/*
 * File:        SigmoidFunction.java
 * Author:      Justin Basilico
 * Course:      PO CS 152: Neural Networks
 * Assignment:  Final Project
 * Updated:     2001.12.18
 * Created:     2001.12.04
 *
 * Description:
 * This file contains the SigmoidFunction class, which implements a sigmoid
 * activation function.
 *
 * Copyright:    Justin Basilico (2001).
 */

/**
 * SigmoidFunction class
 *
 * This class implements an ActivationFunction which is a sigmoid activation
 * function. The sigmoid function in an input x calculates
 *
 *    1 / ( 1 + e^-x )
 *
 * The method for calculating the activation for an unit based on the total
 * input to the unit is the getUnitActivation(double) method.
 *
 * Since there is no internal data in this class, rather than creating new
 * instances of SigmoidFunctions, you should just use the static
 * SigmoidFunction.INSTANCE, which is just an instance of the class that can
 * be used statically for calculating values with this ActivationFunction.
 *
 * @author  Justin Basilico
 * @version 2001.12.18
 * @see     ActivationFunction
 * @see     SigmoidFunction.INSTANCE
 * @see     SigmoidFunction.getUnitActivation(double)
 */
public class SigmoidFunction
    extends Object
    implements ActivationFunction
{
    /** INSTANCE
     * This is an instance of a SigmoidFunction. Since the SigmoidFunction has
     * no internal data, only one instance of it needs to exist in memory.
     * This is it. */
    public static SigmoidFunction
                            INSTANCE = new SigmoidFunction();

    /**
     * SigmoidFunction default constructor
     *
     * Creates new SigmoidFunction instance. You should probably just use the
     * SigmodFunction.INSTANCE because it has no internal data.
     *
     * @see    SigmoidFunction.INSTANCE
     */
    public SigmoidFunction()
    {
    }

    /**
     * getUnitActivation
     *
     * This method takes a double input that is the total input to a unit in a
     * neural network and returns the double activation value for that unit
     * from the given input.
     *
     * This activation function returns the sigmoid value of the input which
     * is defined for input x as:
     *
     *     1 / ( 1 + e^-x )
     *
     * @param  input The value of the input to a unit, as a double.
     * @return The activation for the unit given the input value, as a double.
     * @see    ActivationFunction.getUnitActivation(double)
     */
    public double getUnitActivation(
        double              input)          // Total input to unit.
    {
        // Return the sigmoid function applied to the input.
        return 1 / (1 + Math.exp(-input));
    }
}