/**
* 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.similarity;
import org.apache.mahout.cf.taste.common.Refreshable;
import org.apache.mahout.cf.taste.common.TasteException;
/**
* <p>
* Implementations of this interface define a notion of similarity between two users. Implementations should
* return values in the range -1.0 to 1.0, with 1.0 representing perfect similarity.
* </p>
*
* @see ItemSimilarity
*/
public interface UserSimilarity extends Refreshable {
/**
* <p>
* Returns the degree of similarity, of two users, based on the their preferences.
* </p>
*
* @param userID1 first user ID
* @param userID2 second user ID
* @return similarity between the users, in [-1,1] or {@link Double#NaN} similarity is unknown
* @throws org.apache.mahout.cf.taste.common.NoSuchUserException
* if either user is known to be non-existent in the data
* @throws TasteException if an error occurs while accessing the data
*/
double userSimilarity(long userID1, long userID2) throws TasteException;
// Should we implement userSimilarities() like ItemSimilarity.itemSimilarities()?
/**
* <p>
* Attaches a {@link PreferenceInferrer} to the implementation.
* </p>
*
* @param inferrer {@link PreferenceInferrer}
*/
void setPreferenceInferrer(PreferenceInferrer inferrer);
}