/* 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,
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   * See the License for the specific language governing permissions and
   * limitations under the License.
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  package org.orekit.estimation.measurements.modifiers;
  
  import java.util.Arrays;
  import java.util.List;
  
  import org.hipparchus.Field;
  import org.hipparchus.CalculusFieldElement;
  import org.hipparchus.analysis.differentiation.Gradient;
  import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
  import org.hipparchus.geometry.euclidean.threed.Vector3D;
  import org.orekit.attitudes.InertialProvider;
  import org.orekit.estimation.measurements.EstimatedMeasurement;
  import org.orekit.estimation.measurements.EstimationModifier;
  import org.orekit.estimation.measurements.GroundStation;
  import org.orekit.estimation.measurements.TurnAroundRange;
  import org.orekit.models.earth.troposphere.DiscreteTroposphericModel;
  import org.orekit.propagation.FieldSpacecraftState;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.utils.Differentiation;
  import org.orekit.utils.ParameterDriver;
  import org.orekit.utils.ParameterFunction;
  
  /** Class modifying theoretical turn-around TurnAroundRange measurement with tropospheric delay.
   * The effect of tropospheric correction on the TurnAroundRange is directly computed
   * through the computation of the tropospheric delay.
   *
   * In general, for GNSS, VLBI, ... there is hardly any frequency dependence in the delay.
   * For SLR techniques however, the frequency dependence is sensitive.
   *
   * @author Maxime Journot
   * @since 9.0
   */
  public class TurnAroundRangeTroposphericDelayModifier implements EstimationModifier<TurnAroundRange> {
  
      /** Tropospheric delay model. */
      private final DiscreteTroposphericModel tropoModel;
  
      /** Constructor.
       *
       * @param model  Tropospheric delay model appropriate for the current TurnAroundRange measurement method.
       */
      public TurnAroundRangeTroposphericDelayModifier(final DiscreteTroposphericModel model) {
          tropoModel = model;
      }
  
      /** Compute the measurement error due to Troposphere.
       * @param station station
       * @param state spacecraft state
       * @return the measurement error due to Troposphere
       */
      private double rangeErrorTroposphericModel(final GroundStation station, final SpacecraftState state) {
          //
          final Vector3D position = state.getPVCoordinates().getPosition();
  
          // elevation
          final double elevation = station.getBaseFrame().getElevation(position,
                                                                       state.getFrame(),
                                                                       state.getDate());
  
          // only consider measures above the horizon
          if (elevation > 0) {
              // Delay in meters
              final double delay = tropoModel.pathDelay(elevation, station.getBaseFrame().getPoint(), tropoModel.getParameters(), state.getDate());
  
              return delay;
          }
  
          return 0;
      }
  
      /** Compute the measurement error due to Troposphere.
       * @param <T> type of the element
       * @param station station
       * @param state spacecraft state
       * @param parameters tropospheric model parameters
       * @return the measurement error due to Troposphere
       */
      private <T extends CalculusFieldElement<T>> T rangeErrorTroposphericModel(final GroundStation station,
                                                                            final FieldSpacecraftState<T> state,
                                                                            final T[] parameters) {
          // Field
          final Field<T> field = state.getDate().getField();
          final T zero         = field.getZero();
 
         //
         final FieldVector3D<T> position = state.getPVCoordinates().getPosition();
         final T dsElevation             = station.getBaseFrame().getElevation(position,
                                                                               state.getFrame(),
                                                                               state.getDate());
 
         // only consider measures above the horizon
         if (dsElevation.getReal() > 0) {
             // Delay in meters
             final T delay = tropoModel.pathDelay(dsElevation, station.getBaseFrame().getPoint(field), parameters, state.getDate());
 
             return delay;
         }
 
         return zero;
     }
 
     /** Compute the Jacobian of the delay term wrt state using
     * automatic differentiation.
     *
     * @param derivatives tropospheric delay derivatives
     *
     * @return Jacobian of the delay wrt state
     */
     private double[][] rangeErrorJacobianState(final double[] derivatives) {
         final double[][] finiteDifferencesJacobian = new double[1][6];
         System.arraycopy(derivatives, 0, finiteDifferencesJacobian[0], 0, 6);
         return finiteDifferencesJacobian;
     }
 
 
     /** Compute the derivative of the delay term wrt parameters.
      *
      * @param station ground station
      * @param driver driver for the station offset parameter
      * @param state spacecraft state
      * @return derivative of the delay wrt station offset parameter
      */
     private double rangeErrorParameterDerivative(final GroundStation station,
                                                  final ParameterDriver driver,
                                                  final SpacecraftState state) {
 
         final ParameterFunction rangeError = new ParameterFunction() {
             /** {@inheritDoc} */
             @Override
             public double value(final ParameterDriver parameterDriver) {
                 return rangeErrorTroposphericModel(station, state);
             }
         };
 
         final ParameterFunction rangeErrorDerivative = Differentiation.differentiate(rangeError, 3, 10.0 * driver.getScale());
 
         return rangeErrorDerivative.value(driver);
 
     }
 
     /** Compute the derivative of the delay term wrt parameters using
     * automatic differentiation.
     *
     * @param derivatives tropospheric delay derivatives
     * @param freeStateParameters dimension of the state.
     * @return derivative of the delay wrt tropospheric model parameters
     */
     private double[] rangeErrorParameterDerivative(final double[] derivatives, final int freeStateParameters) {
         // 0 ... freeStateParameters - 1 -> derivatives of the delay wrt state
         // freeStateParameters ... n     -> derivatives of the delay wrt tropospheric parameters
         final int dim = derivatives.length - freeStateParameters;
         final double[] rangeError = new double[dim];
 
         for (int i = 0; i < dim; i++) {
             rangeError[i] = derivatives[freeStateParameters + i];
         }
 
         return rangeError;
     }
 
     /** {@inheritDoc} */
     @Override
     public List<ParameterDriver> getParametersDrivers() {
         return tropoModel.getParametersDrivers();
     }
 
     /** {@inheritDoc} */
     @Override
     public void modify(final EstimatedMeasurement<TurnAroundRange> estimated) {
         final TurnAroundRange measurement      = estimated.getObservedMeasurement();
         final GroundStation   primaryStation   = measurement.getPrimaryStation();
         final GroundStation   secondaryStation = measurement.getSecondaryStation();
         final SpacecraftState state            = estimated.getStates()[0];
 
         final double[] oldValue = estimated.getEstimatedValue();
 
         // Update estimated derivatives with Jacobian of the measure wrt state
         final TroposphericGradientConverter converter =
                 new TroposphericGradientConverter(state, 6, new InertialProvider(state.getFrame()));
         final FieldSpacecraftState<Gradient> gState = converter.getState(tropoModel);
         final Gradient[] gParameters = converter.getParameters(gState, tropoModel);
         final Gradient   primaryGDelay        = rangeErrorTroposphericModel(primaryStation, gState, gParameters);
         final Gradient   secondaryGDelay      = rangeErrorTroposphericModel(secondaryStation, gState, gParameters);
         final double[]   primaryDerivatives   = primaryGDelay.getGradient();
         final double[]   secondaryDerivatives = secondaryGDelay.getGradient();
 
         final double[][] primaryDjac      = rangeErrorJacobianState(primaryDerivatives);
         final double[][] secondaryDjac    = rangeErrorJacobianState(secondaryDerivatives);
         final double[][] stateDerivatives = estimated.getStateDerivatives(0);
         for (int irow = 0; irow < stateDerivatives.length; ++irow) {
             for (int jcol = 0; jcol < stateDerivatives[0].length; ++jcol) {
                 stateDerivatives[irow][jcol] += primaryDjac[irow][jcol] + secondaryDjac[irow][jcol];
             }
         }
         estimated.setStateDerivatives(0, stateDerivatives);
 
         int indexPrimary = 0;
         for (final ParameterDriver driver : getParametersDrivers()) {
             if (driver.isSelected()) {
                 // update estimated derivatives with derivative of the modification wrt tropospheric parameters
                 double parameterDerivative = estimated.getParameterDerivatives(driver)[0];
                 final double[] derivatives = rangeErrorParameterDerivative(primaryDerivatives, converter.getFreeStateParameters());
                 parameterDerivative += derivatives[indexPrimary];
                 estimated.setParameterDerivatives(driver, parameterDerivative);
                 indexPrimary += 1;
             }
 
         }
 
         int indexSecondary = 0;
         for (final ParameterDriver driver : getParametersDrivers()) {
             if (driver.isSelected()) {
                 // update estimated derivatives with derivative of the modification wrt tropospheric parameters
                 double parameterDerivative = estimated.getParameterDerivatives(driver)[0];
                 final double[] derivatives = rangeErrorParameterDerivative(secondaryDerivatives, converter.getFreeStateParameters());
                 parameterDerivative += derivatives[indexSecondary];
                 estimated.setParameterDerivatives(driver, parameterDerivative);
                 indexSecondary += 1;
             }
 
         }
 
         // Update derivatives with respect to primary station position
         for (final ParameterDriver driver : Arrays.asList(primaryStation.getClockOffsetDriver(),
                                                           primaryStation.getEastOffsetDriver(),
                                                           primaryStation.getNorthOffsetDriver(),
                                                           primaryStation.getZenithOffsetDriver())) {
             if (driver.isSelected()) {
                 double parameterDerivative = estimated.getParameterDerivatives(driver)[0];
                 parameterDerivative += rangeErrorParameterDerivative(primaryStation, driver, state);
                 estimated.setParameterDerivatives(driver, parameterDerivative);
             }
         }
 
         // Update derivatives with respect to secondary station position
         for (final ParameterDriver driver : Arrays.asList(secondaryStation.getEastOffsetDriver(),
                                                           secondaryStation.getNorthOffsetDriver(),
                                                           secondaryStation.getZenithOffsetDriver())) {
             if (driver.isSelected()) {
                 double parameterDerivative = estimated.getParameterDerivatives(driver)[0];
                 parameterDerivative += rangeErrorParameterDerivative(secondaryStation, driver, state);
                 estimated.setParameterDerivatives(driver, parameterDerivative);
             }
         }
 
         // Update estimated value taking into account the tropospheric delay.
         // The tropospheric delay is directly added to the TurnAroundRange.
         final double[] newValue = oldValue.clone();
         newValue[0] = newValue[0] + primaryGDelay.getReal() + secondaryGDelay.getReal();
         estimated.setEstimatedValue(newValue);
 
     }
 
 }