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    * contributor license agreements.  See the NOTICE file distributed with
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    * 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
    * the License.  You may obtain a copy of the License at
    *
    *   http://www.apache.org/licenses/LICENSE-2.0
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   * Unless required by applicable law or agreed to in writing, software
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  package org.orekit.forces.empirical;
  
  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.List;
  import java.util.NavigableSet;
  import java.util.stream.Stream;
  
  import org.hipparchus.Field;
  import org.hipparchus.CalculusFieldElement;
  import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
  import org.hipparchus.geometry.euclidean.threed.Vector3D;
  import org.hipparchus.util.MathArrays;
  import org.orekit.attitudes.Attitude;
  import org.orekit.attitudes.AttitudeProvider;
  import org.orekit.attitudes.FieldAttitude;
  import org.orekit.forces.AbstractForceModel;
  import org.orekit.propagation.FieldSpacecraftState;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.propagation.events.EventDetector;
  import org.orekit.propagation.events.FieldEventDetector;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.time.FieldAbsoluteDate;
  import org.orekit.utils.ParameterDriver;
  import org.orekit.utils.TimeSpanMap;
  import org.orekit.utils.TimeSpanMap.Span;
  import org.orekit.utils.TimeSpanMap.Transition;
  
  /** Time span parametric acceleration model.
   *  <p>
   *  This class is closely related to {@link org.orekit.forces.empirical.ParametricAcceleration ParametricAcceleration} class.<br>
   *  The difference is that it has a {@link TimeSpanMap} of {@link AccelerationModel} objects as attribute
   *  instead of a single {@link AccelerationModel} object. <br>
   *  The idea behind this model is to allow the user to design a parametric acceleration model that can see its physical parameters
   *  change with time, at dates chosen by the user. <br>
   *  </p>
   *  <p>
   *  This is a behavior that can be sought in precise orbit determination.<br>
   *  Indeed for this type of application, the empirical parameters must be revalued at
   *  each new orbit.
   *  </p>
   *  <b>Usage</b>:<ul>
   *  <li><u>Construction</u>: constructor takes an acceleration direction, an attitude mode (or an inertial flag) and
   *  an AccelerationModel model.<br>
   *  This last model will be your initial AccelerationModel model and it will be initially valid for the whole time line.<br>
   *  The real validity of this first entry will be truncated as other AccelerationModel models are added.
   *  <li><u>Time spans</u>: AccelerationModel models are added using methods {@link #addAccelerationModelValidAfter(AccelerationModel, AbsoluteDate)}
   *   or {@link #addAccelerationModelValidBefore(AccelerationModel, AbsoluteDate)}.<br>
   *   Recommendations are the same than the ones in {@link TimeSpanMap}, meaning: <ul>
   *   <li>As an entry is added, it truncates the validity of the neighboring entries already present in the map;
   *   <li><b>The transition dates should be entered only once</b>. Repeating a transition date will lead to unexpected result and is not supported;
   *   <li>It is advised to order your AccelerationModel models chronologically when adding them to avoid any confusion.
   *   </ul>
   *   <li><u>Naming the parameter drivers</u>: It is strongly advised to give a custom name to the {@link ParameterDriver}(s)
   *   of each AccelerationModel model that is added to the object. This will allow you keeping track of the evolution of your models.<br>
   *   Different names are mandatory to differentiate the different drivers.<br>
   *   Since there is no default name for acceleration model parameters, you must handle the driver names to consider
   *   different names when adding a new acceleration model.
   *   </ul>
   * @author Bryan Cazabonne
   * @since 10.3
   */
  public class TimeSpanParametricAcceleration extends AbstractForceModel {
  
      /** Prefix for dates before in the parameter drivers' name. */
      public static final String DATE_BEFORE = " - Before ";
  
      /** Prefix for dates after in the parameter drivers' name. */
      public static final String DATE_AFTER = " - After ";
  
      /** Direction of the acceleration in defining frame. */
      private final Vector3D direction;
  
      /** Flag for inertial acceleration direction. */
      private final boolean isInertial;
  
      /** The attitude to override, if set. */
      private final AttitudeProvider attitudeOverride;
  
      /** TimeSpanMap of AccelerationModel objects. */
      private final TimeSpanMap<AccelerationModel> accelerationModelTimeSpanMap;
  
      /** Simple constructor.
      * @param direction acceleration direction in overridden spacecraft frame
      * @param isInertial if true, direction is defined in the same inertial
      * frame used for propagation (i.e. {@link SpacecraftState#getFrame()}),
      * otherwise direction is defined in spacecraft frame (i.e. using the
      * propagation {@link
      * org.orekit.propagation.Propagator#setAttitudeProvider(AttitudeProvider)
      * attitude law})
      * @param accelerationModel acceleration model used to compute the contribution of the empirical acceleration
      */
     public TimeSpanParametricAcceleration(final Vector3D direction,
                                           final boolean isInertial,
                                           final AccelerationModel accelerationModel) {
         this(direction, isInertial, null, accelerationModel);
     }
 
     /** Simple constructor.
      * @param direction acceleration direction in overridden spacecraft frame
      * frame used for propagation (i.e. {@link SpacecraftState#getFrame()}),
      * otherwise direction is defined in spacecraft frame (i.e. using the
      * propagation {@link
      * org.orekit.propagation.Propagator#setAttitudeProvider(AttitudeProvider)
      * attitude law})
      * @param attitudeOverride provider for attitude used to compute acceleration
      * @param accelerationModel acceleration model used to compute the contribution of the empirical acceleration
      */
     public TimeSpanParametricAcceleration(final Vector3D direction,
                                           final AttitudeProvider attitudeOverride,
                                           final AccelerationModel accelerationModel) {
         this(direction, false, attitudeOverride, accelerationModel);
     }
 
     /** Simple constructor.
      * @param direction acceleration direction in overridden spacecraft frame
      * @param isInertial if true, direction is defined in the same inertial
      * frame used for propagation (i.e. {@link SpacecraftState#getFrame()}),
      * otherwise direction is defined in spacecraft frame (i.e. using the
      * propagation {@link
      * org.orekit.propagation.Propagator#setAttitudeProvider(AttitudeProvider)
      * attitude law})
      * @param attitudeOverride provider for attitude used to compute acceleration
      * @param accelerationModel acceleration model used to compute the contribution of the empirical acceleration
      */
     private TimeSpanParametricAcceleration(final Vector3D direction,
                                            final boolean isInertial,
                                            final AttitudeProvider attitudeOverride,
                                            final AccelerationModel accelerationModel) {
         this.direction                    = direction;
         this.isInertial                   = isInertial;
         this.attitudeOverride             = attitudeOverride;
         this.accelerationModelTimeSpanMap = new TimeSpanMap<>(accelerationModel);
     }
 
     /** {@inheritDoc} */
     @Override
     public void init(final SpacecraftState initialState, final AbsoluteDate target) {
         accelerationModelTimeSpanMap.forEach(accelerationModel -> accelerationModel.init(initialState, target));
     }
 
     /** Add an AccelerationModel entry valid before a limit date.<br>
      * <p>
      * Using <code>addAccelerationModelValidBefore(entry, t)</code> will make <code>entry</code>
      * valid in ]-∞, t[ (note the open bracket).
      * <p>
      * <b>WARNING</b>: Since there is no default name for acceleration model parameters,
      * the user must handle itself the driver names to consider different names
      * (i.e. different parameters) when adding a new acceleration model.
      * @param accelerationModel AccelerationModel entry
      * @param latestValidityDate date before which the entry is valid
      * (must be different from <b>all</b> dates already used for transitions)
      */
     public void addAccelerationModelValidBefore(final AccelerationModel accelerationModel, final AbsoluteDate latestValidityDate) {
         accelerationModelTimeSpanMap.addValidBefore(accelerationModel, latestValidityDate);
     }
 
     /** Add a AccelerationModel entry valid after a limit date.<br>
      * <p>
      * Using <code>addAccelerationModelValidAfter(entry, t)</code> will make <code>entry</code>
      * valid in [t, +∞[ (note the closed bracket).
      * <p>
      * <b>WARNING</b>: Since there is no default name for acceleration model parameters,
      * the user must handle itself the driver names to consider different names
      * (i.e. different parameters) when adding a new acceleration model.
      * @param accelerationModel AccelerationModel entry
      * @param earliestValidityDate date after which the entry is valid
      * (must be different from <b>all</b> dates already used for transitions)
      */
     public void addAccelerationModelValidAfter(final AccelerationModel accelerationModel, final AbsoluteDate earliestValidityDate) {
         accelerationModelTimeSpanMap.addValidAfter(accelerationModel, earliestValidityDate);
     }
 
     /** Get the {@link AccelerationModel} model valid at a date.
      * @param date the date of validity
      * @return the AccelerationModel model valid at date
      */
     public AccelerationModel getAccelerationModel(final AbsoluteDate date) {
         return accelerationModelTimeSpanMap.get(date);
     }
 
     /** Get the {@link AccelerationModel} {@link Span} containing a specified date.
      * @param date date belonging to the desired time span
      * @return the AccelerationModel time span containing the specified date
      */
     public Span<AccelerationModel> getAccelerationModelSpan(final AbsoluteDate date) {
         return accelerationModelTimeSpanMap.getSpan(date);
     }
 
     /** Extract a range of the {@link AccelerationModel} map.
      * <p>
      * The object returned will be a new independent instance that will contain
      * only the transitions that lie in the specified range.
      * </p>
      * See the {@link TimeSpanMap#extractRange TimeSpanMap.extractRange method} for more.
      * @param start earliest date at which a transition is included in the range
      * (may be set to {@link AbsoluteDate#PAST_INFINITY} to keep all early transitions)
      * @param end latest date at which a transition is included in the r
      * (may be set to {@link AbsoluteDate#FUTURE_INFINITY} to keep all late transitions)
      * @return a new TimeSpanMap instance of AccelerationModel with all transitions restricted to the specified range
      */
     public TimeSpanMap<AccelerationModel> extractAccelerationModelRange(final AbsoluteDate start, final AbsoluteDate end) {
         return accelerationModelTimeSpanMap.extractRange(start, end);
     }
 
     /** Get the {@link Transition}s of the acceleration model time span map.
      * @return the {@link Transition}s for the acceleration model time span map
      */
     public NavigableSet<Transition<AccelerationModel>> getTransitions() {
         return accelerationModelTimeSpanMap.getTransitions();
     }
 
     /** {@inheritDoc} */
     @Override
     public boolean dependsOnPositionOnly() {
         return isInertial;
     }
 
     /** {@inheritDoc} */
     @Override
     public Vector3D acceleration(final SpacecraftState state,
                                  final double[] parameters) {
 
         // Date
         final AbsoluteDate date = state.getDate();
 
         // Compute inertial direction
         final Vector3D inertialDirection;
         if (isInertial) {
             // the acceleration direction is already defined in the inertial frame
             inertialDirection = direction;
         } else {
             final Attitude attitude;
             if (attitudeOverride == null) {
                 // the acceleration direction is defined in spacecraft frame as set by the propagator
                 attitude = state.getAttitude();
             } else {
                 // the acceleration direction is defined in a dedicated frame
                 attitude = attitudeOverride.getAttitude(state.getOrbit(), date, state.getFrame());
             }
             inertialDirection = attitude.getRotation().applyInverseTo(direction);
         }
 
         // Extract the proper parameters valid at date from the input array
         final double[] extractedParameters = extractParameters(parameters, date);
 
         // Compute and return the parametric acceleration
         return new Vector3D(getAccelerationModel(date).signedAmplitude(state, extractedParameters), inertialDirection);
 
     }
 
     /** {@inheritDoc} */
     @Override
     public <T extends CalculusFieldElement<T>> FieldVector3D<T> acceleration(final FieldSpacecraftState<T> state,
                                                                          final T[] parameters) {
 
         // Date
         final FieldAbsoluteDate<T> date = state.getDate();
 
         // Compute inertial direction
         final FieldVector3D<T> inertialDirection;
         if (isInertial) {
             // the acceleration direction is already defined in the inertial frame
             inertialDirection = new FieldVector3D<>(state.getDate().getField(), direction);
         } else {
             final FieldAttitude<T> attitude;
             if (attitudeOverride == null) {
                 // the acceleration direction is defined in spacecraft frame as set by the propagator
                 attitude = state.getAttitude();
             } else {
                 // the acceleration direction is defined in a dedicated frame
                 attitude = attitudeOverride.getAttitude(state.getOrbit(), date, state.getFrame());
             }
             inertialDirection = attitude.getRotation().applyInverseTo(direction);
         }
 
         // Extract the proper parameters valid at date from the input array
         final T[] extractedParameters = extractParameters(parameters, date);
 
         // Compute and return the parametric acceleration
         return new FieldVector3D<>(getAccelerationModel(date.toAbsoluteDate()).signedAmplitude(state, extractedParameters), inertialDirection);
 
     }
 
     /** {@inheritDoc} */
     @Override
     public Stream<EventDetector> getEventsDetectors() {
         return Stream.empty();
     }
 
     /** {@inheritDoc} */
     @Override
     public <T extends CalculusFieldElement<T>> Stream<FieldEventDetector<T>> getFieldEventsDetectors(final Field<T> field) {
         return Stream.empty();
     }
 
     /** {@inheritDoc}
      * <p>
      * All the parameter drivers of all AccelerationModel models are returned in an array.
      * Models are ordered chronologically.
      * </p>
      */
     @Override
     public List<ParameterDriver> getParametersDrivers() {
 
         // Get all transitions from the TimeSpanMap
         final List<ParameterDriver> listParameterDrivers = new ArrayList<>();
         final NavigableSet<Transition<AccelerationModel>> accelerationModelTransitions =  getTransitions();
 
         // Loop on the transitions
         for (Transition<AccelerationModel> transition : accelerationModelTransitions) {
             // Add all the "before" parameter drivers of each transition
             for (ParameterDriver driver : transition.getBefore().getParametersDrivers()) {
                 // Add the driver only if the name does not exist already
                 if (!findByName(listParameterDrivers, driver.getName())) {
                     listParameterDrivers.add(driver);
                 }
             }
         }
 
         // Finally, add the "after" parameter drivers of the last transition
         for (ParameterDriver driver : accelerationModelTransitions.last().getAfter().getParametersDrivers()) {
             // Adds only if the name does not exist already
             if (!findByName(listParameterDrivers, driver.getName())) {
                 listParameterDrivers.add(driver);
             }
         }
 
         // Return an array of parameter drivers with no duplicated name
         return Collections.unmodifiableList(listParameterDrivers);
 
     }
 
     /** Extract the proper parameter drivers' values from the array in input of the
      * {@link #acceleration(SpacecraftState, double[]) acceleration} method.
      *  Parameters are filtered given an input date.
      * @param parameters the input parameters array
      * @param date the date
      * @return the parameters given the date
      */
     public double[] extractParameters(final double[] parameters, final AbsoluteDate date) {
 
         // Get the acceleration model parameter drivers of the date
         final List<ParameterDriver> empiricalParameterDriver = getAccelerationModel(date).getParametersDrivers();
 
         // Find out the indexes of the parameters in the whole array of parameters
         final List<ParameterDriver> allParameters = getParametersDrivers();
         final double[] outParameters = new double[empiricalParameterDriver.size()];
         int index = 0;
         for (int i = 0; i < allParameters.size(); i++) {
             final String driverName = allParameters.get(i).getName();
             for (ParameterDriver accDriver : empiricalParameterDriver) {
                 if (accDriver.getName().equals(driverName)) {
                     outParameters[index++] = parameters[i];
                 }
             }
         }
         return outParameters;
     }
 
     /** Extract the proper parameter drivers' values from the array in input of the
      * {@link #acceleration(FieldSpacecraftState, CalculusFieldElement[]) acceleration} method.
      *  Parameters are filtered given an input date.
      * @param parameters the input parameters array
      * @param date the date
      * @param <T> extends CalculusFieldElement
      * @return the parameters given the date
      */
     public <T extends CalculusFieldElement<T>> T[] extractParameters(final T[] parameters,
                                                                  final FieldAbsoluteDate<T> date) {
 
         // Get the acceleration parameter drivers of the date
         final List<ParameterDriver> empiricalParameterDriver = getAccelerationModel(date.toAbsoluteDate()).getParametersDrivers();
 
         // Find out the indexes of the parameters in the whole array of parameters
         final List<ParameterDriver> allParameters = getParametersDrivers();
         final T[] outParameters = MathArrays.buildArray(date.getField(), empiricalParameterDriver.size());
         int index = 0;
         for (int i = 0; i < allParameters.size(); i++) {
             final String driverName = allParameters.get(i).getName();
             for (ParameterDriver accDriver : empiricalParameterDriver) {
                 if (accDriver.getName().equals(driverName)) {
                     outParameters[index++] = parameters[i];
                 }
             }
         }
         return outParameters;
     }
 
     /** Find if a parameter driver with a given name already exists in a list of parameter drivers.
      * @param driversList the list of parameter drivers
      * @param name the parameter driver's name to filter with
      * @return true if the name was found, false otherwise
      */
     private boolean findByName(final List<ParameterDriver> driversList, final String name) {
         for (final ParameterDriver d : driversList) {
             if (d.getName().equals(name)) {
                 return true;
             }
         }
         return false;
     }
 
 }