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/*
 * LensKit, an open source recommender systems toolkit.
 * Copyright 2010-2014 LensKit Contributors.  See CONTRIBUTORS.md.
 * 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;

import org.grouplens.lenskit.scored.ScoredId;
import org.lenskit.api.ItemRecommender;

import javax.annotation.Nullable;
import java.util.List;
import java.util.Set;


/**
 * The interface for recommendation based on the items only. The difference from {@link
 * ItemRecommender} is that the input is only the item or list of items instead of user specific
 * information. This interface can be used to provide a Find Similar Items / People Also Liked
 * feature.
 *
 * @see ItemRecommender
 * @author GroupLens Research
 * @compat Public
 * @since 0.10
 */
public interface GlobalItemRecommender {
    /**
     * Recommend all possible items for a basket of items using the default exclude set.
     *
     * @param items The items value.
     * @return The recommended items.
     * @see #globalRecommend(Set, int, Set, Set)
     */
    List globalRecommend(Set items);

    /**
     * Recommend up to n items for a basket of items using the default exclude set.
     *
     * @param items The items value.
     * @param n     The number of recommendations to return.
     * @return The recommended items.
     * @see #globalRecommend(Set, int, Set, Set)
     */
    List globalRecommend(Set items, int n);

    /**
     * Recommend all possible items for a basket of items from a set of candidates using the default
     * exclude set.
     *
     * @param items      The items value.
     * @param candidates The candidate set (can be null to represent the universe).
     * @return The recommended items.
     * @see #globalRecommend(Set, int, Set, Set)
     */
    List globalRecommend(Set items, @Nullable Set candidates);

    /**
     * Produce a set of recommendations for the item. This is the most general recommendation
     * method, allowing the recommendations to be constrained by both a candidate set and an exclude
     * set. The exclude set is applied to the candidate set, so the final effective candidate set is
     * candidates minus exclude.
     *
     * @param items      The items value
     * @param n          The number of ratings to return. If negative, no specific size is requested.
     * @param candidates A set of candidate items which can be recommended. If {@code null}, all
     *                   items are considered candidates.
     * @param exclude    A set of items to be excluded. If {@code null}, a default exclude set is
     *                   used.
     * @return A list of recommended items. If the recommender cannot assign meaningful scores, the
     *         scores will be {@link Double#NaN}.
     */
    List globalRecommend(Set items, int n, @Nullable Set candidates,
                                   @Nullable Set exclude);
}