content: package mrdp.ch2;
  
  import java.io.DataInput;
  import java.io.DataOutput;
  import java.io.IOException;
  import java.text.ParseException;
  import java.text.SimpleDateFormat;
  import java.util.Date;
  import java.util.Map;
  
  import mrdp.utils.MRDPUtils;
  
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.fs.Path;
  import org.apache.hadoop.io.IntWritable;
  import org.apache.hadoop.io.Text;
  import org.apache.hadoop.io.Writable;
  import org.apache.hadoop.mapreduce.Job;
  import org.apache.hadoop.mapreduce.Mapper;
  import org.apache.hadoop.mapreduce.Reducer;
  import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
  import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
  import org.apache.hadoop.util.GenericOptionsParser;
  
  public class AverageDriver {
  
  	public static class SOAverageMapper extends
  			Mapper<Object, Text, IntWritable, CountAverageTuple> {
  
  		private IntWritable outHour = new IntWritable();
  		private CountAverageTuple outCountAverage = new CountAverageTuple();
  
  		private final static SimpleDateFormat frmt = new SimpleDateFormat(
  				"yyyy-MM-dd'T'HH:mm:ss.SSS");
  
  		@SuppressWarnings("deprecation")
  		@Override
  		public void map(Object key, Text value, Context context)
  				throws IOException, InterruptedException {
  
  			// Parse the input string into a nice map
  			Map<String, String> parsed = MRDPUtils.transformXmlToMap(value
  					.toString());
  
  			// Grab the "CreationDate" field,
  			// since it is what we are grouping by
  			String strDate = parsed.get("CreationDate");
  
  			// Grab the comment to find the length
  			String text = parsed.get("Text");
  
  			// .get will return null if the key is not there
  			if (strDate == null || text == null) {
  				// skip this record
  				return;
  			}
  
  			try {
  				// get the hour this comment was posted in
  				Date creationDate = frmt.parse(strDate);
  				outHour.set(creationDate.getHours());
  
  				// get the comment length
  				outCountAverage.setCount(1);
  				outCountAverage.setAverage(text.length());
  
  				// write out the user ID with min max dates and count
  				context.write(outHour, outCountAverage);
  
  			} catch (ParseException e) {
  				System.err.println(e.getMessage());
  				return;
  			}
  		}
  	}
  
  	public static class SOAverageReducer
  			extends
  			Reducer<IntWritable, CountAverageTuple, IntWritable, CountAverageTuple> {
  		private CountAverageTuple result = new CountAverageTuple();
  
  		@Override
  		public void reduce(IntWritable key, Iterable<CountAverageTuple> values,
  				Context context) throws IOException, InterruptedException {
  
  			float sum = 0;
  			float count = 0;
  
  			// Iterate through all input values for this key
  			for (CountAverageTuple val : values) {
  				sum += val.getCount() * val.getAverage();
  				count += val.getCount();
  			}
  
  			result.setCount(count);
  			result.setAverage(sum / count);
  
  			context.write(key, result);
  		}
  	}
  
  	public static void main(String[] args) throws Exception {
  		Configuration conf = new Configuration();
  		String[] otherArgs = new GenericOptionsParser(conf, args)
  				.getRemainingArgs();
  		if (otherArgs.length != 2) {
  			System.err.println("Usage: AverageDriver <in> <out>");
  			System.exit(2);
  		}
  		Job job = new Job(conf, "StackOverflow Average Comment Length");
  		job.setJarByClass(AverageDriver.class);
  		job.setMapperClass(SOAverageMapper.class);
  		job.setCombinerClass(SOAverageReducer.class);
  		job.setReducerClass(SOAverageReducer.class);
  		job.setOutputKeyClass(IntWritable.class);
  		job.setOutputValueClass(CountAverageTuple.class);
  		FileInputFormat.addInputPath(job, new Path(otherArgs[0]));
  		FileOutputFormat.setOutputPath(job, new Path(otherArgs[1]));
  		System.exit(job.waitForCompletion(true) ? 0 : 1);
  	}
  
  	public static class CountAverageTuple implements Writable {
  		private float count = 0f;
  		private float average = 0f;
  
  		public float getCount() {
  			return count;
  		}
  
  		public void setCount(float count) {
  			this.count = count;
  		}
  
  		public float getAverage() {
  			return average;
  		}
  
  		public void setAverage(float average) {
  			this.average = average;
  		}
  
  		@Override
  		public void readFields(DataInput in) throws IOException {
  			count = in.readFloat();
  			average = in.readFloat();
  		}
  
  		@Override
  		public void write(DataOutput out) throws IOException {
  			out.writeFloat(count);
  			out.writeFloat(average);
  		}
  
  		@Override
  		public String toString() {
  			return count + "\t" + average;
  		}
  	}
  }
  