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/**
* 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;
/**
*
* Implementations of this interface define a notion of similarity between two items. Implementations should
* return values in the range -1.0 to 1.0, with 1.0 representing perfect similarity.
*
*
* @see UserSimilarity
*/
public interface ItemSimilarity extends Refreshable {
/**
*
* Returns the degree of similarity, of two items, based on the preferences that users have expressed for
* the items.
*
*
* @param itemID1 first item ID
* @param itemID2 second item ID
* @return similarity between the items, in [-1,1] or {@link Double#NaN} similarity is unknown
* @throws org.apache.mahout.cf.taste.common.NoSuchItemException
* if either item is known to be non-existent in the data
* @throws TasteException if an error occurs while accessing the data
*/
double itemSimilarity(long itemID1, long itemID2) throws TasteException;
/**
* A bulk-get version of {@link #itemSimilarity(long, long)}.
*
* @param itemID1 first item ID
* @param itemID2s second item IDs to compute similarity with
* @return similarity between itemID1 and other items
* @throws org.apache.mahout.cf.taste.common.NoSuchItemException
* if any item is known to be non-existent in the data
* @throws TasteException if an error occurs while accessing the data
*/
double[] itemSimilarities(long itemID1, long[] itemID2s) throws TasteException;
/**
* @return all IDs of similar items, in no particular order
*/
long[] allSimilarItemIDs(long itemID) throws TasteException;
}
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