FieldEventState.java

  1. /*
  2.  * Licensed to the Apache Software Foundation (ASF) under one or more
  3.  * contributor license agreements.  See the NOTICE file distributed with
  4.  * this work for additional information regarding copyright ownership.
  5.  * The ASF licenses this file to You under the Apache License, Version 2.0
  6.  * (the "License"); you may not use this file except in compliance with
  7.  * the License.  You may obtain a copy of the License at
  8.  *
  9.  *      http://www.apache.org/licenses/LICENSE-2.0
  10.  *
  11.  * Unless required by applicable law or agreed to in writing, software
  12.  * distributed under the License is distributed on an "AS IS" BASIS,
  13.  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  14.  * See the License for the specific language governing permissions and
  15.  * limitations under the License.
  16.  */
  17. package org.orekit.propagation.events;

  19. import java.util.function.DoubleFunction;

  21. import org.hipparchus.CalculusFieldElement;
  22. import org.hipparchus.Field;
  23. import org.hipparchus.analysis.UnivariateFunction;
  24. import org.hipparchus.analysis.solvers.BracketedUnivariateSolver;
  25. import org.hipparchus.analysis.solvers.BracketedUnivariateSolver.Interval;
  26. import org.hipparchus.analysis.solvers.BracketingNthOrderBrentSolver;
  27. import org.hipparchus.exception.MathRuntimeException;
  28. import org.hipparchus.ode.events.Action;
  29. import org.hipparchus.util.FastMath;
  30. import org.hipparchus.util.Precision;
  31. import org.orekit.errors.OrekitException;
  32. import org.orekit.errors.OrekitInternalError;
  33. import org.orekit.errors.OrekitMessages;
  34. import org.orekit.propagation.FieldSpacecraftState;
  35. import org.orekit.propagation.sampling.FieldOrekitStepInterpolator;
  36. import org.orekit.time.FieldAbsoluteDate;

  38. /** This class handles the state for one {@link FieldEventDetector
  39.  * event detector} during integration steps.
  40.  *
  41.  * <p>This class is heavily based on the class with the same name from the
  42.  * Hipparchus library. The changes performed consist in replacing
  43.  * raw types (double and double arrays) with space dynamics types
  44.  * ({@link FieldAbsoluteDate}, {@link FieldSpacecraftState}).</p>
  45.  * <p>Each time the propagator proposes a step, the event detector
  46.  * should be checked. This class handles the state of one detector
  47.  * during one propagation step, with references to the state at the
  48.  * end of the preceding step. This information is used to determine if
  49.  * the detector should trigger an event or not during the proposed
  50.  * step (and hence the step should be reduced to ensure the event
  51.  * occurs at a bound rather than inside the step).</p>
  52.  * @author Luc Maisonobe
  53.  * @param <D> class type for the generic version
  54.  */
  55. public class FieldEventState<D extends FieldEventDetector<T>, T extends CalculusFieldElement<T>> {

  57.     /** Event detector. */
  58.     private D detector;

  60.     /** Time of the previous call to g. */
  61.     private FieldAbsoluteDate<T> lastT;

  63.     /** Value from the previous call to g. */
  64.     private T lastG;

  66.     /** Time at the beginning of the step. */
  67.     private FieldAbsoluteDate<T> t0;

  69.     /** Value of the event detector at the beginning of the step. */
  70.     private T g0;

  72.     /** Simulated sign of g0 (we cheat when crossing events). */
  73.     private boolean g0Positive;

  75.     /** Indicator of event expected during the step. */
  76.     private boolean pendingEvent;

  78.     /** Occurrence time of the pending event. */
  79.     private FieldAbsoluteDate<T> pendingEventTime;

  81.     /**
  82.      * Time to stop propagation if the event is a stop event. Used to enable stopping at
  83.      * an event and then restarting after that event.
  84.      */
  85.     private FieldAbsoluteDate<T> stopTime;

  87.     /** Time after the current event. */
  88.     private FieldAbsoluteDate<T> afterEvent;

  90.     /** Value of the g function after the current event. */
  91.     private T afterG;

  93.     /** The earliest time considered for events. */
  94.     private FieldAbsoluteDate<T> earliestTimeConsidered;

  96.     /** Integration direction. */
  97.     private boolean forward;

  99.     /** Variation direction around pending event.
  100.      *  (this is considered with respect to the integration direction)
  101.      */
  102.     private boolean increasing;

  104.     /** Simple constructor.
  105.      * @param detector monitored event detector
  106.      */
  107.     public FieldEventState(final D detector) {

  109.         this.detector = detector;

  111.         // some dummy values ...
  112.         final Field<T> field   = detector.getMaxCheckInterval().getField();
  113.         final T nan            = field.getZero().add(Double.NaN);
  114.         lastT                  = FieldAbsoluteDate.getPastInfinity(field);
  115.         lastG                  = nan;
  116.         t0                     = null;
  117.         g0                     = nan;
  118.         g0Positive             = true;
  119.         pendingEvent           = false;
  120.         pendingEventTime       = null;
  121.         stopTime               = null;
  122.         increasing             = true;
  123.         earliestTimeConsidered = null;
  124.         afterEvent             = null;
  125.         afterG                 = nan;

  127.     }

  129.     /** Get the underlying event detector.
  130.      * @return underlying event detector
  131.      */
  132.     public D getEventDetector() {
  133.         return detector;
  134.     }

  136.     /** Initialize event handler at the start of a propagation.
  137.      * <p>
  138.      * This method is called once at the start of the propagation. It
  139.      * may be used by the event handler to initialize some internal data
  140.      * if needed.
  141.      * </p>
  142.      * @param s0 initial state
  143.      * @param t target time for the integration
  144.      *
  145.      */
  146.     public void init(final FieldSpacecraftState<T> s0,
  147.                      final FieldAbsoluteDate<T> t) {
  148.         detector.init(s0, t);
  149.         final Field<T> field = detector.getMaxCheckInterval().getField();
  150.         lastT = FieldAbsoluteDate.getPastInfinity(field);
  151.         lastG = field.getZero().add(Double.NaN);
  152.     }

  154.     /** Compute the value of the switching function.
  155.      * This function must be continuous (at least in its roots neighborhood),
  156.      * as the integrator will need to find its roots to locate the events.
  157.      * @param s the current state information: date, kinematics, attitude
  158.      * @return value of the switching function
  159.      */
  160.     private T g(final FieldSpacecraftState<T> s) {
  161.         if (!s.getDate().equals(lastT)) {
  162.             lastT = s.getDate();
  163.             lastG = detector.g(s);
  164.         }
  165.         return lastG;
  166.     }

  168.     /** Reinitialize the beginning of the step.
  169.      * @param interpolator interpolator valid for the current step
  170.      */
  171.     public void reinitializeBegin(final FieldOrekitStepInterpolator<T> interpolator) {
  172.         forward = interpolator.isForward();
  173.         final FieldSpacecraftState<T> s0 = interpolator.getPreviousState();
  174.         this.t0 = s0.getDate();
  175.         g0 = g(s0);
  176.         while (g0.getReal() == 0) {
  177.             // extremely rare case: there is a zero EXACTLY at interval start
  178.             // we will use the sign slightly after step beginning to force ignoring this zero
  179.             // try moving forward by half a convergence interval
  180.             final T dt = detector.getThreshold().multiply(forward ? 0.5 : -0.5);
  181.             FieldAbsoluteDate<T> startDate = t0.shiftedBy(dt);
  182.             // if convergence is too small move an ulp
  183.             if (t0.equals(startDate)) {
  184.                 startDate = nextAfter(startDate);
  185.             }
  186.             t0 = startDate;
  187.             g0 = g(interpolator.getInterpolatedState(t0));
  188.         }
  189.         g0Positive = g0.getReal() > 0;
  190.         // "last" event was increasing
  191.         increasing = g0Positive;
  192.     }

  194.     /** Evaluate the impact of the proposed step on the event detector.
  195.      * @param interpolator step interpolator for the proposed step
  196.      * @return true if the event detector triggers an event before
  197.      * the end of the proposed step (this implies the step should be
  198.      * rejected)
  199.      * @exception MathRuntimeException if an event cannot be located
  200.      */
  201.     public boolean evaluateStep(final FieldOrekitStepInterpolator<T> interpolator)
  202.         throws MathRuntimeException {
  203.         forward = interpolator.isForward();
  204.         final FieldSpacecraftState<T> s1 = interpolator.getCurrentState();
  205.         final FieldAbsoluteDate<T> t1 = s1.getDate();
  206.         final T dt = t1.durationFrom(t0);
  207.         if (FastMath.abs(dt.getReal()) < detector.getThreshold().getReal()) {
  208.             // we cannot do anything on such a small step, don't trigger any events
  209.             return false;
  210.         }
  211.         // number of points to check in the current step
  212.         final int n = FastMath.max(1, (int) FastMath.ceil(FastMath.abs(dt.getReal()) / detector.getMaxCheckInterval().getReal()));
  213.         final T h = dt.divide(n);


  216.         FieldAbsoluteDate<T> ta = t0;
  217.         T ga = g0;
  218.         for (int i = 0; i < n; ++i) {

  220.             // evaluate handler value at the end of the substep
  221.             final FieldAbsoluteDate<T> tb = (i == n - 1) ? t1 : t0.shiftedBy(h.multiply(i + 1));
  222.             final T gb = g(interpolator.getInterpolatedState(tb));

  224.             // check events occurrence
  225.             if (gb.getReal() == 0.0 || (g0Positive ^ (gb.getReal() > 0))) {
  226.                 // there is a sign change: an event is expected during this step
  227.                 if (findRoot(interpolator, ta, ga, tb, gb)) {
  228.                     return true;
  229.                 }
  230.             } else {
  231.                 // no sign change: there is no event for now
  232.                 ta = tb;
  233.                 ga = gb;
  234.             }
  235.         }

  237.         // no event during the whole step
  238.         pendingEvent     = false;
  239.         pendingEventTime = null;
  240.         return false;

  242.     }

  244.     /**
  245.      * Find a root in a bracketing interval.
  246.      *
  247.      * <p> When calling this method one of the following must be true. Either ga == 0, gb
  248.      * == 0, (ga < 0  and gb > 0), or (ga > 0 and gb < 0).
  249.      *
  250.      * @param interpolator that covers the interval.
  251.      * @param ta           earliest possible time for root.
  252.      * @param ga           g(ta).
  253.      * @param tb           latest possible time for root.
  254.      * @param gb           g(tb).
  255.      * @return if a zero crossing was found.
  256.      */
  257.     private boolean findRoot(final FieldOrekitStepInterpolator<T> interpolator,
  258.                              final FieldAbsoluteDate<T> ta, final T ga,
  259.                              final FieldAbsoluteDate<T> tb, final T gb) {

  261.         final T zero = ga.getField().getZero();

  263.         // check there appears to be a root in [ta, tb]
  264.         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);
  265.         final T convergence = detector.getThreshold();
  266.         final int maxIterationCount = detector.getMaxIterationCount();
  267.         final BracketedUnivariateSolver<UnivariateFunction> solver =
  268.                 new BracketingNthOrderBrentSolver(0, convergence.getReal(), 0, 5);

  270.         // prepare loop below
  271.         FieldAbsoluteDate<T> loopT = ta;
  272.         T loopG = ga;

  274.         // event time, just at or before the actual root.
  275.         FieldAbsoluteDate<T> beforeRootT = null;
  276.         T beforeRootG = zero.add(Double.NaN);
  277.         // time on the other side of the root.
  278.         // Initialized the the loop below executes once.
  279.         FieldAbsoluteDate<T> afterRootT = ta;
  280.         T afterRootG = zero;

  282.         // check for some conditions that the root finders don't like
  283.         // these conditions cannot not happen in the loop below
  284.         // the ga == 0.0 case is handled by the loop below
  285.         if (ta.equals(tb)) {
  286.             // both non-zero but times are the same. Probably due to reset state
  287.             beforeRootT = ta;
  288.             beforeRootG = ga;
  289.             afterRootT = shiftedBy(beforeRootT, convergence);
  290.             afterRootG = g(interpolator.getInterpolatedState(afterRootT));
  291.         } else if (ga.getReal() != 0.0 && gb.getReal() == 0.0) {
  292.             // hard: ga != 0.0 and gb == 0.0
  293.             // look past gb by up to convergence to find next sign
  294.             // throw an exception if g(t) = 0.0 in [tb, tb + convergence]
  295.             beforeRootT = tb;
  296.             beforeRootG = gb;
  297.             afterRootT = shiftedBy(beforeRootT, convergence);
  298.             afterRootG = g(interpolator.getInterpolatedState(afterRootT));
  299.         } else if (ga.getReal() != 0.0) {
  300.             final T newGa = g(interpolator.getInterpolatedState(ta));
  301.             if (ga.getReal() > 0 != newGa.getReal() > 0) {
  302.                 // both non-zero, step sign change at ta, possibly due to reset state
  303.                 final FieldAbsoluteDate<T> nextT = minTime(shiftedBy(ta, convergence), tb);
  304.                 final T                    nextG = g(interpolator.getInterpolatedState(nextT));
  305.                 if (nextG.getReal() > 0.0 == g0Positive) {
  306.                     // the sign change between ga and newGa just moved the root less than one convergence
  307.                     // threshold later, we are still in a regular search for another root before tb,
  308.                     // we just need to fix the bracketing interval
  309.                     // (see issue https://github.com/Hipparchus-Math/hipparchus/issues/184)
  310.                     loopT = nextT;
  311.                     loopG = nextG;
  312.                 } else {
  313.                     beforeRootT = ta;
  314.                     beforeRootG = newGa;
  315.                     afterRootT  = nextT;
  316.                     afterRootG  = nextG;
  317.                 }
  318.             }
  319.         }

  321.         // loop to skip through "fake" roots, i.e. where g(t) = g'(t) = 0.0
  322.         // executed once if we didn't hit a special case above
  323.         while ((afterRootG.getReal() == 0.0 || afterRootG.getReal() > 0.0 == g0Positive) &&
  324.                 strictlyAfter(afterRootT, tb)) {
  325.             if (loopG.getReal() == 0.0) {
  326.                 // ga == 0.0 and gb may or may not be 0.0
  327.                 // handle the root at ta first
  328.                 beforeRootT = loopT;
  329.                 beforeRootG = loopG;
  330.                 afterRootT = minTime(shiftedBy(beforeRootT, convergence), tb);
  331.                 afterRootG = g(interpolator.getInterpolatedState(afterRootT));
  332.             } else {
  333.                 // both non-zero, the usual case, use a root finder.
  334.                 // time zero for evaluating the function f. Needs to be final
  335.                 final FieldAbsoluteDate<T> fT0 = loopT;
  336.                 final double tbDouble = tb.durationFrom(fT0).getReal();
  337.                 final double middle   = 0.5 * tbDouble;
  338.                 final DoubleFunction<FieldAbsoluteDate<T>> date = dt -> {
  339.                     // use either fT0 or tb as the base time for shifts
  340.                     // in order to ensure we reproduce exactly those times
  341.                     // using only one reference time like fT0 would imply
  342.                     // to use ft0.shiftedBy(tbDouble), which may be different
  343.                     // from tb due to numerical noise (see issue 921)
  344.                     if (forward == dt <= middle) {
  345.                         // use start of interval as reference
  346.                         return fT0.shiftedBy(dt);
  347.                     } else {
  348.                         // use end of interval as reference
  349.                         return tb.shiftedBy(dt - tbDouble);
  350.                     }
  351.                 };
  352.                 final UnivariateFunction f = dt -> g(interpolator.getInterpolatedState(date.apply(dt))).getReal();
  353.                 if (forward) {
  354.                     try {
  355.                         final Interval interval =
  356.                                 solver.solveInterval(maxIterationCount, f, 0, tbDouble);
  357.                         beforeRootT = date.apply(interval.getLeftAbscissa());
  358.                         beforeRootG = zero.add(interval.getLeftValue());
  359.                         afterRootT  = date.apply(interval.getRightAbscissa());
  360.                         afterRootG  = zero.add(interval.getRightValue());
  361.                         // CHECKSTYLE: stop IllegalCatch check
  362.                     } catch (RuntimeException e) {
  363.                         // CHECKSTYLE: resume IllegalCatch check
  364.                         throw new OrekitException(e, OrekitMessages.FIND_ROOT,
  365.                                 detector, loopT, loopG, tb, gb, lastT, lastG);
  366.                     }
  367.                 } else {
  368.                     try {
  369.                         final Interval interval =
  370.                                 solver.solveInterval(maxIterationCount, f, tbDouble, 0);
  371.                         beforeRootT = date.apply(interval.getRightAbscissa());
  372.                         beforeRootG = zero.add(interval.getRightValue());
  373.                         afterRootT  = date.apply(interval.getLeftAbscissa());
  374.                         afterRootG  = zero.add(interval.getLeftValue());
  375.                         // CHECKSTYLE: stop IllegalCatch check
  376.                     } catch (RuntimeException e) {
  377.                         // CHECKSTYLE: resume IllegalCatch check
  378.                         throw new OrekitException(e, OrekitMessages.FIND_ROOT,
  379.                                 detector, tb, gb, loopT, loopG, lastT, lastG);
  380.                     }
  381.                 }
  382.             }
  383.             // tolerance is set to less than 1 ulp
  384.             // assume tolerance is 1 ulp
  385.             if (beforeRootT.equals(afterRootT)) {
  386.                 afterRootT = nextAfter(afterRootT);
  387.                 afterRootG = g(interpolator.getInterpolatedState(afterRootT));
  388.             }
  389.             // check loop is making some progress
  390.             check(forward && afterRootT.compareTo(beforeRootT) > 0 ||
  391.                   !forward && afterRootT.compareTo(beforeRootT) < 0);
  392.             // setup next iteration
  393.             loopT = afterRootT;
  394.             loopG = afterRootG;
  395.         }

  397.         // figure out the result of root finding, and return accordingly
  398.         if (afterRootG.getReal() == 0.0 || afterRootG.getReal() > 0.0 == g0Positive) {
  399.             // loop gave up and didn't find any crossing within this step
  400.             return false;
  401.         } else {
  402.             // real crossing
  403.             check(beforeRootT != null && !Double.isNaN(beforeRootG.getReal()));
  404.             // variation direction, with respect to the integration direction
  405.             increasing = !g0Positive;
  406.             pendingEventTime = beforeRootT;
  407.             stopTime = beforeRootG.getReal() == 0.0 ? beforeRootT : afterRootT;
  408.             pendingEvent = true;
  409.             afterEvent = afterRootT;
  410.             afterG = afterRootG;

  412.             // check increasing set correctly
  413.             check(afterG.getReal() > 0 == increasing);
  414.             check(increasing == gb.getReal() >= ga.getReal());

  416.             return true;
  417.         }

  419.     }

  421.     /**
  422.      * Get the next number after the given number in the current propagation direction.
  423.      *
  424.      * @param t input time
  425.      * @return t +/- 1 ulp depending on the direction.
  426.      */
  427.     private FieldAbsoluteDate<T> nextAfter(final FieldAbsoluteDate<T> t) {
  428.         return t.shiftedBy(forward ? +Precision.EPSILON : -Precision.EPSILON);
  429.     }


  432.     /** Get the occurrence time of the event triggered in the current
  433.      * step.
  434.      * @return occurrence time of the event triggered in the current
  435.      * step.
  436.      */
  437.     public FieldAbsoluteDate<T> getEventDate() {
  438.         return pendingEventTime;
  439.     }

  441.     /**
  442.      * Try to accept the current history up to the given time.
  443.      *
  444.      * <p> It is not necessary to call this method before calling {@link
  445.      * #doEvent(FieldSpacecraftState)} with the same state. It is necessary to call this
  446.      * method before you call {@link #doEvent(FieldSpacecraftState)} on some other event
  447.      * detector.
  448.      *
  449.      * @param state        to try to accept.
  450.      * @param interpolator to use to find the new root, if any.
  451.      * @return if the event detector has an event it has not detected before that is on or
  452.      * before the same time as {@code state}. In other words {@code false} means continue
  453.      * on while {@code true} means stop and handle my event first.
  454.      */
  455.     public boolean tryAdvance(final FieldSpacecraftState<T> state,
  456.                               final FieldOrekitStepInterpolator<T> interpolator) {
  457.         final FieldAbsoluteDate<T> t = state.getDate();
  458.         // check this is only called before a pending event.
  459.         check(!pendingEvent || !strictlyAfter(pendingEventTime, t));

  461.         final boolean meFirst;

  463.         if (strictlyAfter(t, earliestTimeConsidered)) {
  464.             // just found an event and we know the next time we want to search again
  465.             meFirst = false;
  466.         } else {
  467.             // check g function to see if there is a new event
  468.             final T g = g(state);
  469.             final boolean positive = g.getReal() > 0;

  471.             if (positive == g0Positive) {
  472.                 // g function has expected sign
  473.                 g0 = g; // g0Positive is the same
  474.                 meFirst = false;
  475.             } else {
  476.                 // found a root we didn't expect -> find precise location
  477.                 final FieldAbsoluteDate<T> oldPendingEventTime = pendingEventTime;
  478.                 final boolean foundRoot = findRoot(interpolator, t0, g0, t, g);
  479.                 // make sure the new root is not the same as the old root, if one exists
  480.                 meFirst = foundRoot && !pendingEventTime.equals(oldPendingEventTime);
  481.             }
  482.         }

  484.         if (!meFirst) {
  485.             // advance t0 to the current time so we can't find events that occur before t
  486.             t0 = t;
  487.         }

  489.         return meFirst;
  490.     }

  492.     /**
  493.      * Notify the user's listener of the event. The event occurs wholly within this method
  494.      * call including a call to {@link FieldEventDetector#resetState(FieldSpacecraftState)}
  495.      * if necessary.
  496.      *
  497.      * @param state the state at the time of the event. This must be at the same time as
  498.      *              the current value of {@link #getEventDate()}.
  499.      * @return the user's requested action and the new state if the action is {@link
  500.      * Action#RESET_STATE}. Otherwise
  501.      * the new state is {@code state}. The stop time indicates what time propagation should
  502.      * stop if the action is {@link Action#STOP}.
  503.      * This guarantees the integration will stop on or after the root, so that integration
  504.      * may be restarted safely.
  505.      */
  506.     public EventOccurrence<T> doEvent(final FieldSpacecraftState<T> state) {
  507.         // check event is pending and is at the same time
  508.         check(pendingEvent);
  509.         check(state.getDate().equals(this.pendingEventTime));

  511.         final Action action = detector.eventOccurred(state, increasing == forward);
  512.         final FieldSpacecraftState<T> newState;
  513.         if (action == Action.RESET_STATE) {
  514.             newState = detector.resetState(state);
  515.         } else {
  516.             newState = state;
  517.         }
  518.         // clear pending event
  519.         pendingEvent     = false;
  520.         pendingEventTime = null;
  521.         // setup for next search
  522.         earliestTimeConsidered = afterEvent;
  523.         t0 = afterEvent;
  524.         g0 = afterG;
  525.         g0Positive = increasing;
  526.         // check g0Positive set correctly
  527.         check(g0.getReal() == 0.0 || g0Positive == g0.getReal() > 0);
  528.         return new EventOccurrence<T>(action, newState, stopTime);
  529.     }

  531.     /**
  532.      * Shift a time value along the current integration direction: {@link #forward}.
  533.      *
  534.      * @param t     the time to shift.
  535.      * @param delta the amount to shift.
  536.      * @return t + delta if forward, else t - delta. If the result has to be rounded it
  537.      * will be rounded to be before the true value of t + delta.
  538.      */
  539.     private FieldAbsoluteDate<T> shiftedBy(final FieldAbsoluteDate<T> t, final T delta) {
  540.         if (forward) {
  541.             final FieldAbsoluteDate<T> ret = t.shiftedBy(delta);
  542.             if (ret.durationFrom(t).getReal() > delta.getReal()) {
  543.                 return ret.shiftedBy(-Precision.EPSILON);
  544.             } else {
  545.                 return ret;
  546.             }
  547.         } else {
  548.             final FieldAbsoluteDate<T> ret = t.shiftedBy(delta.negate());
  549.             if (t.durationFrom(ret).getReal() > delta.getReal()) {
  550.                 return ret.shiftedBy(+Precision.EPSILON);
  551.             } else {
  552.                 return ret;
  553.             }
  554.         }
  555.     }

  557.     /**
  558.      * Get the time that happens first along the current propagation direction: {@link
  559.      * #forward}.
  560.      *
  561.      * @param a first time
  562.      * @param b second time
  563.      * @return min(a, b) if forward, else max (a, b)
  564.      */
  565.     private FieldAbsoluteDate<T> minTime(final FieldAbsoluteDate<T> a, final FieldAbsoluteDate<T> b) {
  566.         return (forward ^ (a.compareTo(b) > 0)) ? a : b;
  567.     }

  569.     /**
  570.      * Check the ordering of two times.
  571.      *
  572.      * @param t1 the first time.
  573.      * @param t2 the second time.
  574.      * @return true if {@code t2} is strictly after {@code t1} in the propagation
  575.      * direction.
  576.      */
  577.     private boolean strictlyAfter(final FieldAbsoluteDate<T> t1, final FieldAbsoluteDate<T> t2) {
  578.         if (t1 == null || t2 == null) {
  579.             return false;
  580.         } else {
  581.             return forward ? t1.compareTo(t2) < 0 : t2.compareTo(t1) < 0;
  582.         }
  583.     }

  585.     /**
  586.      * Same as keyword assert, but throw a {@link MathRuntimeException}.
  587.      *
  588.      * @param condition to check
  589.      * @throws MathRuntimeException if {@code condition} is false.
  590.      */
  591.     private void check(final boolean condition) throws MathRuntimeException {
  592.         if (!condition) {
  593.             throw new OrekitInternalError(null);
  594.         }
  595.     }

  597.     /**
  598.      * Class to hold the data related to an event occurrence that is needed to decide how
  599.      * to modify integration.
  600.      */
  601.     public static class EventOccurrence<T extends CalculusFieldElement<T>> {

  603.         /** User requested action. */
  604.         private final Action action;
  605.         /** New state for a reset action. */
  606.         private final FieldSpacecraftState<T> newState;
  607.         /** The time to stop propagation if the action is a stop event. */
  608.         private final FieldAbsoluteDate<T> stopDate;

  610.         /**
  611.          * Create a new occurrence of an event.
  612.          *
  613.          * @param action   the user requested action.
  614.          * @param newState for a reset event. Should be the current state unless the
  615.          *                 action is {@link Action#RESET_STATE}.
  616.          * @param stopDate to stop propagation if the action is {@link Action#STOP}. Used
  617.          *                 to move the stop time to just after the root.
  618.          */
  619.         EventOccurrence(final Action action,
  620.                         final FieldSpacecraftState<T> newState,
  621.                         final FieldAbsoluteDate<T> stopDate) {
  622.             this.action = action;
  623.             this.newState = newState;
  624.             this.stopDate = stopDate;
  625.         }

  627.         /**
  628.          * Get the user requested action.
  629.          *
  630.          * @return the action.
  631.          */
  632.         public Action getAction() {
  633.             return action;
  634.         }

  636.         /**
  637.          * Get the new state for a reset action.
  638.          *
  639.          * @return the new state.
  640.          */
  641.         public FieldSpacecraftState<T> getNewState() {
  642.             return newState;
  643.         }

  645.         /**
  646.          * Get the new time for a stop action.
  647.          *
  648.          * @return when to stop propagation.
  649.          */
  650.         public FieldAbsoluteDate<T> getStopDate() {
  651.             return stopDate;
  652.         }

  654.     }

  656.     /**Get PendingEvent.
  657.      * @return if there is a pending event or not
  658.      * */

  660.     public boolean getPendingEvent() {
  661.         return pendingEvent;
  662.     }

  664. }
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