/* Copyright 2022-2025 Romain Serra
    * 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.control.indirect.adjoint.cost;
  
  
  import org.hipparchus.geometry.euclidean.threed.Vector3D;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.propagation.events.EventDetectionSettings;
  import org.orekit.propagation.events.EventDetector;
  
  import java.util.stream.Stream;
  
  /**
   * Class for fuel cost with Cartesian coordinates.
   * It is the integral over time of the Euclidean norm of the thrust vector.
   *
   * @author Romain Serra
   * @see CartesianCost
   * @since 13.0
   */
  public class CartesianFuelCost extends AbstractCartesianCost {
  
      /** Maximum value of thrust force Euclidean norm. */
      private final double maximumThrustMagnitude;
  
      /** Detection settings for singularity detection. */
      private final EventDetectionSettings eventDetectionSettings;
  
      /**
       * Constructor with default detection settings.
       * @param name name
       * @param massFlowRateFactor mass flow rate factor
       * @param maximumThrustMagnitude maximum thrust magnitude
       */
      public CartesianFuelCost(final String name, final double massFlowRateFactor, final double maximumThrustMagnitude) {
          this(name, massFlowRateFactor, maximumThrustMagnitude, EventDetectionSettings.getDefaultEventDetectionSettings());
      }
  
      /**
       * Constructor.
       * @param name name
       * @param massFlowRateFactor mass flow rate factor
       * @param maximumThrustMagnitude maximum thrust magnitude
       * @param eventDetectionSettings singularity event detection settings
       */
      public CartesianFuelCost(final String name, final double massFlowRateFactor, final double maximumThrustMagnitude,
                               final EventDetectionSettings eventDetectionSettings) {
          super(name, massFlowRateFactor);
          this.maximumThrustMagnitude = maximumThrustMagnitude;
          this.eventDetectionSettings = eventDetectionSettings;
      }
  
      /** Getter for maximum thrust magnitude.
       * @return maximum thrust
       */
      public double getMaximumThrustMagnitude() {
          return maximumThrustMagnitude;
      }
  
      /**
       * Getter for event detection settings.
       * @return detection settings.
       */
      public EventDetectionSettings getEventDetectionSettings() {
          return eventDetectionSettings;
      }
  
      /**
       * Evaluate switching function (whose sign determines the bang-bang control profile).
       * @param adjointVariables adjoint vector
       * @param mass mass
       * @return value of switch function
       */
      private double evaluateSwitchFunction(final double[] adjointVariables, final double mass) {
          double switchFunction = getAdjointVelocityNorm(adjointVariables) / mass - 1.;
          if (getAdjointDimension() > 6) {
              switchFunction -= adjointVariables[6] * getMassFlowRateFactor();
          }
          return switchFunction;
      }
  
      /**
       * Computes the direction of thrust.
       * @param adjointVariables adjoint vector
       * @return thrust direction
      */
     private Vector3D getThrustDirection(final double[] adjointVariables) {
         return new Vector3D(adjointVariables[3], adjointVariables[4], adjointVariables[5]).normalize();
     }
 
     /** {@inheritDoc} */
     @Override
     public Vector3D getThrustAccelerationVector(final double[] adjointVariables, final double mass) {
         final double switchFunction = evaluateSwitchFunction(adjointVariables, mass);
         if (switchFunction > 0.) {
             return getThrustDirection(adjointVariables).scalarMultiply(maximumThrustMagnitude / mass);
         } else {
             return Vector3D.ZERO;
         }
     }
 
     /** {@inheritDoc} */
     @Override
     public void updateAdjointDerivatives(final double[] adjointVariables, final double mass,
                                          final double[] adjointDerivatives) {
         if (getAdjointDimension() > 6) {
             final double switchFunction = evaluateSwitchFunction(adjointVariables, mass);
             if (switchFunction > 0.) {
                 adjointDerivatives[6] += getAdjointVelocityNorm(adjointVariables) * maximumThrustMagnitude / (mass * mass);
             }
         }
     }
 
     /** {@inheritDoc} */
     @Override
     public double getHamiltonianContribution(final double[] adjointVariables, final double mass) {
         final Vector3D thrustForce = getThrustAccelerationVector(adjointVariables, mass).scalarMultiply(mass);
         return -thrustForce.getNorm();
     }
 
     /** {@inheritDoc} */
     @Override
     public Stream<EventDetector> getEventDetectors() {
         return Stream.of(new FuelCostSwitchDetector(eventDetectionSettings));
     }
 
     /**
      * Event detector for bang-bang switches.
      */
     private class FuelCostSwitchDetector extends ControlSwitchDetector {
 
         /**
          * Constructor.
          * @param detectionSettings detection settings.
          */
         FuelCostSwitchDetector(final EventDetectionSettings detectionSettings) {
             super(detectionSettings);
         }
 
         /** {@inheritDoc} */
         @Override
         public double g(final SpacecraftState state) {
             final double[] adjoint = state.getAdditionalState(getAdjointName());
             return evaluateSwitchFunction(adjoint, state.getMass());
         }
     }
 
 }