/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.mahout.cf.taste.impl.recommender;

import org.apache.mahout.cf.taste.common.Refreshable;
import org.apache.mahout.cf.taste.common.TasteException;
import org.apache.mahout.cf.taste.impl.common.FastIDSet;

/**
 * <p>
 * Returns the "similarity" between two clusters of users, according to some definition of similarity.
 * Subclassses define different notions of similarity.
 * </p>
 * 
 * @see TreeClusteringRecommender
 */
public interface ClusterSimilarity extends Refreshable {
  
  /**
   * @param cluster1
   *          first cluster of user IDs
   * @param cluster2
   *          second cluster of user IDs
   * @return "distance" between clusters; a bigger value means less similarity
   * @throws TasteException
   *           if an error occurs while computing similarity, such as errors accessing an underlying
   *           {@link org.apache.mahout.cf.taste.model.DataModel}
   * @throws IllegalArgumentException
   *           if either argument is null or empty
   */
  double getSimilarity(FastIDSet cluster1, FastIDSet cluster2) throws TasteException;
  
}
