/*
 * 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 java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Set;

import javax.annotation.Nullable;

import com.google.common.annotations.GwtCompatible;

/**
 * A collection that supports order-independent equality, like {@link Set}, but
 * may have duplicate elements. A multiset is also sometimes called a
 * <i>bag</i>.
 *
 * <p>
 * Elements of a multiset that are equal to one another are referred to as
 * <i>occurrences</i> of the same single element. The total number of
 * occurrences of an element in a multiset is called the <i>count</i> of that
 * element (the terms "frequency" and "multiplicity" are equivalent, but not
 * used in this API). Since the count of an element is represented as an {@code
 * int}, a multiset may never contain more than {@link Integer#MAX_VALUE}
 * occurrences of any one element.
 *
 * <p>
 * {@code Multiset} refines the specifications of several methods from
 * {@code Collection}. It also defines an additional query operation,
 * {@link #count}, which returns the count of an element. There are five new
 * bulk-modification operations, for example {@link #add(Object, int)}, to add
 * or remove multiple occurrences of an element at once, or to set the count of
 * an element to a specific value. These modification operations are optional,
 * but implementations which support the standard collection operations
 * {@link #add(Object)} or {@link #remove(Object)} are encouraged to implement
 * the related methods as well. Finally, two collection views are provided:
 * {@link #elementSet} contains the distinct elements of the multiset "with
 * duplicates collapsed", and {@link #entrySet} is similar but contains
 * {@link Entry Multiset.Entry} instances, each providing both a distinct
 * element and the count of that element.
 *
 * <p>
 * In addition to these required methods, implementations of {@code
 * Multiset} are expected to provide two {@code static} creation methods:
 * {@code create()}, returning an empty multiset, and {@code
 * create(Iterable<? extends E>)}, returning a multiset containing the given
 * initial elements. This is simply a refinement of {@code Collection}'s
 * constructor recommendations, reflecting the new developments of Java 5.
 *
 * <p>
 * As with other collection types, the modification operations are optional, and
 * should throw {@link UnsupportedOperationException} when they are not
 * implemented. Most implementations should support either all add operations or
 * none of them, all removal operations or none of them, and if and only if all
 * of these are supported, the {@code setCount} methods as well.
 *
 * <p>
 * A multiset uses {@link Object#equals} to determine whether two instances
 * should be considered "the same," <i>unless specified otherwise</i> by the
 * implementation.
 *
 * <p>
 * Common implementations include {@link ImmutableMultiset},
 * {@link HashMultiset}, and {@link ConcurrentHashMultiset}.
 *
 * <p>
 * If your values may be zero, negative, or outside the range of an int, you may
 * wish to use {@link com.google.common.util.concurrent.AtomicLongMap} instead.
 * Note, however, that unlike {@code Multiset}, {@code AtomicLongMap} does not
 * automatically remove zeros.
 * 
 * <p>
 * See the Guava User Guide article on <a href=
 * "http://code.google.com/p/guava-libraries/wiki/NewCollectionTypesExplained#Multiset">
 * {@code Multiset}</a>.
 *
 * @author Kevin Bourrillion
 * @since 2.0 (imported from Google Collections Library)
 */
@GwtCompatible
public interface Multiset<E> extends Collection<E> {
	// Query Operations

	/**
	 * Returns the number of occurrences of an element in this multiset (the
	 * <i>count</i> of the element). Note that for an {@link Object#equals}-based
	 * multiset, this gives the same result as {@link Collections#frequency} (which
	 * would presumably perform more poorly).
	 *
	 * <p>
	 * <b>Note:</b> the utility method {@link Iterables#frequency} generalizes this
	 * operation; it correctly delegates to this method when dealing with a
	 * multiset, but it can also accept any other iterable type.
	 *
	 * @param element the element to count occurrences of
	 * @return the number of occurrences of the element in this multiset; possibly
	 *         zero but never negative
	 */
	int count(@Nullable Object element);

	// Bulk Operations

	/**
	 * Adds a number of occurrences of an element to this multiset. Note that if
	 * {@code occurrences == 1}, this method has the identical effect to
	 * {@link #add(Object)}. This method is functionally equivalent (except in the
	 * case of overflow) to the call {@code addAll(Collections.nCopies(element,
	 * occurrences))}, which would presumably perform much more poorly.
	 *
	 * @param element     the element to add occurrences of; may be null only if
	 *                    explicitly allowed by the implementation
	 * @param occurrences the number of occurrences of the element to add. May be
	 *                    zero, in which case no change will be made.
	 * @return the count of the element before the operation; possibly zero
	 * @throws IllegalArgumentException if {@code occurrences} is negative, or if
	 *                                  this operation would result in more than
	 *                                  {@link Integer#MAX_VALUE} occurrences of the
	 *                                  element
	 * @throws NullPointerException     if {@code element} is null and this
	 *                                  implementation does not permit null
	 *                                  elements. Note that if {@code
	 *     occurrences}              is zero, the implementation may opt to
	 *                                  return normally.
	 */
	int add(@Nullable E element, int occurrences);

	/**
	 * Removes a number of occurrences of the specified element from this multiset.
	 * If the multiset contains fewer than this number of occurrences to begin with,
	 * all occurrences will be removed. Note that if {@code occurrences == 1}, this
	 * is functionally equivalent to the call {@code remove(element)}.
	 *
	 * @param element     the element to conditionally remove occurrences of
	 * @param occurrences the number of occurrences of the element to remove. May be
	 *                    zero, in which case no change will be made.
	 * @return the count of the element before the operation; possibly zero
	 * @throws IllegalArgumentException if {@code occurrences} is negative
	 */
	int remove(@Nullable Object element, int occurrences);

	/**
	 * Adds or removes the necessary occurrences of an element such that the element
	 * attains the desired count.
	 *
	 * @param element the element to add or remove occurrences of; may be null only
	 *                if explicitly allowed by the implementation
	 * @param count   the desired count of the element in this multiset
	 * @return the count of the element before the operation; possibly zero
	 * @throws IllegalArgumentException if {@code count} is negative
	 * @throws NullPointerException     if {@code element} is null and this
	 *                                  implementation does not permit null
	 *                                  elements. Note that if {@code
	 *     count}                    is zero, the implementor may optionally
	 *                                  return zero instead.
	 */
	int setCount(E element, int count);

	/**
	 * Conditionally sets the count of an element to a new value, as described in
	 * {@link #setCount(Object, int)}, provided that the element has the expected
	 * current count. If the current count is not {@code oldCount}, no change is
	 * made.
	 *
	 * @param element  the element to conditionally set the count of; may be null
	 *                 only if explicitly allowed by the implementation
	 * @param oldCount the expected present count of the element in this multiset
	 * @param newCount the desired count of the element in this multiset
	 * @return {@code true} if the condition for modification was met. This implies
	 *         that the multiset was indeed modified, unless
	 *         {@code oldCount == newCount}.
	 * @throws IllegalArgumentException if {@code oldCount} or {@code newCount} is
	 *                                  negative
	 * @throws NullPointerException     if {@code element} is null and the
	 *                                  implementation does not permit null
	 *                                  elements. Note that if {@code
	 *     oldCount}                 and {@code newCount} are both zero, the
	 *                                  implementor may optionally return
	 *                                  {@code true} instead.
	 */
	boolean setCount(E element, int oldCount, int newCount);

	// Views

	/**
	 * Returns the set of distinct elements contained in this multiset. The element
	 * set is backed by the same data as the multiset, so any change to either is
	 * immediately reflected in the other. The order of the elements in the element
	 * set is unspecified.
	 *
	 * <p>
	 * If the element set supports any removal operations, these necessarily cause
	 * <b>all</b> occurrences of the removed element(s) to be removed from the
	 * multiset. Implementations are not expected to support the add operations,
	 * although this is possible.
	 *
	 * <p>
	 * A common use for the element set is to find the number of distinct elements
	 * in the multiset: {@code elementSet().size()}.
	 *
	 * @return a view of the set of distinct elements in this multiset
	 */
	Set<E> elementSet();

	/**
	 * Returns a view of the contents of this multiset, grouped into {@code
	 * Multiset.Entry} instances, each providing an element of the multiset and the
	 * count of that element. This set contains exactly one entry for each distinct
	 * element in the multiset (thus it always has the same size as the
	 * {@link #elementSet}). The order of the elements in the element set is
	 * unspecified.
	 *
	 * <p>
	 * The entry set is backed by the same data as the multiset, so any change to
	 * either is immediately reflected in the other. However, multiset changes may
	 * or may not be reflected in any {@code Entry} instances already retrieved from
	 * the entry set (this is implementation-dependent). Furthermore,
	 * implementations are not required to support modifications to the entry set at
	 * all, and the {@code Entry} instances themselves don't even have methods for
	 * modification. See the specific implementation class for more details on how
	 * its entry set handles modifications.
	 *
	 * @return a set of entries representing the data of this multiset
	 */
	Set<Entry<E>> entrySet();

	/**
	 * An unmodifiable element-count pair for a multiset. The
	 * {@link Multiset#entrySet} method returns a view of the multiset whose
	 * elements are of this class. A multiset implementation may return Entry
	 * instances that are either live "read-through" views to the Multiset, or
	 * immutable snapshots. Note that this type is unrelated to the similarly-named
	 * type {@code Map.Entry}.
	 *
	 * @since 2.0 (imported from Google Collections Library)
	 */
	interface Entry<E> {

		/**
		 * Returns the multiset element corresponding to this entry. Multiple calls to
		 * this method always return the same instance.
		 *
		 * @return the element corresponding to this entry
		 */
		E getElement();

		/**
		 * Returns the count of the associated element in the underlying multiset. This
		 * count may either be an unchanging snapshot of the count at the time the entry
		 * was retrieved, or a live view of the current count of the element in the
		 * multiset, depending on the implementation. Note that in the former case, this
		 * method can never return zero, while in the latter, it will return zero if all
		 * occurrences of the element were since removed from the multiset.
		 *
		 * @return the count of the element; never negative
		 */
		int getCount();

		/**
		 * {@inheritDoc}
		 *
		 * <p>
		 * Returns {@code true} if the given object is also a multiset entry and the two
		 * entries represent the same element and count. That is, two entries {@code a}
		 * and {@code b} are equal if:
		 * 
		 * <pre>
		 *    {@code
		 *
		 *   Objects.equal(a.getElement(), b.getElement())
		 *       && a.getCount() == b.getCount()}
		 * </pre>
		 */
		@Override
		// TODO(kevinb): check this wrt TreeMultiset?
		boolean equals(Object o);

		/**
		 * {@inheritDoc}
		 *
		 * <p>
		 * The hash code of a multiset entry for element {@code element} and count
		 * {@code count} is defined as:
		 * 
		 * <pre>
		 *    {@code
		 *
		 *   ((element == null) ? 0 : element.hashCode()) ^ count}
		 * </pre>
		 */
		@Override
		int hashCode();

		/**
		 * Returns the canonical string representation of this entry, defined as
		 * follows. If the count for this entry is one, this is simply the string
		 * representation of the corresponding element. Otherwise, it is the string
		 * representation of the element, followed by the three characters {@code
		 * " x "} (space, letter x, space), followed by the count.
		 */
		@Override
		String toString();
	}

	// Comparison and hashing

	/**
	 * Compares the specified object with this multiset for equality. Returns
	 * {@code true} if the given object is also a multiset and contains equal
	 * elements with equal counts, regardless of order.
	 */
	@Override
	// TODO(kevinb): caveats about equivalence-relation?
	boolean equals(@Nullable Object object);

	/**
	 * Returns the hash code for this multiset. This is defined as the sum of
	 * 
	 * <pre>
	 *    {@code
	 *
	 *   ((element == null) ? 0 : element.hashCode()) ^ count(element)}
	 * </pre>
	 *
	 * <p>
	 * over all distinct elements in the multiset. It follows that a multiset and
	 * its entry set always have the same hash code.
	 */
	@Override
	int hashCode();

	/**
	 * {@inheritDoc}
	 *
	 * <p>
	 * It is recommended, though not mandatory, that this method return the result
	 * of invoking {@link #toString} on the {@link #entrySet}, yielding a result
	 * such as {@code [a x 3, c, d x 2, e]}.
	 */
	@Override
	String toString();

	// Refined Collection Methods

	/**
	 * {@inheritDoc}
	 *
	 * <p>
	 * Elements that occur multiple times in the multiset will appear multiple times
	 * in this iterator, though not necessarily sequentially.
	 */
	@Override
	Iterator<E> iterator();

	/**
	 * Determines whether this multiset contains the specified element.
	 *
	 * <p>
	 * This method refines {@link Collection#contains} to further specify that it
	 * <b>may not</b> throw an exception in response to {@code element} being null
	 * or of the wrong type.
	 *
	 * @param element the element to check for
	 * @return {@code true} if this multiset contains at least one occurrence of the
	 *         element
	 */
	@Override
	boolean contains(@Nullable Object element);

	/**
	 * Returns {@code true} if this multiset contains at least one occurrence of
	 * each element in the specified collection.
	 *
	 * <p>
	 * This method refines {@link Collection#containsAll} to further specify that it
	 * <b>may not</b> throw an exception in response to any of {@code
	 * elements} being null or of the wrong type.
	 *
	 * <p>
	 * <b>Note:</b> this method does not take into account the occurrence count of
	 * an element in the two collections; it may still return {@code
	 * true} even if {@code elements} contains several occurrences of an element and
	 * this multiset contains only one. This is no different than any other
	 * collection type like {@link List}, but it may be unexpected to the user of a
	 * multiset.
	 *
	 * @param elements the collection of elements to be checked for containment in
	 *                 this multiset
	 * @return {@code true} if this multiset contains at least one occurrence of
	 *         each element contained in {@code elements}
	 * @throws NullPointerException if {@code elements} is null
	 */
	@Override
	boolean containsAll(Collection<?> elements);

	/**
	 * Adds a single occurrence of the specified element to this multiset.
	 *
	 * <p>
	 * This method refines {@link Collection#add}, which only <i>ensures</i> the
	 * presence of the element, to further specify that a successful call must
	 * always increment the count of the element, and the overall size of the
	 * collection, by one.
	 *
	 * @param element the element to add one occurrence of; may be null only if
	 *                explicitly allowed by the implementation
	 * @return {@code true} always, since this call is required to modify the
	 *         multiset, unlike other {@link Collection} types
	 * @throws NullPointerException     if {@code element} is null and this
	 *                                  implementation does not permit null elements
	 * @throws IllegalArgumentException if {@link Integer#MAX_VALUE} occurrences of
	 *                                  {@code element} are already contained in
	 *                                  this multiset
	 */
	@Override
	boolean add(E element);

	/**
	 * Removes a <i>single</i> occurrence of the specified element from this
	 * multiset, if present.
	 *
	 * <p>
	 * This method refines {@link Collection#remove} to further specify that it
	 * <b>may not</b> throw an exception in response to {@code element} being null
	 * or of the wrong type.
	 *
	 * @param element the element to remove one occurrence of
	 * @return {@code true} if an occurrence was found and removed
	 */
	@Override
	boolean remove(@Nullable Object element);

	/**
	 * {@inheritDoc}
	 *
	 * <p>
	 * <b>Note:</b> This method ignores how often any element might appear in
	 * {@code c}, and only cares whether or not an element appears at all. If you
	 * wish to remove one occurrence in this multiset for every occurrence in
	 * {@code c}, see {@link Multisets#removeOccurrences(Multiset, Multiset)}.
	 * 
	 * <p>
	 * This method refines {@link Collection#removeAll} to further specify that it
	 * <b>may not</b> throw an exception in response to any of {@code elements}
	 * being null or of the wrong type.
	 */
	@Override
	boolean removeAll(Collection<?> c);

	/**
	 * {@inheritDoc}
	 *
	 * <p>
	 * <b>Note:</b> This method ignores how often any element might appear in
	 * {@code c}, and only cares whether or not an element appears at all. If you
	 * wish to remove one occurrence in this multiset for every occurrence in
	 * {@code c}, see {@link Multisets#retainOccurrences(Multiset, Multiset)}.
	 * 
	 * <p>
	 * This method refines {@link Collection#retainAll} to further specify that it
	 * <b>may not</b> throw an exception in response to any of {@code elements}
	 * being null or of the wrong type.
	 * 
	 * @see Multisets#retainOccurrences(Multiset, Multiset)
	 */
	@Override
	boolean retainAll(Collection<?> c);
}
