/* Contributed to the public domain
    * 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.gravity;
  
  import java.util.Collections;
  import java.util.List;
  
  import org.hipparchus.CalculusFieldElement;
  import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
  import org.hipparchus.geometry.euclidean.threed.Vector3D;
  import org.hipparchus.util.FastMath;
  import org.orekit.annotation.DefaultDataContext;
  import org.orekit.bodies.CelestialBody;
  import org.orekit.data.DataContext;
  import org.orekit.forces.ForceModel;
  import org.orekit.propagation.FieldSpacecraftState;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.utils.Constants;
  import org.orekit.utils.ExtendedPositionProvider;
  import org.orekit.utils.FieldPVCoordinates;
  import org.orekit.utils.PVCoordinates;
  import org.orekit.utils.ParameterDriver;
  
  /**
   * De Sitter post-Newtonian correction force due to general relativity.
   * <p>
   * De Sitter term causes a precession of the orbital plane at a rate of 19 mas per year.
   * </p>
   * @see "Petit, G. and Luzum, B. (eds.), IERS Conventions (2010), Chapter 10,
   * General relativistic models for space-time coordinates and equations of motion (2010)"
   *
   * @author Bryan Cazabonne
   * @since 10.3
   */
  public class DeSitterRelativity implements ForceModel {
  
      /** Suffix for parameter name for attraction coefficient enabling Jacobian processing. */
      public static final String ATTRACTION_COEFFICIENT_SUFFIX = " attraction coefficient";
  
      /** Central attraction scaling factor.
       * <p>
       * We use a power of 2 to avoid numeric noise introduction
       * in the multiplications/divisions sequences.
       * </p>
       */
      private static final double MU_SCALE = FastMath.scalb(1.0, 32);
  
      /** The Sun. */
      private final CelestialBody sun;
  
      /** The Earth. */
      private final ExtendedPositionProvider earth;
  
      /** Driver for gravitational parameter. */
      private final ParameterDriver gmParameterDriver;
  
      /**
       * Constructor.
       * <p>It uses the {@link DataContext#getDefault()} to initialize the celestial bodies.</p>
       */
      @DefaultDataContext
      public DeSitterRelativity() {
          this(DataContext.getDefault().getCelestialBodies().getEarth(),
               DataContext.getDefault().getCelestialBodies().getSun());
      }
  
      /**
       * Simple constructor.
       * @param earth the Earth
       * @param sun the Sun
       */
      public DeSitterRelativity(final CelestialBody earth, final CelestialBody sun) {
          gmParameterDriver = new ParameterDriver(sun.getName() + ATTRACTION_COEFFICIENT_SUFFIX,
                                                  sun.getGM(), MU_SCALE,
                                                  0.0, Double.POSITIVE_INFINITY);
          this.earth = earth;
          this.sun   = sun;
      }
  
      /**
       * Get the sun model used to compute De Sitter effect.
       * @return the sun model
       */
      public CelestialBody getSun() {
          return sun;
     }
 
     /**
      * Get the Earth model used to compute De Sitter effect.
      * @return the earth model
      */
     public ExtendedPositionProvider getEarth() {
         return earth;
     }
 
     /** {@inheritDoc} */
     @Override
     public boolean dependsOnPositionOnly() {
         return false;
     }
 
     /** {@inheritDoc} */
     @Override
     public Vector3D acceleration(final SpacecraftState s, final double[] parameters) {
 
         // Useful constant
         final double c2 = Constants.SPEED_OF_LIGHT * Constants.SPEED_OF_LIGHT;
 
         // Sun's gravitational parameter
         final double gm = parameters[0];
 
         // Satellite velocity with respect to the Earth
         final PVCoordinates pvSat = s.getPVCoordinates();
         final Vector3D vSat = pvSat.getVelocity();
 
         // Coordinates of the Earth with respect to the Sun
         final PVCoordinates pvEarth = earth.getPVCoordinates(s.getDate(), sun.getInertiallyOrientedFrame());
         final Vector3D pEarth = pvEarth.getPosition();
         final Vector3D vEarth = pvEarth.getVelocity();
 
         // Radius
         final double r  = pEarth.getNorm();
         final double r3 = r * r * r;
 
         // Eq. 10.12
         return new Vector3D((-3.0 * gm) / (c2 * r3), vEarth.crossProduct(pEarth).crossProduct(vSat));
     }
 
     /** {@inheritDoc} */
     @Override
     public <T extends CalculusFieldElement<T>> FieldVector3D<T> acceleration(final FieldSpacecraftState<T> s,
                                                                          final T[] parameters) {
 
         // Useful constant
         final double c2 = Constants.SPEED_OF_LIGHT * Constants.SPEED_OF_LIGHT;
 
         // Sun's gravitational parameter
         final T gm = parameters[0];
 
         // Satellite velocity with respect to the Earth
         final FieldPVCoordinates<T> pvSat = s.getPVCoordinates();
         final FieldVector3D<T> vSat = pvSat.getVelocity();
 
         // Coordinates of the Earth with respect to the Sun
         final FieldPVCoordinates<T> pvEarth = earth.getPVCoordinates(s.getDate(), sun.getInertiallyOrientedFrame());
         final FieldVector3D<T> pEarth = pvEarth.getPosition();
         final FieldVector3D<T> vEarth = pvEarth .getVelocity();
 
         // Radius
         final T r  = pEarth.getNorm();
         final T r3 = r.multiply(r).multiply(r);
 
         // Eq. 10.12
         return new FieldVector3D<>(gm.multiply(-3.0).divide(r3.multiply(c2)), vEarth.crossProduct(pEarth).crossProduct(vSat));
     }
 
     /** {@inheritDoc} */
     @Override
     public List<ParameterDriver> getParametersDrivers() {
         return Collections.singletonList(gmParameterDriver);
     }
 
 }