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/*
 * LensKit, an open source recommender systems toolkit.
 * Copyright 2010-2013 Regents of the University of Minnesota and contributors
 * Work on LensKit has been funded by the National Science Foundation under
 * grants IIS 05-34939, 08-08692, 08-12148, and 10-17697.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as
 * published by the Free Software Foundation; either version 2.1 of the
 * License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
 * details.
 *
 * You should have received a copy of the GNU General Public License along with
 * this program; if not, write to the Free Software Foundation, Inc., 51
 * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 */
package org.grouplens.lenskit.knn.user;

import it.unimi.dsi.fastutil.longs.Long2ObjectMap;
import it.unimi.dsi.fastutil.longs.LongSet;
import org.grouplens.grapht.annotation.DefaultImplementation;
import org.grouplens.lenskit.data.event.Event;
import org.grouplens.lenskit.data.history.UserHistory;

import javax.annotation.Nonnull;
import java.util.Collection;

/**
 * Interface for neighborhood-finding strategies. These strategies are used by
 * {@link UserUserItemScorer} to find neighbors for generating scores.
 *
 * @author GroupLens Research
 */
@DefaultImplementation(SimpleNeighborhoodFinder.class)
public interface NeighborhoodFinder {
    /**
     * Find neighboring users for particular items. {@var user} and the
     * returned rating vectors are unnormalized.  Any normalization
     * used by the neighborhood finder is only for comparing neighbors.
     *
     * @param user  The user's event history.
     * @param items The items we're trying to recommend, or {@code null} to get
     *              get neighborhoods for all possible items.
     * @return A map from item IDs to user neighborhoods for all items for which
     *         we can find neighboring users.
     */
    Long2ObjectMap> findNeighbors(
            @Nonnull UserHistory user, @Nonnull LongSet items);
}