/**
 * 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 org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
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.map.OpenIntObjectHashMap;

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

final class ALSUtils {

  private ALSUtils() {}

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

  static OpenIntObjectHashMap<Vector> readMatrixByRows(Path dir, Configuration conf) {
    OpenIntObjectHashMap<Vector> matrix = new OpenIntObjectHashMap<Vector>();

    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().clone();
      matrix.put(rowIndex, row);
    }
    return matrix;
  }
}