/* Copyright 2010-2011 Centre National d'Études Spatiales
 * Licensed to CS Systèmes d'Information (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.stream.Stream;

import org.hipparchus.Field;
import org.hipparchus.RealFieldElement;
import org.hipparchus.analysis.differentiation.DSFactory;
import org.hipparchus.analysis.differentiation.DerivativeStructure;
import org.hipparchus.geometry.euclidean.threed.FieldRotation;
import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.hipparchus.util.FastMath;
import org.orekit.errors.OrekitException;
import org.orekit.errors.OrekitInternalError;
import org.orekit.forces.AbstractForceModel;
import org.orekit.frames.Frame;
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.propagation.numerical.FieldTimeDerivativesEquations;
import org.orekit.propagation.numerical.TimeDerivativesEquations;
import org.orekit.time.AbsoluteDate;
import org.orekit.utils.ParameterDriver;
import org.orekit.utils.ParameterObserver;

/** Force model for Newtonian central body attraction.
 * @author Luc Maisonobe
 */
public class NewtonianAttraction extends AbstractForceModel {

    /** 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);

    /** Drivers for force model parameters. */
    private final ParameterDriver[] parametersDrivers;

    /** Central attraction coefficient (m^3/s^2). */
    private double mu;

    /** Factory for the DerivativeStructure instances. */
    private final DSFactory factory;

   /** Simple constructor.
     * @param mu central attraction coefficient (m^3/s^2)
     */
    public NewtonianAttraction(final double mu) {
        this.parametersDrivers = new ParameterDriver[1];
        try {
            parametersDrivers[0] = new ParameterDriver(NewtonianAttraction.CENTRAL_ATTRACTION_COEFFICIENT,
                                                       mu, MU_SCALE,
                                                       0.0, Double.POSITIVE_INFINITY);
            parametersDrivers[0].addObserver(new ParameterObserver() {
                /** {@inheritDoc} */
                @Override
                public void valueChanged(final double previousValue, final ParameterDriver driver) {
                    NewtonianAttraction.this.mu = driver.getValue();
                }
            });
        } catch (OrekitException oe) {
            // this should never occur as valueChanged above never throws an exception
            throw new OrekitInternalError(oe);
        };

        this.mu = mu;
        this.factory = new DSFactory(1, 1);
    }

    /** {@inheritDoc} */
    public FieldVector3D<DerivativeStructure> accelerationDerivatives(final AbsoluteDate date, final Frame frame,
                                                                      final FieldVector3D<DerivativeStructure> position, final FieldVector3D<DerivativeStructure> velocity,
                                                                      final FieldRotation<DerivativeStructure> rotation, final DerivativeStructure mass)
        throws OrekitException {

        final DerivativeStructure r2 = position.getNormSq();
        return new FieldVector3D<DerivativeStructure>(r2.sqrt().multiply(r2).reciprocal().multiply(-mu), position);

    }

    /** {@inheritDoc} */
    public FieldVector3D<DerivativeStructure> accelerationDerivatives(final SpacecraftState s, final String paramName)
        throws OrekitException {

        complainIfNotSupported(paramName);

        final Vector3D            position = s.getPVCoordinates().getPosition();
        final double              r2       = position.getNormSq();
        final DerivativeStructure muds     = factory.variable(0, mu);
        return new FieldVector3D<DerivativeStructure>(muds.divide(-r2 * FastMath.sqrt(r2)), position);

    }

    /** Get the central attraction coefficient μ.
     * @return mu central attraction coefficient (m³/s²)
     */
    public double getMu() {
        return mu;
    }

    /** {@inheritDoc} */
    public void addContribution(final SpacecraftState s, final TimeDerivativesEquations adder)
        throws OrekitException {
        adder.addKeplerContribution(mu);
    }

    /** {@inheritDoc} */
    public <T extends RealFieldElement<T>> void addContribution(final FieldSpacecraftState<T> s, final FieldTimeDerivativesEquations<T> adder)
        throws OrekitException {
        adder.addKeplerContribution(mu);
    }

    /** {@inheritDoc} */
    public Stream<EventDetector> getEventsDetectors() {
        return Stream.empty();
    }

    @Override
    /** {@inheritDoc} */
    public <T extends RealFieldElement<T>> Stream<FieldEventDetector<T>> getFieldEventsDetectors(final Field<T> field) {
        return Stream.empty();
    }

    /** {@inheritDoc} */
    public ParameterDriver[] getParametersDrivers() {
        return parametersDrivers.clone();
    }

}