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.ArrayList;
  import java.util.Collections;
  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 MedianStdDevDriver {
  
  	public static class SOMedianStdDevMapper extends
  			Mapper<Object, Text, IntWritable, IntWritable> {
  
  		private IntWritable outHour = new IntWritable();
  		private IntWritable outCommentLength = new IntWritable();
  
  		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
  				outCommentLength.set(text.length());
  
  				// write out the user ID with min max dates and count
  				context.write(outHour, outCommentLength);
  
  			} catch (ParseException e) {
  				System.err.println(e.getMessage());
  				return;
  			}
  		}
  	}
  
  	public static class SOMedianStdDevReducer extends
  			Reducer<IntWritable, IntWritable, IntWritable, MedianStdDevTuple> {
  		private MedianStdDevTuple result = new MedianStdDevTuple();
  		private ArrayList<Float> commentLengths = new ArrayList<Float>();
  
  		@Override
  		public void reduce(IntWritable key, Iterable<IntWritable> values,
  				Context context) throws IOException, InterruptedException {
  
  			float sum = 0;
  			float count = 0;
  			commentLengths.clear();
  			result.setStdDev(0);
  			
  			// Iterate through all input values for this key
  			for (IntWritable val : values) {
  				commentLengths.add((float) val.get());
  				sum += val.get();
  				++count;
  			}
  
  			// sort commentLengths to calculate median
  			Collections.sort(commentLengths);
  
  			// if commentLengths is an even value, average middle two elements
  			if (count % 2 == 0) {
  				result.setMedian((commentLengths.get((int) count / 2 - 1) + commentLengths
  						.get((int) count / 2)) / 2.0f);
  			} else {
  				// else, set median to middle value
  				result.setMedian(commentLengths.get((int) count / 2));
  			}
  
  			// calculate standard deviation
  			float mean = sum / count;
  
  			float sumOfSquares = 0.0f;
  			for (Float f : commentLengths) {
  				sumOfSquares += (f - mean) * (f - mean);
  			}
  
  			result.setStdDev((float) Math.sqrt(sumOfSquares / (count - 1)));
  
  			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: MedianStdDevDriver <in> <out>");
  			System.exit(2);
  		}
  		Job job = new Job(conf,
  				"StackOverflow Comment Length Median StdDev By Hour");
  		job.setJarByClass(MedianStdDevDriver.class);
  		job.setMapperClass(SOMedianStdDevMapper.class);
  		job.setReducerClass(SOMedianStdDevReducer.class);
  		job.setMapOutputKeyClass(IntWritable.class);
  		job.setMapOutputValueClass(IntWritable.class);
  		job.setOutputKeyClass(IntWritable.class);
  		job.setOutputValueClass(MedianStdDevTuple.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 MedianStdDevTuple implements Writable {
  		private float median = 0;
  		private float stddev = 0f;
  
  		public float getMedian() {
  			return median;
  		}
  
  		public void setMedian(float median) {
  			this.median = median;
  		}
  
  		public float getStdDev() {
  			return stddev;
  		}
  
  		public void setStdDev(float stddev) {
  			this.stddev = stddev;
  		}
  
  		@Override
  		public void readFields(DataInput in) throws IOException {
  			median = in.readFloat();
  			stddev = in.readFloat();
  		}
  
  		@Override
  		public void write(DataOutput out) throws IOException {
  			out.writeFloat(median);
  			out.writeFloat(stddev);
  		}
  
  		@Override
  		public String toString() {
  			return median + "\t" + stddev;
  		}
  	}
  }
  