EventState.java
/*
* 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,
* 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.propagation.events;
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.util.FastMath;
import org.hipparchus.util.Precision;
import org.orekit.errors.OrekitInternalError;
import org.orekit.propagation.SpacecraftState;
import org.orekit.propagation.events.handlers.EventHandler;
import org.orekit.propagation.sampling.OrekitStepInterpolator;
import org.orekit.time.AbsoluteDate;
import java.io.Serializable;
/** This class handles the state for one {@link EventDetector
* 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 AbsoluteDate}, {@link SpacecraftState}).</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 <T> class type for the generic version
*/
public class EventState<T extends EventDetector> implements Serializable {
/** Serializable version identifier. */
private static final long serialVersionUID = 4489391420715269318L;
/** Event detector. */
private T detector;
/** Time of the previous call to g. */
private AbsoluteDate lastT;
/** Value from the previous call to g. */
private double lastG;
/** Time at the beginning of the step. */
private AbsoluteDate t0;
/** Value of the event detector at the beginning of the step. */
private double 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 AbsoluteDate 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 AbsoluteDate stopTime;
/** Time after the current event. */
private AbsoluteDate afterEvent;
/** Value of the g function after the current event. */
private double afterG;
/** The earliest time considered for events. */
private AbsoluteDate 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 EventState(final T detector) {
this.detector = detector;
// some dummy values ...
lastT = AbsoluteDate.PAST_INFINITY;
lastG = Double.NaN;
t0 = null;
g0 = Double.NaN;
g0Positive = true;
pendingEvent = false;
pendingEventTime = null;
stopTime = null;
increasing = true;
earliestTimeConsidered = null;
afterEvent = null;
afterG = Double.NaN;
}
/** Get the underlying event detector.
* @return underlying event detector
*/
public T 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 SpacecraftState s0,
final AbsoluteDate t) {
detector.init(s0, t);
lastT = AbsoluteDate.PAST_INFINITY;
lastG = 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 double g(final SpacecraftState 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 OrekitStepInterpolator interpolator) {
forward = interpolator.isForward();
final SpacecraftState s0 = interpolator.getPreviousState();
this.t0 = s0.getDate();
g0 = g(s0);
while (g0 == 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 double dt = (forward ? 0.5 : -0.5) * detector.getThreshold();
AbsoluteDate 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 > 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 OrekitStepInterpolator interpolator)
throws MathRuntimeException {
forward = interpolator.isForward();
final SpacecraftState s1 = interpolator.getCurrentState();
final AbsoluteDate t1 = s1.getDate();
final double dt = t1.durationFrom(t0);
if (FastMath.abs(dt) < detector.getThreshold()) {
// 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) / detector.getMaxCheckInterval()));
final double h = dt / n;
AbsoluteDate ta = t0;
double ga = g0;
for (int i = 0; i < n; ++i) {
// evaluate handler value at the end of the substep
final AbsoluteDate tb = (i == n - 1) ? t1 : t0.shiftedBy((i + 1) * h);
final double gb = g(interpolator.getInterpolatedState(tb));
// check events occurrence
if (gb == 0.0 || (g0Positive ^ (gb > 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 OrekitStepInterpolator interpolator,
final AbsoluteDate ta, final double ga,
final AbsoluteDate tb, final double gb) {
// check there appears to be a root in [ta, tb]
check(ga == 0.0 || gb == 0.0 || (ga > 0.0 && gb < 0.0) || (ga < 0.0 && gb > 0.0));
final double convergence = detector.getThreshold();
final int maxIterationCount = detector.getMaxIterationCount();
final BracketedUnivariateSolver<UnivariateFunction> solver =
new BracketingNthOrderBrentSolver(0, convergence, 0, 5);
// event time, just at or before the actual root.
AbsoluteDate beforeRootT = null;
double beforeRootG = Double.NaN;
// time on the other side of the root.
// Initialized the the loop below executes once.
AbsoluteDate afterRootT = ta;
double afterRootG = 0.0;
// 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 != 0.0 && gb == 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 != 0.0) {
final double newGa = g(interpolator.getInterpolatedState(ta));
if (ga > 0 != newGa > 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
AbsoluteDate loopT = ta;
double loopG = ga;
while ((afterRootG == 0.0 || afterRootG > 0.0 == g0Positive) &&
strictlyAfter(afterRootT, tb)) {
if (loopG == 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 AbsoluteDate fT0 = loopT;
final UnivariateFunction f = dt -> {
return g(interpolator.getInterpolatedState(fT0.shiftedBy(dt)));
};
// tb as a double for use in f
final double tbDouble = tb.durationFrom(fT0);
if (forward) {
final Interval interval =
solver.solveInterval(maxIterationCount, f, 0, tbDouble);
beforeRootT = fT0.shiftedBy(interval.getLeftAbscissa());
beforeRootG = interval.getLeftValue();
afterRootT = fT0.shiftedBy(interval.getRightAbscissa());
afterRootG = interval.getRightValue();
} else {
final Interval interval =
solver.solveInterval(maxIterationCount, f, tbDouble, 0);
beforeRootT = fT0.shiftedBy(interval.getRightAbscissa());
beforeRootG = interval.getRightValue();
afterRootT = fT0.shiftedBy(interval.getLeftAbscissa());
afterRootG = 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 == 0.0 || afterRootG > 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));
// variation direction, with respect to the integration direction
increasing = !g0Positive;
pendingEventTime = beforeRootT;
stopTime = beforeRootG == 0.0 ? beforeRootT : afterRootT;
pendingEvent = true;
afterEvent = afterRootT;
afterG = afterRootG;
// check increasing set correctly
check(afterG > 0 == increasing);
check(increasing == gb >= ga);
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 AbsoluteDate nextAfter(final AbsoluteDate 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 AbsoluteDate 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(SpacecraftState)} with the same state. It is necessary to call this
* method before you call {@link #doEvent(SpacecraftState)} 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 SpacecraftState state,
final OrekitStepInterpolator interpolator) {
// check this is only called before a pending event.
check(!(pendingEvent && strictlyAfter(pendingEventTime, state.getDate())));
final AbsoluteDate 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 double g = g(state);
final boolean positive = g > 0;
// check for new root, pendingEventTime may be null if there is not pending event
if ((g == 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 EventDetector#resetState(SpacecraftState)}
* 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
* org.orekit.propagation.events.handlers.EventHandler.Action#RESET_STATE Action.RESET_STATE}.
* Otherwise the new state is {@code state}. The stop time indicates what time propagation
* should stop if the action is {@link
* org.orekit.propagation.events.handlers.EventHandler.Action#STOP Action.STOP}.
* This guarantees the integration will stop on or after the root, so that integration
* may be restarted safely.
*/
public EventOccurrence doEvent(final SpacecraftState state) {
// check event is pending and is at the same time
check(pendingEvent);
check(state.getDate().equals(this.pendingEventTime));
final EventHandler.Action action = detector.eventOccurred(state, increasing == forward);
final SpacecraftState newState;
if (action == EventHandler.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 == 0.0 || g0Positive == (g0 > 0));
return new EventOccurrence(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 AbsoluteDate shiftedBy(final AbsoluteDate t, final double delta) {
if (forward) {
final AbsoluteDate ret = t.shiftedBy(delta);
if (ret.durationFrom(t) > delta) {
return ret.shiftedBy(-Precision.EPSILON);
} else {
return ret;
}
} else {
final AbsoluteDate ret = t.shiftedBy(-delta);
if (t.durationFrom(ret) > delta) {
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 AbsoluteDate minTime(final AbsoluteDate a, final AbsoluteDate 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 AbsoluteDate t1, final AbsoluteDate 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 {
/** User requested action. */
private final EventHandler.Action action;
/** New state for a reset action. */
private final SpacecraftState newState;
/** The time to stop propagation if the action is a stop event. */
private final AbsoluteDate 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 EventHandler.Action action,
final SpacecraftState newState,
final AbsoluteDate stopDate) {
this.action = action;
this.newState = newState;
this.stopDate = stopDate;
}
/**
* Get the user requested action.
*
* @return the action.
*/
public EventHandler.Action getAction() {
return action;
}
/**
* Get the new state for a reset action.
*
* @return the new state.
*/
public SpacecraftState getNewState() {
return newState;
}
/**
* Get the new time for a stop action.
*
* @return when to stop propagation.
*/
public AbsoluteDate getStopDate() {
return stopDate;
}
}
}