImmutableSet<E> of() {
		return (ImmutableSet<E>) EmptyImmutableSet.INSTANCE;
	}

	/**
	 * Returns an immutable set containing a single element. This set behaves and
	 * performs comparably to {@link Collections#singleton}, but will not accept a
	 * null element. It is preferable mainly for consistency and maintainability of
	 * your code.
	 */
	public static <E> ImmutableSet<E> of(E element) {
		return new SingletonImmutableSet<E>(element);
	}

	/**
	 * Returns an immutable set containing the given elements, in order. Repeated
	 * occurrences of an element (according to {@link Object#equals}) after the
	 * first are ignored.
	 *
	 * @throws NullPointerException if any element is null
	 */
	public static <E> ImmutableSet<E> of(E e1, E e2) {
		return construct(2, e1, e2);
	}

	/**
	 * Returns an immutable set containing the given elements, in order. Repeated
	 * occurrences of an element (according to {@link Object#equals}) after the
	 * first are ignored.
	 *
	 * @throws NullPointerException if any element is null
	 */
	public static <E> ImmutableSet<E> of(E e1, E e2, E e3) {
		return construct(3, e1, e2, e3);
	}

	/**
	 * Returns an immutable set containing the given elements, in order. Repeated
	 * occurrences of an element (according to {@link Object#equals}) after the
	 * first are ignored.
	 *
	 * @throws NullPointerException if any element is null
	 */
	public static <E> ImmutableSet<E> of(E e1, E e2, E e3, E e4) {
		return construct(4, e1, e2, e3, e4);
	}

	/**
	 * Returns an immutable set containing the given elements, in order. Repeated
	 * occurrences of an element (according to {@link Object#equals}) after the
	 * first are ignored.
	 *
	 * @throws NullPointerException if any element is null
	 */
	public static <E> ImmutableSet<E> of(E e1, E e2, E e3, E e4, E e5) {
		return construct(5, e1, e2, e3, e4, e5);
	}

	/**
	 * Returns an immutable set containing the given elements, in order. Repeated
	 * occurrences of an element (according to {@link Object#equals}) after the
	 * first are ignored.
	 *
	 * @throws NullPointerException if any element is null
	 * @since 3.0 (source-compatible since 2.0)
	 */
	public static <E> ImmutableSet<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E... others) {
		final int paramCount = 6;
		Object[] elements = new Object[paramCount + others.length];
		elements[0] = e1;
		elements[1] = e2;
		elements[2] = e3;
		elements[3] = e4;
		elements[4] = e5;
		elements[5] = e6;
		System.arraycopy(others, 0, elements, paramCount, others.length);
		return construct(elements.length, elements);
	}

	/**
	 * Constructs an {@code ImmutableSet} from the first {@code n} elements of the
	 * specified array. If {@code k} is the size of the returned
	 * {@code ImmutableSet}, then the unique elements of {@code elements} will be in
	 * the first {@code k} positions, and {@code elements[i] == null} for
	 * {@code k <= i < n}.
	 *
	 * <p>
	 * This may modify {@code elements}. Additionally, if
	 * {@code n == elements.length} and {@code elements} contains no duplicates,
	 * {@code elements} may be used without copying in the returned
	 * {@code ImmutableSet}, in which case it may no longer be modified.
	 *
	 * <p>
	 * {@code elements} may contain only values of type {@code E}.
	 *
	 * @throws NullPointerException if any of the first {@code n} elements of
	 *                              {@code elements} is null
	 */
	private static <E> ImmutableSet<E> construct(int n, Object... elements) {
		switch (n) {
		case 0:
			return of();
		case 1:
			@SuppressWarnings("unchecked") // safe; elements contains only E's
			E elem = (E) elements[0];
			return of(elem);
		default:
			// continue below to handle the general case
		}
		int tableSize = chooseTableSize(n);
		Object[] table = new Object[tableSize];
		int mask = tableSize - 1;
		int hashCode = 0;
		int uniques = 0;
		for (int i = 0; i < n; i++) {
			Object element = checkElementNotNull(elements[i], i);
			int hash = element.hashCode();
			for (int j = Hashing.smear(hash);; j++) {
				int index = j & mask;
				Object value = table[index];
				if (value == null) {
					// Came to an empty slot. Put the element here.
					elements[uniques++] = element;
					table[index] = element;
					hashCode += hash;
					break;
				} else if (value.equals(element)) {
					break;
				}
			}
		}
		Arrays.fill(elements, uniques, n, null);
		if (uniques == 1) {
			// There is only one element or elements are all duplicates
			@SuppressWarnings("unchecked") // we are careful to only pass in E
			E element = (E) elements[0];
			return new SingletonImmutableSet<E>(element, hashCode);
		} else if (tableSize != chooseTableSize(uniques)) {
			// Resize the table when the array includes too many duplicates.
			// when this happens, we have already made a copy
			return construct(uniques, elements);
		} else {
			Object[] uniqueElements = (uniques < elements.length) ? ObjectArrays.arraysCopyOf(elements, uniques)
					: elements;
			return new RegularImmutableSet<E>(uniqueElements, hashCode, table, mask);
		}
	}

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

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

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

	/**
	 * Returns an array size suitable for the backing array of a hash table that
	 * uses open addressing with linear probing in its implementation. The returned
	 * size is the smallest power of two that can hold setSize elements with the
	 * desired load factor.
	 *
	 * <p>
	 * Do not call this method with setSize < 2.
	 */
	@VisibleForTesting
	static int chooseTableSize(int setSize) {
		// Correct the size for open addressing to match desired load factor.
		if (setSize < CUTOFF) {
			// Round up to the next highest power of 2.
			int tableSize = Integer.highestOneBit(setSize - 1) << 1;
			while (tableSize * DESIRED_LOAD_FACTOR < setSize) {
				tableSize <<= 1;
			}
			return tableSize;
		}

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

	/**
	 * Returns an immutable set containing the given elements, in order. Repeated
	 * occurrences of an element (according to {@link Object#equals}) after the
	 * first are ignored.
	 *
	 * @throws NullPointerException if any of {@code elements} is null
	 * @since 3.0
	 */
	public static <E> ImmutableSet<E> copyOf(E[] elements) {
		switch (elements.length) {
		case 0:
			return of();
		case 1:
			return of(elements[0]);
		default:
			return construct(elements.length, elements.clone());
		}
	}

	/**
	 * Returns an immutable set containing the given elements, in order. Repeated
	 * occurrences of an element (according to {@link Object#equals}) after the
	 * first are ignored. This method iterates over {@code elements} at most once.
	 *
	 * <p>
	 * Note that if {@code s} is a {@code Set<String>}, then {@code
	 * ImmutableSet.copyOf(s)} returns an {@code ImmutableSet<String>} containing
	 * each of the strings in {@code s}, while {@code ImmutableSet.of(s)} returns a
	 * {@code ImmutableSet<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.
	 *
	 * @throws NullPointerException if any of {@code elements} is null
	 */
	public static <E> ImmutableSet<E> copyOf(Iterable<? extends E> elements) {
		return (elements instanceof Collection) ? copyOf(Collections2.cast(elements)) : copyOf(elements.iterator());
	}

	/**
	 * Returns an immutable set containing the given elements, in order. Repeated
	 * occurrences of an element (according to {@link Object#equals}) after the
	 * first are ignored.
	 *
	 * @throws NullPointerException if any of {@code elements} is null
	 */
	public static <E> ImmutableSet<E> copyOf(Iterator<? extends E> elements) {
		// We special-case for 0 or 1 elements, but anything further is madness.
		if (!elements.hasNext()) {
			return of();
		}
		E first = elements.next();
		if (!elements.hasNext()) {
			return of(first);
		} else {
			return new ImmutableSet.Builder<E>().add(first).addAll(elements).build();
		}
	}

	/**
	 * Returns an immutable set containing the given elements, in order. Repeated
	 * occurrences of an element (according to {@link Object#equals}) after the
	 * first are ignored. This method iterates over {@code elements} at most once.
	 *
	 * <p>
	 * Note that if {@code s} is a {@code Set<String>}, then {@code
	 * ImmutableSet.copyOf(s)} returns an {@code ImmutableSet<String>} containing
	 * each of the strings in {@code s}, while {@code ImmutableSet.of(s)} returns a
	 * {@code ImmutableSet<Set<String>>} containing one element (the given set
	 * itself).
	 *
	 * <p>
	 * <b>Note:</b> Despite what the method name suggests, {@code copyOf} will
	 * return constant-space views, rather than linear-space copies, of some inputs
	 * known to be immutable. For some other immutable inputs, such as key sets of
	 * an {@code ImmutableMap}, it still performs a copy in order to avoid holding
	 * references to the values of the map. The heuristics used in this decision are
	 * undocumented and subject to change except that:
	 * <ul>
	 * <li>A full copy will be done of any {@code ImmutableSortedSet}.</li>
	 * <li>{@code ImmutableSet.copyOf()} is idempotent with respect to pointer
	 * equality.</li>
	 * </ul>
	 *
	 * <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 any of {@code elements} is null
	 * @since 7.0 (source-compatible since 2.0)
	 */
	public static <E> ImmutableSet<E> copyOf(Collection<? extends E> elements) {
		/*
		 * TODO(user): consider checking for ImmutableAsList here TODO(user): consider
		 * checking for Multiset here
		 */
		if (elements instanceof ImmutableSet && !(elements instanceof ImmutableSortedSet)) {
			@SuppressWarnings("unchecked") // all supported methods are covariant
			ImmutableSet<E> set = (ImmutableSet<E>) elements;
			if (!set.isPartialView()) {
				return set;
			}
		} else if (elements instanceof EnumSet) {
			return copyOfEnumSet((EnumSet) elements);
		}
		Object[] array = elements.toArray();
		return construct(array.length, array);
	}

	private static <E extends Enum<E>> ImmutableSet<E> copyOfEnumSet(EnumSet<E> enumSet) {
		return ImmutableEnumSet.asImmutable(EnumSet.copyOf(enumSet));
	}

	ImmutableSet() {
	}

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

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

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

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

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

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

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

		private static final long serialVersionUID = 0;
	}

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

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

	/**
	 * A builder for creating immutable set instances, especially {@code public
	 * static final} sets ("constant sets"). Example:
	 * 
	 * <pre>
	 * {
	 * 	&#64;code
	 *
	 * 	public static final ImmutableSet<Color> GOOGLE_COLORS = new ImmutableSet.Builder<Color>()
	 * 			.addAll(WEBSAFE_COLORS).add(new Color(0, 191, 255)).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 class Builder<E> extends ImmutableCollection.ArrayBasedBuilder<E> {

		/**
		 * Creates a new builder. The returned builder is equivalent to the builder
		 * generated by {@link ImmutableSet#builder}.
		 */
		public Builder() {
			this(DEFAULT_INITIAL_CAPACITY);
		}

		Builder(int capacity) {
			super(capacity);
		}

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

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

		/**
		 * Returns a newly-created {@code ImmutableSet} based on the contents of the
		 * {@code Builder}.
		 */
		@Override
		public ImmutableSet<E> build() {
			ImmutableSet<E> result = construct(size, contents);
			// construct has the side effect of deduping contents, so we update size
			// accordingly.
			size = result.size();
			return result;
		}
	}
}