/* 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.gravity;
  
  import java.util.Collections;
  import java.util.List;
  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.FastMath;
  import org.orekit.bodies.CelestialBodies;
  import org.orekit.bodies.CelestialBody;
  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.utils.ParameterDriver;
  
  /** Third body attraction force model.
   *
   * @author Fabien Maussion
   * @author Véronique Pommier-Maurussane
   */
  public class ThirdBodyAttraction extends AbstractForceModel {
  
      /** 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);
  
      /** Drivers for third body attraction coefficient. */
      private final ParameterDriver gmParameterDriver;
  
      /** The body to consider. */
      private final CelestialBody body;
  
      /** Simple constructor.
       * @param body the third body to consider
       * (ex: {@link CelestialBodies#getSun()} or
       * {@link CelestialBodies#getMoon()})
       */
      public ThirdBodyAttraction(final CelestialBody body) {
          gmParameterDriver = new ParameterDriver(body.getName() + ATTRACTION_COEFFICIENT_SUFFIX,
                                                  body.getGM(), MU_SCALE,
                                                  0.0, Double.POSITIVE_INFINITY);
  
          this.body = body;
      }
  
      /** {@inheritDoc} */
      @Override
      public boolean dependsOnPositionOnly() {
          return true;
      }
  
      /** {@inheritDoc} */
      @Override
      public Vector3D acceleration(final SpacecraftState s, final double[] parameters) {
  
          final double gm = parameters[0];
  
          // compute bodies separation vectors and squared norm
          final Vector3D centralToBody = body.getPVCoordinates(s.getDate(), s.getFrame()).getPosition();
          final double r2Central       = centralToBody.getNormSq();
          final Vector3D satToBody     = centralToBody.subtract(s.getPVCoordinates().getPosition());
          final double r2Sat           = satToBody.getNormSq();
  
          // compute relative acceleration
          return new Vector3D(gm / (r2Sat * FastMath.sqrt(r2Sat)), satToBody,
                             -gm / (r2Central * FastMath.sqrt(r2Central)), centralToBody);
  
      }
  
      /** {@inheritDoc} */
      @Override
     public <T extends CalculusFieldElement<T>> FieldVector3D<T> acceleration(final FieldSpacecraftState<T> s,
                                                                          final T[] parameters) {
 
         final T gm = parameters[0];
 
         // compute bodies separation vectors and squared norm
         final FieldVector3D<T> centralToBody = new FieldVector3D<>(s.getA().getField(),
                                                                    body.getPVCoordinates(s.getDate().toAbsoluteDate(), s.getFrame()).getPosition());
         final T                r2Central     = centralToBody.getNormSq();
         final FieldVector3D<T> satToBody     = centralToBody.subtract(s.getPVCoordinates().getPosition());
         final T                r2Sat         = satToBody.getNormSq();
 
         // compute relative acceleration
         return new FieldVector3D<>(r2Sat.multiply(r2Sat.sqrt()).reciprocal().multiply(gm), satToBody,
                                    r2Central.multiply(r2Central.sqrt()).reciprocal().multiply(gm).negate(), centralToBody);
 
     }
 
     /** {@inheritDoc} */
     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} */
     @Override
     public List<ParameterDriver> getParametersDrivers() {
         return Collections.singletonList(gmParameterDriver);
     }
 
 }