/* Copyright 2002-2021 CS GROUP
    * Licensed to CS GROUP (CS) under one or more
    * 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
    * 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.orekit.forces.drag;
  
  import java.util.List;
  import java.util.stream.Stream;
  
  import org.hipparchus.Field;
  import org.hipparchus.CalculusFieldElement;
  import org.hipparchus.analysis.differentiation.DerivativeStructure;
  import org.hipparchus.analysis.differentiation.Gradient;
  import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
  import org.hipparchus.geometry.euclidean.threed.Vector3D;
  import org.orekit.frames.Frame;
  import org.orekit.models.earth.atmosphere.Atmosphere;
  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;
  
  
  /** Atmospheric drag force model.
   *
   * The drag acceleration is computed as follows :
   *
   * γ = (1/2 * ρ * V² * S / Mass) * DragCoefVector
   *
   * With DragCoefVector = {C<sub>x</sub>, C<sub>y</sub>, C<sub>z</sub>} and S given by the user through the interface
   * {@link DragSensitive}
   *
   * @author &Eacute;douard Delente
   * @author Fabien Maussion
   * @author V&eacute;ronique Pommier-Maurussane
   * @author Pascal Parraud
   */
  
  public class DragForce extends AbstractDragForceModel {
  
      /** Atmospheric model. */
      private final Atmosphere atmosphere;
  
      /** Spacecraft. */
      private final DragSensitive spacecraft;
  
      /** Simple constructor.
       * @param atmosphere atmospheric model
       * @param spacecraft the object physical and geometrical information
       */
      public DragForce(final Atmosphere atmosphere, final DragSensitive spacecraft) {
          super(atmosphere);
          this.atmosphere = atmosphere;
          this.spacecraft = spacecraft;
      }
  
      /** {@inheritDoc} */
      @Override
      public Vector3D acceleration(final SpacecraftState s, final double[] parameters) {
  
          final AbsoluteDate date     = s.getDate();
          final Frame        frame    = s.getFrame();
          final Vector3D     position = s.getPVCoordinates().getPosition();
  
          final double rho    = atmosphere.getDensity(date, position, frame);
          final Vector3D vAtm = atmosphere.getVelocity(date, position, frame);
          final Vector3D relativeVelocity = vAtm.subtract(s.getPVCoordinates().getVelocity());
  
          return spacecraft.dragAcceleration(date, frame, position, s.getAttitude().getRotation(),
                                             s.getMass(), rho, relativeVelocity, parameters);
  
      }
  
      /** {@inheritDoc} */
      @SuppressWarnings("unchecked")
      @Override
      public <T extends CalculusFieldElement<T>> FieldVector3D<T> acceleration(final FieldSpacecraftState<T> s,
                                                                           final T[] parameters) {
  
          final FieldAbsoluteDate<T> date     = s.getDate();
          final Frame                frame    = s.getFrame();
          final FieldVector3D<T>     position = s.getPVCoordinates().getPosition();
  
          // Density and its derivatives
         final T rho;
 
         // Check for faster computation dedicated to derivatives with respect to state
         // Using finite differences instead of automatic differentiation as it seems to be much
         // faster for the drag's derivatives' computation
         if (isGradientStateDerivative(s)) {
             rho =  (T) this.getGradientDensityWrtStateUsingFiniteDifferences(date.toAbsoluteDate(), frame, (FieldVector3D<Gradient>) position);
         } else if (isDSStateDerivative(s)) {
             rho = (T) this.getDSDensityWrtStateUsingFiniteDifferences(date.toAbsoluteDate(), frame, (FieldVector3D<DerivativeStructure>) position);
         } else {
             rho = atmosphere.getDensity(date, position, frame);
         }
 
         // Spacecraft relative velocity with respect to the atmosphere
         final FieldVector3D<T> vAtm = atmosphere.getVelocity(date, position, frame);
         final FieldVector3D<T> relativeVelocity = vAtm.subtract(s.getPVCoordinates().getVelocity());
 
         // Drag acceleration along with its derivatives
         return spacecraft.dragAcceleration(date, frame, position, s.getAttitude().getRotation(),
                                            s.getMass(), rho, relativeVelocity, parameters);
 
     }
 
     /** {@inheritDoc} */
     @Override
     public List<ParameterDriver> getParametersDrivers() {
         return spacecraft.getDragParametersDrivers();
     }
 
     /** {@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();
     }
 
     /** Get the atmospheric model.
      * @return atmosphere model
      */
     public Atmosphere getAtmosphere() {
         return atmosphere;
     }
 
     /** Get spacecraft that are sensitive to atmospheric drag forces.
      * @return drag sensitive spacecraft model
      */
     public DragSensitive getSpacecraft() {
         return spacecraft;
     }
 }