FieldODEEventHandler.java

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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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package org.hipparchus.ode.events;

import org.hipparchus.CalculusFieldElement;
import org.hipparchus.ode.FieldODEState;
import org.hipparchus.ode.FieldODEStateAndDerivative;

/** This interface represents a handler for discrete events triggered
 * during ODE integration.
 *
 * <p>Some events can be triggered at discrete times as an ODE problem
 * is solved. This occurs for example when the integration process
 * should be stopped as some state is reached (G-stop facility) when the
 * precise date is unknown a priori, or when the derivatives have
 * states boundaries crossings.
 * </p>
 *
 * <p>These events are defined as occurring when a <code>g</code>
 * switching function sign changes.</p>
 *
 * <p>Since events are only problem-dependent and are triggered by the
 * independent <i>time</i> variable and the state vector, they can
 * occur at virtually any time, unknown in advance. The integrators will
 * take care to avoid sign changes inside the steps, they will reduce
 * the step size when such an event is detected in order to put this
 * event exactly at the end of the current step. This guarantees that
 * step interpolation (which always has a one step scope) is relevant
 * even in presence of discontinuities. This is independent from the
 * stepsize control provided by integrators that monitor the local
 * error (this event handling feature is available for all integrators,
 * including fixed step ones).</p>
 *
 * <p>
 * Note that prior to Hipparchus 3.0, some of the methods that are now in
 * {@link FieldODEEventDetector} were in this interface (and the remaining
 * ones were in the defunct {@code FieldEventHandlerConfiguration} interface).
 * The interfaces have been reorganized to allow different objects to be
 * used in event detection and event handling, hence allowing users to
 * reuse predefined events detectors with custom handlers.
 * </p>
 * @see org.hipparchus.ode.events
 * @param <T> the type of the field elements
 */
public interface FieldODEEventHandler<T extends CalculusFieldElement<T>>  {

    /** Initialize event handler 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 event handler to initialize some internal data
     * if needed.
     * </p>
     * <p>
     * The default implementation does nothing
     * </p>
     * @param initialState initial time, state vector and derivative
     * @param finalTime target time for the integration
     * @param detector event detector related to the event handler
     */
    default void init(FieldODEStateAndDerivative<T> initialState, T finalTime, FieldODEEventDetector<T> detector) {
        // nothing by default
    }

    /** Handle an event and choose what to do next.

     * <p>This method is called when the integrator has accepted a step
     * ending exactly on a sign change of the function, just <em>after</em>
     * the step handler itself is called (see below for scheduling). It
     * allows the user to update his internal data to acknowledge the fact
     * the event has been handled (for example setting a flag in the {@link
     * org.hipparchus.ode.FieldOrdinaryDifferentialEquation
     * differential equations} to switch the derivatives computation in
     * case of discontinuity), or to direct the integrator to either stop
     * or continue integration, possibly with a reset state or derivatives.</p>
     *
     * <ul>
     *   <li>if {@link Action#STOP} is returned, the integration will be stopped,</li>
     *   <li>if {@link Action#RESET_STATE} is returned, the {@link #resetState
     *   resetState} method will be called once the step handler has
     *   finished its task, and the integrator will also recompute the
     *   derivatives,</li>
     *   <li>if {@link Action#RESET_DERIVATIVES} is returned, the integrator
     *   will recompute the derivatives,
     *   <li>if {@link Action#RESET_EVENTS} is returned, the integrator
     *   will recheck all event handlers,
     *   <li>if {@link Action#CONTINUE} is returned, no specific action will
     *   be taken (apart from having called this method) and integration
     *   will continue.</li>
     * </ul>
     *
     * <p>The scheduling between this method and the {@link
     * org.hipparchus.ode.sampling.FieldODEStepHandler FieldODEStepHandler} method {@link
     * org.hipparchus.ode.sampling.FieldODEStepHandler#handleStep
     * handleStep(interpolator, isLast)} is to call {@code handleStep} first and this method afterwards
     * (this scheduling changed as of Hipparchus 2.0). This scheduling allows user code
     * called by this method and user code called by step handlers to get values
     * of the independent time variable consistent with integration direction.</p>
     *
     * @param state current value of the independent <i>time</i> variable, state vector
     * and derivative
     * @param detector detector that triggered the event
     * @param increasing if true, the value of the switching function increases
     * when times increases around event (note that increase is measured with respect
     * to physical time, not with respect to integration which may go backward in time)
     * @return indication of what the integrator should do next, this
     * value must be one of {@link Action#STOP}, {@link Action#RESET_STATE},
     * {@link Action#RESET_DERIVATIVES}, {@link Action#RESET_EVENTS}, or
     * {@link Action#CONTINUE}
     */
    Action eventOccurred(FieldODEStateAndDerivative<T> state, FieldODEEventDetector<T> detector, boolean increasing);

    /** Reset the state prior to continue the integration.

     * <p>This method is called after the step handler has returned and
     * before the next step is started, but only when {@link
     * #eventOccurred(FieldODEStateAndDerivative, FieldODEEventDetector, boolean) eventOccurred}
     * has itself returned the {@link Action#RESET_STATE} indicator. It allows the user to reset
     * the state vector for the next step, without perturbing the step handler of the
     * finishing step.</p>
     * <p>The default implementation returns its argument.</p>
     * @param detector detector that triggered the event
     * @param state current value of the independent <i>time</i> variable, state vector
     * and derivative
     * @return reset state (note that it does not include the derivatives, they will
     * be added automatically by the integrator afterwards)
     */
    default FieldODEState<T> resetState(FieldODEEventDetector<T> detector, FieldODEStateAndDerivative<T> state) {
        return state;
    }

}