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    * 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
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    *
    *   http://www.apache.org/licenses/LICENSE-2.0
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   * 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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  package org.orekit.propagation.numerical;
  
  import org.hipparchus.linear.RealMatrix;
  import org.hipparchus.util.FastMath;
  import org.junit.jupiter.api.Assertions;
  import org.orekit.propagation.MatricesHarvester;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.propagation.integration.AdditionalDerivativesProvider;
  import org.orekit.propagation.sampling.OrekitStepHandler;
  import org.orekit.propagation.sampling.OrekitStepInterpolator;
  import org.orekit.time.AbsoluteDate;
  
  class PickUpHandler implements OrekitStepHandler, StateTransitionMatrixGenerator.PartialsObserver {
  
      private final NumericalPropagator propagator;
      private final MatricesHarvester harvester;
      private final AbsoluteDate pickUpDate;
      private final String accParamName;
      private final String columnName;
      private StateTransitionMatrixGenerator stmGenerator;
      private SpacecraftState s0;
      private RealMatrix dYdY0;
      private RealMatrix dYdP;
      private double[] accPartial;
  
      public PickUpHandler(final NumericalPropagator propagator, final AbsoluteDate pickUpDate,
                           final String accParamName, final String columnName) {
          this.propagator   = propagator;
          this.harvester    = propagator.setupMatricesComputation("stm", null, null);
          this.pickUpDate   = pickUpDate;
          this.accParamName = accParamName;
          this.columnName   = columnName;
          this.s0           = null;
          this.accPartial   = null;
          if (columnName == null) {
              Assertions.assertTrue(harvester.getJacobiansColumnsNames().isEmpty());
          } else {
              Assertions.assertEquals(1, harvester.getJacobiansColumnsNames().size());
              Assertions.assertEquals(columnName, harvester.getJacobiansColumnsNames().get(0));
          }
      }
  
      public SpacecraftState getState() {
          return s0;
      }
  
      public RealMatrix getStm() {
          return dYdY0;
      }
  
      public RealMatrix getdYdP() {
          return dYdP;
      }
  
      public double[] getAccPartial() {
          return accPartial.clone();
      }
  
      public void init(SpacecraftState s0, AbsoluteDate t) {
          // as the generators are only created on the fly at propagation start
          // we retrieve the STM generator here
          for (final AdditionalDerivativesProvider provider : propagator.getAdditionalDerivativesProviders()) {
              if (provider instanceof StateTransitionMatrixGenerator) {
                  stmGenerator = (StateTransitionMatrixGenerator) provider;
                  stmGenerator.addObserver(accParamName, this);
              }
          }
      }
  
      public void handleStep(OrekitStepInterpolator interpolator) {
          if (pickUpDate != null) {
              // we want to pick up some intermediate Jacobians
              double dt0 = pickUpDate.durationFrom(interpolator.getPreviousState().getDate());
              double dt1 = pickUpDate.durationFrom(interpolator.getCurrentState().getDate());
              if (dt0 * dt1 > 0) {
                  // the current step does not cover the pickup date
                  return;
              } else {
                  checkState(interpolator.getInterpolatedState(pickUpDate));
              }
          }
      }
  
     public void finish(SpacecraftState finalState) {
         if (s0 == null) {
             checkState(finalState);
         }
     }
 
     public void partialsComputed(final SpacecraftState state, final double[] newFactor, final double[] newAccelerationPartials) {
         if (accParamName != null &&
             (pickUpDate == null || FastMath.abs(pickUpDate.durationFrom(state.getDate())) < 1.0e-6)) {
             accPartial = newAccelerationPartials.clone();
         }
     }
 
     private void checkState(final SpacecraftState state) {
         stmGenerator.combinedDerivatives(state); // just for the side effect of calling partialsComputed
         Assertions.assertEquals(columnName == null ? 1 : 2, state.getAdditionalDataValues().size());
         dYdY0 = harvester.getStateTransitionMatrix(state);
         dYdP  = harvester.getParametersJacobian(state); // may be null
         s0    = state;
     }
 
 }