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  package org.orekit.bodies;
  
  import org.hipparchus.analysis.differentiation.Gradient;
  import org.hipparchus.analysis.differentiation.GradientField;
  import org.hipparchus.analysis.differentiation.UnivariateDerivative2;
  import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
  import org.hipparchus.geometry.euclidean.threed.Vector3D;
  import org.junit.jupiter.api.Assertions;
  import org.junit.jupiter.api.BeforeEach;
  import org.junit.jupiter.api.Test;
  import org.orekit.Utils;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.time.FieldAbsoluteDate;
  import org.orekit.time.TimeScale;
  import org.orekit.time.TimeScalesFactory;
  import org.orekit.utils.FieldPVCoordinates;
  import org.orekit.utils.PVCoordinates;
  
  import java.util.Arrays;
  
  class PosVelChebyshevTest {
  
      private static final double DURATION = 10.;
  
      @BeforeEach
      public void setUp() {
          Utils.setDataRoot("regular-data/de405-ephemerides");
      }
  
      @Test
      void testGetPosition() {
          // Given
          final PosVelChebyshev chebyshev = createPolynomial();
          final AbsoluteDate date = chebyshev.getDate().shiftedBy(DURATION / 2);
          // When
          final double[] actualPositionArray = chebyshev.getPosition(date).toArray();
          // Then
          final double[] expectedPositionArray = chebyshev.getPositionVelocityAcceleration(date).getPosition().toArray();
          for (int i = 0; i < expectedPositionArray.length; i++) {
              Assertions.assertEquals(expectedPositionArray[i], actualPositionArray[i]);
          }
      }
  
      @Test
      void testFieldGetPositionWithGradient() {
          // Given
          final PosVelChebyshev chebyshev = createPolynomial();
          final AbsoluteDate date = chebyshev.getDate().shiftedBy(DURATION / 2);
          final int freeParameters = 1;
          final GradientField field = GradientField.getField(freeParameters);
          final Gradient variable = Gradient.variable(freeParameters, 0, 0);
          final FieldAbsoluteDate<Gradient> fieldDate = new FieldAbsoluteDate<>(field, date).shiftedBy(variable);
          // When
          final Gradient[] actualPositionArray = chebyshev.getPosition(fieldDate).toArray();
          // Then
          final Gradient[] expectedPositionArray = chebyshev.getPositionVelocityAcceleration(fieldDate).getPosition().toArray();
          for (int i = 0; i < expectedPositionArray.length; i++) {
              Assertions.assertEquals(expectedPositionArray[i], actualPositionArray[i]);
          }
      }
  
      @Test
      void testFieldWithGradientVersusNonField() {
          // Given
          final PosVelChebyshev chebyshev = createPolynomial();
          final AbsoluteDate date = chebyshev.getDate().shiftedBy(DURATION / 2);
          final int freeParameters = 1;
          final GradientField field = GradientField.getField(freeParameters);
          final Gradient variable = Gradient.variable(freeParameters, 0, 0);
          final FieldAbsoluteDate<Gradient> fieldDate = new FieldAbsoluteDate<>(field, date).shiftedBy(variable);
          // When
          final FieldPVCoordinates<Gradient> fieldPVCoordinates = chebyshev.getPositionVelocityAcceleration(fieldDate);
          // Then
          final PVCoordinates expectedPVCoordinates = chebyshev.getPositionVelocityAcceleration(fieldDate.toAbsoluteDate());
          final PVCoordinates actualPVCoordinates = fieldPVCoordinates.toPVCoordinates();
          Assertions.assertEquals(expectedPVCoordinates.getPosition(), actualPVCoordinates.getPosition());
          Assertions.assertEquals(expectedPVCoordinates.getVelocity(), actualPVCoordinates.getVelocity());
          Assertions.assertEquals(expectedPVCoordinates.getAcceleration(), actualPVCoordinates.getAcceleration());
      }
  
      @Test
      void testVelocityAndAccelerationAreDerivativesOfPosition() {
         // Given
         final PosVelChebyshev chebyshev = createPolynomial();
         final AbsoluteDate date = chebyshev.getDate().shiftedBy(DURATION / 2);
         final UnivariateDerivative2 variable = new UnivariateDerivative2(0., 1., 0.);
         final FieldAbsoluteDate<UnivariateDerivative2> fieldDate =
                 new FieldAbsoluteDate<>(variable.getField(), date).shiftedBy(variable);
         // When
         final FieldPVCoordinates<UnivariateDerivative2> fieldPVCoordinates = chebyshev.getPositionVelocityAcceleration(fieldDate);
         // Then
         final PVCoordinates expectedPVCoordinates = chebyshev.getPositionVelocityAcceleration(fieldDate.toAbsoluteDate());
         final Vector3D expectedVelocity = expectedPVCoordinates.getVelocity();
         final Vector3D expectedAcceleration = expectedPVCoordinates.getAcceleration();
         final FieldVector3D<UnivariateDerivative2> fieldPosition = fieldPVCoordinates.getPosition();
         final double tolerance = 1e-15;
         Assertions.assertEquals(expectedVelocity.getX(), fieldPosition.getX().getDerivative(1), tolerance);
         Assertions.assertEquals(expectedVelocity.getY(), fieldPosition.getY().getDerivative(1), tolerance);
         Assertions.assertEquals(expectedVelocity.getZ(), fieldPosition.getZ().getDerivative(1), tolerance);
         Assertions.assertEquals(expectedAcceleration.getX(), fieldPosition.getX().getDerivative(2), tolerance);
         Assertions.assertEquals(expectedAcceleration.getY(), fieldPosition.getY().getDerivative(2), tolerance);
         Assertions.assertEquals(expectedAcceleration.getZ(), fieldPosition.getZ().getDerivative(2), tolerance);
     }
 
     private PosVelChebyshev createPolynomial() {
         final AbsoluteDate start = AbsoluteDate.ARBITRARY_EPOCH;
         final TimeScale timeScale = TimeScalesFactory.getTAI();
         final int degree = 10;
         final double[] firstComponents = new double[degree];
         final double[] secondComponents = new double[degree];
         final double[] thirdComponents = new double[degree];
         final double arbitraryCoefficient = 1.;
         Arrays.fill(firstComponents, arbitraryCoefficient);
         Arrays.fill(secondComponents, arbitraryCoefficient);
         Arrays.fill(thirdComponents, arbitraryCoefficient);
         return new PosVelChebyshev(start, timeScale, DURATION, firstComponents, secondComponents, thirdComponents);
     }
 }