text: package mrdp.ch3;
  
  import java.io.*;
  import java.util.Random;
  
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.fs.*;
  import org.apache.hadoop.io.*;
  import org.apache.hadoop.mapreduce.Job;
  import org.apache.hadoop.mapreduce.Mapper;
  import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
  import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
  import org.apache.hadoop.util.GenericOptionsParser;
  
  public class SimpleRandomSampling {
  
  	public static class SRSMapper extends
  			Mapper<Object, Text, NullWritable, Text> {
  
  		private Random rands = new Random();
  		private Double percentage;
  
  		@Override
  		protected void setup(Context context) throws IOException,
  				InterruptedException {
  			// retrieve the percentage that is passed in via the configuration
  			// like this: conf.set("filter_percentage", .5); for .5%
  			String strPercentage = context.getConfiguration().get(
  					"filter_percentage");
  
  			percentage = Double.parseDouble(strPercentage) / 100.0;
  		}
  
  		@Override
  		public void map(Object key, Text value, Context context)
  				throws IOException, InterruptedException {
  
  			if (rands.nextDouble() < percentage) {
  				context.write(NullWritable.get(), value);
  			}
  		}
  	}
  
  	public static void main(String[] args) throws Exception {
  		Configuration conf = new Configuration();
  		String[] otherArgs = new GenericOptionsParser(conf, args)
  				.getRemainingArgs();
  		if (otherArgs.length != 3) {
  			System.err.println("Usage: SRS <percentage> <in> <out>");
  			System.exit(2);
  		}
  		conf.set("filter_percentage", otherArgs[0]);
  
  		Job job = new Job(conf, "SRS");
  		job.setJarByClass(SimpleRandomSampling.class);
  		job.setMapperClass(SRSMapper.class);
  		job.setOutputKeyClass(NullWritable.class);
  		job.setOutputValueClass(Text.class);
  		job.setNumReduceTasks(0); // Set number of reducers to zero
  		FileInputFormat.addInputPath(job, new Path(otherArgs[1]));
  		FileOutputFormat.setOutputPath(job, new Path(otherArgs[2]));
  		System.exit(job.waitForCompletion(true) ? 0 : 1);
  	}
  }
  