org.codelibs.elasticsearch.taste.similarity.ItemSimilarity Maven / Gradle / Ivy

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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.codelibs.elasticsearch.taste.similarity;

import org.codelibs.elasticsearch.taste.common.Refreshable;

/**
 * 
 * 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.codelibs.elasticsearch.taste.exception.NoSuchItemException
     *  if either item is known to be non-existent in the data
     */
    double itemSimilarity(long itemID1, long itemID2);

    /**
     * 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.codelibs.elasticsearch.taste.exception.NoSuchItemException
     *  if any item is known to be non-existent in the data
     */
    double[] itemSimilarities(long itemID1, long[] itemID2s);

    /**
     * @return all IDs of similar items, in no particular order
     */
    long[] allSimilarItemIDs(long itemID);
}