src/main/java/org/apache/horn/funcs/Sigmoid.java

/**
 * 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.DoubleFunction;

/**
 * The Sigmoid function
 * 
 * <pre>
 * f(x) = 1 / (1 + e^{-x})
 * </pre>
 */
public class Sigmoid extends DoubleFunction {

  @Override
  public double apply(double value) {
    if(value > 100) { // to avoid overflow and underflow
      return 0.9999;
    } else if (value < -100) {
      return 0.0001;
    }
    return 1.0 / (1 + Math.exp(-value));
  }

  @Override
  public double applyDerivative(double value) {
    return apply(value) * (1 - apply(value));
  }

}