/**
 * 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.hadoop.mapred;

import java.io.IOException;

import java.util.Iterator;

import org.apache.hadoop.io.Closeable;
import org.apache.hadoop.io.Writable;
import org.apache.hadoop.io.WritableComparable;

/** Reduces a set of intermediate values which share a key to a smaller set of
 * values.  Input values are the grouped output of a {@link Mapper}. */
public interface Reducer extends JobConfigurable, Closeable {
  /** Combines values for a given key.  Output values must be of the same type
   * as input values.  Input keys must not be altered.  Typically all values
   * are combined into zero or one value.  Output pairs are collected with
   * calls to {@link OutputCollector#collect(WritableComparable,Writable)}.
   *
   * @param key the key
   * @param values the values to combine
   * @param output to collect combined values
   */
  void reduce(WritableComparable key, Iterator values,
              OutputCollector output, Reporter reporter)
    throws IOException;

}
