/*
 * 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;

import org.encog.mathutil.BoundMath;
import org.encog.ml.factory.MLActivationFactory;
import org.encog.util.obj.ActivationUtil;

/**
 * The sigmoid activation function takes on a sigmoidal shape. Only positive
 * numbers are generated. Do not use this activation function if negative number
 * output is desired.
 */
public class ActivationSigmoid implements ActivationFunction {

	/**
	 * Serial id for this class.
	 */
	private static final long serialVersionUID = 5622349801036468572L;

	/**
	 * The parameters.
	 */
	private final double[] params;

	/**
	 * Construct a basic sigmoid function, with a slope of 1.
	 */
	public ActivationSigmoid() {
		this.params = new double[0];
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public final void activationFunction(final double[] x, final int start,
			final int size) {
		for (int i = start; i < start + size; i++) {
			x[i] = 1.0 / (1.0 + BoundMath.exp(-1 * x[i]));
		}
	}

	/**
	 * @return The object cloned;
	 */
	@Override
	public final ActivationFunction clone() {
		return new ActivationSigmoid();
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public final double derivativeFunction(final double b, final double a) {
		return a * (1.0 - a);
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public final String[] getParamNames() {
		final String[] results = {};
		return results;
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public final double[] getParams() {
		return this.params;
	}

	/**
	 * @return True, sigmoid has a derivative.
	 */
	@Override
	public final boolean hasDerivative() {
		return true;
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public final void setParam(final int index, final double value) {
		this.params[index] = value;
	}
	
	/**
	 * {@inheritDoc}
	 */
	@Override
	public String getFactoryCode() {
		return ActivationUtil.generateActivationFactory(MLActivationFactory.AF_SIGMOID, this);
	}
}
