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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.semianalytical.dsst.utilities.hansen;
  
  import org.hipparchus.util.FastMath;
  import org.junit.jupiter.api.Assertions;
  import org.junit.jupiter.api.Test;
  
  public class HansenThirdBodyLinearTest {
  
      private static double hansen(int n, int s, double chi) {
          if (n == 0 && s == 0) {
              return 1.0;
          } else if (n == 0 && s == 1) {
              return -1.0;
          } else if (n == 1 && s == 0) {
              return 1 + 0.5 * (1 - 1.0 / (chi * chi));
          } else if (n >= 1 && n == s - 1) {
              return (-(2 * (double)s - 1) / (double)s ) * hansen(s - 2, s - 1, chi);
          } else if (n >= 1 && n == s) {
              return ((2 * (double)s + 1) / ((double)s + 1) ) * hansen(s - 1, s, chi);
          } else {
              return ((2 * (double)n + 1) / ((double)n + 1)) * hansen(n - 1, s, chi) -
                     ((((double)n + (double)s) * ((double)n - (double)s)) / ((double)n * ((double)n + 1) * chi * chi)) * hansen(n - 2, s, chi);
          }
      }
  
      @Test
      public void testLinearVsRecursive00() {
          doTestLinearVsRecursive(0.0, 1.1e-12);
      }
  
      @Test
      public void testLinearVsRecursive01() {
          doTestLinearVsRecursive(0.1, 2.7e-13);
      }
  
      @Test
      public void testLinearVsRecursive02() {
          doTestLinearVsRecursive(0.2, 9.5e-14);
      }
  
      @Test
      public void testLinearVsRecursive03() {
          doTestLinearVsRecursive(0.3, 5.6e-14);
      }
  
      @Test
      public void testLinearVsRecursive04() {
          doTestLinearVsRecursive(0.4, 1.5e-14);
      }
  
      @Test
      public void testLinearVsRecursive05() {
          doTestLinearVsRecursive(0.5, 5.9e-15);
      }
  
      @Test
      public void testLinearVsRecursive06() {
          doTestLinearVsRecursive(0.6, 3.7e-15);
      }
  
      @Test
      public void testLinearVsRecursive07() {
          doTestLinearVsRecursive(0.7, 1.7e-15);
      }
  
      @Test
      public void testLinearVsRecursive08() {
          doTestLinearVsRecursive(0.8, 9.5e-16);
      }
  
      @Test
      public void testLinearVsRecursive09() {
          doTestLinearVsRecursive(0.9, 8.3e-16);
      }
  
      private void doTestLinearVsRecursive(final double ecc, final double tol) {
          final int N = 22;
          final double chi = 1.0 / FastMath.sqrt(1 - ecc * ecc);
          final HansenThirdBodyLinear[] htbl = new HansenThirdBodyLinear[N + 1];
  
          for (int s = 0; s <= N; s++) {
              htbl[s] = new HansenThirdBodyLinear(N, s);
              htbl[s].computeInitValues(1 / chi, 1 / (chi * chi), 1 / (chi * chi * chi));
         }
 
         double maxRelativeError = 0;
         for (int s = 0; s <= N; s++) {
             for (int n = FastMath.max(2, s); n <= N; n++) {
                 final double hansenRec = hansen(n, s, chi);
                 final double hansenLin = htbl[s].getValue(n, 1 / chi);
                 final double relativeError = FastMath.abs((hansenLin - hansenRec) / hansenRec);
                 maxRelativeError = FastMath.max(maxRelativeError, relativeError);
             }
         }
 
         Assertions.assertEquals(0.0, maxRelativeError, tol);
 
     }
 
 
 }