/* Copyright 2010-2011 Centre National d'Études Spatiales
    * 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.Field;
  import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
  import org.hipparchus.geometry.euclidean.threed.Vector3D;
  import org.hipparchus.util.FastMath;
  import org.orekit.forces.ForceModel;
  import org.orekit.propagation.FieldSpacecraftState;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.propagation.numerical.FieldTimeDerivativesEquations;
  import org.orekit.propagation.numerical.TimeDerivativesEquations;
  import org.orekit.time.TimeInterval;
  import org.orekit.utils.drivers.ParameterDriver;
  
  /** Force model for Newtonian central body attraction.
   * @author Luc Maisonobe
   */
  public class NewtonianAttraction implements ForceModel {
  
      /** Name of the single parameter of this model: the central attraction coefficient. */
      public static final String CENTRAL_ATTRACTION_COEFFICIENT = "central 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);
  
      /** Driver for gravitational parameter. */
      private final ParameterDriver gmParameterDriver;
  
     /** Simple constructor.
       * @param mu central attraction coefficient (m^3/s^2)
       */
      public NewtonianAttraction(final double mu) {
          gmParameterDriver = new ParameterDriver(NewtonianAttraction.CENTRAL_ATTRACTION_COEFFICIENT,
                                                  mu, MU_SCALE,
                                                  0.0, Double.POSITIVE_INFINITY, TimeInterval.UNLIMITED);
      }
  
      /** {@inheritDoc} */
      @Override
      public boolean dependsOnPositionOnly() {
          return true;
      }
  
      /** Get the central attraction coefficient μ.
       * @return mu central attraction coefficient (m³/s²)
       */
      public double getMu() {
          return gmParameterDriver.getValue();
      }
  
      /** Get the central attraction coefficient μ.
       * @param <T> the type of the field element
       * @param field field to which the state belongs
       * @return mu central attraction coefficient (m³/s²)
       */
      public <T extends CalculusFieldElement<T>> T getMu(final Field<T> field) {
          final T zero = field.getZero();
          return zero.newInstance(gmParameterDriver.getValue());
      }
  
      /** {@inheritDoc} */
      @Override
      public void addContribution(final SpacecraftState s, final TimeDerivativesEquations adder) {
          adder.addKeplerContribution(getMu());
      }
  
      /** {@inheritDoc} */
      @Override
      public <T extends CalculusFieldElement<T>> void addContribution(final FieldSpacecraftState<T> s,
                                                                      final FieldTimeDerivativesEquations<T> adder) {
          final Field<T> field = s.getDate().getField();
          adder.addKeplerContribution(getMu(field));
      }
  
     /** {@inheritDoc} */
     @Override
     public Vector3D acceleration(final SpacecraftState s, final double[] parameters) {
         final double mu = parameters[0];
         final double r2 = s.getPosition().getNorm2Sq();
         return new Vector3D(-mu / (FastMath.sqrt(r2) * r2), s.getPosition());
     }
 
     /** {@inheritDoc} */
     @Override
     public <T extends CalculusFieldElement<T>> FieldVector3D<T> acceleration(final FieldSpacecraftState<T> s,
                                                                          final T[] parameters) {
         final T mu = parameters[0];
         final T r2 = s.getPosition().getNorm2Sq();
         return new FieldVector3D<>(r2.sqrt().multiply(r2).reciprocal().multiply(mu).negate(), s.getPosition());
     }
 
     /** {@inheritDoc} */
     @Override
     public List<ParameterDriver> getParametersDrivers() {
         return Collections.singletonList(gmParameterDriver);
     }
 
 }