FieldEventState.java
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* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF 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 org.hipparchus.Field;
import org.hipparchus.RealFieldElement;
import org.hipparchus.analysis.UnivariateFunction;
import org.hipparchus.analysis.solvers.BracketedUnivariateSolver;
import org.hipparchus.analysis.solvers.BracketedUnivariateSolver.Interval;
import org.hipparchus.analysis.solvers.BracketingNthOrderBrentSolver;
import org.hipparchus.exception.MathRuntimeException;
import org.hipparchus.ode.events.Action;
import org.hipparchus.util.FastMath;
import org.hipparchus.util.Precision;
import org.orekit.errors.OrekitInternalError;
import org.orekit.propagation.FieldSpacecraftState;
import org.orekit.propagation.sampling.FieldOrekitStepInterpolator;
import org.orekit.time.FieldAbsoluteDate;
/** This class handles the state for one {@link FieldEventDetector
* event detector} during integration steps.
*
* <p>This class is heavily based on the class with the same name from the
* Hipparchus library. The changes performed consist in replacing
* raw types (double and double arrays) with space dynamics types
* ({@link FieldAbsoluteDate}, {@link FieldSpacecraftState}).</p>
* <p>Each time the propagator proposes a step, the event detector
* should be checked. This class handles the state of one detector
* during one propagation step, with references to the state at the
* end of the preceding step. This information is used to determine if
* the detector should trigger an event or not during the proposed
* step (and hence the step should be reduced to ensure the event
* occurs at a bound rather than inside the step).</p>
* @author Luc Maisonobe
* @param <D> class type for the generic version
*/
public class FieldEventState<D extends FieldEventDetector<T>, T extends RealFieldElement<T>> {
/** Event detector. */
private D detector;
/** Time of the previous call to g. */
private FieldAbsoluteDate<T> lastT;
/** Value from the previous call to g. */
private T lastG;
/** Time at the beginning of the step. */
private FieldAbsoluteDate<T> t0;
/** Value of the event detector at the beginning of the step. */
private T g0;
/** Simulated sign of g0 (we cheat when crossing events). */
private boolean g0Positive;
/** Indicator of event expected during the step. */
private boolean pendingEvent;
/** Occurrence time of the pending event. */
private FieldAbsoluteDate<T> pendingEventTime;
/**
* Time to stop propagation if the event is a stop event. Used to enable stopping at
* an event and then restarting after that event.
*/
private FieldAbsoluteDate<T> stopTime;
/** Time after the current event. */
private FieldAbsoluteDate<T> afterEvent;
/** Value of the g function after the current event. */
private T afterG;
/** The earliest time considered for events. */
private FieldAbsoluteDate<T> earliestTimeConsidered;
/** Integration direction. */
private boolean forward;
/** Variation direction around pending event.
* (this is considered with respect to the integration direction)
*/
private boolean increasing;
/** Simple constructor.
* @param detector monitored event detector
*/
public FieldEventState(final D detector) {
this.detector = detector;
// some dummy values ...
final Field<T> field = detector.getMaxCheckInterval().getField();
final T nan = field.getZero().add(Double.NaN);
lastT = FieldAbsoluteDate.getPastInfinity(field);
lastG = nan;
t0 = null;
g0 = nan;
g0Positive = true;
pendingEvent = false;
pendingEventTime = null;
stopTime = null;
increasing = true;
earliestTimeConsidered = null;
afterEvent = null;
afterG = nan;
}
/** Get the underlying event detector.
* @return underlying event detector
*/
public D getEventDetector() {
return detector;
}
/** Initialize event handler at the start of a propagation.
* <p>
* This method is called once at the start of the propagation. It
* may be used by the event handler to initialize some internal data
* if needed.
* </p>
* @param s0 initial state
* @param t target time for the integration
*
*/
public void init(final FieldSpacecraftState<T> s0,
final FieldAbsoluteDate<T> t) {
detector.init(s0, t);
final Field<T> field = detector.getMaxCheckInterval().getField();
lastT = FieldAbsoluteDate.getPastInfinity(field);
lastG = field.getZero().add(Double.NaN);
}
/** Compute the value of the switching function.
* This function must be continuous (at least in its roots neighborhood),
* as the integrator will need to find its roots to locate the events.
* @param s the current state information: date, kinematics, attitude
* @return value of the switching function
*/
private T g(final FieldSpacecraftState<T> s) {
if (!s.getDate().equals(lastT)) {
lastT = s.getDate();
lastG = detector.g(s);
}
return lastG;
}
/** Reinitialize the beginning of the step.
* @param interpolator interpolator valid for the current step
*/
public void reinitializeBegin(final FieldOrekitStepInterpolator<T> interpolator) {
forward = interpolator.isForward();
final FieldSpacecraftState<T> s0 = interpolator.getPreviousState();
this.t0 = s0.getDate();
g0 = g(s0);
while (g0.getReal() == 0) {
// extremely rare case: there is a zero EXACTLY at interval start
// we will use the sign slightly after step beginning to force ignoring this zero
// try moving forward by half a convergence interval
final T dt = detector.getThreshold().multiply(forward ? 0.5 : -0.5);
FieldAbsoluteDate<T> startDate = t0.shiftedBy(dt);
// if convergence is too small move an ulp
if (t0.equals(startDate)) {
startDate = nextAfter(startDate);
}
t0 = startDate;
g0 = g(interpolator.getInterpolatedState(t0));
}
g0Positive = g0.getReal() > 0;
// "last" event was increasing
increasing = g0Positive;
}
/** Evaluate the impact of the proposed step on the event detector.
* @param interpolator step interpolator for the proposed step
* @return true if the event detector triggers an event before
* the end of the proposed step (this implies the step should be
* rejected)
* @exception MathRuntimeException if an event cannot be located
*/
public boolean evaluateStep(final FieldOrekitStepInterpolator<T> interpolator)
throws MathRuntimeException {
forward = interpolator.isForward();
final FieldSpacecraftState<T> s1 = interpolator.getCurrentState();
final FieldAbsoluteDate<T> t1 = s1.getDate();
final T dt = t1.durationFrom(t0);
if (FastMath.abs(dt.getReal()) < detector.getThreshold().getReal()) {
// we cannot do anything on such a small step, don't trigger any events
return false;
}
// number of points to check in the current step
final int n = FastMath.max(1, (int) FastMath.ceil(FastMath.abs(dt.getReal()) / detector.getMaxCheckInterval().getReal()));
final T h = dt.divide(n);
FieldAbsoluteDate<T> ta = t0;
T ga = g0;
for (int i = 0; i < n; ++i) {
// evaluate handler value at the end of the substep
final FieldAbsoluteDate<T> tb = (i == n - 1) ? t1 : t0.shiftedBy(h.multiply(i + 1));
final T gb = g(interpolator.getInterpolatedState(tb));
// check events occurrence
if (gb.getReal() == 0.0 || (g0Positive ^ (gb.getReal() > 0))) {
// there is a sign change: an event is expected during this step
if (findRoot(interpolator, ta, ga, tb, gb)) {
return true;
}
} else {
// no sign change: there is no event for now
ta = tb;
ga = gb;
}
}
// no event during the whole step
pendingEvent = false;
pendingEventTime = null;
return false;
}
/**
* Find a root in a bracketing interval.
*
* <p> When calling this method one of the following must be true. Either ga == 0, gb
* == 0, (ga < 0 and gb > 0), or (ga > 0 and gb < 0).
*
* @param interpolator that covers the interval.
* @param ta earliest possible time for root.
* @param ga g(ta).
* @param tb latest possible time for root.
* @param gb g(tb).
* @return if a zero crossing was found.
*/
private boolean findRoot(final FieldOrekitStepInterpolator<T> interpolator,
final FieldAbsoluteDate<T> ta, final T ga,
final FieldAbsoluteDate<T> tb, final T gb) {
final T zero = ga.getField().getZero();
// check there appears to be a root in [ta, tb]
check(ga.getReal() == 0.0 || gb.getReal() == 0.0 || (ga.getReal() > 0.0 && gb.getReal() < 0.0) || (ga.getReal() < 0.0 && gb.getReal() > 0.0));
final T convergence = detector.getThreshold();
final int maxIterationCount = detector.getMaxIterationCount();
final BracketedUnivariateSolver<UnivariateFunction> solver =
new BracketingNthOrderBrentSolver(0, convergence.getReal(), 0, 5);
// event time, just at or before the actual root.
FieldAbsoluteDate<T> beforeRootT = null;
T beforeRootG = zero.add(Double.NaN);
// time on the other side of the root.
// Initialized the the loop below executes once.
FieldAbsoluteDate<T> afterRootT = ta;
T afterRootG = zero;
// check for some conditions that the root finders don't like
// these conditions cannot not happen in the loop below
// the ga == 0.0 case is handled by the loop below
if (ta.equals(tb)) {
// both non-zero but times are the same. Probably due to reset state
beforeRootT = ta;
beforeRootG = ga;
afterRootT = shiftedBy(beforeRootT, convergence);
afterRootG = g(interpolator.getInterpolatedState(afterRootT));
} else if (ga.getReal() != 0.0 && gb.getReal() == 0.0) {
// hard: ga != 0.0 and gb == 0.0
// look past gb by up to convergence to find next sign
// throw an exception if g(t) = 0.0 in [tb, tb + convergence]
beforeRootT = tb;
beforeRootG = gb;
afterRootT = shiftedBy(beforeRootT, convergence);
afterRootG = g(interpolator.getInterpolatedState(afterRootT));
} else if (ga.getReal() != 0.0) {
final T newGa = g(interpolator.getInterpolatedState(ta));
if (ga.getReal() > 0 != newGa.getReal() > 0) {
// both non-zero, step sign change at ta, possibly due to reset state
beforeRootT = ta;
beforeRootG = newGa;
afterRootT = minTime(shiftedBy(beforeRootT, convergence), tb);
afterRootG = g(interpolator.getInterpolatedState(afterRootT));
}
}
// loop to skip through "fake" roots, i.e. where g(t) = g'(t) = 0.0
// executed once if we didn't hit a special case above
FieldAbsoluteDate<T> loopT = ta;
T loopG = ga;
while ((afterRootG.getReal() == 0.0 || afterRootG.getReal() > 0.0 == g0Positive) &&
strictlyAfter(afterRootT, tb)) {
if (loopG.getReal() == 0.0) {
// ga == 0.0 and gb may or may not be 0.0
// handle the root at ta first
beforeRootT = loopT;
beforeRootG = loopG;
afterRootT = minTime(shiftedBy(beforeRootT, convergence), tb);
afterRootG = g(interpolator.getInterpolatedState(afterRootT));
} else {
// both non-zero, the usual case, use a root finder.
// time zero for evaluating the function f. Needs to be final
final FieldAbsoluteDate<T> fT0 = loopT;
final UnivariateFunction f = dt -> {
return g(interpolator.getInterpolatedState(fT0.shiftedBy(dt))).getReal();
};
// tb as a double for use in f
final T tbDouble = tb.durationFrom(fT0);
if (forward) {
final Interval interval =
solver.solveInterval(maxIterationCount, f, 0, tbDouble.getReal());
beforeRootT = fT0.shiftedBy(interval.getLeftAbscissa());
beforeRootG = zero.add(interval.getLeftValue());
afterRootT = fT0.shiftedBy(interval.getRightAbscissa());
afterRootG = zero.add(interval.getRightValue());
} else {
final Interval interval =
solver.solveInterval(maxIterationCount, f, tbDouble.getReal(), 0);
beforeRootT = fT0.shiftedBy(interval.getRightAbscissa());
beforeRootG = zero.add(interval.getRightValue());
afterRootT = fT0.shiftedBy(interval.getLeftAbscissa());
afterRootG = zero.add(interval.getLeftValue());
}
}
// tolerance is set to less than 1 ulp
// assume tolerance is 1 ulp
if (beforeRootT.equals(afterRootT)) {
afterRootT = nextAfter(afterRootT);
afterRootG = g(interpolator.getInterpolatedState(afterRootT));
}
// check loop is making some progress
check((forward && afterRootT.compareTo(beforeRootT) > 0) ||
(!forward && afterRootT.compareTo(beforeRootT) < 0));
// setup next iteration
loopT = afterRootT;
loopG = afterRootG;
}
// figure out the result of root finding, and return accordingly
if (afterRootG.getReal() == 0.0 || afterRootG.getReal() > 0.0 == g0Positive) {
// loop gave up and didn't find any crossing within this step
return false;
} else {
// real crossing
check(beforeRootT != null && !Double.isNaN(beforeRootG.getReal()));
// variation direction, with respect to the integration direction
increasing = !g0Positive;
pendingEventTime = beforeRootT;
stopTime = beforeRootG.getReal() == 0.0 ? beforeRootT : afterRootT;
pendingEvent = true;
afterEvent = afterRootT;
afterG = afterRootG;
// check increasing set correctly
check(afterG.getReal() > 0 == increasing);
check(increasing == gb.getReal() >= ga.getReal());
return true;
}
}
/**
* Get the next number after the given number in the current propagation direction.
*
* @param t input time
* @return t +/- 1 ulp depending on the direction.
*/
private FieldAbsoluteDate<T> nextAfter(final FieldAbsoluteDate<T> t) {
return t.shiftedBy(forward ? +Precision.EPSILON : -Precision.EPSILON);
}
/** Get the occurrence time of the event triggered in the current
* step.
* @return occurrence time of the event triggered in the current
* step.
*/
public FieldAbsoluteDate<T> getEventDate() {
return pendingEventTime;
}
/**
* Try to accept the current history up to the given time.
*
* <p> It is not necessary to call this method before calling {@link
* #doEvent(FieldSpacecraftState)} with the same state. It is necessary to call this
* method before you call {@link #doEvent(FieldSpacecraftState)} on some other event
* detector.
*
* @param state to try to accept.
* @param interpolator to use to find the new root, if any.
* @return if the event detector has an event it has not detected before that is on or
* before the same time as {@code state}. In other words {@code false} means continue
* on while {@code true} means stop and handle my event first.
*/
public boolean tryAdvance(final FieldSpacecraftState<T> state,
final FieldOrekitStepInterpolator<T> interpolator) {
// check this is only called before a pending event.
check(!(pendingEvent && strictlyAfter(pendingEventTime, state.getDate())));
final FieldAbsoluteDate<T> t = state.getDate();
// just found an event and we know the next time we want to search again
if (strictlyAfter(t, earliestTimeConsidered)) {
return false;
}
final T g = g(state);
final boolean positive = g.getReal() > 0;
// check for new root, pendingEventTime may be null if there is not pending event
if ((g.getReal() == 0.0 && t.equals(pendingEventTime)) || positive == g0Positive) {
// at a root we already found, or g function has expected sign
t0 = t;
g0 = g; // g0Positive is the same
return false;
} else {
// found a root we didn't expect -> find precise location
return findRoot(interpolator, t0, g0, t, g);
}
}
/**
* Notify the user's listener of the event. The event occurs wholly within this method
* call including a call to {@link FieldEventDetector#resetState(FieldSpacecraftState)}
* if necessary.
*
* @param state the state at the time of the event. This must be at the same time as
* the current value of {@link #getEventDate()}.
* @return the user's requested action and the new state if the action is {@link
* Action#RESET_STATE}. Otherwise
* the new state is {@code state}. The stop time indicates what time propagation should
* stop if the action is {@link Action#STOP}.
* This guarantees the integration will stop on or after the root, so that integration
* may be restarted safely.
*/
public EventOccurrence<T> doEvent(final FieldSpacecraftState<T> state) {
// check event is pending and is at the same time
check(pendingEvent);
check(state.getDate().equals(this.pendingEventTime));
final Action action = detector.eventOccurred(state, increasing == forward);
final FieldSpacecraftState<T> newState;
if (action == Action.RESET_STATE) {
newState = detector.resetState(state);
} else {
newState = state;
}
// clear pending event
pendingEvent = false;
pendingEventTime = null;
// setup for next search
earliestTimeConsidered = afterEvent;
t0 = afterEvent;
g0 = afterG;
g0Positive = increasing;
// check g0Positive set correctly
check(g0.getReal() == 0.0 || g0Positive == (g0.getReal() > 0));
return new EventOccurrence<T>(action, newState, stopTime);
}
/**
* Shift a time value along the current integration direction: {@link #forward}.
*
* @param t the time to shift.
* @param delta the amount to shift.
* @return t + delta if forward, else t - delta. If the result has to be rounded it
* will be rounded to be before the true value of t + delta.
*/
private FieldAbsoluteDate<T> shiftedBy(final FieldAbsoluteDate<T> t, final T delta) {
if (forward) {
final FieldAbsoluteDate<T> ret = t.shiftedBy(delta);
if (ret.durationFrom(t).getReal() > delta.getReal()) {
return ret.shiftedBy(-Precision.EPSILON);
} else {
return ret;
}
} else {
final FieldAbsoluteDate<T> ret = t.shiftedBy(delta.negate());
if (t.durationFrom(ret).getReal() > delta.getReal()) {
return ret.shiftedBy(+Precision.EPSILON);
} else {
return ret;
}
}
}
/**
* Get the time that happens first along the current propagation direction: {@link
* #forward}.
*
* @param a first time
* @param b second time
* @return min(a, b) if forward, else max (a, b)
*/
private FieldAbsoluteDate<T> minTime(final FieldAbsoluteDate<T> a, final FieldAbsoluteDate<T> b) {
return (forward ^ (a.compareTo(b) > 0)) ? a : b;
}
/**
* Check the ordering of two times.
*
* @param t1 the first time.
* @param t2 the second time.
* @return true if {@code t2} is strictly after {@code t1} in the propagation
* direction.
*/
private boolean strictlyAfter(final FieldAbsoluteDate<T> t1, final FieldAbsoluteDate<T> t2) {
if (t1 == null || t2 == null) {
return false;
} else {
return forward ? t1.compareTo(t2) < 0 : t2.compareTo(t1) < 0;
}
}
/**
* Same as keyword assert, but throw a {@link MathRuntimeException}.
*
* @param condition to check
* @throws MathRuntimeException if {@code condition} is false.
*/
private void check(final boolean condition) throws MathRuntimeException {
if (!condition) {
throw new OrekitInternalError(null);
}
}
/**
* Class to hold the data related to an event occurrence that is needed to decide how
* to modify integration.
*/
public static class EventOccurrence<T extends RealFieldElement<T>> {
/** User requested action. */
private final Action action;
/** New state for a reset action. */
private final FieldSpacecraftState<T> newState;
/** The time to stop propagation if the action is a stop event. */
private final FieldAbsoluteDate<T> stopDate;
/**
* Create a new occurrence of an event.
*
* @param action the user requested action.
* @param newState for a reset event. Should be the current state unless the
* action is {@link Action#RESET_STATE}.
* @param stopDate to stop propagation if the action is {@link Action#STOP}. Used
* to move the stop time to just after the root.
*/
EventOccurrence(final Action action,
final FieldSpacecraftState<T> newState,
final FieldAbsoluteDate<T> stopDate) {
this.action = action;
this.newState = newState;
this.stopDate = stopDate;
}
/**
* Get the user requested action.
*
* @return the action.
*/
public Action getAction() {
return action;
}
/**
* Get the new state for a reset action.
*
* @return the new state.
*/
public FieldSpacecraftState<T> getNewState() {
return newState;
}
/**
* Get the new time for a stop action.
*
* @return when to stop propagation.
*/
public FieldAbsoluteDate<T> getStopDate() {
return stopDate;
}
}
/**Get PendingEvent.
* @return if there is a pending event or not
* */
public boolean getPendingEvent() {
return pendingEvent;
}
}