EventEnablingPredicateFilter.java

/* Copyright 2002-2020 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,
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 * See the License for the specific language governing permissions and
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package org.orekit.propagation.events;

import java.util.Arrays;

import org.hipparchus.ode.events.Action;
import org.orekit.propagation.SpacecraftState;
import org.orekit.propagation.events.handlers.EventHandler;
import org.orekit.time.AbsoluteDate;

/** Wrapper used to detect events only when enabled by an external predicated function.
 *
 * <p>General {@link EventDetector events} are defined implicitly
 * by a {@link EventDetector#g(SpacecraftState) g function} crossing
 * zero. This implies that during an orbit propagation, events are
 * triggered at all zero crossings.
 * </p>
 *
 * <p>Sometimes, users would like to enable or disable events by themselves,
 * for example to trigger them only for certain orbits, or to check elevation
 * maximums only when elevation itself is positive (i.e. they want to
 * discard elevation maximums below ground). In these cases, looking precisely
 * for all events location and triggering events that will later be ignored
 * is a waste of computing time.</p>
 *
 * <p>Users can wrap a regular {@link EventDetector event detector} in
 * an instance of this class and provide this wrapping instance to
 * a {@link org.orekit.propagation.Propagator}
 * in order to avoid wasting time looking for uninteresting events.
 * The wrapper will intercept the calls to the {@link
 * EventDetector#g(SpacecraftState) g function} and to the {@link
 * EventDetector#eventOccurred(SpacecraftState, boolean)
 * eventOccurred} method in order to ignore uninteresting events. The
 * wrapped regular {@link EventDetector event detector} will the see only
 * the interesting events, i.e. either only events that occur when a
 * user-provided event enabling predicate function is true, ignoring all events
 * that occur when the event enabling predicate function is false. The number of
 * calls to the {@link EventDetector#g(SpacecraftState) g function} will also be
 * reduced.</p>
 * @see EventSlopeFilter
 * @since 7.1
 */

public class EventEnablingPredicateFilter<T extends EventDetector>
    extends AbstractDetector<EventEnablingPredicateFilter<T>> {

    /** Number of past transformers updates stored. */
    private static final int HISTORY_SIZE = 100;

    /** Wrapped event detector. */
    private final T rawDetector;

    /** Enabling predicate function. */
    private final EnablingPredicate<? super T> enabler;

    /** Transformers of the g function. */
    private final Transformer[] transformers;

    /** Update time of the transformers. */
    private final AbsoluteDate[] updates;

    /** Indicator for forward integration. */
    private boolean forward;

    /** Extreme time encountered so far. */
    private AbsoluteDate extremeT;

    /** Detector function value at extremeT. */
    private double extremeG;

    /** Wrap an {@link EventDetector event detector}.
     * @param rawDetector event detector to wrap
     * @param enabler event enabling predicate function to use
     */
    public EventEnablingPredicateFilter(final T rawDetector,
                                        final EnablingPredicate<? super T> enabler) {
        this(rawDetector.getMaxCheckInterval(), rawDetector.getThreshold(),
             rawDetector.getMaxIterationCount(), new LocalHandler<T>(),
             rawDetector, enabler);
    }

    /** Private constructor with full parameters.
     * <p>
     * This constructor is private as users are expected to use the builder
     * API with the various {@code withXxx()} methods to set up the instance
     * in a readable manner without using a huge amount of parameters.
     * </p>
     * @param maxCheck maximum checking interval (s)
     * @param threshold convergence threshold (s)
     * @param maxIter maximum number of iterations in the event time search
     * @param handler event handler to call at event occurrences
     * @param rawDetector event detector to wrap
     * @param enabler event enabling function to use
     */
    private EventEnablingPredicateFilter(final double maxCheck, final double threshold,
                                         final int maxIter, final EventHandler<? super EventEnablingPredicateFilter<T>> handler,
                                         final T rawDetector,
                                         final EnablingPredicate<? super T> enabler) {
        super(maxCheck, threshold, maxIter, handler);
        this.rawDetector  = rawDetector;
        this.enabler      = enabler;
        this.transformers = new Transformer[HISTORY_SIZE];
        this.updates      = new AbsoluteDate[HISTORY_SIZE];
    }

    /** {@inheritDoc} */
    @Override
    protected EventEnablingPredicateFilter<T> create(final double newMaxCheck, final double newThreshold,
                                                     final int newMaxIter,
                                                     final EventHandler<? super EventEnablingPredicateFilter<T>> newHandler) {
        return new EventEnablingPredicateFilter<T>(newMaxCheck, newThreshold, newMaxIter, newHandler, rawDetector, enabler);
    }

    /**  {@inheritDoc} */
    public void init(final SpacecraftState s0,
                     final AbsoluteDate t) {
        super.init(s0, t);

        // delegate to raw detector
        rawDetector.init(s0, t);

        // initialize events triggering logic
        forward  = t.compareTo(s0.getDate()) >= 0;
        extremeT = forward ? AbsoluteDate.PAST_INFINITY : AbsoluteDate.FUTURE_INFINITY;
        extremeG = Double.NaN;
        Arrays.fill(transformers, Transformer.UNINITIALIZED);
        Arrays.fill(updates, extremeT);

    }

    /**  {@inheritDoc} */
    public double g(final SpacecraftState s) {

        final double  rawG      = rawDetector.g(s);
        final boolean isEnabled = enabler.eventIsEnabled(s, rawDetector, rawG);
        if (Double.isNaN(extremeG)) {
            extremeG = rawG;
        }

        // search which transformer should be applied to g
        if (forward) {
            final int last = transformers.length - 1;
            if (extremeT.compareTo(s.getDate()) < 0) {
                // we are at the forward end of the history

                // check if enabled status has changed
                final Transformer previous = transformers[last];
                final Transformer next     = selectTransformer(previous, extremeG, isEnabled);
                if (next != previous) {
                    // there is a status change somewhere between extremeT and t.
                    // the new transformer is valid for t (this is how we have just computed
                    // it above), but it is in fact valid on both sides of the change, so
                    // it was already valid before t and even up to previous time. We store
                    // the switch at extremeT for safety, to ensure the previous transformer
                    // is not applied too close of the root
                    System.arraycopy(updates,      1, updates,      0, last);
                    System.arraycopy(transformers, 1, transformers, 0, last);
                    updates[last]      = extremeT;
                    transformers[last] = next;
                }

                extremeT = s.getDate();
                extremeG = rawG;

                // apply the transform
                return next.transformed(rawG);

            } else {
                // we are in the middle of the history

                // select the transformer
                for (int i = last; i > 0; --i) {
                    if (updates[i].compareTo(s.getDate()) <= 0) {
                        // apply the transform
                        return transformers[i].transformed(rawG);
                    }
                }

                return transformers[0].transformed(rawG);

            }
        } else {
            if (s.getDate().compareTo(extremeT) < 0) {
                // we are at the backward end of the history

                // check if a new rough root has been crossed
                final Transformer previous = transformers[0];
                final Transformer next     = selectTransformer(previous, extremeG, isEnabled);
                if (next != previous) {
                    // there is a status change somewhere between extremeT and t.
                    // the new transformer is valid for t (this is how we have just computed
                    // it above), but it is in fact valid on both sides of the change, so
                    // it was already valid before t and even up to previous time. We store
                    // the switch at extremeT for safety, to ensure the previous transformer
                    // is not applied too close of the root
                    System.arraycopy(updates,      0, updates,      1, updates.length - 1);
                    System.arraycopy(transformers, 0, transformers, 1, transformers.length - 1);
                    updates[0]      = extremeT;
                    transformers[0] = next;
                }

                extremeT = s.getDate();
                extremeG = rawG;

                // apply the transform
                return next.transformed(rawG);

            } else {
                // we are in the middle of the history

                // select the transformer
                for (int i = 0; i < updates.length - 1; ++i) {
                    if (s.getDate().compareTo(updates[i]) <= 0) {
                        // apply the transform
                        return transformers[i].transformed(rawG);
                    }
                }

                return transformers[updates.length - 1].transformed(rawG);

            }
        }

    }

    /** Get next function transformer in the specified direction.
     * @param previous transformer active on the previous point with respect
     * to integration direction (may be null if no previous point is known)
     * @param previousG value of the g function at the previous point
     * @param isEnabled if true the event should be enabled now
     * @return next transformer transformer
     */
    private Transformer selectTransformer(final Transformer previous, final double previousG, final boolean isEnabled) {
        if (isEnabled) {
            // we need to select a transformer that can produce zero crossings,
            // so it is either Transformer.PLUS or Transformer.MINUS
            switch (previous) {
                case UNINITIALIZED :
                    return Transformer.PLUS; // this initial choice is arbitrary, it could have been Transformer.MINUS
                case MIN :
                    return previousG >= 0 ? Transformer.MINUS : Transformer.PLUS;
                case MAX :
                    return previousG >= 0 ? Transformer.PLUS : Transformer.MINUS;
                default :
                    return previous;
            }
        } else {
            // we need to select a transformer that cannot produce any zero crossings,
            // so it is either Transformer.MAX or Transformer.MIN
            switch (previous) {
                case UNINITIALIZED :
                    return Transformer.MAX; // this initial choice is arbitrary, it could have been Transformer.MIN
                case PLUS :
                    return previousG >= 0 ? Transformer.MAX : Transformer.MIN;
                case MINUS :
                    return previousG >= 0 ? Transformer.MIN : Transformer.MAX;
                default :
                    return previous;
            }
        }
    }

    /** Local handler. */
    private static class LocalHandler<T extends EventDetector> implements EventHandler<EventEnablingPredicateFilter<T>> {

        /** {@inheritDoc} */
        public Action eventOccurred(final SpacecraftState s, final EventEnablingPredicateFilter<T> ef, final boolean increasing) {
            final Transformer transformer = ef.forward ? ef.transformers[ef.transformers.length - 1] : ef.transformers[0];
            return ef.rawDetector.eventOccurred(s, transformer == Transformer.PLUS ? increasing : !increasing);
        }

        /** {@inheritDoc} */
        @Override
        public SpacecraftState resetState(final EventEnablingPredicateFilter<T> ef, final SpacecraftState oldState) {
            return ef.rawDetector.resetState(oldState);
        }

    }

}