/* Copyright 2002-2025 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
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   */
  package org.orekit.forces.gravity;
  
  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.propagation.FieldSpacecraftState;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.utils.ExtendedPositionProvider;
  
  /** Third body attraction force model.
   *
   * @author Fabien Maussion
   * @author Véronique Pommier-Maurussane
   */
  public class ThirdBodyAttraction extends AbstractBodyAttraction {
  
      /** Simple constructor.
       * @param positionProvider extended position provider for the body to consider
       * @param name name of the body
       * @param mu body gravitational constant
       * @since 13.0
       */
      public ThirdBodyAttraction(final ExtendedPositionProvider positionProvider, final String name, final double mu) {
          super(positionProvider, name, mu);
      }
  
      /** Constructor.
       * @param body the third body to consider
       * (ex: {@link CelestialBodies#getSun()} or
       * {@link CelestialBodies#getMoon()})
       */
      public ThirdBodyAttraction(final CelestialBody body) {
          this(body, body.getName(), body.getGM());
      }
  
      /** {@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 = getBodyPosition(s.getDate(), s.getFrame());
          final double r2Central       = centralToBody.getNormSq();
          final Vector3D satToBody     = centralToBody.subtract(s.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 = getBodyPosition(s.getDate(), s.getFrame());
          final T                r2Central     = centralToBody.getNormSq();
          final FieldVector3D<T> satToBody     = centralToBody.subtract(s.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);
  
      }
  
  }