ShortPeriodicsInterpolatedCoefficient.java

  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;

  19. import org.hipparchus.analysis.interpolation.HermiteInterpolator;
  20. import org.hipparchus.util.FastMath;
  21. import org.orekit.time.AbsoluteDate;

  23. import java.util.ArrayList;

  25. /** Interpolated short periodics coefficients.
  26.  * <p>
  27.  * Representation of a coefficient that need to be interpolated over time.
  28.  * </p><p>
  29.  * The short periodics coefficients can be interpolated for faster computation.
  30.  * This class stores computed values of the coefficients through the method
  31.  * {@link #addGridPoint} and gives an interpolated result through the method
  32.  * {@link #value}.
  33.  * </p>
  34.  * @author Nicolas Bernard
  35.  *
  36.  */
  37. public class ShortPeriodicsInterpolatedCoefficient {

  39.     /**Values of the already computed coefficients.*/
  40.     private ArrayList<double[]> values;

  42.     /**Grid points.*/
  43.     private ArrayList<AbsoluteDate> abscissae;

  45.     /**Number of points used in the interpolation.*/
  46.     private int interpolationPoints;

  48.     /**Index of the latest closest neighbor.*/
  49.     private int latestClosestNeighbor;

  51.     /**Simple constructor.
  52.      * @param interpolationPoints number of points used in the interpolation
  53.      */
  54.     public ShortPeriodicsInterpolatedCoefficient(final int interpolationPoints) {
  55.         this.interpolationPoints = interpolationPoints;
  56.         this.abscissae = new ArrayList<>();
  57.         this.values = new ArrayList<>();
  58.         this.latestClosestNeighbor = 0;
  59.     }

  61.     /**Compute the value of the coefficient.
  62.      * @param date date at which the coefficient should be computed
  63.      * @return value of the coefficient
  64.      */
  65.     public double[] value(final AbsoluteDate date) {
  66.         //Get the closest points from the input date
  67.         final int[] neighbors = getNeighborsIndices(date);

  69.         //Creation and set up of the interpolator
  70.         final HermiteInterpolator interpolator = new HermiteInterpolator();
  71.         for (int i : neighbors) {
  72.             interpolator.addSamplePoint(abscissae.get(i).durationFrom(date), values.get(i));
  73.         }

  75.         //interpolation
  76.         return interpolator.value(0.0);

  78.     }

  80.     /**Find the closest available points from the specified date.
  81.      * @param date date of interest
  82.      * @return indices corresponding to the closest points on the time scale
  83.      */
  84.     private int[] getNeighborsIndices(final AbsoluteDate date) {
  85.         final int sizeofNeighborhood = FastMath.min(interpolationPoints, abscissae.size());
  86.         final int[] neighborsIndices = new int[sizeofNeighborhood];

  88.         //If the size of the complete sample is less than
  89.         //the desired number of interpolation points,
  90.         //then the entire sample is considered as the neighborhood
  91.         if (interpolationPoints >= abscissae.size()) {
  92.             for (int i = 0; i < sizeofNeighborhood; i++) {
  93.                 neighborsIndices[i] = i;
  94.             }
  95.         } else {
  96.             // get indices around closest neighbor
  97.             int inf = getClosestNeighbor(date);
  98.             int sup = inf + 1;

  100.             while (sup - inf < interpolationPoints) {
  101.                 if (inf == 0) { //This means that we have reached the earliest date
  102.                     sup++;
  103.                 } else if (sup >= abscissae.size()) { //This means that we have reached the latest date
  104.                     inf--;
  105.                 } else { //the choice is made between the two next neighbors
  106.                     final double lowerNeighborDistance = FastMath.abs(abscissae.get(inf - 1).durationFrom(date));
  107.                     final double upperNeighborDistance = FastMath.abs(abscissae.get(sup).durationFrom(date));

  109.                     if (lowerNeighborDistance <= upperNeighborDistance) {
  110.                         inf--;
  111.                     } else {
  112.                         sup++;
  113.                     }
  114.                 }
  115.             }

  117.             for (int i = 0; i < interpolationPoints; ++i) {
  118.                 neighborsIndices[i] = inf + i;
  119.             }

  121.         }

  123.         return neighborsIndices;
  124.     }

  126.     /**Find the closest point from a specific date amongst the available points.
  127.      * @param date date of interest
  128.      * @return index of the closest abscissa from the date of interest
  129.      */
  130.     private int getClosestNeighbor(final AbsoluteDate date) {
  131.         //the starting point is the latest result of a call to this method.
  132.         //Indeed, as this class is meant to be called during an integration process
  133.         //with an input date evolving often continuously in time, there is a high
  134.         //probability that the result will be the same as for last call of
  135.         //this method.
  136.         final int closestNeighbor;

  138.         //case where the date is before the available points
  139.         if (date.compareTo(abscissae.get(0)) <= 0) {
  140.             closestNeighbor = 0;
  141.         }
  142.         //case where the date is after the available points
  143.         else if (date.compareTo(abscissae.get(abscissae.size() - 1)) >= 0) {
  144.             closestNeighbor = abscissae.size() - 1;
  145.         }
  146.         //general case: one is looking for the two consecutives entries that surround the input date
  147.         //then one choose the closest one
  148.         else {
  149.             int lowerBorder = latestClosestNeighbor;
  150.             int upperBorder = latestClosestNeighbor;

  152.             final int searchDirection = date.compareTo(abscissae.get(latestClosestNeighbor));
  153.             if (searchDirection > 0) {
  154.                 upperBorder++;
  155.                 while (date.compareTo(abscissae.get(upperBorder)) > 0) {
  156.                     upperBorder++;
  157.                     lowerBorder++;
  158.                 }
  159.             }
  160.             else {
  161.                 lowerBorder--;
  162.                 while (date.compareTo(abscissae.get(lowerBorder)) < 0) {
  163.                     upperBorder--;
  164.                     lowerBorder--;
  165.                 }
  166.             }

  168.             final double lowerDistance = FastMath.abs(date.durationFrom(abscissae.get(lowerBorder)));
  169.             final double upperDistance = FastMath.abs(date.durationFrom(abscissae.get(upperBorder)));

  171.             closestNeighbor = (lowerDistance < upperDistance) ? lowerBorder : upperBorder;
  172.         }

  174.         //The result is stored in order to speed up the next call to the function
  175.         //Indeed, it is highly likely that the requested result will be the same
  176.         this.latestClosestNeighbor = closestNeighbor;
  177.         return closestNeighbor;
  178.     }

  180.     /** Clear the recorded values from the interpolation grid.
  181.      */
  182.     public void clearHistory() {
  183.         abscissae.clear();
  184.         values.clear();
  185.     }

  187.     /** Add a point to the interpolation grid.
  188.      * @param date abscissa of the point
  189.      * @param value value of the element
  190.      */
  191.     public void addGridPoint(final AbsoluteDate date, final double[] value) {
  192.         //If the grid is empty, the value is directly added to both arrays
  193.         if (abscissae.isEmpty()) {
  194.             abscissae.add(date);
  195.             values.add(value);
  196.         }
  197.         //If the grid already contains this point, only its value is changed
  198.         else if (abscissae.contains(date)) {
  199.             values.set(abscissae.indexOf(date), value);
  200.         }
  201.         //If the grid does not contain this point, the position of the point
  202.         //in the grid is computed first
  203.         else {
  204.             final int closestNeighbor = getClosestNeighbor(date);
  205.             final int index = (date.compareTo(abscissae.get(closestNeighbor)) < 0) ? closestNeighbor : closestNeighbor + 1;
  206.             abscissae.add(index, date);
  207.             values.add(index, value);
  208.         }
  209.     }
  210. }
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