/*
    * 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 com.google.common.annotations.GwtCompatible;
  
  import java.util.Collection;
  import java.util.Collections;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Set;
  
  import javax.annotation.Nullable;
  
  /**
   * 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);
 }