OrdinaryDifferentialEquation.java

/*
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 * contributor license agreements.  See the NOTICE file distributed with
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 * The Hipparchus project licenses this file to You under the Apache License, Version 2.0
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 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
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 * distributed under the License is distributed on an "AS IS" BASIS,
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package org.hipparchus.ode;

/** 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 ODEIntegrator#integrate(OrdinaryDifferentialEquation,
 * ODEState, double)} 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 ODEIntegrator
 * @see FirstOrderConverter
 * @see SecondOrderODE
 *
 */
public interface OrdinaryDifferentialEquation {

    /** 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>
     * <p>
     * The default implementation does nothing.
     * </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
     */
    default void init(double t0, double[] y0, double finalTime) {
        // do nothing by default
    }

    /** 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
     */
    double[] computeDerivatives(double t, double[] y);

}