/**
 * 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.neighborhood;

import org.apache.mahout.cf.taste.common.TasteException;
import org.apache.mahout.cf.taste.impl.common.LongPrimitiveIterator;
import org.apache.mahout.cf.taste.impl.common.SamplingLongPrimitiveIterator;
import org.apache.mahout.cf.taste.impl.recommender.TopItems;
import org.apache.mahout.cf.taste.model.DataModel;
import org.apache.mahout.cf.taste.similarity.UserSimilarity;

import com.google.common.base.Preconditions;

/**
 * <p>
 * Computes a neighborhood consisting of the nearest n users to a given user. "Nearest" is defined by the
 * given {@link UserSimilarity}.
 * </p>
 */
public final class NearestNUserNeighborhood extends AbstractUserNeighborhood {
  
  private final int n;
  private final double minSimilarity;
  
  /**
   * @param n neighborhood size; capped at the number of users in the data model
   * @throws IllegalArgumentException
   *           if {@code n < 1}, or userSimilarity or dataModel are {@code null}
   */
  public NearestNUserNeighborhood(int n, UserSimilarity userSimilarity, DataModel dataModel) throws TasteException {
    this(n, Double.NEGATIVE_INFINITY, userSimilarity, dataModel, 1.0);
  }
  
  /**
   * @param n neighborhood size; capped at the number of users in the data model
   * @param minSimilarity minimal similarity required for neighbors
   * @throws IllegalArgumentException
   *           if {@code n < 1}, or userSimilarity or dataModel are {@code null}
   */
  public NearestNUserNeighborhood(int n,
                                  double minSimilarity,
                                  UserSimilarity userSimilarity,
                                  DataModel dataModel) throws TasteException {
    this(n, minSimilarity, userSimilarity, dataModel, 1.0);
  }
  
  /**
   * @param n neighborhood size; capped at the number of users in the data model
   * @param minSimilarity minimal similarity required for neighbors
   * @param samplingRate percentage of users to consider when building neighborhood -- decrease to trade quality for
   *   performance
   * @throws IllegalArgumentException
   *           if {@code n < 1} or samplingRate is NaN or not in (0,1], or userSimilarity or dataModel are
   *           {@code null}
   */
  public NearestNUserNeighborhood(int n,
                                  double minSimilarity,
                                  UserSimilarity userSimilarity,
                                  DataModel dataModel,
                                  double samplingRate) throws TasteException {
    super(userSimilarity, dataModel, samplingRate);
    Preconditions.checkArgument(n >= 1, "n must be at least 1");
    int numUsers = dataModel.getNumUsers();
    this.n = n > numUsers ? numUsers : n;
    this.minSimilarity = minSimilarity;
  }
  
  @Override
  public long[] getUserNeighborhood(long userID) throws TasteException {
    
    DataModel dataModel = getDataModel();
    UserSimilarity userSimilarityImpl = getUserSimilarity();
    
    TopItems.Estimator<Long> estimator = new Estimator(userSimilarityImpl, userID, minSimilarity);
    
    LongPrimitiveIterator userIDs = SamplingLongPrimitiveIterator.maybeWrapIterator(dataModel.getUserIDs(),
      getSamplingRate());
    
    return TopItems.getTopUsers(n, userIDs, null, estimator);
  }
  
  @Override
  public String toString() {
    return "NearestNUserNeighborhood";
  }
  
  private static final class Estimator implements TopItems.Estimator<Long> {
    private final UserSimilarity userSimilarityImpl;
    private final long theUserID;
    private final double minSim;
    
    private Estimator(UserSimilarity userSimilarityImpl, long theUserID, double minSim) {
      this.userSimilarityImpl = userSimilarityImpl;
      this.theUserID = theUserID;
      this.minSim = minSim;
    }
    
    @Override
    public double estimate(Long userID) throws TasteException {
      if (userID == theUserID) {
        return Double.NaN;
      }
      double sim = userSimilarityImpl.userSimilarity(theUserID, userID);
      return sim >= minSim ? sim : Double.NaN;
    }
  }
}