/*
    * 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.giraph.reducers.impl;
  
  import java.io.DataInput;
  import java.io.DataOutput;
  import java.io.IOException;
  
  import org.apache.commons.lang3.tuple.Pair;
  import org.apache.giraph.reducers.ReduceOperation;
  import org.apache.giraph.utils.WritableUtils;
  import org.apache.giraph.writable.tuple.PairWritable;
  import org.apache.hadoop.io.Writable;
  import com.google.common.base.Preconditions;
  
  /**
   * Combines two individual reducers, to create a single reducer of pairs that
   * reduces each of them individually.
   *
   * @param <S1> First single value type
   * @param <R1> First reduced value type
   * @param <S2> Second single value type
   * @param <R2> Second reduced value type
   */
  public class PairReduce<S1, R1 extends Writable, S2, R2 extends Writable>
      implements ReduceOperation<Pair<S1, S2>, PairWritable<R1, R2>> {
    /** First reduceOp */
    private ReduceOperation<S1, R1> reduce1;
    /** Second reduceOp */
    private ReduceOperation<S2, R2> reduce2;
  
    /** Constructor */
    public PairReduce() {
    }
  
    /**
     * Constructor
     * @param reduce1 First reduceOp
     * @param reduce2 Second reduceOp
     */
    public PairReduce(
        ReduceOperation<S1, R1> reduce1, ReduceOperation<S2, R2> reduce2) {
      this.reduce1 = reduce1;
      this.reduce2 = reduce2;
    }
  
  
    @Override
    public PairWritable<R1, R2> createInitialValue() {
      return new PairWritable<>(
          reduce1.createInitialValue(), reduce2.createInitialValue());
    }
  
    @Override
    public PairWritable<R1, R2> reduce(
        PairWritable<R1, R2> curValue, Pair<S1, S2> valueToReduce) {
      Preconditions.checkState(
          curValue.getLeft() ==
          reduce1.reduce(curValue.getLeft(), valueToReduce.getLeft()));
      Preconditions.checkState(
          curValue.getRight() ==
          reduce2.reduce(curValue.getRight(), valueToReduce.getRight()));
      return curValue;
    }
  
    @Override
    public PairWritable<R1, R2> reduceMerge(
        PairWritable<R1, R2> curValue, PairWritable<R1, R2> valueToReduce) {
      Preconditions.checkState(
          curValue.getLeft() ==
          reduce1.reduceMerge(curValue.getLeft(), valueToReduce.getLeft()));
      Preconditions.checkState(
          curValue.getRight() ==
          reduce2.reduceMerge(curValue.getRight(), valueToReduce.getRight()));
      return curValue;
    }
  
    @Override
    public void write(DataOutput out) throws IOException {
      WritableUtils.writeWritableObject(reduce1, out);
      WritableUtils.writeWritableObject(reduce2, out);
    }
  
    @Override
   public void readFields(DataInput in) throws IOException {
     reduce1 = WritableUtils.readWritableObject(in, null);
     reduce2 = WritableUtils.readWritableObject(in, null);
   }
 }