Multiset

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  MOVED TO : https://github.com/claudemartin/multiset

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package ch.claude_martin;

import static java.util.Arrays.asList;
import static java.util.Objects.requireNonNull;

import java.io.Serializable;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.util.*;
import java.util.AbstractMap.SimpleImmutableEntry;
import java.util.Map.Entry;
import java.util.function.*;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;

import javax.annotation.concurrent.NotThreadSafe;

/**
 * Multiset in Java. It doesn't maintain sorting or insertion order. But it's not a regular set,
 * because you can add an element more than once. If an element is already present that equals the
 * element to be inserted then it's multiplicity (count) is incremented by one. <code>null</code> is
 * allowed as an element.
 *
 * <p>
 * An element with multiplicity zero is completely removed from the data structure and will not
 * occur as a tuple of {@code (element, 0)}. This needs to be considered when using methods such as
 * {@link #setMultiplicities(ToIntBiFunction)}, {@link #forEach(ObjIntConsumer)}, {@link #entries()}
 * , and {@link #merge(Multiset, IntBinaryOperator)}.
 *
 * <p>
 * <a href="https://humanoidreadable.wordpress.com/2015/02/20/multiset-in-java/">blog entry</a>
 * <p>
 * Copyright: You can use this for whatever you like! No restrictions.
 *
 * @author Claude Martin
 *
 * @param <T>
 *          The type of the elements
 */
@NotThreadSafe
public final class Multiset<T> extends AbstractCollection<T> implements Serializable {
  private static final long serialVersionUID = -7083567870279959503L;

  /** The backing map. */
  final Map<T, Integer> map;
  /** View as map. Lazy. This is the same as {@link #map} if and only if this is unmodifiable. */
  private transient Map<T, Integer> view = null;

  /**
   * Total size of this multiset.
   *
   * @implNote Modification must always be performend on the {@link #map} first. This will fail if
   *           this multiset {@link #isUnmodifiable() is unmodifiable}.
   */
  int size = 0;

  public Multiset() {
    this.map = new HashMap<>();
  }

  /** Creates an unmodifiable Multiset. */
  private Multiset(final Multiset<T> multiset, final boolean unmodifiable) {
    assert unmodifiable;
    this.map = this.view = Collections.unmodifiableMap(multiset.map);
    this.size = multiset.size;
    this.checkSize();
    return;
  }

  /** Creates an empty Multiset. */
  private Multiset(final boolean empty) {
    this.map = this.view = Collections.emptyMap();
    this.size = 0;
  }

  /** Creates a new Multiset and adds all elements from the given collection. */
  public Multiset(final Collection<? extends T> coll) {
    requireNonNull(coll, "coll");
    this.map = new HashMap<>(coll.size());
    for (final T t : coll)
      this.map.merge(t, 1, Integer::sum);
    this.size = coll.size();
    this.checkSize();
  }

  /** Creates a new Multiset and adds all elements from the given Multiset. */
  public Multiset(final Multiset<? extends T> multiset) {
    requireNonNull(multiset, "multiset");
    multiset.checkSize();
    this.map = new HashMap<>(multiset.map);
    this.size = multiset.size;
    this.checkSize();
  }

  /**
   * Creates a new {@link Multiset} containing all given elements.
   *
   * @param elements
   *          The elements
   * @return <code>new Multiset<>(asList(elements));</code>
   */
  @SafeVarargs
  public static <T> Multiset<T> of(final T... elements) {
    requireNonNull(elements, "elements");
    return new Multiset<>(asList(elements));
  }

  /**
   * Creates a new {@link Multiset} containing all given elements, with the multiplicity defined by
   * the given function.
   *
   * @param set
   *          The elements
   * @param m
   *          The multiplicity
   * @return new {@link Multiset} greated from the given data.
   */
  public static <T> Multiset<T> of(final Set<? extends T> set, final ToIntFunction<? super T> m) {
    requireNonNull(set, "set");
    requireNonNull(m, "m");
    final Multiset<T> result = new Multiset<>();
    for (final T t : set)
      result.setMultiplicity(t, m.applyAsInt(t));
    return result;
  }

  /**
   * Adds the object and returns true.
   *
   * @see #insert(Object)
   */
  @Override
  public boolean add(final T t) {
    this.insert(t);
    return true;
  }

  /**
   * Sets the multiplicity of a certain key. The key is removed if m is 0 or added if it was 0.
   * Therefore this may lead to a modification of the underlying data structure. The
   * {@link #iterator() Iterator} supports @link {@link Iterator#remove() removal} of elements
   * during iteration.
   *
   * @param element
   *          the element
   * @param m
   *          new multiplicity
   * @returns the old multiplicity.
   */
  public int setMultiplicity(final T element, final int m) {
    if (m < 0)
      throw new IllegalArgumentException("m < 0");
    final int i = this.map.getOrDefault(element, 0);
    if (m == i)
      return i;
    if (m == 0)
      this.map.remove(element);
    else
      this.map.put(element, m);
    this.size = this.size - i + m;
    this.checkSize();
    return i;
  }

  /**
   * Sets the multiplicity of a certain key, only if currently set to the specified value.
   *
   * @param element
   *          the element
   * @param oldMultiplicity
   *          old multiplicity
   * @param newMultiplicity
   *          new multiplicity
   *
   * @return {@code true} if the value was replaced
   *
   * @see Map#replace(Object, Object, Object)
   */
  public boolean setMultiplicity(final T element, final int oldMultiplicity,
      final int newMultiplicity) {
    requireNonNull(element, "obj");
    if (oldMultiplicity < 0 || newMultiplicity < 0)
      throw new IllegalArgumentException("negative multiplicity");
    if (this.getMultiplicity(element) == oldMultiplicity) {
      this.setMultiplicity(element, newMultiplicity);
      return true;
    }
    return false;
  }

  /**
   * Adds the object and returns the new multiplicity.
   *
   * @see #add(Object)
   */
  public int insert(final T t) {
    final Integer result = this.map.merge(t, 1, Integer::sum);
    this.size++;
    this.checkSize();
    return result;
  }

  /**
   * Removes a single instance of the specified element from this multiset. This decrements the
   * multiplicity by one. Does nothing if it wasn't in the set. To remove all equal objects you must
   * call <code>{@link #setMultiplicity(Object, int) setMultiplicity(t, 0)}</code>.
   */
  @Override
  @SuppressWarnings("unchecked")
  public boolean remove(final Object t) {
    final int value = this.map.getOrDefault(t, 0);
    switch (value) {
    case 0:
      return false;
    case 1:
      this.map.remove(t);
      break;
    default:
      assert value > 1 : "invalid value";
      this.map.put((T) t, value - 1);
      break;
    }
    this.size--;
    this.checkSize();
    return true;
  }

  /**
   * Reset the multiplicity for each element in this {@link Multiset}.
   *
   * <p>
   * Note that elements with a multiplicity of zero will not be processed, as they are not contained
   * in the multiset.
   *
   * @param m
   *          function for new multiplicities
   * @return true, if any multiplicity of any element was altered.
   */
  public boolean setMultiplicities(final ToIntBiFunction<? super T, Integer> m) {
    requireNonNull(m, "m");
    if (this.isEmpty())
      return false;
    boolean modified = false;
    final Iterator<Entry<T, Integer>> itr = this.map.entrySet().iterator();
    while (itr.hasNext()) {
      final Entry<T, Integer> next = itr.next();
      final T t = next.getKey();
      final int oldm = next.getValue();
      final int newM = m.applyAsInt(t, oldm);
      if (oldm != newM) {
        if (newM == 0)
          itr.remove();
        else
          next.setValue(newM);
        this.size += -oldm + newM;
        modified = true;
      }
    }
    this.checkSize();
    return modified;
  }

  @SuppressWarnings({ "unchecked" })
  @Override
  public boolean removeAll(final Collection<?> c) {
    requireNonNull(c, "c");
    if (c instanceof Multiset)
      return this.removeAll((Multiset<? extends T>) c);
    return super.removeAll(c);
  }

  /**
   * @see #removeAll(Collection)
   */
  public boolean removeAll(final Multiset<?> set) {
    requireNonNull(set, "set");
    boolean result = false;
    if (set.isEmpty())
      return false;
    final Iterator<Entry<T, Integer>> itr = this.map.entrySet().iterator();
    while (itr.hasNext()) {
      final Entry<T, Integer> next = itr.next();
      if (set.map.containsKey(next.getKey())) {
        itr.remove();
        this.size -= next.getValue();
        result = true;
      }
    }
    this.checkSize();
    return result;
  }

  @Override
  public boolean retainAll(final Collection<?> c) {
    requireNonNull(c, "c");
    if (c instanceof Multiset)
      return this.retainAll((Multiset<?>) c);
    return this.asSet().retainAll(c);
  }

  /**
   * @see #retainAll(Collection)
   */
  public boolean retainAll(final Multiset<?> set) {
    requireNonNull(set, "set");
    if (set == this)
      return false;
    return this.asSet().retainAll(set.asSet());
  }

  /**
   * Get the multiplicity or the given object. If the object is not in this multiset then the
   * multiplicity is 0.
   *
   * @return the multiplicity.
   */
  public int getMultiplicity(final Object t) {
    return this.map.getOrDefault(t, 0);
  }

  Iterator<T> iterator(final Iterator<Entry<T, Integer>> entries) {
    return new Iterator<T>() {
      int c = 0;
      T t = null;

      @Override
      public T next() {
        if (!this.hasNext())
          throw new NoSuchElementException();
        if (this.c == 0) {
          final Entry<T, Integer> e = entries.next();
          this.t = e.getKey();
          this.c = e.getValue();
        }
        this.c--;
        return this.t;
      }

      @Override
      public boolean hasNext() {
        return this.c > 0 || entries.hasNext();
      }

      @Override
      public void remove() {
        final int multiplicity = Multiset.this.getMultiplicity(this.t);
        if (multiplicity == 1) {
          entries.remove();
          Multiset.this.size--;
        } else
          Multiset.this.setMultiplicity(this.t, multiplicity - 1);
      }
    };
  }

  /**
   * Iterator of all elements. Each element is iterated as many times as it is in this multiset. The
   * {@link Iterator} supports the {@link Iterator#remove() remove} method.
   */
  @Override
  public Iterator<T> iterator() {
    this.checkSize();
    return this.iterator(this.map.entrySet().iterator());
  }

  @Override
  public Spliterator<T> spliterator() {
    return Spliterators.spliterator(this, 0);
  }

  /** The class invariant. */
  void checkSize() {
    assert this.map.values().stream().mapToInt(i -> i).sum() == this.size : this.getClass()
        .getSimpleName() + ": wrong size";
  }

  /**
   * {@inheritDoc}
   * <p>
   * This returns the total number of elements, including repeated memberships (cardinality). To get
   * the amount of discinct elements use {@link #asMap()}.{@link Map#size() size()}.
   * */
  @Override
  public int size() {
    this.checkSize();
    return this.size;
  }

  /**
   * Returns a copy of the underlying set of elements. Changes to this set do not alter the
   * Multiset.
   *
   * <p>
   * If using the formal definition <code>(A, m)</code> of a multiset, the returned set is
   * <code>A</code> and <code>multiset::getMultiplicity</code> is <code>m</code>.
   *
   * @see #asSet()
   * @see #entries()
   */
  public Set<T> toSet() {
    return new HashSet<>(this.map.keySet());
  }

  /**
   * Conventiance mehtod to create a string representation of this multiset.
   * <p>
   * Example: {@code ms.toString((t, i) -> t + " x " + i, ", ", "[ ", " ]")}
   *
   * @param f
   *          Mapping function for each entry (element and multiplicity)
   * @param delimiter
   *          the delimiter to be used between each entry
   * @param prefix
   *          the sequence of characters to be used at the beginning of the joined result
   * @param suffix
   *          the sequence of characters to be used at the end of the joined result
   * @return string representation of this multiset
   */
  public String toString(final BiFunction<T, Integer, String> f, final CharSequence delimiter,
      final CharSequence prefix, final CharSequence suffix) {
    requireNonNull(f, "f");
    requireNonNull(delimiter, "delimiter");
    requireNonNull(prefix, "prefix");
    requireNonNull(suffix, "suffix");

    return this.map.entrySet().stream().map(e -> f.apply(e.getKey(), e.getValue()))
        .collect(Collectors.joining(delimiter, prefix, suffix));
  }

  /**
   * As a new, sorted List.
   *
   * <p>
   * To sort by multiplicity, descending:<br>
   * <code>multiset.toList((a, b) -> b.getValue() - a.getValue())</code>
   */
  public List<T> toList(final Comparator<Map.Entry<T, Integer>> comparator) {
    if (null == comparator)
      return this.stream().collect(Collectors.toList());
    return this.entries().sorted(comparator)
        .flatMap(e -> IntStream.range(0, e.getValue()).<T> mapToObj(x -> e.getKey()))
        .collect(Collectors.toList());
  }

  /**
   * Streams each element and it's multiplicity. The returned entries are immutable. Mutable entries
   * can be accessed using: {@code this.asMap().entrySet()}
   */
  @SuppressWarnings("unchecked")
  public Stream<Map.Entry<T, Integer>> entries() {
    final Stream<Entry<T, Integer>> stream = this.map.entrySet().stream();
    if (this.isUnmodifiable())
      return stream;
    return stream.map(SimpleImmutableEntry::new);
  }

  @Override
  public boolean addAll(final Collection<? extends T> c) {
    requireNonNull(c, "c");
    if (c instanceof Multiset)
      return this.addAll((Multiset<? extends T>) c);
    return super.addAll(c);
  }

  /** @see #addAll(Collection) */
  public boolean addAll(final Multiset<? extends T> ms) {
    for (final Entry<? extends T, Integer> e : ms.map.entrySet()) {
      final Integer val = e.getValue();
      this.map.merge(e.getKey(), val, Integer::sum);
      this.size += val;
    }
    this.checkSize();
    return ms.size() != 0;
  }

  /**
   * Two {@link Multiset}s are equal if both contain the same elements with the same multiplicities.
   */
  @Override
  public boolean equals(final Object obj) {
    if (obj instanceof Multiset) {
      final Multiset<?> other = (Multiset<?>) obj;
      return this.size == other.size && this.map.equals(other.map);
    }
    return false;
  }

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

  @Override
  public void clear() {
    if (this.size == 0)
      return;
    this.map.clear();
    this.size = 0;
    this.checkSize();
  }

  /**
   * Creates a clone. The cloned multiset is modifiable.
   */
  @Override
  protected Object clone() {
    return new Multiset<>(this);
  }

  @Override
  public boolean containsAll(final Collection<?> c) {
    requireNonNull(c, "c");
    if (c instanceof Multiset)
      this.map.keySet().containsAll(((Multiset<?>) c).map.keySet());
    return this.map.keySet().containsAll(c);
  }

  @Override
  public boolean isEmpty() {
    return this.size == 0;
  }

  /** Process each object only once, with the multiplicity given as a second parameter. */
  public void forEach(final ObjIntConsumer<T> action) {
    this.map.forEach(action::accept);
  }

  @Override
  public boolean contains(final Object obj) {
    return this.map.containsKey(obj);
  }

  /**
   * The intersection of two multisets. This is equal to
   * <code>this.{@link #merge(Multiset, IntBinaryOperator) merge}(set, {@link Math#min(int, int) Math::min})</code>
   * . However, the type of the elements in the provided multiset does not matter.
   */
  @SuppressWarnings("unchecked")
  public Multiset<T> intersect(final Multiset<?> set) {
    requireNonNull(set, "set");
    return this.merge((Multiset<T>) set, Math::min);
  }

  /**
   * The union of two multisets. This is equal to
   * <code>this.{@link #merge(Multiset, IntBinaryOperator) merge}(set, {@link Integer#sum(int, int) Integer::sum})</code>
   *
   * <p>
   * Note: A different definition of <i>union</i> would use {@link Math#max(int, int)} with
   * {@link #merge(Multiset, IntBinaryOperator)}.
   *
   * @see #addAll(Collection)
   */
  public Multiset<T> union(final Multiset<? extends T> set) {
    requireNonNull(set, "set");
    return this.merge(set, Integer::sum);
  }

  public <S> Multiset<S> union(final Multiset<? extends S> set, final Class<S> supertype) {
    requireNonNull(set, "set");
    requireNonNull(supertype, "supertype");
    return this.merge(set, Integer::sum, supertype);
  }

  /**
   * The relative complement of this and a given multiset.
   *
   * @see #removeAll(Collection)
   */
  public Multiset<T> minus(final Multiset<? extends T> set) {
    requireNonNull(set, "set");
    final Multiset<T> result = new Multiset<>(this);
    if (set.isEmpty())
      return result;
    result.removeAll(set);
    return result;
  }

  /**
   * Create new multiset from this and another multiset. The given operation will calculate the new
   * multiplicity for all elements.
   *
   * <p>
   * This pseudo-code illustrates how the operation is used to create a new multiset. In this case
   * the result is equal to the result of {@link #union(Multiset)}. <code><pre>
   * mset1 = [ a, a, a, b ];
   * mset2 = [ b, c ];
   * union = m1.merge(m2, Integer::sum);
   * // m3 = [ a, a, a, b, b, c ]
   * </pre></code>
   */
  public Multiset<T> merge(final Multiset<? extends T> set, final IntBinaryOperator operation) {
    requireNonNull(set, "set");
    requireNonNull(operation, "operation");
    final Multiset<T> result = new Multiset<>(this);
    if (set.isEmpty() && this.isEmpty())
      return result;
    Stream.concat(this.map.keySet().stream(), set.map.keySet().stream()).distinct().forEach(t -> {
      result.setMultiplicity(t, //
          operation.applyAsInt(this.getMultiplicity(t), set.getMultiplicity(t)));
    });
    result.checkSize();
    return result;
  }

  /**
   * Create new multiset from this and another multiset. The given operation will calculate the new
   * multiplicity for all elements.
   *
   * @implNote Only with assertions enabled there will be a check that all elements are assignable
   *           to the given supertype.
   * @param set
   *          The other multiset
   * @param operation
   *          the operation for the multiplicities
   * @param supertype
   *          a supertype of both multisets
   */
  @SuppressWarnings("unchecked")
  public <S> Multiset<S> merge(final Multiset<? extends S> set, final IntBinaryOperator operation,
      final Class<S> supertype) {
    requireNonNull(set, "set");
    requireNonNull(operation, "operation");
    requireNonNull(supertype, "supertype");
    assert this.asSet().stream().allMatch(e -> supertype.isAssignableFrom(e.getClass()));
    assert set.asSet().stream().allMatch(e -> supertype.isAssignableFrom(e.getClass()));
    return ((Multiset<S>) this).merge(set, operation);
  }

  private void readObject(final java.io.ObjectInputStream stream) throws Exception {
    requireNonNull(stream, "stream").defaultReadObject();
    this.checkSize();
  }

  /**
   * Returnes an unmodifiable view of the specified {@link Multiset}.
   *
   * @param multiset
   *          the multiset for which an unmodifiable view is to be returned.
   * @return an unmodifiable view of the specified map.
   */
  public static <T> Multiset<T> unmodifiableMultiset(final Multiset<T> multiset) {
    requireNonNull(multiset, "multiset");
    if (multiset.isUnmodifiable())
      return multiset;
    if (multiset.isEmpty())
      return emptyMultiset();
    return new Multiset<>(multiset, true);
  }

  /**
   * Checks whether this is backed by an unmodifiable map. An unmodifiable {@link Multiset} always
   * has the same reference set for {@link #map} and {@link #view}.
   *
   * @see #unmodifiableMultiset(Multiset)
   */
  public boolean isUnmodifiable() {
    return this.map == this.view;
  }

  /**
   * A map-view of this {@link Multiset}. Changes are reflected on the {@link Multiset}. However,
   * when using streams or iterators it is not allowed to set the multiplicity to 0. Instead the
   * entry needs to be {@link Iterator#remove() removed}.
   * <p>
   * Methods such as {@link Map#put}, {@link Map#clear}, and {@link Map#contains} delegated to the
   * backing map of the {@link Multiset}.
   *
   * @see #asSet()
   * @see #entries()
   * */
  public Map<T, Integer> asMap() {
    if (this.view != null)
      return this.view;
    return this.view = new AbstractMap<T, Integer>() {
      private Set<java.util.Map.Entry<T, Integer>> entrySet;

      @Override
      public Integer put(final T key, final Integer value) {
        return Multiset.this.setMultiplicity(key, value);
      }

      @Override
      public Set<java.util.Map.Entry<T, Integer>> entrySet() {
        if (this.entrySet != null)
          return this.entrySet;
        return this.entrySet = new AbstractSet<Map.Entry<T, Integer>>() {

          @Override
          public Iterator<java.util.Map.Entry<T, Integer>> iterator() {
            final Iterator<java.util.Map.Entry<T, Integer>> itr = Multiset.this.map.entrySet()
                .iterator();
            return new Iterator<Map.Entry<T, Integer>>() {
              private Entry<T, Integer> last = null;

              @Override
              public boolean hasNext() {
                return itr.hasNext();
              }

              @Override
              public java.util.Map.Entry<T, Integer> next() {
                return this.last = new Entry<T, Integer>() {
                  final Entry<T, Integer> next = itr.next();
                  final T t = this.next.getKey();
                  int v = this.next.getValue();

                  @Override
                  public T getKey() {
                    return this.t;
                  }

                  @Override
                  public Integer getValue() {
                    return this.v;
                  }

                  @Override
                  public Integer setValue(final Integer value) {
                    requireNonNull(value, "value");
                    final int val = value.intValue();
                    if (val < 0)
                      throw new IllegalArgumentException("value=" + val);
                    if (val == 0)
                      throw new IllegalArgumentException(
                          "Can't set multiplicity to 0. Use remove() instead!");
                    final int old = this.v;
                    Multiset.this.setMultiplicity(this.t, val);
                    this.v = val;
                    return old;
                  }

                  @Override
                  public final int hashCode() {
                    return Objects.hashCode(this.t) ^ this.v;
                  }

                  @Override
                  public final boolean equals(final Object o) {
                    if (o == this)
                      return true;
                    if (o instanceof Map.Entry) {
                      final Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
                      if (Objects.equals(this.t, e.getKey())
                          && Objects.equals(this.v, e.getValue()))
                        return true;
                    }
                    return false;
                  }

                  @Override
                  public String toString() {
                    return this.t + "=" + this.v;
                  }
                };
              }

              @Override
              public void remove() {
                if (this.last == null)
                  throw new NoSuchElementException();
                itr.remove();
                Multiset.this.size -= this.last.getValue();
                Multiset.this.checkSize();
              }
            };

          }

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

          @Override
          public boolean isEmpty() {
            return Multiset.this.map.isEmpty();
          }

          @Override
          public boolean contains(final Object o) {
            return Multiset.this.map.keySet().contains(o);
          }

          @Override
          public boolean add(final java.util.Map.Entry<T, Integer> e) {
            // NOT SUPPORTED!!!
            return Multiset.this.map.entrySet().add(e);
          }

          @Override
          public boolean remove(final Object o) {
            return Multiset.this.map.entrySet().remove(o);
          }

          @Override
          public boolean containsAll(final Collection<?> c) {
            return Multiset.this.map.entrySet().containsAll(c);
          }

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

        };
      }
    };

  }

  /**
   * Returns a set of all elements, without multiplicity. The set is backed by the multiset, so
   * changes to the multiset are reflected in the set, and vice-versa. Removing one element from the
   * set removes all equal elements from the multiset. It does not support the <code>add</code> or
   * <code>addAll</code>.
   *
   * <p>
   * If using the formal definition <code>(A, m)</code> of a multiset, the returned set is
   * <code>A</code> and <code>multiset::getMultiplicity</code> is <code>m</code>.
   *
   * @see #toSet()
   * @see #asMap()
   * @see #entries()
   * @return set-view of all elements
   */
  public Set<T> asSet() {
    return this.asMap().keySet();
  }

  /**
   * Creates a synchronized (thread-safe) collection and a synchronized map, both backed by the
   * specified multiset. The collection behaves just like the specified multiset, while the map
   * corresponds to {@link #asMap()}. Both data structures are modifiable and use the created
   * collection as the mutex for synchronization.
   *
   * @see Collections#synchronizedCollection(Collection)
   *
   * @param set
   *          the multiset
   * @param consumer
   *          accepts the collection and the map.
   * @throws RuntimeException
   *           if the security manager does not allow to create a SynchronizedMap by reflection.
   */
  @SuppressWarnings("unchecked")
  public static <T> void synchronizedMultiset(final Multiset<T> set,
      final BiConsumer<Collection<T>, Map<T, Integer>> consumer) throws RuntimeException {
    requireNonNull(set, "set");
    requireNonNull(consumer, "consumer");
    try {
      final Collection<T> collection = Collections.synchronizedCollection(set);
      final Class<? extends Map<T, Integer>> cls = (Class<? extends Map<T, Integer>>) Class
          .forName("java.util.Collections$SynchronizedMap");
      final Constructor<? extends Map<T, Integer>> ctor = //
          cls.getDeclaredConstructor(Map.class, Object.class);
      ctor.setAccessible(true);
      final Map<T, Integer> map = ctor.newInstance(set.asMap(), collection);
      consumer.accept(collection, map);
    } catch (InstantiationException | IllegalAccessException | IllegalArgumentException
        | InvocationTargetException | NoSuchMethodException | SecurityException
        | ClassNotFoundException ex) {
      throw new RuntimeException("Can't create synchronized Multiset.", ex);
    }
  }

  static Multiset<?> emptyMultiset;

  /** Returns an empty multiset (unmodifiable). */
  @SuppressWarnings("unchecked")
  public static <T> Multiset<T> emptyMultiset() {
    if (null == emptyMultiset)
      return (Multiset<T>) (emptyMultiset = new Multiset<T>(true));
    return (Multiset<T>) emptyMultiset;
  }
}