/*
 * 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.commons.math.ode;

/**
 * This interface represents a second order differential equations set.
 * 
 * <p>
 * This interface should be implemented by all real second order differential
 * equation problems before they can be handled by the integrators
 * {@link SecondOrderIntegrator#integrate} method.
 * </p>
 *
 * <p>
 * A second order differential equations problem, as seen by an integrator is
 * the second time derivative <code>d2Y/dt^2</code> of a state vector
 * <code>Y</code>, both being one dimensional arrays. From the integrator point
 * of view, this derivative depends only on the current time <code>t</code>, on
 * the state vector <code>Y</code> and on the first time derivative of the state
 * vector.
 * </p>
 *
 * <p>
 * For real problems, the derivative depends also on parameters that do not
 * belong to the state vector (dynamical model constants for example). These
 * constants are completely outside of the scope of this interface, the classes
 * that implement it are allowed to handle them as they want.
 * </p>
 *
 * @see SecondOrderIntegrator
 * @see FirstOrderConverter
 * @see FirstOrderDifferentialEquations
 * @version $Revision$ $Date$
 * @since 1.2
 */

public interface SecondOrderDifferentialEquations {

	/**
	 * Get the dimension of the problem.
	 * 
	 * @return dimension of the problem
	 */
	public int getDimension();

	/**
	 * Get the current time derivative of the state vector.
	 * 
	 * @param t
	 *            current value of the independent <I>time</I> variable
	 * @param y
	 *            array containing the current value of the state vector
	 * @param yDot
	 *            array containing the current value of the first derivative of
	 *            the state vector
	 * @param yDDot
	 *            placeholder array where to put the second time derivative of
	 *            the state vector
	 * @throws DerivativeException
	 *             this exception is propagated to the caller if the underlying
	 *             user function triggers one
	 */
	public void computeSecondDerivatives(double t, double[] y, double[] yDot, double[] yDDot)
			throws DerivativeException;

}
