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The Apache Commons Math project is a library of lightweight, self-contained mathematics and statistics components addressing the most common practical problems not immediately available in the Java programming language or commons-lang.

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/*
 * 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.math3.ode;

/**
 * This interface allows users to add secondary differential equations to a primary
 * set of differential equations.
 * 
 * In some cases users may need to integrate some problem-specific equations along
 * with a primary set of differential equations. One example is optimal control where
 * adjoined parameters linked to the minimized hamiltonian must be integrated.
 * 
 * 
 * This interface allows users to add such equations to a primary set of {@link
 * FirstOrderDifferentialEquations first order differential equations}
 * thanks to the {@link
 * ExpandableStatefulODE#addSecondaryEquations(SecondaryEquations)}
 * method.
 * 
 * @see ExpandableStatefulODE
 * @version $Id: SecondaryEquations.java 1244107 2012-02-14 16:17:55Z erans $
 * @since 3.0
 */
public interface SecondaryEquations {

    /** Get the dimension of the secondary state parameters.
     * @return dimension of the secondary state parameters
     */
    int getDimension();

    /** Compute the derivatives related to the secondary state parameters.
     * @param t current value of the independent time variable
     * @param primary array containing the current value of the primary state vector
     * @param primaryDot array containing the derivative of the primary state vector
     * @param secondary array containing the current value of the secondary state vector
     * @param secondaryDot placeholder array where to put the derivative of the secondary state vector
     */
    void computeDerivatives(double t, double[] primary, double[] primaryDot,
                            double[] secondary, double[] secondaryDot);

}