AbstractTimeInterpolator.java

/* Copyright 2002-2023 CS GROUP
 * Licensed to CS GROUP (CS) under one or more
 * 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,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.orekit.time;

import org.hipparchus.util.FastMath;
import org.orekit.errors.OrekitIllegalArgumentException;
import org.orekit.errors.OrekitMessages;
import org.orekit.utils.ImmutableTimeStampedCache;

import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

/**
 * Abstract class for time interpolator.
 *
 * @param <T> interpolated time stamped type
 *
 * @author Vincent Cucchietti
 */
public abstract class AbstractTimeInterpolator<T extends TimeStamped> implements TimeInterpolator<T> {

    /** Default extrapolation time threshold: 1ms. */
    public static final double DEFAULT_EXTRAPOLATION_THRESHOLD_SEC = 1e-3;

    /** Default number of interpolation points. */
    public static final int DEFAULT_INTERPOLATION_POINTS = 2;

    /** The extrapolation threshold beyond which the propagation will fail. */
    private final double extrapolationThreshold;

    /** Neighbor size. */
    private final int interpolationPoints;

    /**
     * Constructor.
     *
     * @param interpolationPoints number of interpolation points
     * @param extrapolationThreshold extrapolation threshold beyond which the propagation will fail
     */
    public AbstractTimeInterpolator(final int interpolationPoints, final double extrapolationThreshold) {
        this.interpolationPoints    = interpolationPoints;
        this.extrapolationThreshold = extrapolationThreshold;
    }

    /**
     * Method checking if given interpolator is compatible with given sample size.
     *
     * @param interpolator interpolator
     * @param sampleSize sample size
     */
    public static void checkInterpolatorCompatibilityWithSampleSize(
            final TimeInterpolator<? extends TimeStamped> interpolator,
            final int sampleSize) {

        // Retrieve all sub-interpolators (or a singleton list with given interpolator if there are no sub-interpolators)
        final List<TimeInterpolator<? extends TimeStamped>> subInterpolators = interpolator.getSubInterpolators();
        for (final TimeInterpolator<? extends TimeStamped> subInterpolator : subInterpolators) {
            if (sampleSize < subInterpolator.getNbInterpolationPoints()) {
                throw new OrekitIllegalArgumentException(OrekitMessages.NOT_ENOUGH_DATA, sampleSize);
            }
        }
    }

    /** {@inheritDoc} */
    @Override
    public T interpolate(final AbsoluteDate interpolationDate, final Stream<T> sample) {
        return interpolate(interpolationDate, sample.collect(Collectors.toList()));
    }

    /** {@inheritDoc}. */
    @Override
    public T interpolate(final AbsoluteDate interpolationDate, final Collection<T> sample) {
        final InterpolationData interpolationData = new InterpolationData(interpolationDate, sample);
        return interpolate(interpolationData);
    }

    /** {@inheritDoc} */
    public List<TimeInterpolator<? extends TimeStamped>> getSubInterpolators() {
        return Collections.singletonList(this);
    }

    /** {@inheritDoc} */
    public int getNbInterpolationPoints() {
        return interpolationPoints;
    }

    /** {@inheritDoc} */
    public double getExtrapolationThreshold() {
        return extrapolationThreshold;
    }

    /**
     * Add all lowest level sub interpolators to the sub interpolator list.
     *
     * @param subInterpolator optional sub interpolator to add
     * @param subInterpolators list of sub interpolators
     */
    protected void addOptionalSubInterpolatorIfDefined(final TimeInterpolator<? extends TimeStamped> subInterpolator,
                                                       final List<TimeInterpolator<? extends TimeStamped>> subInterpolators) {
        // Add all lowest level sub interpolators
        if (subInterpolator != null) {
            subInterpolators.addAll(subInterpolator.getSubInterpolators());
        }
    }

    /**
     * Interpolate instance from given interpolation data.
     *
     * @param interpolationData interpolation data
     *
     * @return interpolated instance from given interpolation data.
     */
    protected abstract T interpolate(InterpolationData interpolationData);

    /**
     * Get the time parameter which lies between [0:1] by normalizing the difference between interpolating time and previous
     * date by the Δt between tabulated values.
     *
     * @param interpolatingTime time at which we want to interpolate a value (between previous and next tabulated dates)
     * @param previousDate previous tabulated value date
     * @param nextDate next tabulated value date
     *
     * @return time parameter which lies between [0:1]
     */
    protected double getTimeParameter(final AbsoluteDate interpolatingTime,
                                      final AbsoluteDate previousDate,
                                      final AbsoluteDate nextDate) {
        return interpolatingTime.durationFrom(previousDate) / nextDate.getDate().durationFrom(previousDate);
    }

    /**
     * Nested class used to store interpolation data.
     * <p>
     * It makes the interpolator thread safe.
     */
    public class InterpolationData {

        /** Interpolation date. */
        private final AbsoluteDate interpolationDate;

        /** Cached samples. */
        private final ImmutableTimeStampedCache<T> cachedSamples;

        /** Neighbor list around interpolation date. */
        private final List<T> neighborList;

        /**
         * Constructor.
         *
         * @param interpolationDate interpolation date
         * @param sample time stamped sample
         */
        protected InterpolationData(final AbsoluteDate interpolationDate, final Collection<T> sample) {
            // Handle specific case that is not handled by the immutable time stamped cache constructor
            if (sample.size() < 2) {
                throw new OrekitIllegalArgumentException(OrekitMessages.NOT_ENOUGH_DATA, sample.size());
            }

            // Create immutable time stamped cache
            this.cachedSamples = new ImmutableTimeStampedCache<>(interpolationPoints, sample);

            // Find neighbors
            final AbsoluteDate central         = getCentralDate(interpolationDate);
            final Stream<T>    neighborsStream = cachedSamples.getNeighbors(central);

            // Convert to unmodifiable list
            this.neighborList = Collections.unmodifiableList(neighborsStream.collect(Collectors.toList()));

            // Store interpolation date
            this.interpolationDate = interpolationDate;
        }

        /**
         * Get the central date to use to find neighbors while taking into account extrapolation threshold.
         *
         * @param date interpolation date
         *
         * @return central date to use to find neighbors
         */
        protected AbsoluteDate getCentralDate(final AbsoluteDate date) {
            final AbsoluteDate central;
            final AbsoluteDate minDate = cachedSamples.getEarliest().getDate();
            final AbsoluteDate maxDate = cachedSamples.getLatest().getDate();

            if (date.compareTo(minDate) < 0 &&
                    FastMath.abs(date.durationFrom(minDate)) <= extrapolationThreshold) {
                // avoid TimeStampedCacheException as we are still within the tolerance before minDate
                central = minDate;
            } else if (date.compareTo(maxDate) > 0 &&
                    FastMath.abs(date.durationFrom(maxDate)) <= extrapolationThreshold) {
                // avoid TimeStampedCacheException as we are still within the tolerance after maxDate
                central = maxDate;
            } else {
                central = date;
            }

            return central;
        }

        /** Get interpolation date.
         * @return interpolation date
         */
        public AbsoluteDate getInterpolationDate() {
            return interpolationDate;
        }

        /** Get cached samples.
         * @return cached samples
         */
        public ImmutableTimeStampedCache<T> getCachedSamples() {
            return cachedSamples;
        }

        /** Get neighbor list.
         * @return neighbor list
         */
        public List<T> getNeighborList() {
            return neighborList;
        }

    }
}