/*
 * 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.checkNotNull;
import static com.google.common.collect.CollectPreconditions.checkNonnegative;
import static com.google.common.collect.CollectPreconditions.checkRemove;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Function;
import com.google.common.base.Predicate;
import com.google.common.base.Predicates;
import com.google.common.base.Supplier;
import com.google.common.collect.Maps.EntryTransformer;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.j2objc.annotations.Weak;
import com.google.j2objc.annotations.WeakOuter;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.AbstractCollection;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.SortedSet;
import javax.annotation.Nullable;

/**
 * Provides static methods acting on or generating a {@code Multimap}.
 *
 * <p>See the Guava User Guide article on <a href=
 * "https://github.com/google/guava/wiki/CollectionUtilitiesExplained#multimaps">
 * {@code Multimaps}</a>.
 *
 * @author Jared Levy
 * @author Robert Konigsberg
 * @author Mike Bostock
 * @author Louis Wasserman
 * @since 2.0
 */
@GwtCompatible(emulated = true)
public final class Multimaps {
  private Multimaps() {}

  /**
   * Creates a new {@code Multimap} backed by {@code map}, whose internal value
   * collections are generated by {@code factory}.
   *
   * <b>Warning: do not use</b> this method when the collections returned by
   * {@code factory} implement either {@link List} or {@code Set}! Use the more
   * specific method {@link #newListMultimap}, {@link #newSetMultimap} or {@link
   * #newSortedSetMultimap} instead, to avoid very surprising behavior from
   * {@link Multimap#equals}.
   *
   * <p>The {@code factory}-generated and {@code map} classes determine the
   * multimap iteration order. They also specify the behavior of the
   * {@code equals}, {@code hashCode}, and {@code toString} methods for the
   * multimap and its returned views. However, the multimap's {@code get}
   * method returns instances of a different class than {@code factory.get()}
   * does.
   *
   * <p>The multimap is serializable if {@code map}, {@code factory}, the
   * collections generated by {@code factory}, and the multimap contents are all
   * serializable.
   *
   * <p>The multimap is not threadsafe when any concurrent operations update the
   * multimap, even if {@code map} and the instances generated by
   * {@code factory} are. Concurrent read operations will work correctly. To
   * allow concurrent update operations, wrap the multimap with a call to
   * {@link #synchronizedMultimap}.
   *
   * <p>Call this method only when the simpler methods
   * {@link ArrayListMultimap#create()}, {@link HashMultimap#create()},
   * {@link LinkedHashMultimap#create()}, {@link LinkedListMultimap#create()},
   * {@link TreeMultimap#create()}, and
   * {@link TreeMultimap#create(Comparator, Comparator)} won't suffice.
   *
   * <p>Note: the multimap assumes complete ownership over of {@code map} and
   * the collections returned by {@code factory}. Those objects should not be
   * manually updated and they should not use soft, weak, or phantom references.
   *
   * @param map place to store the mapping from each key to its corresponding
   *     values
   * @param factory supplier of new, empty collections that will each hold all
   *     values for a given key
   * @throws IllegalArgumentException if {@code map} is not empty
   */
  public static <K, V> Multimap<K, V> newMultimap(
      Map<K, Collection<V>> map, final Supplier<? extends Collection<V>> factory) {
    return new CustomMultimap<>(map, factory);
  }

  private static class CustomMultimap<K, V> extends AbstractMapBasedMultimap<K, V> {
    transient Supplier<? extends Collection<V>> factory;

    CustomMultimap(Map<K, Collection<V>> map, Supplier<? extends Collection<V>> factory) {
      super(map);
      this.factory = checkNotNull(factory);
    }

    @Override
    protected Collection<V> createCollection() {
      return factory.get();
    }

    // can't use Serialization writeMultimap and populateMultimap methods since
    // there's no way to generate the empty backing map.

    /** @serialData the factory and the backing map */
    @GwtIncompatible // java.io.ObjectOutputStream
    private void writeObject(ObjectOutputStream stream) throws IOException {
      stream.defaultWriteObject();
      stream.writeObject(factory);
      stream.writeObject(backingMap());
    }

    @GwtIncompatible // java.io.ObjectInputStream
    @SuppressWarnings("unchecked") // reading data stored by writeObject
    private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
      stream.defaultReadObject();
      factory = (Supplier<? extends Collection<V>>) stream.readObject();
      Map<K, Collection<V>> map = (Map<K, Collection<V>>) stream.readObject();
      setMap(map);
    }

    @GwtIncompatible // java serialization not supported
    private static final long serialVersionUID = 0;
  }

  /**
   * Creates a new {@code ListMultimap} that uses the provided map and factory.
   * It can generate a multimap based on arbitrary {@link Map} and {@link List}
   * classes.
   *
   * <p>The {@code factory}-generated and {@code map} classes determine the
   * multimap iteration order. They also specify the behavior of the
   * {@code equals}, {@code hashCode}, and {@code toString} methods for the
   * multimap and its returned views. The multimap's {@code get}, {@code
   * removeAll}, and {@code replaceValues} methods return {@code RandomAccess}
   * lists if the factory does. However, the multimap's {@code get} method
   * returns instances of a different class than does {@code factory.get()}.
   *
   * <p>The multimap is serializable if {@code map}, {@code factory}, the
   * lists generated by {@code factory}, and the multimap contents are all
   * serializable.
   *
   * <p>The multimap is not threadsafe when any concurrent operations update the
   * multimap, even if {@code map} and the instances generated by
   * {@code factory} are. Concurrent read operations will work correctly. To
   * allow concurrent update operations, wrap the multimap with a call to
   * {@link #synchronizedListMultimap}.
   *
   * <p>Call this method only when the simpler methods
   * {@link ArrayListMultimap#create()} and {@link LinkedListMultimap#create()}
   * won't suffice.
   *
   * <p>Note: the multimap assumes complete ownership over of {@code map} and
   * the lists returned by {@code factory}. Those objects should not be manually
   * updated, they should be empty when provided, and they should not use soft,
   * weak, or phantom references.
   *
   * @param map place to store the mapping from each key to its corresponding
   *     values
   * @param factory supplier of new, empty lists that will each hold all values
   *     for a given key
   * @throws IllegalArgumentException if {@code map} is not empty
   */
  public static <K, V> ListMultimap<K, V> newListMultimap(
      Map<K, Collection<V>> map, final Supplier<? extends List<V>> factory) {
    return new CustomListMultimap<>(map, factory);
  }

  private static class CustomListMultimap<K, V> extends AbstractListMultimap<K, V> {
    transient Supplier<? extends List<V>> factory;

    CustomListMultimap(Map<K, Collection<V>> map, Supplier<? extends List<V>> factory) {
      super(map);
      this.factory = checkNotNull(factory);
    }

    @Override
    protected List<V> createCollection() {
      return factory.get();
    }

    /** @serialData the factory and the backing map */
    @GwtIncompatible // java.io.ObjectOutputStream
    private void writeObject(ObjectOutputStream stream) throws IOException {
      stream.defaultWriteObject();
      stream.writeObject(factory);
      stream.writeObject(backingMap());
    }

    @GwtIncompatible // java.io.ObjectInputStream
    @SuppressWarnings("unchecked") // reading data stored by writeObject
    private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
      stream.defaultReadObject();
      factory = (Supplier<? extends List<V>>) stream.readObject();
      Map<K, Collection<V>> map = (Map<K, Collection<V>>) stream.readObject();
      setMap(map);
    }

    @GwtIncompatible // java serialization not supported
    private static final long serialVersionUID = 0;
  }

  /**
   * Creates a new {@code SetMultimap} that uses the provided map and factory.
   * It can generate a multimap based on arbitrary {@link Map} and {@link Set}
   * classes.
   *
   * <p>The {@code factory}-generated and {@code map} classes determine the
   * multimap iteration order. They also specify the behavior of the
   * {@code equals}, {@code hashCode}, and {@code toString} methods for the
   * multimap and its returned views. However, the multimap's {@code get}
   * method returns instances of a different class than {@code factory.get()}
   * does.
   *
   * <p>The multimap is serializable if {@code map}, {@code factory}, the
   * sets generated by {@code factory}, and the multimap contents are all
   * serializable.
   *
   * <p>The multimap is not threadsafe when any concurrent operations update the
   * multimap, even if {@code map} and the instances generated by
   * {@code factory} are. Concurrent read operations will work correctly. To
   * allow concurrent update operations, wrap the multimap with a call to
   * {@link #synchronizedSetMultimap}.
   *
   * <p>Call this method only when the simpler methods
   * {@link HashMultimap#create()}, {@link LinkedHashMultimap#create()},
   * {@link TreeMultimap#create()}, and
   * {@link TreeMultimap#create(Comparator, Comparator)} won't suffice.
   *
   * <p>Note: the multimap assumes complete ownership over of {@code map} and
   * the sets returned by {@code factory}. Those objects should not be manually
   * updated and they should not use soft, weak, or phantom references.
   *
   * @param map place to store the mapping from each key to its corresponding
   *     values
   * @param factory supplier of new, empty sets that will each hold all values
   *     for a given key
   * @throws IllegalArgumentException if {@code map} is not empty
   */
  public static <K, V> SetMultimap<K, V> newSetMultimap(
      Map<K, Collection<V>> map, final Supplier<? extends Set<V>> factory) {
    return new CustomSetMultimap<>(map, factory);
  }

  private static class CustomSetMultimap<K, V> extends AbstractSetMultimap<K, V> {
    transient Supplier<? extends Set<V>> factory;

    CustomSetMultimap(Map<K, Collection<V>> map, Supplier<? extends Set<V>> factory) {
      super(map);
      this.factory = checkNotNull(factory);
    }

    @Override
    protected Set<V> createCollection() {
      return factory.get();
    }

    /** @serialData the factory and the backing map */
    @GwtIncompatible // java.io.ObjectOutputStream
    private void writeObject(ObjectOutputStream stream) throws IOException {
      stream.defaultWriteObject();
      stream.writeObject(factory);
      stream.writeObject(backingMap());
    }

    @GwtIncompatible // java.io.ObjectInputStream
    @SuppressWarnings("unchecked") // reading data stored by writeObject
    private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
      stream.defaultReadObject();
      factory = (Supplier<? extends Set<V>>) stream.readObject();
      Map<K, Collection<V>> map = (Map<K, Collection<V>>) stream.readObject();
      setMap(map);
    }

    @GwtIncompatible // not needed in emulated source
    private static final long serialVersionUID = 0;
  }

  /**
   * Creates a new {@code SortedSetMultimap} that uses the provided map and
   * factory. It can generate a multimap based on arbitrary {@link Map} and
   * {@link SortedSet} classes.
   *
   * <p>The {@code factory}-generated and {@code map} classes determine the
   * multimap iteration order. They also specify the behavior of the
   * {@code equals}, {@code hashCode}, and {@code toString} methods for the
   * multimap and its returned views. However, the multimap's {@code get}
   * method returns instances of a different class than {@code factory.get()}
   * does.
   *
   * <p>The multimap is serializable if {@code map}, {@code factory}, the
   * sets generated by {@code factory}, and the multimap contents are all
   * serializable.
   *
   * <p>The multimap is not threadsafe when any concurrent operations update the
   * multimap, even if {@code map} and the instances generated by
   * {@code factory} are. Concurrent read operations will work correctly. To
   * allow concurrent update operations, wrap the multimap with a call to
   * {@link #synchronizedSortedSetMultimap}.
   *
   * <p>Call this method only when the simpler methods
   * {@link TreeMultimap#create()} and
   * {@link TreeMultimap#create(Comparator, Comparator)} won't suffice.
   *
   * <p>Note: the multimap assumes complete ownership over of {@code map} and
   * the sets returned by {@code factory}. Those objects should not be manually
   * updated and they should not use soft, weak, or phantom references.
   *
   * @param map place to store the mapping from each key to its corresponding
   *     values
   * @param factory supplier of new, empty sorted sets that will each hold
   *     all values for a given key
   * @throws IllegalArgumentException if {@code map} is not empty
   */
  public static <K, V> SortedSetMultimap<K, V> newSortedSetMultimap(
      Map<K, Collection<V>> map, final Supplier<? extends SortedSet<V>> factory) {
    return new CustomSortedSetMultimap<>(map, factory);
  }

  private static class CustomSortedSetMultimap<K, V> extends AbstractSortedSetMultimap<K, V> {
    transient Supplier<? extends SortedSet<V>> factory;
    transient Comparator<? super V> valueComparator;

    CustomSortedSetMultimap(Map<K, Collection<V>> map, Supplier<? extends SortedSet<V>> factory) {
      super(map);
      this.factory = checkNotNull(factory);
      valueComparator = factory.get().comparator();
    }

    @Override
    protected SortedSet<V> createCollection() {
      return factory.get();
    }

    @Override
    public Comparator<? super V> valueComparator() {
      return valueComparator;
    }

    /** @serialData the factory and the backing map */
    @GwtIncompatible // java.io.ObjectOutputStream
    private void writeObject(ObjectOutputStream stream) throws IOException {
      stream.defaultWriteObject();
      stream.writeObject(factory);
      stream.writeObject(backingMap());
    }

    @GwtIncompatible // java.io.ObjectInputStream
    @SuppressWarnings("unchecked") // reading data stored by writeObject
    private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
      stream.defaultReadObject();
      factory = (Supplier<? extends SortedSet<V>>) stream.readObject();
      valueComparator = factory.get().comparator();
      Map<K, Collection<V>> map = (Map<K, Collection<V>>) stream.readObject();
      setMap(map);
    }

    @GwtIncompatible // not needed in emulated source
    private static final long serialVersionUID = 0;
  }

  /**
   * Copies each key-value mapping in {@code source} into {@code dest}, with
   * its key and value reversed.
   *
   * <p>If {@code source} is an {@link ImmutableMultimap}, consider using
   * {@link ImmutableMultimap#inverse} instead.
   *
   * @param source any multimap
   * @param dest the multimap to copy into; usually empty
   * @return {@code dest}
   */
  @CanIgnoreReturnValue
  public static <K, V, M extends Multimap<K, V>> M invertFrom(
      Multimap<? extends V, ? extends K> source, M dest) {
    checkNotNull(dest);
    for (Map.Entry<? extends V, ? extends K> entry : source.entries()) {
      dest.put(entry.getValue(), entry.getKey());
    }
    return dest;
  }

  /**
   * Returns a synchronized (thread-safe) multimap backed by the specified
   * multimap. In order to guarantee serial access, it is critical that
   * <b>all</b> access to the backing multimap is accomplished through the
   * returned multimap.
   *
   * <p>It is imperative that the user manually synchronize on the returned
   * multimap when accessing any of its collection views: <pre>   {@code
   *
   *   Multimap<K, V> multimap = Multimaps.synchronizedMultimap(
   *       HashMultimap.<K, V>create());
   *   ...
   *   Collection<V> values = multimap.get(key);  // Needn't be in synchronized block
   *   ...
   *   synchronized (multimap) {  // Synchronizing on multimap, not values!
   *     Iterator<V> i = values.iterator(); // Must be in synchronized block
   *     while (i.hasNext()) {
   *       foo(i.next());
   *     }
   *   }}</pre>
   *
   * <p>Failure to follow this advice may result in non-deterministic behavior.
   *
   * <p>Note that the generated multimap's {@link Multimap#removeAll} and
   * {@link Multimap#replaceValues} methods return collections that aren't
   * synchronized.
   *
   * <p>The returned multimap will be serializable if the specified multimap is
   * serializable.
   *
   * @param multimap the multimap to be wrapped in a synchronized view
   * @return a synchronized view of the specified multimap
   */
  public static <K, V> Multimap<K, V> synchronizedMultimap(Multimap<K, V> multimap) {
    return Synchronized.multimap(multimap, null);
  }

  /**
   * Returns an unmodifiable view of the specified multimap. Query operations on
   * the returned multimap "read through" to the specified multimap, and
   * attempts to modify the returned multimap, either directly or through the
   * multimap's views, result in an {@code UnsupportedOperationException}.
   *
   * <p>Note that the generated multimap's {@link Multimap#removeAll} and
   * {@link Multimap#replaceValues} methods return collections that are
   * modifiable.
   *
   * <p>The returned multimap will be serializable if the specified multimap is
   * serializable.
   *
   * @param delegate the multimap for which an unmodifiable view is to be
   *     returned
   * @return an unmodifiable view of the specified multimap
   */
  public static <K, V> Multimap<K, V> unmodifiableMultimap(Multimap<K, V> delegate) {
    if (delegate instanceof UnmodifiableMultimap || delegate instanceof ImmutableMultimap) {
      return delegate;
    }
    return new UnmodifiableMultimap<>(delegate);
  }

  /**
   * Simply returns its argument.
   *
   * @deprecated no need to use this
   * @since 10.0
   */
  @Deprecated
  public static <K, V> Multimap<K, V> unmodifiableMultimap(ImmutableMultimap<K, V> delegate) {
    return checkNotNull(delegate);
  }

  private static class UnmodifiableMultimap<K, V> extends ForwardingMultimap<K, V>
      implements Serializable {
    final Multimap<K, V> delegate;
    transient Collection<Entry<K, V>> entries;
    transient Multiset<K> keys;
    transient Set<K> keySet;
    transient Collection<V> values;
    transient Map<K, Collection<V>> map;

    UnmodifiableMultimap(final Multimap<K, V> delegate) {
      this.delegate = checkNotNull(delegate);
    }

    @Override
    protected Multimap<K, V> delegate() {
      return delegate;
    }

    @Override
    public void clear() {
      throw new UnsupportedOperationException();
    }

    @Override
    public Map<K, Collection<V>> asMap() {
      Map<K, Collection<V>> result = map;
      if (result == null) {
        result =
            map =
                Collections.unmodifiableMap(
                    Maps.transformValues(
                        delegate.asMap(),
                        new Function<Collection<V>, Collection<V>>() {
                          @Override
                          public Collection<V> apply(Collection<V> collection) {
                            return unmodifiableValueCollection(collection);
                          }
                        }));
      }
      return result;
    }

    @Override
    public Collection<Entry<K, V>> entries() {
      Collection<Entry<K, V>> result = entries;
      if (result == null) {
        entries = result = unmodifiableEntries(delegate.entries());
      }
      return result;
    }

    @Override
    public Collection<V> get(K key) {
      return unmodifiableValueCollection(delegate.get(key));
    }

    @Override
    public Multiset<K> keys() {
      Multiset<K> result = keys;
      if (result == null) {
        keys = result = Multisets.unmodifiableMultiset(delegate.keys());
      }
      return result;
    }

    @Override
    public Set<K> keySet() {
      Set<K> result = keySet;
      if (result == null) {
        keySet = result = Collections.unmodifiableSet(delegate.keySet());
      }
      return result;
    }

    @Override
    public boolean put(K key, V value) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean putAll(K key, Iterable<? extends V> values) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean putAll(Multimap<? extends K, ? extends V> multimap) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean remove(Object key, Object value) {
      throw new UnsupportedOperationException();
    }

    @Override
    public Collection<V> removeAll(Object key) {
      throw new UnsupportedOperationException();
    }

    @Override
    public Collection<V> replaceValues(K key, Iterable<? extends V> values) {
      throw new UnsupportedOperationException();
    }

    @Override
    public Collection<V> values() {
      Collection<V> result = values;
      if (result == null) {
        values = result = Collections.unmodifiableCollection(delegate.values());
      }
      return result;
    }

    private static final long serialVersionUID = 0;
  }

  private static class UnmodifiableListMultimap<K, V> extends UnmodifiableMultimap<K, V>
      implements ListMultimap<K, V> {
    UnmodifiableListMultimap(ListMultimap<K, V> delegate) {
      super(delegate);
    }

    @Override
    public ListMultimap<K, V> delegate() {
      return (ListMultimap<K, V>) super.delegate();
    }

    @Override
    public List<V> get(K key) {
      return Collections.unmodifiableList(delegate().get(key));
    }

    @Override
    public List<V> removeAll(Object key) {
      throw new UnsupportedOperationException();
    }

    @Override
    public List<V> replaceValues(K key, Iterable<? extends V> values) {
      throw new UnsupportedOperationException();
    }

    private static final long serialVersionUID = 0;
  }

  private static class UnmodifiableSetMultimap<K, V> extends UnmodifiableMultimap<K, V>
      implements SetMultimap<K, V> {
    UnmodifiableSetMultimap(SetMultimap<K, V> delegate) {
      super(delegate);
    }

    @Override
    public SetMultimap<K, V> delegate() {
      return (SetMultimap<K, V>) super.delegate();
    }

    @Override
    public Set<V> get(K key) {
      /*
       * Note that this doesn't return a SortedSet when delegate is a
       * SortedSetMultiset, unlike (SortedSet<V>) super.get().
       */
      return Collections.unmodifiableSet(delegate().get(key));
    }

    @Override
    public Set<Map.Entry<K, V>> entries() {
      return Maps.unmodifiableEntrySet(delegate().entries());
    }

    @Override
    public Set<V> removeAll(Object key) {
      throw new UnsupportedOperationException();
    }

    @Override
    public Set<V> replaceValues(K key, Iterable<? extends V> values) {
      throw new UnsupportedOperationException();
    }

    private static final long serialVersionUID = 0;
  }

  private static class UnmodifiableSortedSetMultimap<K, V> extends UnmodifiableSetMultimap<K, V>
      implements SortedSetMultimap<K, V> {
    UnmodifiableSortedSetMultimap(SortedSetMultimap<K, V> delegate) {
      super(delegate);
    }

    @Override
    public SortedSetMultimap<K, V> delegate() {
      return (SortedSetMultimap<K, V>) super.delegate();
    }

    @Override
    public SortedSet<V> get(K key) {
      return Collections.unmodifiableSortedSet(delegate().get(key));
    }

    @Override
    public SortedSet<V> removeAll(Object key) {
      throw new UnsupportedOperationException();
    }

    @Override
    public SortedSet<V> replaceValues(K key, Iterable<? extends V> values) {
      throw new UnsupportedOperationException();
    }

    @Override
    public Comparator<? super V> valueComparator() {
      return delegate().valueComparator();
    }

    private static final long serialVersionUID = 0;
  }

  /**
   * Returns a synchronized (thread-safe) {@code SetMultimap} backed by the
   * specified multimap.
   *
   * <p>You must follow the warnings described in {@link #synchronizedMultimap}.
   *
   * <p>The returned multimap will be serializable if the specified multimap is
   * serializable.
   *
   * @param multimap the multimap to be wrapped
   * @return a synchronized view of the specified multimap
   */
  public static <K, V> SetMultimap<K, V> synchronizedSetMultimap(SetMultimap<K, V> multimap) {
    return Synchronized.setMultimap(multimap, null);
  }

  /**
   * Returns an unmodifiable view of the specified {@code SetMultimap}. Query
   * operations on the returned multimap "read through" to the specified
   * multimap, and attempts to modify the returned multimap, either directly or
   * through the multimap's views, result in an
   * {@code UnsupportedOperationException}.
   *
   * <p>Note that the generated multimap's {@link Multimap#removeAll} and
   * {@link Multimap#replaceValues} methods return collections that are
   * modifiable.
   *
   * <p>The returned multimap will be serializable if the specified multimap is
   * serializable.
   *
   * @param delegate the multimap for which an unmodifiable view is to be
   *     returned
   * @return an unmodifiable view of the specified multimap
   */
  public static <K, V> SetMultimap<K, V> unmodifiableSetMultimap(SetMultimap<K, V> delegate) {
    if (delegate instanceof UnmodifiableSetMultimap || delegate instanceof ImmutableSetMultimap) {
      return delegate;
    }
    return new UnmodifiableSetMultimap<>(delegate);
  }

  /**
   * Simply returns its argument.
   *
   * @deprecated no need to use this
   * @since 10.0
   */
  @Deprecated
  public static <K, V> SetMultimap<K, V> unmodifiableSetMultimap(
      ImmutableSetMultimap<K, V> delegate) {
    return checkNotNull(delegate);
  }

  /**
   * Returns a synchronized (thread-safe) {@code SortedSetMultimap} backed by
   * the specified multimap.
   *
   * <p>You must follow the warnings described in {@link #synchronizedMultimap}.
   *
   * <p>The returned multimap will be serializable if the specified multimap is
   * serializable.
   *
   * @param multimap the multimap to be wrapped
   * @return a synchronized view of the specified multimap
   */
  public static <K, V> SortedSetMultimap<K, V> synchronizedSortedSetMultimap(
      SortedSetMultimap<K, V> multimap) {
    return Synchronized.sortedSetMultimap(multimap, null);
  }

  /**
   * Returns an unmodifiable view of the specified {@code SortedSetMultimap}.
   * Query operations on the returned multimap "read through" to the specified
   * multimap, and attempts to modify the returned multimap, either directly or
   * through the multimap's views, result in an
   * {@code UnsupportedOperationException}.
   *
   * <p>Note that the generated multimap's {@link Multimap#removeAll} and
   * {@link Multimap#replaceValues} methods return collections that are
   * modifiable.
   *
   * <p>The returned multimap will be serializable if the specified multimap is
   * serializable.
   *
   * @param delegate the multimap for which an unmodifiable view is to be
   *     returned
   * @return an unmodifiable view of the specified multimap
   */
  public static <K, V> SortedSetMultimap<K, V> unmodifiableSortedSetMultimap(
      SortedSetMultimap<K, V> delegate) {
    if (delegate instanceof UnmodifiableSortedSetMultimap) {
      return delegate;
    }
    return new UnmodifiableSortedSetMultimap<>(delegate);
  }

  /**
   * Returns a synchronized (thread-safe) {@code ListMultimap} backed by the
   * specified multimap.
   *
   * <p>You must follow the warnings described in {@link #synchronizedMultimap}.
   *
   * @param multimap the multimap to be wrapped
   * @return a synchronized view of the specified multimap
   */
  public static <K, V> ListMultimap<K, V> synchronizedListMultimap(ListMultimap<K, V> multimap) {
    return Synchronized.listMultimap(multimap, null);
  }

  /**
   * Returns an unmodifiable view of the specified {@code ListMultimap}. Query
   * operations on the returned multimap "read through" to the specified
   * multimap, and attempts to modify the returned multimap, either directly or
   * through the multimap's views, result in an
   * {@code UnsupportedOperationException}.
   *
   * <p>Note that the generated multimap's {@link Multimap#removeAll} and
   * {@link Multimap#replaceValues} methods return collections that are
   * modifiable.
   *
   * <p>The returned multimap will be serializable if the specified multimap is
   * serializable.
   *
   * @param delegate the multimap for which an unmodifiable view is to be
   *     returned
   * @return an unmodifiable view of the specified multimap
   */
  public static <K, V> ListMultimap<K, V> unmodifiableListMultimap(ListMultimap<K, V> delegate) {
    if (delegate instanceof UnmodifiableListMultimap || delegate instanceof ImmutableListMultimap) {
      return delegate;
    }
    return new UnmodifiableListMultimap<>(delegate);
  }

  /**
   * Simply returns its argument.
   *
   * @deprecated no need to use this
   * @since 10.0
   */
  @Deprecated
  public static <K, V> ListMultimap<K, V> unmodifiableListMultimap(
      ImmutableListMultimap<K, V> delegate) {
    return checkNotNull(delegate);
  }

  /**
   * Returns an unmodifiable view of the specified collection, preserving the
   * interface for instances of {@code SortedSet}, {@code Set}, {@code List} and
   * {@code Collection}, in that order of preference.
   *
   * @param collection the collection for which to return an unmodifiable view
   * @return an unmodifiable view of the collection
   */
  private static <V> Collection<V> unmodifiableValueCollection(Collection<V> collection) {
    if (collection instanceof SortedSet) {
      return Collections.unmodifiableSortedSet((SortedSet<V>) collection);
    } else if (collection instanceof Set) {
      return Collections.unmodifiableSet((Set<V>) collection);
    } else if (collection instanceof List) {
      return Collections.unmodifiableList((List<V>) collection);
    }
    return Collections.unmodifiableCollection(collection);
  }

  /**
   * Returns an unmodifiable view of the specified collection of entries. The
   * {@link Entry#setValue} operation throws an {@link
   * UnsupportedOperationException}. If the specified collection is a {@code
   * Set}, the returned collection is also a {@code Set}.
   *
   * @param entries the entries for which to return an unmodifiable view
   * @return an unmodifiable view of the entries
   */
  private static <K, V> Collection<Entry<K, V>> unmodifiableEntries(
      Collection<Entry<K, V>> entries) {
    if (entries instanceof Set) {
      return Maps.unmodifiableEntrySet((Set<Entry<K, V>>) entries);
    }
    return new Maps.UnmodifiableEntries<>(Collections.unmodifiableCollection(entries));
  }

  /**
   * Returns {@link ListMultimap#asMap multimap.asMap()}, with its type
   * corrected from {@code Map<K, Collection<V>>} to {@code Map<K, List<V>>}.
   *
   * @since 15.0
   */
  @Beta
  @SuppressWarnings("unchecked")
  // safe by specification of ListMultimap.asMap()
  public static <K, V> Map<K, List<V>> asMap(ListMultimap<K, V> multimap) {
    return (Map<K, List<V>>) (Map<K, ?>) multimap.asMap();
  }

  /**
   * Returns {@link SetMultimap#asMap multimap.asMap()}, with its type corrected
   * from {@code Map<K, Collection<V>>} to {@code Map<K, Set<V>>}.
   *
   * @since 15.0
   */
  @Beta
  @SuppressWarnings("unchecked")
  // safe by specification of SetMultimap.asMap()
  public static <K, V> Map<K, Set<V>> asMap(SetMultimap<K, V> multimap) {
    return (Map<K, Set<V>>) (Map<K, ?>) multimap.asMap();
  }

  /**
   * Returns {@link SortedSetMultimap#asMap multimap.asMap()}, with its type
   * corrected from {@code Map<K, Collection<V>>} to
   * {@code Map<K, SortedSet<V>>}.
   *
   * @since 15.0
   */
  @Beta
  @SuppressWarnings("unchecked")
  // safe by specification of SortedSetMultimap.asMap()
  public static <K, V> Map<K, SortedSet<V>> asMap(SortedSetMultimap<K, V> multimap) {
    return (Map<K, SortedSet<V>>) (Map<K, ?>) multimap.asMap();
  }

  /**
   * Returns {@link Multimap#asMap multimap.asMap()}. This is provided for
   * parity with the other more strongly-typed {@code asMap()} implementations.
   *
   * @since 15.0
   */
  @Beta
  public static <K, V> Map<K, Collection<V>> asMap(Multimap<K, V> multimap) {
    return multimap.asMap();
  }

  /**
   * Returns a multimap view of the specified map. The multimap is backed by the
   * map, so changes to the map are reflected in the multimap, and vice versa.
   * If the map is modified while an iteration over one of the multimap's
   * collection views is in progress (except through the iterator's own {@code
   * remove} operation, or through the {@code setValue} operation on a map entry
   * returned by the iterator), the results of the iteration are undefined.
   *
   * <p>The multimap supports mapping removal, which removes the corresponding
   * mapping from the map. It does not support any operations which might add
   * mappings, such as {@code put}, {@code putAll} or {@code replaceValues}.
   *
   * <p>The returned multimap will be serializable if the specified map is
   * serializable.
   *
   * @param map the backing map for the returned multimap view
   */
  public static <K, V> SetMultimap<K, V> forMap(Map<K, V> map) {
    return new MapMultimap<>(map);
  }

  /** @see Multimaps#forMap */
  private static class MapMultimap<K, V> extends AbstractMultimap<K, V>
      implements SetMultimap<K, V>, Serializable {
    final Map<K, V> map;

    MapMultimap(Map<K, V> map) {
      this.map = checkNotNull(map);
    }

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

    @Override
    public boolean containsKey(Object key) {
      return map.containsKey(key);
    }

    @Override
    public boolean containsValue(Object value) {
      return map.containsValue(value);
    }

    @Override
    public boolean containsEntry(Object key, Object value) {
      return map.entrySet().contains(Maps.immutableEntry(key, value));
    }

    @Override
    public Set<V> get(final K key) {
      return new Sets.ImprovedAbstractSet<V>() {
        @Override
        public Iterator<V> iterator() {
          return new Iterator<V>() {
            int i;

            @Override
            public boolean hasNext() {
              return (i == 0) && map.containsKey(key);
            }

            @Override
            public V next() {
              if (!hasNext()) {
                throw new NoSuchElementException();
              }
              i++;
              return map.get(key);
            }

            @Override
            public void remove() {
              checkRemove(i == 1);
              i = -1;
              map.remove(key);
            }
          };
        }

        @Override
        public int size() {
          return map.containsKey(key) ? 1 : 0;
        }
      };
    }

    @Override
    public boolean put(K key, V value) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean putAll(K key, Iterable<? extends V> values) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean putAll(Multimap<? extends K, ? extends V> multimap) {
      throw new UnsupportedOperationException();
    }

    @Override
    public Set<V> replaceValues(K key, Iterable<? extends V> values) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean remove(Object key, Object value) {
      return map.entrySet().remove(Maps.immutableEntry(key, value));
    }

    @Override
    public Set<V> removeAll(Object key) {
      Set<V> values = new HashSet<V>(2);
      if (!map.containsKey(key)) {
        return values;
      }
      values.add(map.remove(key));
      return values;
    }

    @Override
    public void clear() {
      map.clear();
    }

    @Override
    public Set<K> keySet() {
      return map.keySet();
    }

    @Override
    public Collection<V> values() {
      return map.values();
    }

    @Override
    public Set<Entry<K, V>> entries() {
      return map.entrySet();
    }

    @Override
    Iterator<Entry<K, V>> entryIterator() {
      return map.entrySet().iterator();
    }

    @Override
    Map<K, Collection<V>> createAsMap() {
      return new AsMap<>(this);
    }

    @Override
    public int hashCode() {
      return map.hashCode();
    }

    private static final long serialVersionUID = 7845222491160860175L;
  }

  /**
   * Returns a view of a multimap where each value is transformed by a function.
   * All other properties of the multimap, such as iteration order, are left
   * intact. For example, the code: <pre>   {@code
   *
   * Multimap<String, Integer> multimap =
   *     ImmutableSetMultimap.of("a", 2, "b", -3, "b", -3, "a", 4, "c", 6);
   * Function<Integer, String> square = new Function<Integer, String>() {
   *     public String apply(Integer in) {
   *       return Integer.toString(in * in);
   *     }
   * };
   * Multimap<String, String> transformed =
   *     Multimaps.transformValues(multimap, square);
   *   System.out.println(transformed);}</pre>
   *
   * ... prints {@code {a=[4, 16], b=[9, 9], c=[36]}}.
   *
   * <p>Changes in the underlying multimap are reflected in this view.
   * Conversely, this view supports removal operations, and these are reflected
   * in the underlying multimap.
   *
   * <p>It's acceptable for the underlying multimap to contain null keys, and
   * even null values provided that the function is capable of accepting null
   * input.  The transformed multimap might contain null values, if the function
   * sometimes gives a null result.
   *
   * <p>The returned multimap is not thread-safe or serializable, even if the
   * underlying multimap is.  The {@code equals} and {@code hashCode} methods
   * of the returned multimap are meaningless, since there is not a definition
   * of {@code equals} or {@code hashCode} for general collections, and
   * {@code get()} will return a general {@code Collection} as opposed to a
   * {@code List} or a {@code Set}.
   *
   * <p>The function is applied lazily, invoked when needed. This is necessary
   * for the returned multimap to be a view, but it means that the function will
   * be applied many times for bulk operations like
   * {@link Multimap#containsValue} and {@code Multimap.toString()}. For this to
   * perform well, {@code function} should be fast. To avoid lazy evaluation
   * when the returned multimap doesn't need to be a view, copy the returned
   * multimap into a new multimap of your choosing.
   *
   * @since 7.0
   */
  public static <K, V1, V2> Multimap<K, V2> transformValues(
      Multimap<K, V1> fromMultimap, final Function<? super V1, V2> function) {
    checkNotNull(function);
    EntryTransformer<K, V1, V2> transformer = Maps.asEntryTransformer(function);
    return transformEntries(fromMultimap, transformer);
  }

  /**
   * Returns a view of a multimap whose values are derived from the original
   * multimap's entries. In contrast to {@link #transformValues}, this method's
   * entry-transformation logic may depend on the key as well as the value.
   *
   * <p>All other properties of the transformed multimap, such as iteration
   * order, are left intact. For example, the code: <pre>   {@code
   *
   *   SetMultimap<String, Integer> multimap =
   *       ImmutableSetMultimap.of("a", 1, "a", 4, "b", -6);
   *   EntryTransformer<String, Integer, String> transformer =
   *       new EntryTransformer<String, Integer, String>() {
   *         public String transformEntry(String key, Integer value) {
   *            return (value >= 0) ? key : "no" + key;
   *         }
   *       };
   *   Multimap<String, String> transformed =
   *       Multimaps.transformEntries(multimap, transformer);
   *   System.out.println(transformed);}</pre>
   *
   * ... prints {@code {a=[a, a], b=[nob]}}.
   *
   * <p>Changes in the underlying multimap are reflected in this view.
   * Conversely, this view supports removal operations, and these are reflected
   * in the underlying multimap.
   *
   * <p>It's acceptable for the underlying multimap to contain null keys and
   * null values provided that the transformer is capable of accepting null
   * inputs. The transformed multimap might contain null values if the
   * transformer sometimes gives a null result.
   *
   * <p>The returned multimap is not thread-safe or serializable, even if the
   * underlying multimap is.  The {@code equals} and {@code hashCode} methods
   * of the returned multimap are meaningless, since there is not a definition
   * of {@code equals} or {@code hashCode} for general collections, and
   * {@code get()} will return a general {@code Collection} as opposed to a
   * {@code List} or a {@code Set}.
   *
   * <p>The transformer is applied lazily, invoked when needed. This is
   * necessary for the returned multimap to be a view, but it means that the
   * transformer will be applied many times for bulk operations like {@link
   * Multimap#containsValue} and {@link Object#toString}. For this to perform
   * well, {@code transformer} should be fast. To avoid lazy evaluation when the
   * returned multimap doesn't need to be a view, copy the returned multimap
   * into a new multimap of your choosing.
   *
   * <p><b>Warning:</b> This method assumes that for any instance {@code k} of
   * {@code EntryTransformer} key type {@code K}, {@code k.equals(k2)} implies
   * that {@code k2} is also of type {@code K}. Using an {@code
   * EntryTransformer} key type for which this may not hold, such as {@code
   * ArrayList}, may risk a {@code ClassCastException} when calling methods on
   * the transformed multimap.
   *
   * @since 7.0
   */
  public static <K, V1, V2> Multimap<K, V2> transformEntries(
      Multimap<K, V1> fromMap, EntryTransformer<? super K, ? super V1, V2> transformer) {
    return new TransformedEntriesMultimap<>(fromMap, transformer);
  }

  private static class TransformedEntriesMultimap<K, V1, V2> extends AbstractMultimap<K, V2> {
    final Multimap<K, V1> fromMultimap;
    final EntryTransformer<? super K, ? super V1, V2> transformer;

    TransformedEntriesMultimap(
        Multimap<K, V1> fromMultimap,
        final EntryTransformer<? super K, ? super V1, V2> transformer) {
      this.fromMultimap = checkNotNull(fromMultimap);
      this.transformer = checkNotNull(transformer);
    }

    Collection<V2> transform(K key, Collection<V1> values) {
      Function<? super V1, V2> function = Maps.asValueToValueFunction(transformer, key);
      if (values instanceof List) {
        return Lists.transform((List<V1>) values, function);
      } else {
        return Collections2.transform(values, function);
      }
    }

    @Override
    Map<K, Collection<V2>> createAsMap() {
      return Maps.transformEntries(
          fromMultimap.asMap(),
          new EntryTransformer<K, Collection<V1>, Collection<V2>>() {
            @Override
            public Collection<V2> transformEntry(K key, Collection<V1> value) {
              return transform(key, value);
            }
          });
    }

    @Override
    public void clear() {
      fromMultimap.clear();
    }

    @Override
    public boolean containsKey(Object key) {
      return fromMultimap.containsKey(key);
    }

    @Override
    Iterator<Entry<K, V2>> entryIterator() {
      return Iterators.transform(
          fromMultimap.entries().iterator(), Maps.<K, V1, V2>asEntryToEntryFunction(transformer));
    }

    @Override
    public Collection<V2> get(final K key) {
      return transform(key, fromMultimap.get(key));
    }

    @Override
    public boolean isEmpty() {
      return fromMultimap.isEmpty();
    }

    @Override
    public Set<K> keySet() {
      return fromMultimap.keySet();
    }

    @Override
    public Multiset<K> keys() {
      return fromMultimap.keys();
    }

    @Override
    public boolean put(K key, V2 value) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean putAll(K key, Iterable<? extends V2> values) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean putAll(Multimap<? extends K, ? extends V2> multimap) {
      throw new UnsupportedOperationException();
    }

    @SuppressWarnings("unchecked")
    @Override
    public boolean remove(Object key, Object value) {
      return get((K) key).remove(value);
    }

    @SuppressWarnings("unchecked")
    @Override
    public Collection<V2> removeAll(Object key) {
      return transform((K) key, fromMultimap.removeAll(key));
    }

    @Override
    public Collection<V2> replaceValues(K key, Iterable<? extends V2> values) {
      throw new UnsupportedOperationException();
    }

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

    @Override
    Collection<V2> createValues() {
      return Collections2.transform(
          fromMultimap.entries(), Maps.<K, V1, V2>asEntryToValueFunction(transformer));
    }
  }

  /**
   * Returns a view of a {@code ListMultimap} where each value is transformed by
   * a function. All other properties of the multimap, such as iteration order,
   * are left intact. For example, the code: <pre>   {@code
   *
   *   ListMultimap<String, Integer> multimap
   *        = ImmutableListMultimap.of("a", 4, "a", 16, "b", 9);
   *   Function<Integer, Double> sqrt =
   *       new Function<Integer, Double>() {
   *         public Double apply(Integer in) {
   *           return Math.sqrt((int) in);
   *         }
   *       };
   *   ListMultimap<String, Double> transformed = Multimaps.transformValues(map,
   *       sqrt);
   *   System.out.println(transformed);}</pre>
   *
   * ... prints {@code {a=[2.0, 4.0], b=[3.0]}}.
   *
   * <p>Changes in the underlying multimap are reflected in this view.
   * Conversely, this view supports removal operations, and these are reflected
   * in the underlying multimap.
   *
   * <p>It's acceptable for the underlying multimap to contain null keys, and
   * even null values provided that the function is capable of accepting null
   * input.  The transformed multimap might contain null values, if the function
   * sometimes gives a null result.
   *
   * <p>The returned multimap is not thread-safe or serializable, even if the
   * underlying multimap is.
   *
   * <p>The function is applied lazily, invoked when needed. This is necessary
   * for the returned multimap to be a view, but it means that the function will
   * be applied many times for bulk operations like
   * {@link Multimap#containsValue} and {@code Multimap.toString()}. For this to
   * perform well, {@code function} should be fast. To avoid lazy evaluation
   * when the returned multimap doesn't need to be a view, copy the returned
   * multimap into a new multimap of your choosing.
   *
   * @since 7.0
   */
  public static <K, V1, V2> ListMultimap<K, V2> transformValues(
      ListMultimap<K, V1> fromMultimap, final Function<? super V1, V2> function) {
    checkNotNull(function);
    EntryTransformer<K, V1, V2> transformer = Maps.asEntryTransformer(function);
    return transformEntries(fromMultimap, transformer);
  }

  /**
   * Returns a view of a {@code ListMultimap} whose values are derived from the
   * original multimap's entries. In contrast to
   * {@link #transformValues(ListMultimap, Function)}, this method's
   * entry-transformation logic may depend on the key as well as the value.
   *
   * <p>All other properties of the transformed multimap, such as iteration
   * order, are left intact. For example, the code: <pre>   {@code
   *
   *   Multimap<String, Integer> multimap =
   *       ImmutableMultimap.of("a", 1, "a", 4, "b", 6);
   *   EntryTransformer<String, Integer, String> transformer =
   *       new EntryTransformer<String, Integer, String>() {
   *         public String transformEntry(String key, Integer value) {
   *           return key + value;
   *         }
   *       };
   *   Multimap<String, String> transformed =
   *       Multimaps.transformEntries(multimap, transformer);
   *   System.out.println(transformed);}</pre>
   *
   * ... prints {@code {"a"=["a1", "a4"], "b"=["b6"]}}.
   *
   * <p>Changes in the underlying multimap are reflected in this view.
   * Conversely, this view supports removal operations, and these are reflected
   * in the underlying multimap.
   *
   * <p>It's acceptable for the underlying multimap to contain null keys and
   * null values provided that the transformer is capable of accepting null
   * inputs. The transformed multimap might contain null values if the
   * transformer sometimes gives a null result.
   *
   * <p>The returned multimap is not thread-safe or serializable, even if the
   * underlying multimap is.
   *
   * <p>The transformer is applied lazily, invoked when needed. This is
   * necessary for the returned multimap to be a view, but it means that the
   * transformer will be applied many times for bulk operations like {@link
   * Multimap#containsValue} and {@link Object#toString}. For this to perform
   * well, {@code transformer} should be fast. To avoid lazy evaluation when the
   * returned multimap doesn't need to be a view, copy the returned multimap
   * into a new multimap of your choosing.
   *
   * <p><b>Warning:</b> This method assumes that for any instance {@code k} of
   * {@code EntryTransformer} key type {@code K}, {@code k.equals(k2)} implies
   * that {@code k2} is also of type {@code K}. Using an {@code
   * EntryTransformer} key type for which this may not hold, such as {@code
   * ArrayList}, may risk a {@code ClassCastException} when calling methods on
   * the transformed multimap.
   *
   * @since 7.0
   */
  public static <K, V1, V2> ListMultimap<K, V2> transformEntries(
      ListMultimap<K, V1> fromMap, EntryTransformer<? super K, ? super V1, V2> transformer) {
    return new TransformedEntriesListMultimap<>(fromMap, transformer);
  }

  private static final class TransformedEntriesListMultimap<K, V1, V2>
      extends TransformedEntriesMultimap<K, V1, V2> implements ListMultimap<K, V2> {

    TransformedEntriesListMultimap(
        ListMultimap<K, V1> fromMultimap, EntryTransformer<? super K, ? super V1, V2> transformer) {
      super(fromMultimap, transformer);
    }

    @Override
    List<V2> transform(K key, Collection<V1> values) {
      return Lists.transform((List<V1>) values, Maps.asValueToValueFunction(transformer, key));
    }

    @Override
    public List<V2> get(K key) {
      return transform(key, fromMultimap.get(key));
    }

    @SuppressWarnings("unchecked")
    @Override
    public List<V2> removeAll(Object key) {
      return transform((K) key, fromMultimap.removeAll(key));
    }

    @Override
    public List<V2> replaceValues(K key, Iterable<? extends V2> values) {
      throw new UnsupportedOperationException();
    }
  }

  /**
   * Creates an index {@code ImmutableListMultimap} that contains the results of
   * applying a specified function to each item in an {@code Iterable} of
   * values. Each value will be stored as a value in the resulting multimap,
   * yielding a multimap with the same size as the input iterable. The key used
   * to store that value in the multimap will be the result of calling the
   * function on that value. The resulting multimap is created as an immutable
   * snapshot. In the returned multimap, keys appear in the order they are first
   * encountered, and the values corresponding to each key appear in the same
   * order as they are encountered.
   *
   * <p>For example, <pre>   {@code
   *
   *   List<String> badGuys =
   *       Arrays.asList("Inky", "Blinky", "Pinky", "Pinky", "Clyde");
   *   Function<String, Integer> stringLengthFunction = ...;
   *   Multimap<Integer, String> index =
   *       Multimaps.index(badGuys, stringLengthFunction);
   *   System.out.println(index);}</pre>
   *
   * <p>prints <pre>   {@code
   *
   *   {4=[Inky], 6=[Blinky], 5=[Pinky, Pinky, Clyde]}}</pre>
   *
   * <p>The returned multimap is serializable if its keys and values are all
   * serializable.
   *
   * @param values the values to use when constructing the {@code
   *     ImmutableListMultimap}
   * @param keyFunction the function used to produce the key for each value
   * @return {@code ImmutableListMultimap} mapping the result of evaluating the
   *     function {@code keyFunction} on each value in the input collection to
   *     that value
   * @throws NullPointerException if any element of {@code values} is {@code null}, or if {@code
   *     keyFunction} produces {@code null} for any key
   */
  public static <K, V> ImmutableListMultimap<K, V> index(
      Iterable<V> values, Function<? super V, K> keyFunction) {
    return index(values.iterator(), keyFunction);
  }

  /**
   * Creates an index {@code ImmutableListMultimap} that contains the results of
   * applying a specified function to each item in an {@code Iterator} of
   * values. Each value will be stored as a value in the resulting multimap,
   * yielding a multimap with the same size as the input iterator. The key used
   * to store that value in the multimap will be the result of calling the
   * function on that value. The resulting multimap is created as an immutable
   * snapshot. In the returned multimap, keys appear in the order they are first
   * encountered, and the values corresponding to each key appear in the same
   * order as they are encountered.
   *
   * <p>For example, <pre>   {@code
   *
   *   List<String> badGuys =
   *       Arrays.asList("Inky", "Blinky", "Pinky", "Pinky", "Clyde");
   *   Function<String, Integer> stringLengthFunction = ...;
   *   Multimap<Integer, String> index =
   *       Multimaps.index(badGuys.iterator(), stringLengthFunction);
   *   System.out.println(index);}</pre>
   *
   * <p>prints <pre>   {@code
   *
   *   {4=[Inky], 6=[Blinky], 5=[Pinky, Pinky, Clyde]}}</pre>
   *
   * <p>The returned multimap is serializable if its keys and values are all
   * serializable.
   *
   * @param values the values to use when constructing the {@code
   *     ImmutableListMultimap}
   * @param keyFunction the function used to produce the key for each value
   * @return {@code ImmutableListMultimap} mapping the result of evaluating the
   *     function {@code keyFunction} on each value in the input collection to
   *     that value
   * @throws NullPointerException if any element of {@code values} is {@code null}, or if {@code
   *     keyFunction} produces {@code null} for any key
   * @since 10.0
   */
  public static <K, V> ImmutableListMultimap<K, V> index(
      Iterator<V> values, Function<? super V, K> keyFunction) {
    checkNotNull(keyFunction);
    ImmutableListMultimap.Builder<K, V> builder = ImmutableListMultimap.builder();
    while (values.hasNext()) {
      V value = values.next();
      checkNotNull(value, values);
      builder.put(keyFunction.apply(value), value);
    }
    return builder.build();
  }

  static class Keys<K, V> extends AbstractMultiset<K> {
    @Weak final Multimap<K, V> multimap;

    Keys(Multimap<K, V> multimap) {
      this.multimap = multimap;
    }

    @Override
    Iterator<Multiset.Entry<K>> entryIterator() {
      return new TransformedIterator<Map.Entry<K, Collection<V>>, Multiset.Entry<K>>(
          multimap.asMap().entrySet().iterator()) {
        @Override
        Multiset.Entry<K> transform(final Map.Entry<K, Collection<V>> backingEntry) {
          return new Multisets.AbstractEntry<K>() {
            @Override
            public K getElement() {
              return backingEntry.getKey();
            }

            @Override
            public int getCount() {
              return backingEntry.getValue().size();
            }
          };
        }
      };
    }

    @Override
    int distinctElements() {
      return multimap.asMap().size();
    }

    @Override
    Set<Multiset.Entry<K>> createEntrySet() {
      return new KeysEntrySet();
    }

    @WeakOuter
    class KeysEntrySet extends Multisets.EntrySet<K> {
      @Override
      Multiset<K> multiset() {
        return Keys.this;
      }

      @Override
      public Iterator<Multiset.Entry<K>> iterator() {
        return entryIterator();
      }

      @Override
      public int size() {
        return distinctElements();
      }

      @Override
      public boolean isEmpty() {
        return multimap.isEmpty();
      }

      @Override
      public boolean contains(@Nullable Object o) {
        if (o instanceof Multiset.Entry) {
          Multiset.Entry<?> entry = (Multiset.Entry<?>) o;
          Collection<V> collection = multimap.asMap().get(entry.getElement());
          return collection != null && collection.size() == entry.getCount();
        }
        return false;
      }

      @Override
      public boolean remove(@Nullable Object o) {
        if (o instanceof Multiset.Entry) {
          Multiset.Entry<?> entry = (Multiset.Entry<?>) o;
          Collection<V> collection = multimap.asMap().get(entry.getElement());
          if (collection != null && collection.size() == entry.getCount()) {
            collection.clear();
            return true;
          }
        }
        return false;
      }
    }

    @Override
    public boolean contains(@Nullable Object element) {
      return multimap.containsKey(element);
    }

    @Override
    public Iterator<K> iterator() {
      return Maps.keyIterator(multimap.entries().iterator());
    }

    @Override
    public int count(@Nullable Object element) {
      Collection<V> values = Maps.safeGet(multimap.asMap(), element);
      return (values == null) ? 0 : values.size();
    }

    @Override
    public int remove(@Nullable Object element, int occurrences) {
      checkNonnegative(occurrences, "occurrences");
      if (occurrences == 0) {
        return count(element);
      }

      Collection<V> values = Maps.safeGet(multimap.asMap(), element);

      if (values == null) {
        return 0;
      }

      int oldCount = values.size();
      if (occurrences >= oldCount) {
        values.clear();
      } else {
        Iterator<V> iterator = values.iterator();
        for (int i = 0; i < occurrences; i++) {
          iterator.next();
          iterator.remove();
        }
      }
      return oldCount;
    }

    @Override
    public void clear() {
      multimap.clear();
    }

    @Override
    public Set<K> elementSet() {
      return multimap.keySet();
    }
  }

  /**
   * A skeleton implementation of {@link Multimap#entries()}.
   */
  abstract static class Entries<K, V> extends AbstractCollection<Map.Entry<K, V>> {
    abstract Multimap<K, V> multimap();

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

    @Override
    public boolean contains(@Nullable Object o) {
      if (o instanceof Map.Entry) {
        Map.Entry<?, ?> entry = (Map.Entry<?, ?>) o;
        return multimap().containsEntry(entry.getKey(), entry.getValue());
      }
      return false;
    }

    @Override
    public boolean remove(@Nullable Object o) {
      if (o instanceof Map.Entry) {
        Map.Entry<?, ?> entry = (Map.Entry<?, ?>) o;
        return multimap().remove(entry.getKey(), entry.getValue());
      }
      return false;
    }

    @Override
    public void clear() {
      multimap().clear();
    }
  }

  /**
   * A skeleton implementation of {@link Multimap#asMap()}.
   */
  static final class AsMap<K, V> extends Maps.ViewCachingAbstractMap<K, Collection<V>> {
    @Weak private final Multimap<K, V> multimap;

    AsMap(Multimap<K, V> multimap) {
      this.multimap = checkNotNull(multimap);
    }

    @Override
    public int size() {
      return multimap.keySet().size();
    }

    @Override
    protected Set<Entry<K, Collection<V>>> createEntrySet() {
      return new EntrySet();
    }

    void removeValuesForKey(Object key) {
      multimap.keySet().remove(key);
    }

    @WeakOuter
    class EntrySet extends Maps.EntrySet<K, Collection<V>> {
      @Override
      Map<K, Collection<V>> map() {
        return AsMap.this;
      }

      @Override
      public Iterator<Entry<K, Collection<V>>> iterator() {
        return Maps.asMapEntryIterator(
            multimap.keySet(),
            new Function<K, Collection<V>>() {
              @Override
              public Collection<V> apply(K key) {
                return multimap.get(key);
              }
            });
      }

      @Override
      public boolean remove(Object o) {
        if (!contains(o)) {
          return false;
        }
        Map.Entry<?, ?> entry = (Map.Entry<?, ?>) o;
        removeValuesForKey(entry.getKey());
        return true;
      }
    }

    @SuppressWarnings("unchecked")
    @Override
    public Collection<V> get(Object key) {
      return containsKey(key) ? multimap.get((K) key) : null;
    }

    @Override
    public Collection<V> remove(Object key) {
      return containsKey(key) ? multimap.removeAll(key) : null;
    }

    @Override
    public Set<K> keySet() {
      return multimap.keySet();
    }

    @Override
    public boolean isEmpty() {
      return multimap.isEmpty();
    }

    @Override
    public boolean containsKey(Object key) {
      return multimap.containsKey(key);
    }

    @Override
    public void clear() {
      multimap.clear();
    }
  }

  /**
   * Returns a multimap containing the mappings in {@code unfiltered} whose keys
   * satisfy a predicate. The returned multimap is a live view of
   * {@code unfiltered}; changes to one affect the other.
   *
   * <p>The resulting multimap's views have iterators that don't support
   * {@code remove()}, but all other methods are supported by the multimap and
   * its views. When adding a key that doesn't satisfy the predicate, the
   * multimap's {@code put()}, {@code putAll()}, and {@code replaceValues()}
   * methods throw an {@link IllegalArgumentException}.
   *
   * <p>When methods such as {@code removeAll()} and {@code clear()} are called on
   * the filtered multimap or its views, only mappings whose keys satisfy the
   * filter will be removed from the underlying multimap.
   *
   * <p>The returned multimap isn't threadsafe or serializable, even if
   * {@code unfiltered} is.
   *
   * <p>Many of the filtered multimap's methods, such as {@code size()}, iterate
   * across every key/value mapping in the underlying multimap and determine
   * which satisfy the filter. When a live view is <i>not</i> needed, it may be
   * faster to copy the filtered multimap and use the copy.
   *
   * <p><b>Warning:</b> {@code keyPredicate} must be <i>consistent with equals</i>,
   * as documented at {@link Predicate#apply}. Do not provide a predicate such
   * as {@code Predicates.instanceOf(ArrayList.class)}, which is inconsistent
   * with equals.
   *
   * @since 11.0
   */
  public static <K, V> Multimap<K, V> filterKeys(
      Multimap<K, V> unfiltered, final Predicate<? super K> keyPredicate) {
    if (unfiltered instanceof SetMultimap) {
      return filterKeys((SetMultimap<K, V>) unfiltered, keyPredicate);
    } else if (unfiltered instanceof ListMultimap) {
      return filterKeys((ListMultimap<K, V>) unfiltered, keyPredicate);
    } else if (unfiltered instanceof FilteredKeyMultimap) {
      FilteredKeyMultimap<K, V> prev = (FilteredKeyMultimap<K, V>) unfiltered;
      return new FilteredKeyMultimap<>(
          prev.unfiltered, Predicates.<K>and(prev.keyPredicate, keyPredicate));
    } else if (unfiltered instanceof FilteredMultimap) {
      FilteredMultimap<K, V> prev = (FilteredMultimap<K, V>) unfiltered;
      return filterFiltered(prev, Maps.<K>keyPredicateOnEntries(keyPredicate));
    } else {
      return new FilteredKeyMultimap<>(unfiltered, keyPredicate);
    }
  }

  /**
   * Returns a multimap containing the mappings in {@code unfiltered} whose keys
   * satisfy a predicate. The returned multimap is a live view of
   * {@code unfiltered}; changes to one affect the other.
   *
   * <p>The resulting multimap's views have iterators that don't support
   * {@code remove()}, but all other methods are supported by the multimap and
   * its views. When adding a key that doesn't satisfy the predicate, the
   * multimap's {@code put()}, {@code putAll()}, and {@code replaceValues()}
   * methods throw an {@link IllegalArgumentException}.
   *
   * <p>When methods such as {@code removeAll()} and {@code clear()} are called on
   * the filtered multimap or its views, only mappings whose keys satisfy the
   * filter will be removed from the underlying multimap.
   *
   * <p>The returned multimap isn't threadsafe or serializable, even if
   * {@code unfiltered} is.
   *
   * <p>Many of the filtered multimap's methods, such as {@code size()}, iterate
   * across every key/value mapping in the underlying multimap and determine
   * which satisfy the filter. When a live view is <i>not</i> needed, it may be
   * faster to copy the filtered multimap and use the copy.
   *
   * <p><b>Warning:</b> {@code keyPredicate} must be <i>consistent with equals</i>,
   * as documented at {@link Predicate#apply}. Do not provide a predicate such
   * as {@code Predicates.instanceOf(ArrayList.class)}, which is inconsistent
   * with equals.
   *
   * @since 14.0
   */
  public static <K, V> SetMultimap<K, V> filterKeys(
      SetMultimap<K, V> unfiltered, final Predicate<? super K> keyPredicate) {
    if (unfiltered instanceof FilteredKeySetMultimap) {
      FilteredKeySetMultimap<K, V> prev = (FilteredKeySetMultimap<K, V>) unfiltered;
      return new FilteredKeySetMultimap<>(
          prev.unfiltered(), Predicates.<K>and(prev.keyPredicate, keyPredicate));
    } else if (unfiltered instanceof FilteredSetMultimap) {
      FilteredSetMultimap<K, V> prev = (FilteredSetMultimap<K, V>) unfiltered;
      return filterFiltered(prev, Maps.<K>keyPredicateOnEntries(keyPredicate));
    } else {
      return new FilteredKeySetMultimap<>(unfiltered, keyPredicate);
    }
  }

  /**
   * Returns a multimap containing the mappings in {@code unfiltered} whose keys
   * satisfy a predicate. The returned multimap is a live view of
   * {@code unfiltered}; changes to one affect the other.
   *
   * <p>The resulting multimap's views have iterators that don't support
   * {@code remove()}, but all other methods are supported by the multimap and
   * its views. When adding a key that doesn't satisfy the predicate, the
   * multimap's {@code put()}, {@code putAll()}, and {@code replaceValues()}
   * methods throw an {@link IllegalArgumentException}.
   *
   * <p>When methods such as {@code removeAll()} and {@code clear()} are called on
   * the filtered multimap or its views, only mappings whose keys satisfy the
   * filter will be removed from the underlying multimap.
   *
   * <p>The returned multimap isn't threadsafe or serializable, even if
   * {@code unfiltered} is.
   *
   * <p>Many of the filtered multimap's methods, such as {@code size()}, iterate
   * across every key/value mapping in the underlying multimap and determine
   * which satisfy the filter. When a live view is <i>not</i> needed, it may be
   * faster to copy the filtered multimap and use the copy.
   *
   * <p><b>Warning:</b> {@code keyPredicate} must be <i>consistent with equals</i>,
   * as documented at {@link Predicate#apply}. Do not provide a predicate such
   * as {@code Predicates.instanceOf(ArrayList.class)}, which is inconsistent
   * with equals.
   *
   * @since 14.0
   */
  public static <K, V> ListMultimap<K, V> filterKeys(
      ListMultimap<K, V> unfiltered, final Predicate<? super K> keyPredicate) {
    if (unfiltered instanceof FilteredKeyListMultimap) {
      FilteredKeyListMultimap<K, V> prev = (FilteredKeyListMultimap<K, V>) unfiltered;
      return new FilteredKeyListMultimap<>(
          prev.unfiltered(), Predicates.<K>and(prev.keyPredicate, keyPredicate));
    } else {
      return new FilteredKeyListMultimap<>(unfiltered, keyPredicate);
    }
  }

  /**
   * Returns a multimap containing the mappings in {@code unfiltered} whose values
   * satisfy a predicate. The returned multimap is a live view of
   * {@code unfiltered}; changes to one affect the other.
   *
   * <p>The resulting multimap's views have iterators that don't support
   * {@code remove()}, but all other methods are supported by the multimap and
   * its views. When adding a value that doesn't satisfy the predicate, the
   * multimap's {@code put()}, {@code putAll()}, and {@code replaceValues()}
   * methods throw an {@link IllegalArgumentException}.
   *
   * <p>When methods such as {@code removeAll()} and {@code clear()} are called on
   * the filtered multimap or its views, only mappings whose value satisfy the
   * filter will be removed from the underlying multimap.
   *
   * <p>The returned multimap isn't threadsafe or serializable, even if
   * {@code unfiltered} is.
   *
   * <p>Many of the filtered multimap's methods, such as {@code size()}, iterate
   * across every key/value mapping in the underlying multimap and determine
   * which satisfy the filter. When a live view is <i>not</i> needed, it may be
   * faster to copy the filtered multimap and use the copy.
   *
   * <p><b>Warning:</b> {@code valuePredicate} must be <i>consistent with
   * equals</i>, as documented at {@link Predicate#apply}. Do not provide a
   * predicate such as {@code Predicates.instanceOf(ArrayList.class)}, which is
   * inconsistent with equals.
   *
   * @since 11.0
   */
  public static <K, V> Multimap<K, V> filterValues(
      Multimap<K, V> unfiltered, final Predicate<? super V> valuePredicate) {
    return filterEntries(unfiltered, Maps.<V>valuePredicateOnEntries(valuePredicate));
  }

  /**
   * Returns a multimap containing the mappings in {@code unfiltered} whose values
   * satisfy a predicate. The returned multimap is a live view of
   * {@code unfiltered}; changes to one affect the other.
   *
   * <p>The resulting multimap's views have iterators that don't support
   * {@code remove()}, but all other methods are supported by the multimap and
   * its views. When adding a value that doesn't satisfy the predicate, the
   * multimap's {@code put()}, {@code putAll()}, and {@code replaceValues()}
   * methods throw an {@link IllegalArgumentException}.
   *
   * <p>When methods such as {@code removeAll()} and {@code clear()} are called on
   * the filtered multimap or its views, only mappings whose value satisfy the
   * filter will be removed from the underlying multimap.
   *
   * <p>The returned multimap isn't threadsafe or serializable, even if
   * {@code unfiltered} is.
   *
   * <p>Many of the filtered multimap's methods, such as {@code size()}, iterate
   * across every key/value mapping in the underlying multimap and determine
   * which satisfy the filter. When a live view is <i>not</i> needed, it may be
   * faster to copy the filtered multimap and use the copy.
   *
   * <p><b>Warning:</b> {@code valuePredicate} must be <i>consistent with
   * equals</i>, as documented at {@link Predicate#apply}. Do not provide a
   * predicate such as {@code Predicates.instanceOf(ArrayList.class)}, which is
   * inconsistent with equals.
   *
   * @since 14.0
   */
  public static <K, V> SetMultimap<K, V> filterValues(
      SetMultimap<K, V> unfiltered, final Predicate<? super V> valuePredicate) {
    return filterEntries(unfiltered, Maps.<V>valuePredicateOnEntries(valuePredicate));
  }

  /**
   * Returns a multimap containing the mappings in {@code unfiltered} that
   * satisfy a predicate. The returned multimap is a live view of
   * {@code unfiltered}; changes to one affect the other.
   *
   * <p>The resulting multimap's views have iterators that don't support
   * {@code remove()}, but all other methods are supported by the multimap and
   * its views. When adding a key/value pair that doesn't satisfy the predicate,
   * multimap's {@code put()}, {@code putAll()}, and {@code replaceValues()}
   * methods throw an {@link IllegalArgumentException}.
   *
   * <p>When methods such as {@code removeAll()} and {@code clear()} are called on
   * the filtered multimap or its views, only mappings whose keys satisfy the
   * filter will be removed from the underlying multimap.
   *
   * <p>The returned multimap isn't threadsafe or serializable, even if
   * {@code unfiltered} is.
   *
   * <p>Many of the filtered multimap's methods, such as {@code size()}, iterate
   * across every key/value mapping in the underlying multimap and determine
   * which satisfy the filter. When a live view is <i>not</i> needed, it may be
   * faster to copy the filtered multimap and use the copy.
   *
   * <p><b>Warning:</b> {@code entryPredicate} must be <i>consistent with
   * equals</i>, as documented at {@link Predicate#apply}.
   *
   * @since 11.0
   */
  public static <K, V> Multimap<K, V> filterEntries(
      Multimap<K, V> unfiltered, Predicate<? super Entry<K, V>> entryPredicate) {
    checkNotNull(entryPredicate);
    if (unfiltered instanceof SetMultimap) {
      return filterEntries((SetMultimap<K, V>) unfiltered, entryPredicate);
    }
    return (unfiltered instanceof FilteredMultimap)
        ? filterFiltered((FilteredMultimap<K, V>) unfiltered, entryPredicate)
        : new FilteredEntryMultimap<K, V>(checkNotNull(unfiltered), entryPredicate);
  }

  /**
   * Returns a multimap containing the mappings in {@code unfiltered} that
   * satisfy a predicate. The returned multimap is a live view of
   * {@code unfiltered}; changes to one affect the other.
   *
   * <p>The resulting multimap's views have iterators that don't support
   * {@code remove()}, but all other methods are supported by the multimap and
   * its views. When adding a key/value pair that doesn't satisfy the predicate,
   * multimap's {@code put()}, {@code putAll()}, and {@code replaceValues()}
   * methods throw an {@link IllegalArgumentException}.
   *
   * <p>When methods such as {@code removeAll()} and {@code clear()} are called on
   * the filtered multimap or its views, only mappings whose keys satisfy the
   * filter will be removed from the underlying multimap.
   *
   * <p>The returned multimap isn't threadsafe or serializable, even if
   * {@code unfiltered} is.
   *
   * <p>Many of the filtered multimap's methods, such as {@code size()}, iterate
   * across every key/value mapping in the underlying multimap and determine
   * which satisfy the filter. When a live view is <i>not</i> needed, it may be
   * faster to copy the filtered multimap and use the copy.
   *
   * <p><b>Warning:</b> {@code entryPredicate} must be <i>consistent with
   * equals</i>, as documented at {@link Predicate#apply}.
   *
   * @since 14.0
   */
  public static <K, V> SetMultimap<K, V> filterEntries(
      SetMultimap<K, V> unfiltered, Predicate<? super Entry<K, V>> entryPredicate) {
    checkNotNull(entryPredicate);
    return (unfiltered instanceof FilteredSetMultimap)
        ? filterFiltered((FilteredSetMultimap<K, V>) unfiltered, entryPredicate)
        : new FilteredEntrySetMultimap<K, V>(checkNotNull(unfiltered), entryPredicate);
  }

  /**
   * Support removal operations when filtering a filtered multimap. Since a
   * filtered multimap has iterators that don't support remove, passing one to
   * the FilteredEntryMultimap constructor would lead to a multimap whose removal
   * operations would fail. This method combines the predicates to avoid that
   * problem.
   */
  private static <K, V> Multimap<K, V> filterFiltered(
      FilteredMultimap<K, V> multimap, Predicate<? super Entry<K, V>> entryPredicate) {
    Predicate<Entry<K, V>> predicate =
        Predicates.<Entry<K, V>>and(multimap.entryPredicate(), entryPredicate);
    return new FilteredEntryMultimap<>(multimap.unfiltered(), predicate);
  }

  /**
   * Support removal operations when filtering a filtered multimap. Since a filtered multimap has
   * iterators that don't support remove, passing one to the FilteredEntryMultimap constructor would
   * lead to a multimap whose removal operations would fail. This method combines the predicates to
   * avoid that problem.
   */
  private static <K, V> SetMultimap<K, V> filterFiltered(
      FilteredSetMultimap<K, V> multimap, Predicate<? super Entry<K, V>> entryPredicate) {
    Predicate<Entry<K, V>> predicate =
        Predicates.<Entry<K, V>>and(multimap.entryPredicate(), entryPredicate);
    return new FilteredEntrySetMultimap<>(multimap.unfiltered(), predicate);
  }

  static boolean equalsImpl(Multimap<?, ?> multimap, @Nullable Object object) {
    if (object == multimap) {
      return true;
    }
    if (object instanceof Multimap) {
      Multimap<?, ?> that = (Multimap<?, ?>) object;
      return multimap.asMap().equals(that.asMap());
    }
    return false;
  }

  // TODO(jlevy): Create methods that filter a SortedSetMultimap.
}