/*
 * Encog(tm) Core v3.2 - Java Version
 * http://www.heatonresearch.com/encog/
 * https://github.com/encog/encog-java-core
 
 * Copyright 2008-2013 Heaton Research, Inc.
 *
 * Licensed 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.
 *   
 * For more information on Heaton Research copyrights, licenses 
 * and trademarks visit:
 * http://www.heatonresearch.com/copyright
 */
package org.encog.engine.network.activation;


/**
 * The bipolar sigmoid activation function is like the regular sigmoid activation function,
 * except Bipolar sigmoid activation function. TheOutput range is -1 to 1 instead of the more normal 0 to 1.
 * 
 * This activation is typically part of a CPPN neural network, such as 
 * HyperNEAT.
 * 
 * The idea for this activation function was developed by  Ken Stanley, of  
 * the University of Texas at Austin.
 * http://www.cs.ucf.edu/~kstanley/
 */
public class ActivationBipolarSteepenedSigmoid implements ActivationFunction {
	
	/**
	 * The serial id.
	 */
	private static final long serialVersionUID = 1L;

	/**
	 * {@inheritDoc}
	 */
	@Override
	public void activationFunction(double[] d, int start, int size) {
		for(int i=start;i<start+size;i++) {
			d[i] = (2.0 / (1.0 + Math.exp(-4.9 * d[i]))) - 1.0;
		}		
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public double derivativeFunction(double b, double a) {
		return 1;
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public boolean hasDerivative() {
		return true;
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public double[] getParams() {
		return ActivationLinear.P;
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public void setParam(int index, double value) {		
		
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public String[] getParamNames() {
		return ActivationLinear.N;
	}
	
	/**
	 * {@inheritDoc}
	 */
	@Override
	public final ActivationFunction clone() {
		return new ActivationBipolarSteepenedSigmoid();
	}
	
	/**
	 * {@inheritDoc}
	 */
	@Override
	public String getFactoryCode() {
		return null;
	}

}
