FieldTransformGenerator.java

/* Copyright 2002-2025 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
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 *
 *   http://www.apache.org/licenses/LICENSE-2.0
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package org.orekit.frames;

import java.util.ArrayList;
import java.util.List;

import org.hipparchus.Field;
import org.hipparchus.CalculusFieldElement;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.utils.GenericTimeStampedCache;
import org.orekit.utils.TimeStampedGenerator;

/** Generator to use field transforms in {@link GenericTimeStampedCache}.
 * @see GenericTimeStampedCache
 * @since 9.0
 * @author Luc Maisonobe
 * @param <T> type of the field elements
 */
public class FieldTransformGenerator<T extends CalculusFieldElement<T>> implements TimeStampedGenerator<FieldTransform<T>> {

    /** Field to which the elements belong. */
    private final Field<T> field;

    /** Number of neighbors. */
    private int neighborsSize;

    /** Underlying provider. */
    private final TransformProvider provider;

    /** Step size. */
    private final double step;

    /** simple constructor.
     * @param field field to which the elements belong
     * @param neighborsSize number of neighbors
     * @param provider underlying provider
     * @param step step size
     */
    public FieldTransformGenerator(final Field<T> field,
                                   final int neighborsSize,
                                   final TransformProvider provider,
                                   final double step) {
        this.field         = field;
        this.neighborsSize = neighborsSize;
        this.provider      = provider;
        this.step          = step;
    }

    /** {@inheritDoc} */
    public List<FieldTransform<T>> generate(final AbsoluteDate existingDate, final AbsoluteDate date) {

        final FieldAbsoluteDate<T> fieldDate = new FieldAbsoluteDate<>(field, date);
        final List<FieldTransform<T>> generated = new ArrayList<>();

        if (existingDate == null) {

            // no prior existing transforms, just generate a first set
            for (int i = 0; i < neighborsSize; ++i) {
                generated.add(provider.getTransform(fieldDate.shiftedBy(i * step)));
            }

        } else {

            // some transforms have already been generated
            // add the missing ones up to specified date
            FieldAbsoluteDate<T> t = new FieldAbsoluteDate<>(field, existingDate);
            if (date.compareTo(t.toAbsoluteDate()) > 0) {
                // forward generation
                do {
                    t = t.shiftedBy(step);
                    generated.add(generated.size(), provider.getTransform(t));
                } while (t.compareTo(fieldDate) <= 0);
            } else {
                // backward generation
                do {
                    t = t.shiftedBy(-step);
                    generated.add(0, provider.getTransform(t));
                } while (t.compareTo(fieldDate) >= 0);
            }

        }

        // return the generated transforms
        return generated;

    }

}