/**
 * Copyright 2010 The Apache Software Foundation
 *
 * 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.hbase.mapreduce;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Writable;
import org.apache.hadoop.mapreduce.InputFormat;
import org.apache.hadoop.mapreduce.InputSplit;
import org.apache.hadoop.mapreduce.JobContext;
import org.apache.hadoop.mapreduce.RecordReader;
import org.apache.hadoop.mapreduce.TaskAttemptContext;

/**
 * Input format that creates as many map tasks as configured in
 * mapred.map.tasks, each provided with a single row of
 * NullWritables. This can be useful when trying to write mappers
 * which don't have any real input (eg when the mapper is simply
 * producing random data as output)
 */
public class NMapInputFormat extends InputFormat<NullWritable, NullWritable> {

  @Override
  public RecordReader<NullWritable, NullWritable> createRecordReader(
      InputSplit split,
      TaskAttemptContext tac) throws IOException, InterruptedException {
    return new SingleRecordReader<NullWritable, NullWritable>(
        NullWritable.get(), NullWritable.get());
  }

  @Override
  public List<InputSplit> getSplits(JobContext context) throws IOException,
      InterruptedException {
    int count = context.getConfiguration().getInt("mapred.map.tasks", 1);
    List<InputSplit> splits = new ArrayList<InputSplit>(count);
    for (int i = 0; i < count; i++) {
      splits.add(new NullInputSplit());
    }
    return splits;
  }

  private static class NullInputSplit extends InputSplit implements Writable {
    @Override
    public long getLength() throws IOException, InterruptedException {
      return 0;
    }

    @Override
    public String[] getLocations() throws IOException, InterruptedException {
      return new String[] {};
    }

    @Override
    public void readFields(DataInput in) throws IOException {
    }

    @Override
    public void write(DataOutput out) throws IOException {
    }
  }
  
  private static class SingleRecordReader<K, V>
    extends RecordReader<K, V> {
    
    private final K key;
    private final V value;
    boolean providedKey = false;

    SingleRecordReader(K key, V value) {
      this.key = key;
      this.value = value;
    }

    @Override
    public void close() {
    }

    @Override
    public K getCurrentKey() {
      return key;
    }

    @Override
    public V getCurrentValue(){
      return value;
    }

    @Override
    public float getProgress() {
      return 0;
    }

    @Override
    public void initialize(InputSplit split, TaskAttemptContext tac) {
    }

    @Override
    public boolean nextKeyValue() {
      if (providedKey) return false;
      providedKey = true;
      return true;
    }
    
  }
}