/**
 * 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.hadoop.als;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

import com.google.common.base.Preconditions;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.LocalFileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.SequenceFile;
import org.apache.mahout.common.HadoopUtil;
import org.apache.mahout.common.Pair;
import org.apache.mahout.common.iterator.sequencefile.PathFilters;
import org.apache.mahout.common.iterator.sequencefile.PathType;
import org.apache.mahout.common.iterator.sequencefile.SequenceFileDirIterable;
import org.apache.mahout.common.iterator.sequencefile.SequenceFileDirValueIterator;
import org.apache.mahout.math.Vector;
import org.apache.mahout.math.VectorWritable;
import org.apache.mahout.math.als.AlternatingLeastSquaresSolver;
import org.apache.mahout.math.map.OpenIntObjectHashMap;

final class ALS {

  private ALS() {}

  static Vector readFirstRow(Path dir, Configuration conf) throws IOException {
    Iterator<VectorWritable> iterator = new SequenceFileDirValueIterator<>(dir, PathType.LIST,
        PathFilters.partFilter(), null, true, conf);
    return iterator.hasNext() ? iterator.next().get() : null;
  }

  public static OpenIntObjectHashMap<Vector> readMatrixByRowsFromDistributedCache(int numEntities,
      Configuration conf) throws IOException {

    IntWritable rowIndex = new IntWritable();
    VectorWritable row = new VectorWritable();


    OpenIntObjectHashMap<Vector> featureMatrix = numEntities > 0
        ? new OpenIntObjectHashMap<Vector>(numEntities) : new OpenIntObjectHashMap<Vector>();

    Path[] cachedFiles = HadoopUtil.getCachedFiles(conf);
    LocalFileSystem localFs = FileSystem.getLocal(conf);

    for (Path cachedFile : cachedFiles) {
      try (SequenceFile.Reader reader = new SequenceFile.Reader(localFs, cachedFile, conf)){
        while (reader.next(rowIndex, row)) {
          featureMatrix.put(rowIndex.get(), row.get());
        }
      }
    }

    Preconditions.checkState(!featureMatrix.isEmpty(), "Feature matrix is empty");
    return featureMatrix;
  }

  public static OpenIntObjectHashMap<Vector> readMatrixByRows(Path dir, Configuration conf) {
    OpenIntObjectHashMap<Vector> matrix = new OpenIntObjectHashMap<>();
    for (Pair<IntWritable,VectorWritable> pair
        : new SequenceFileDirIterable<IntWritable,VectorWritable>(dir, PathType.LIST, PathFilters.partFilter(), conf)) {
      int rowIndex = pair.getFirst().get();
      Vector row = pair.getSecond().get();
      matrix.put(rowIndex, row);
    }
    return matrix;
  }

  public static Vector solveExplicit(VectorWritable ratingsWritable, OpenIntObjectHashMap<Vector> uOrM,
    double lambda, int numFeatures) {
    Vector ratings = ratingsWritable.get();

    List<Vector> featureVectors = new ArrayList<>(ratings.getNumNondefaultElements());
    for (Vector.Element e : ratings.nonZeroes()) {
      int index = e.index();
      featureVectors.add(uOrM.get(index));
    }

    return AlternatingLeastSquaresSolver.solve(featureVectors, ratings, lambda, numFeatures);
  }
}