DSSTForceModel

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 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * CS licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
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 *
 *   http://www.apache.org/licenses/LICENSE-2.0
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package org.orekit.propagation.semianalytical.dsst.forces;

import java.util.List;

import org.hipparchus.Field;
import org.hipparchus.CalculusFieldElement;
import org.hipparchus.util.MathArrays;
import org.orekit.attitudes.AttitudeProvider;
import org.orekit.propagation.FieldSpacecraftState;
import org.orekit.propagation.PropagationType;
import org.orekit.propagation.SpacecraftState;
import org.orekit.propagation.events.EventDetector;
import org.orekit.propagation.events.FieldEventDetector;
import org.orekit.propagation.semianalytical.dsst.utilities.AuxiliaryElements;
import org.orekit.propagation.semianalytical.dsst.utilities.FieldAuxiliaryElements;
import org.orekit.time.AbsoluteDate;
import org.orekit.utils.ParameterDriver;

/** This interface represents a force modifying spacecraft motion for a {@link
 *  org.orekit.propagation.semianalytical.dsst.DSSTPropagator DSSTPropagator}.
 *  <p>
 *  Objects implementing this interface are intended to be added to a {@link
 *  org.orekit.propagation.semianalytical.dsst.DSSTPropagator DSST propagator}
 *  before the propagation is started.
 *  </p>
 *  <p>
 *  The propagator will call at the very beginning of a propagation the {@link
 *  #initializeShortPeriodTerms(AuxiliaryElements, PropagationType, double[])} method allowing
 *  preliminary computation such as truncation if needed.
 *  </p>
 *  <p>
 *  Then the propagator will call at each step:
 *  <ol>
 *  <li>the {@link #getMeanElementRate(SpacecraftState, AuxiliaryElements, double[])} method.
 *  The force model instance will extract all the state data needed to compute
 *  the mean element rates that contribute to the mean state derivative.</li>
 *  <li>the {@link #updateShortPeriodTerms(double[], SpacecraftState...)} method,
 *  if osculating parameters are desired, on a sample of points within the
 *  last step.</li>
 *  </ol>
 *
 * @author Romain Di Constanzo
 * @author Pascal Parraud
 */
public interface DSSTForceModel {

    /**
     * Initialize the force model at the start of propagation.
     * <p> The default implementation of this method does nothing.</p>
     *
     * @param initialState spacecraft state at the start of propagation.
     * @param target       date of propagation. Not equal to {@code initialState.getDate()}.
     * @since 11.0
     */
    default void init(SpacecraftState initialState, AbsoluteDate target) {
    }

    /** Performs initialization prior to propagation for the current force model.
     *  <p>
     *  This method aims at being called at the very beginning of a propagation.
     *  </p>
     *  @param auxiliaryElements auxiliary elements related to the current orbit
     *  @param type type of the elements used during the propagation
     *  @param parameters values of the force model parameters
     *  @return a list of objects that will hold short period terms (the objects
     *  are also retained by the force model, which will update them during propagation)
     */
    List<ShortPeriodTerms> initializeShortPeriodTerms(AuxiliaryElements auxiliaryElements,
                                                      PropagationType type, double[] parameters);

    /** Performs initialization prior to propagation for the current force model.
     *  <p>
     *  This method aims at being called at the very beginning of a propagation.
     *  </p>
     *  @param <T> type of the elements
     *  @param auxiliaryElements auxiliary elements related to the current orbit
     *  @param type type of the elements used during the propagation
     *  @param parameters values of the force model parameters
     *  @return a list of objects that will hold short period terms (the objects
     *  are also retained by the force model, which will update them during propagation)
     */
    <T extends CalculusFieldElement<T>> List<FieldShortPeriodTerms<T>> initializeShortPeriodTerms(FieldAuxiliaryElements<T> auxiliaryElements,
                                                                                              PropagationType type, T[] parameters);

    /** Get force model parameters.
     * @return force model parameters
     * @since 9.0
     */
    default double[] getParameters() {
        final List<ParameterDriver> drivers = getParametersDrivers();
        final double[] parameters = new double[drivers.size()];
        for (int i = 0; i < drivers.size(); ++i) {
            parameters[i] = drivers.get(i).getValue();
        }
        return parameters;
    }

    /** Get force model parameters.
     * @param field field to which the elements belong
     * @param <T> type of the elements
     * @return force model parameters
     * @since 9.0
     */
    default <T extends CalculusFieldElement<T>> T[] getParameters(final Field<T> field) {
        final List<ParameterDriver> drivers = getParametersDrivers();
        final T[] parameters = MathArrays.buildArray(field, drivers.size());
        for (int i = 0; i < drivers.size(); ++i) {
            parameters[i] = field.getZero().add(drivers.get(i).getValue());
        }
        return parameters;
    }

    /** Computes the mean equinoctial elements rates da<sub>i</sub> / dt.
     *
     *  @param state current state information: date, kinematics, attitude
     *  @param auxiliaryElements auxiliary elements related to the current orbit
     *  @param parameters values of the force model parameters
     *  @return the mean element rates dai/dt
     */
    double[] getMeanElementRate(SpacecraftState state,
                                AuxiliaryElements auxiliaryElements, double[] parameters);

    /** Computes the mean equinoctial elements rates da<sub>i</sub> / dt.
     *
     *  @param <T> type of the elements
     *  @param state current state information: date, kinematics, attitude
     *  @param auxiliaryElements auxiliary elements related to the current orbit
     *  @param parameters values of the force model parameters
     *  @return the mean element rates dai/dt
     */
    <T extends CalculusFieldElement<T>> T[] getMeanElementRate(FieldSpacecraftState<T> state,
                                                           FieldAuxiliaryElements<T> auxiliaryElements, T[] parameters);


    /** Get the discrete events related to the model.
     * @return array of events detectors or null if the model is not
     * related to any discrete events
     */
    EventDetector[] getEventsDetectors();

    /** Get the discrete events related to the model.
     * @param <T> type of the elements
     * @param field field used by default
     * @return array of events detectors or null if the model is not
     * related to any discrete events
     */
    <T extends CalculusFieldElement<T>> FieldEventDetector<T>[] getFieldEventsDetectors(Field<T> field);

    /** Register an attitude provider.
     * <p>
     * Register an attitude provider that can be used by the force model.
     * </p>
     * @param provider the {@link AttitudeProvider}
     */
    void registerAttitudeProvider(AttitudeProvider provider);

    /** Update the short period terms.
     * <p>
     * The {@link ShortPeriodTerms short period terms} that will be updated
     * are the ones that were returned during the call to {@link
     * #initializeShortPeriodTerms(AuxiliaryElements, PropagationType, double[])}.
     * </p>
     * @param parameters values of the force model parameters
     * @param meanStates mean states information: date, kinematics, attitude
     */
    void updateShortPeriodTerms(double[] parameters, SpacecraftState... meanStates);

    /** Update the short period terms.
     * <p>
     * The {@link ShortPeriodTerms short period terms} that will be updated
     * are the ones that were returned during the call to {@link
     * #initializeShortPeriodTerms(AuxiliaryElements, PropagationType, double[])}.
     * </p>
     * @param <T> type of the elements
     * @param parameters values of the force model parameters
     * @param meanStates mean states information: date, kinematics, attitude
     */
    @SuppressWarnings("unchecked")
    <T extends CalculusFieldElement<T>> void updateShortPeriodTerms(T[] parameters, FieldSpacecraftState<T>... meanStates);

    /** Get the drivers for force model parameters.
     * @return drivers for force model parameters
     */
    List<ParameterDriver> getParametersDrivers();

}