/**
 * 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.impl.recommender;

import org.apache.mahout.cf.taste.common.TasteException;
import org.apache.mahout.cf.taste.impl.common.FastIDSet;
import org.apache.mahout.cf.taste.model.DataModel;
import org.apache.mahout.cf.taste.neighborhood.UserNeighborhood;
import org.apache.mahout.cf.taste.similarity.UserSimilarity;

/**
 * A variant on {@link GenericUserBasedRecommender} which is appropriate for use when no notion of preference
 * value exists in the data.
 */
public final class GenericBooleanPrefUserBasedRecommender extends GenericUserBasedRecommender {
  
  public GenericBooleanPrefUserBasedRecommender(DataModel dataModel,
                                                UserNeighborhood neighborhood,
                                                UserSimilarity similarity) {
    super(dataModel, neighborhood, similarity);
  }
  
  /**
   * This computation is in a technical sense, wrong, since in the domain of "boolean preference users" where
   * all preference values are 1, this method should only ever return 1.0 or NaN. This isn't terribly useful
   * however since it means results can't be ranked by preference value (all are 1). So instead this returns a
   * sum of similarities to any other user in the neighborhood who has also rated the item.
   */
  @Override
  protected float doEstimatePreference(long theUserID, long[] theNeighborhood, long itemID) throws TasteException {
    if (theNeighborhood.length == 0) {
      return Float.NaN;
    }
    DataModel dataModel = getDataModel();
    UserSimilarity similarity = getSimilarity();
    float totalSimilarity = 0.0f;
    boolean foundAPref = false;
    for (long userID : theNeighborhood) {
      // See GenericItemBasedRecommender.doEstimatePreference() too
      if (userID != theUserID && dataModel.getPreferenceValue(userID, itemID) != null) {
        foundAPref = true;
        totalSimilarity += (float) similarity.userSimilarity(theUserID, userID);
      }
    }
    return foundAPref ? totalSimilarity : Float.NaN;
  }
  
  @Override
  protected FastIDSet getAllOtherItems(long[] theNeighborhood, long theUserID) throws TasteException {
    DataModel dataModel = getDataModel();
    FastIDSet possibleItemIDs = new FastIDSet();
    for (long userID : theNeighborhood) {
      possibleItemIDs.addAll(dataModel.getItemIDsFromUser(userID));
    }
    possibleItemIDs.removeAll(dataModel.getItemIDsFromUser(theUserID));
    return possibleItemIDs;
  }
  
  @Override
  public String toString() {
    return "GenericBooleanPrefUserBasedRecommender";
  }
  
}