/**
* 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.mapreduce.Reducer;
import java.io.IOException;
/**
* Subtracts the partial entropy.
*/
public final class CalculateEntropyReducer
extends Reducer<NullWritable, DoubleWritable, NullWritable, DoubleWritable> {
private static final double LOG_2 = Math.log(2.0);
private final DoubleWritable result = new DoubleWritable();
private long numberItems;
@Override
protected void setup(Context context) throws IOException, InterruptedException {
super.setup(context);
numberItems = Long.parseLong(context.getConfiguration().get(Entropy.NUMBER_ITEMS_PARAM));
}
@Override
protected void reduce(NullWritable key, Iterable<DoubleWritable> values, Context context)
throws IOException, InterruptedException {
double entropy = 0.0;
for (DoubleWritable value : values) {
entropy += value.get();
}
result.set((Math.log(numberItems) - entropy / numberItems) / LOG_2);
context.write(key, result);
}
}