code: /*
   * #%L
   * Simmetrics Core
   * %%
   * Copyright (C) 2014 - 2016 Simmetrics 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.
   * #L%
   */
  package org.simmetrics;
  
  import com.google.common.collect.Multiset;
  import com.google.common.collect.Multisets;
  
  /**
   * Measures the unnormalized dissimilarity between two arbitrary multisets
   * containing elements of the same type. The measurement results in a value
   * between 0 and 1 inclusive. A value of zero indicates that the lists are
   * dissimilar, a value of 1 indicates they are similar.
   * <p>
   * The elements in the multisets have to implement {@link Object#hashCode()} and
   * {@link Object#equals(Object)}.
   * <p>
   * The elements in the multisets may not be null if metric does not support null
   * values.
   * <p>
   * The distance measure should be consistent with equals such that
   * {@code a.equals(b) => distance(a,b) == 0.0}.
   * <p>
   * The distance measure should be reflexive such that
   * {@code distance(a,a) == 0.0}.
   * <p>
   * The distance measure should be symmetric such that
   * {@code distance(a,b) == distance(b,a)}.
   * <p>
   * Implementations may not modify the contents of the multisets. Multisets
   * should be treated as if wrapped by
   * {@link Multisets#unmodifiableMultiset(Multiset)}.
   * 
   * @see <a href="https://en.wikipedia.org/wiki/Metric_(mathematics)">Wikipedia -
   *      Metric</a>
   * 
   * @param <E>
   *            the type of elements contained in the lists
   */
  public interface MultisetDistance<E> extends Distance<Multiset<E>> {
  
  	/**
  	 * Measures the distance between multisets a and b. The measurement results
  	 * in a non-negative value. A value of {@code 0.0} indicates that {@code a}
  	 * and {@code b} are similar.
  	 * <p>
  	 * Results are undefined if {@code a} and {@code b} are based on different
  	 * equivalence relations (as {@code HashMultiset} and {@code TreeMultiset}
  	 * are).
  	 * 
  	 * @param a
  	 *            list a to compare
  	 * @param b
  	 *            list b to compare
  	 * @return a non-negative value
  	 * @throws NullPointerException
  	 *             when either a or b is null
  	 */
  	@Override
  	public float distance(Multiset<E> a, Multiset<E> b);
  
  }
  