/**
* 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.mahout.math.stats.entropy;
import org.apache.hadoop.io.DoubleWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.mahout.math.VarIntWritable;
import java.io.IOException;
/**
* Calculates the entropy for the value with H(x) = x * log(x)
*/
public final class CalculateEntropyMapper extends Mapper<Text, VarIntWritable, NullWritable, DoubleWritable> {
private final DoubleWritable result = new DoubleWritable();
@Override
protected void map(Text key, VarIntWritable value, Context context) throws IOException, InterruptedException {
result.set(value.get() * Math.log(value.get()));
context.write(NullWritable.get(), result);
}
}