/**
 * 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.horn.funcs;

import org.apache.hama.commons.math.DoubleDoubleFunction;

/**
 * for softmaxed output 
 */
public class CategoricalCrossEntropy extends DoubleDoubleFunction {
  
  private static final double epsilon = 1e-8;
  
  @Override
  public double apply(double target, double actual) {
    return -target * Math.log(Math.max(actual, epsilon));
  }

  @Override
  public double applyDerivative(double target, double actual) {
    // o - y
    return -(target - actual);
  }

}