CachedNormalizedSphericalHarmonicsProvider.java
/* Copyright 2002-2023 CS GROUP
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* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* 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
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* 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.forces.gravity.potential;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.stream.Stream;
import org.hipparchus.analysis.interpolation.HermiteInterpolator;
import org.orekit.errors.OrekitException;
import org.orekit.errors.TimeStampedCacheException;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.TimeStamped;
import org.orekit.utils.GenericTimeStampedCache;
import org.orekit.utils.TimeStampedCache;
import org.orekit.utils.TimeStampedGenerator;
/** Caching wrapper for {@link NormalizedSphericalHarmonicsProvider}.
* <p>
* This wrapper improves efficiency of {@link NormalizedSphericalHarmonicsProvider}
* by sampling the values at a user defined rate and using interpolation
* between samples. This is important with providers that have sub-daily
* frequencies and are computing intensive, such as tides fields.
* </p>
* @see NormalizedSphericalHarmonicsProvider
* @see org.orekit.forces.gravity.SolidTides
* @see TimeStampedCache
* @author Luc Maisonobe
* @since 6.1
*/
public class CachedNormalizedSphericalHarmonicsProvider implements NormalizedSphericalHarmonicsProvider {
/** Underlying raw provider. */
private final NormalizedSphericalHarmonicsProvider rawProvider;
/** Number of coefficients in C<sub>n, m</sub> and S<sub>n, m</sub> arrays (counted separately). */
private final int size;
/** Cache. */
private final TimeStampedCache<TimeStampedSphericalHarmonics> cache;
/** Simple constructor.
* @param rawProvider underlying raw provider
* @param step time step between sample points for interpolation
* @param nbPoints number of points to use for interpolation, must be at least 2
* @param maxSlots maximum number of independent cached time slots
* @param maxSpan maximum duration span in seconds of one slot
* (can be set to {@code Double.POSITIVE_INFINITY} if desired)
* @param newSlotInterval time interval above which a new slot is created
* instead of extending an existing one
*/
public CachedNormalizedSphericalHarmonicsProvider(final NormalizedSphericalHarmonicsProvider rawProvider,
final double step, final int nbPoints,
final int maxSlots, final double maxSpan,
final double newSlotInterval) {
this.rawProvider = rawProvider;
final int k = rawProvider.getMaxDegree() + 1;
this.size = (k * (k + 1)) / 2;
cache = new GenericTimeStampedCache<TimeStampedSphericalHarmonics>(nbPoints, maxSlots, maxSpan,
newSlotInterval, new Generator(step));
}
/** {@inheritDoc} */
@Override
public int getMaxDegree() {
return rawProvider.getMaxDegree();
}
/** {@inheritDoc} */
@Override
public int getMaxOrder() {
return rawProvider.getMaxOrder();
}
/** {@inheritDoc} */
@Override
public double getMu() {
return rawProvider.getMu();
}
/** {@inheritDoc} */
@Override
public double getAe() {
return rawProvider.getAe();
}
/** {@inheritDoc} */
@Override
public AbsoluteDate getReferenceDate() {
return rawProvider.getReferenceDate();
}
/** {@inheritDoc} */
@Override
public TideSystem getTideSystem() {
return rawProvider.getTideSystem();
}
/** {@inheritDoc} */
@Override
public NormalizedSphericalHarmonics onDate(final AbsoluteDate date) {
return TimeStampedSphericalHarmonics.interpolate(date, cache.getNeighbors(date));
}
/** Generator for time-stamped spherical harmonics. */
private class Generator implements TimeStampedGenerator<TimeStampedSphericalHarmonics> {
/** Time step between generated sets. */
private final double step;
/** Simple constructor.
* @param step time step between generated sets
*/
Generator(final double step) {
this.step = step;
}
/** {@inheritDoc} */
@Override
public List<TimeStampedSphericalHarmonics> generate(final AbsoluteDate existingDate,
final AbsoluteDate date) {
try {
final List<TimeStampedSphericalHarmonics> generated =
new ArrayList<TimeStampedSphericalHarmonics>();
final double[] cnmsnm = new double[2 * size];
if (existingDate == null) {
// no prior existing transforms, just generate a first set
for (int i = 0; i < cache.getMaxNeighborsSize(); ++i) {
final AbsoluteDate t = date.shiftedBy((i - cache.getMaxNeighborsSize() / 2) * step);
fillArray(rawProvider.onDate(t), cnmsnm);
generated.add(new TimeStampedSphericalHarmonics(t, cnmsnm));
}
} else {
// some coefficients have already been generated
// add the missing ones up to specified date
AbsoluteDate t = existingDate;
if (date.compareTo(t) > 0) {
// forward generation
do {
t = t.shiftedBy(step);
fillArray(rawProvider.onDate(t), cnmsnm);
generated.add(new TimeStampedSphericalHarmonics(t, cnmsnm));
} while (t.compareTo(date) <= 0);
} else {
// backward generation
do {
t = t.shiftedBy(-step);
fillArray(rawProvider.onDate(t), cnmsnm);
generated.add(new TimeStampedSphericalHarmonics(t, cnmsnm));
} while (t.compareTo(date) >= 0);
// ensure forward chronological order
Collections.reverse(generated);
}
}
// return the generated sample
return generated;
} catch (OrekitException oe) {
throw new TimeStampedCacheException(oe);
}
}
/** Fill coefficients array for one entry.
* @param raw the un-interpolated spherical harmonics
* @param cnmsnm arrays to fill in
*/
private void fillArray(final NormalizedSphericalHarmonics raw,
final double[] cnmsnm) {
int index = 0;
for (int n = 0; n <= rawProvider.getMaxDegree(); ++n) {
for (int m = 0; m <= n; ++m) {
cnmsnm[index++] = raw.getNormalizedCnm(n, m);
}
}
for (int n = 0; n <= rawProvider.getMaxDegree(); ++n) {
for (int m = 0; m <= n; ++m) {
cnmsnm[index++] = raw.getNormalizedSnm(n, m);
}
}
}
}
/**
* Internal class for time-stamped spherical harmonics. Instances are created using
* {@link #interpolate(AbsoluteDate, Collection)}
*/
private static class TimeStampedSphericalHarmonics
implements TimeStamped, NormalizedSphericalHarmonics {
/** Current date. */
private final AbsoluteDate date;
/** number of C or S coefficients. */
private final int size;
/** Flattened array for C<sub>n,m</sub> and S<sub>n,m</sub> coefficients. */
private final double[] cnmsnm;
/** Simple constructor.
* @param date current date
* @param cnmsnm flattened array for C<sub>n,m</sub> and S<sub>n,m</sub>
* coefficients. It is copied.
*/
private TimeStampedSphericalHarmonics(final AbsoluteDate date,
final double[] cnmsnm) {
this.date = date;
this.cnmsnm = cnmsnm.clone();
this.size = cnmsnm.length / 2;
}
/** {@inheritDoc} */
@Override
public AbsoluteDate getDate() {
return date;
}
/** {@inheritDoc} */
@Override
public double getNormalizedCnm(final int n, final int m) {
return cnmsnm[(n * (n + 1)) / 2 + m];
}
/** {@inheritDoc} */
@Override
public double getNormalizedSnm(final int n, final int m) {
return cnmsnm[(n * (n + 1)) / 2 + m + size];
}
/** Interpolate spherical harmonics.
* <p>
* The interpolated instance is created by polynomial Hermite interpolation.
* </p>
* @param date interpolation date
* @param sample sample points on which interpolation should be done
* @return a new time-stamped spherical harmonics, interpolated at specified date
*/
public static TimeStampedSphericalHarmonics interpolate(final AbsoluteDate date,
final Stream<TimeStampedSphericalHarmonics> sample) {
// set up an interpolator taking derivatives into account
final HermiteInterpolator interpolator = new HermiteInterpolator();
// add sample points
sample.forEach(tssh -> interpolator.addSamplePoint(tssh.date.durationFrom(date), tssh.cnmsnm));
// build a new interpolated instance
return new TimeStampedSphericalHarmonics(date, interpolator.value(0.0));
}
}
}