/* 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.propagation.events;
  
  import org.hipparchus.analysis.differentiation.UnivariateDerivative1;
  import org.hipparchus.analysis.differentiation.UnivariateDerivative1Field;
  import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.propagation.events.handlers.EventHandler;
  import org.orekit.time.FieldAbsoluteDate;
  import org.orekit.utils.ExtendedPositionProvider;
  import org.orekit.utils.FieldPVCoordinates;
  
  /** Detector of local extrema with angular separation.
   * @author Romain Serra
   * @see AngularSeparationDetector
   * @since 13.1
   */
  public class ExtremumAngularSeparationDetector extends AbstractDetector<ExtremumAngularSeparationDetector> {
  
      /** Beacon at the center of the proximity zone. */
      private final ExtendedPositionProvider beacon;
  
      /** Observer for the spacecraft, that may also see the beacon at the same time if they are too close. */
      private final ExtendedPositionProvider observer;
  
      /** Protected constructor with full parameters.
       * @param detectionSettings detection settings
       * @param handler event handler to call at event occurrences
       * @param beacon beacon at the center of the proximity zone
       * @param observer observer for the spacecraft, that may also see
       * the beacon at the same time if they are too close to each other
       */
      public ExtremumAngularSeparationDetector(final EventDetectionSettings detectionSettings,
                                               final EventHandler handler,
                                               final ExtendedPositionProvider beacon,
                                                  final ExtendedPositionProvider observer) {
          super(detectionSettings, handler);
          this.beacon         = beacon;
          this.observer       = observer;
      }
  
      /** Get the beacon at the center of the proximity zone.
       * @return beacon at the center of the proximity zone
       */
      public ExtendedPositionProvider getBeacon() {
          return beacon;
      }
  
      /** Get the observer for the spacecraft.
       * @return observer for the spacecraft
       */
      public ExtendedPositionProvider getObserver() {
          return observer;
      }
  
      @Override
      protected ExtremumAngularSeparationDetector create(final EventDetectionSettings detectionSettings,
                                                         final EventHandler newHandler) {
          return new ExtremumAngularSeparationDetector(detectionSettings, newHandler, beacon, observer);
      }
  
      @Override
      public double g(final SpacecraftState s) {
          final FieldPVCoordinates<UnivariateDerivative1> pv = s.getPVCoordinates().toUnivariateDerivative1PV();
          final UnivariateDerivative1 dt = new UnivariateDerivative1(0., 1.);
          final FieldAbsoluteDate<UnivariateDerivative1> fieldDate = new FieldAbsoluteDate<>(UnivariateDerivative1Field.getInstance(),
                  s.getDate()).shiftedBy(dt);
          final FieldVector3D<UnivariateDerivative1> bP = beacon.getPosition(fieldDate, s.getFrame());
          final FieldVector3D<UnivariateDerivative1> oP = observer.getPosition(fieldDate, s.getFrame());
          final UnivariateDerivative1 separation = FieldVector3D.angle(pv.getPosition().subtract(oP), bP.subtract(oP));
          return separation.getFirstDerivative();
      }
  }