/*
 * 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.math4.legacy.ode;

import org.apache.commons.math4.legacy.core.RealFieldElement;

/** This interface represents a first order differential equations set.
 *
 * <p>This interface should be implemented by all real first order
 * differential equation problems before they can be handled by the
 * integrators {@link FirstOrderIntegrator#integrate} method.</p>
 *
 * <p>A first order differential equations problem, as seen by an
 * integrator is the time derivative <code>dY/dt</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> and on the state vector
 * <code>Y</code>.</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 FirstOrderFieldIntegrator
 *
 * @param <T> the type of the field elements
 * @since 3.6
 */

public interface FirstOrderFieldDifferentialEquations<T extends RealFieldElement<T>> {

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

    /** Initialize equations at the start of an ODE integration.
     * <p>
     * This method is called once at the start of the integration. It
     * may be used by the equations to initialize some internal data
     * if needed.
     * </p>
     * @param t0 value of the independent <I>time</I> variable at integration start
     * @param y0 array containing the value of the state vector at integration start
     * @param finalTime target time for the integration
     */
    void init(T t0, T[] y0, T finalTime);

    /** 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
     * @return time derivative of the state vector
     */
    T[] computeDerivatives(T t, T[] y);

}