1 /* Copyright 2002-2025 CS GROUP
2 * Licensed to CS GROUP (CS) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * CS licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.orekit.propagation.semianalytical.dsst.utilities;
18
19 import org.hipparchus.complex.Complex;
20
21 import java.util.ArrayList;
22 import java.util.List;
23
24 /** Compute the S<sub>j</sub>(k, h) and the C<sub>j</sub>(k, h) series
25 * and their partial derivatives with respect to k and h.
26 * <p>
27 * Those series are given in Danielson paper by expression 2.5.3-(5):
28 *
29 * <p> C<sub>j</sub>(k, h) + i S<sub>j</sub>(k, h) = (k+ih)<sup>j</sup>
30 *
31 * <p>
32 * The C<sub>j</sub>(k, h) and the S<sub>j</sub>(k, h) elements are store as an
33 * {@link ArrayList} of {@link Complex} number, the C<sub>j</sub>(k, h) being
34 * represented by the real and the S<sub>j</sub>(k, h) by the imaginary part.
35 */
36 public class CjSjCoefficient {
37
38 /** Last computed order j. */
39 private int jLast;
40
41 /** Complex base (k + ih) of the C<sub>j</sub>, S<sub>j</sub> series. */
42 private final Complex kih;
43
44 /** List of computed elements. */
45 private final List<Complex> cjsj;
46
47 /** C<sub>j</sub>(k, h) and S<sub>j</sub>(k, h) constructor.
48 * @param k k value
49 * @param h h value
50 */
51 public CjSjCoefficient(final double k, final double h) {
52 kih = new Complex(k, h);
53 cjsj = new ArrayList<>();
54 cjsj.add(new Complex(1, 0));
55 cjsj.add(kih);
56 jLast = 1;
57 }
58
59 /** Get the C<sub>j</sub> coefficient.
60 * @param j order
61 * @return C<sub>j</sub>
62 */
63 public double getCj(final int j) {
64 if (j > jLast) {
65 // Update to order j
66 updateCjSj(j);
67 }
68 return cjsj.get(j).getReal();
69 }
70
71 /** Get the S<sub>j</sub> coefficient.
72 * @param j order
73 * @return S<sub>j</sub>
74 */
75 public double getSj(final int j) {
76 if (j > jLast) {
77 // Update to order j
78 updateCjSj(j);
79 }
80 return cjsj.get(j).getImaginary();
81 }
82
83 /** Get the dC<sub>j</sub> / dk coefficient.
84 * @param j order
85 * @return dC<sub>j</sub> / d<sub>k</sub>
86 */
87 public double getDcjDk(final int j) {
88 return j == 0 ? 0 : j * getCj(j - 1);
89 }
90
91 /** Get the dS<sub>j</sub> / dk coefficient.
92 * @param j order
93 * @return dS<sub>j</sub> / d<sub>k</sub>
94 */
95 public double getDsjDk(final int j) {
96 return j == 0 ? 0 : j * getSj(j - 1);
97 }
98
99 /** Get the dC<sub>j</sub> / dh coefficient.
100 * @param j order
101 * @return dC<sub>i</sub> / d<sub>k</sub>
102 */
103 public double getDcjDh(final int j) {
104 return j == 0 ? 0 : -j * getSj(j - 1);
105 }
106
107 /** Get the dS<sub>j</sub> / dh coefficient.
108 * @param j order
109 * @return dS<sub>j</sub> / d<sub>h</sub>
110 */
111 public double getDsjDh(final int j) {
112 return j == 0 ? 0 : j * getCj(j - 1);
113 }
114
115 /** Update the cjsj up to order j.
116 * @param j order
117 */
118 private void updateCjSj(final int j) {
119 Complex last = cjsj.get(cjsj.size() - 1);
120 for (int i = jLast; i < j; i++) {
121 final Complex next = last.multiply(kih);
122 cjsj.add(next);
123 last = next;
124 }
125 jLast = j;
126 }
127 }