EventState.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 org.hipparchus.analysis.UnivariateFunction;
  20. import org.hipparchus.analysis.solvers.BracketedUnivariateSolver;
  21. import org.hipparchus.analysis.solvers.BracketedUnivariateSolver.Interval;
  22. import org.hipparchus.analysis.solvers.BracketingNthOrderBrentSolver;
  23. import org.hipparchus.exception.MathRuntimeException;
  24. import org.hipparchus.util.FastMath;
  25. import org.hipparchus.util.Precision;
  26. import org.orekit.errors.OrekitInternalError;
  27. import org.orekit.propagation.SpacecraftState;
  28. import org.orekit.propagation.events.handlers.EventHandler;
  29. import org.orekit.propagation.sampling.OrekitStepInterpolator;
  30. import org.orekit.time.AbsoluteDate;

  32. import java.io.Serializable;

  34. /** This class handles the state for one {@link EventDetector
  35.  * event detector} during integration steps.
  36.  *
  37.  * <p>This class is heavily based on the class with the same name from the
  38.  * Hipparchus library. The changes performed consist in replacing
  39.  * raw types (double and double arrays) with space dynamics types
  40.  * ({@link AbsoluteDate}, {@link SpacecraftState}).</p>
  41.  * <p>Each time the propagator proposes a step, the event detector
  42.  * should be checked. This class handles the state of one detector
  43.  * during one propagation step, with references to the state at the
  44.  * end of the preceding step. This information is used to determine if
  45.  * the detector should trigger an event or not during the proposed
  46.  * step (and hence the step should be reduced to ensure the event
  47.  * occurs at a bound rather than inside the step).</p>
  48.  * @author Luc Maisonobe
  49.  * @param <T> class type for the generic version
  50.  */
  51. public class EventState<T extends EventDetector> implements Serializable {

  53.     /** Serializable version identifier. */
  54.     private static final long serialVersionUID = 4489391420715269318L;

  56.     /** Event detector. */
  57.     private T detector;

  59.     /** Time of the previous call to g. */
  60.     private AbsoluteDate lastT;

  62.     /** Value from the previous call to g. */
  63.     private double lastG;

  65.     /** Time at the beginning of the step. */
  66.     private AbsoluteDate t0;

  68.     /** Value of the event detector at the beginning of the step. */
  69.     private double g0;

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

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

  77.     /** Occurrence time of the pending event. */
  78.     private AbsoluteDate pendingEventTime;

  80.     /**
  81.      * Time to stop propagation if the event is a stop event. Used to enable stopping at
  82.      * an event and then restarting after that event.
  83.      */
  84.     private AbsoluteDate stopTime;

  86.     /** Time after the current event. */
  87.     private AbsoluteDate afterEvent;

  89.     /** Value of the g function after the current event. */
  90.     private double afterG;

  92.     /** The earliest time considered for events. */
  93.     private AbsoluteDate earliestTimeConsidered;

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

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

  103.     /** Simple constructor.
  104.      * @param detector monitored event detector
  105.      */
  106.     public EventState(final T detector) {
  107.         this.detector     = detector;

  109.         // some dummy values ...
  110.         lastT                  = AbsoluteDate.PAST_INFINITY;
  111.         lastG                  = Double.NaN;
  112.         t0                     = null;
  113.         g0                     = Double.NaN;
  114.         g0Positive             = true;
  115.         pendingEvent           = false;
  116.         pendingEventTime       = null;
  117.         stopTime               = null;
  118.         increasing             = true;
  119.         earliestTimeConsidered = null;
  120.         afterEvent             = null;
  121.         afterG                 = Double.NaN;

  123.     }

  125.     /** Get the underlying event detector.
  126.      * @return underlying event detector
  127.      */
  128.     public T getEventDetector() {
  129.         return detector;
  130.     }

  132.     /** Initialize event handler at the start of a propagation.
  133.      * <p>
  134.      * This method is called once at the start of the propagation. It
  135.      * may be used by the event handler to initialize some internal data
  136.      * if needed.
  137.      * </p>
  138.      * @param s0 initial state
  139.      * @param t target time for the integration
  140.      *
  141.      */
  142.     public void init(final SpacecraftState s0,
  143.                      final AbsoluteDate t) {
  144.         detector.init(s0, t);
  145.         lastT = AbsoluteDate.PAST_INFINITY;
  146.         lastG = Double.NaN;
  147.     }

  149.     /** Compute the value of the switching function.
  150.      * This function must be continuous (at least in its roots neighborhood),
  151.      * as the integrator will need to find its roots to locate the events.
  152.      * @param s the current state information: date, kinematics, attitude
  153.      * @return value of the switching function
  154.      */
  155.     private double g(final SpacecraftState s) {
  156.         if (!s.getDate().equals(lastT)) {
  157.             lastT = s.getDate();
  158.             lastG = detector.g(s);
  159.         }
  160.         return lastG;
  161.     }

  163.     /** Reinitialize the beginning of the step.
  164.      * @param interpolator interpolator valid for the current step
  165.      */
  166.     public void reinitializeBegin(final OrekitStepInterpolator interpolator) {
  167.         forward = interpolator.isForward();
  168.         final SpacecraftState s0 = interpolator.getPreviousState();
  169.         this.t0 = s0.getDate();
  170.         g0 = g(s0);
  171.         while (g0 == 0) {
  172.             // extremely rare case: there is a zero EXACTLY at interval start
  173.             // we will use the sign slightly after step beginning to force ignoring this zero
  174.             // try moving forward by half a convergence interval
  175.             final double dt = (forward ? 0.5 : -0.5) * detector.getThreshold();
  176.             AbsoluteDate startDate = t0.shiftedBy(dt);
  177.             // if convergence is too small move an ulp
  178.             if (t0.equals(startDate)) {
  179.                 startDate = nextAfter(startDate);
  180.             }
  181.             t0 = startDate;
  182.             g0 = g(interpolator.getInterpolatedState(t0));
  183.         }
  184.         g0Positive = g0 > 0;
  185.         // "last" event was increasing
  186.         increasing = g0Positive;
  187.     }

  189.     /** Evaluate the impact of the proposed step on the event detector.
  190.      * @param interpolator step interpolator for the proposed step
  191.      * @return true if the event detector triggers an event before
  192.      * the end of the proposed step (this implies the step should be
  193.      * rejected)
  194.      * @exception MathRuntimeException if an event cannot be located
  195.      */
  196.     public boolean evaluateStep(final OrekitStepInterpolator interpolator)
  197.         throws MathRuntimeException {

  199.         forward = interpolator.isForward();
  200.         final SpacecraftState s1 = interpolator.getCurrentState();
  201.         final AbsoluteDate t1 = s1.getDate();
  202.         final double dt = t1.durationFrom(t0);
  203.         if (FastMath.abs(dt) < detector.getThreshold()) {
  204.             // we cannot do anything on such a small step, don't trigger any events
  205.             return false;
  206.         }
  207.         // number of points to check in the current step
  208.         final int n = FastMath.max(1, (int) FastMath.ceil(FastMath.abs(dt) / detector.getMaxCheckInterval()));
  209.         final double h = dt / n;


  212.         AbsoluteDate ta = t0;
  213.         double ga = g0;
  214.         for (int i = 0; i < n; ++i) {

  216.             // evaluate handler value at the end of the substep
  217.             final AbsoluteDate tb = (i == n - 1) ? t1 : t0.shiftedBy((i + 1) * h);
  218.             final double gb = g(interpolator.getInterpolatedState(tb));

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

  232.         }

  234.         // no event during the whole step
  235.         pendingEvent     = false;
  236.         pendingEventTime = null;
  237.         return false;

  239.     }

  241.     /**
  242.      * Find a root in a bracketing interval.
  243.      *
  244.      * <p> When calling this method one of the following must be true. Either ga == 0, gb
  245.      * == 0, (ga < 0  and gb > 0), or (ga > 0 and gb < 0).
  246.      *
  247.      * @param interpolator that covers the interval.
  248.      * @param ta           earliest possible time for root.
  249.      * @param ga           g(ta).
  250.      * @param tb           latest possible time for root.
  251.      * @param gb           g(tb).
  252.      * @return if a zero crossing was found.
  253.      */
  254.     private boolean findRoot(final OrekitStepInterpolator interpolator,
  255.                              final AbsoluteDate ta, final double ga,
  256.                              final AbsoluteDate tb, final double gb) {
  257.         // check there appears to be a root in [ta, tb]
  258.         check(ga == 0.0 || gb == 0.0 || (ga > 0.0 && gb < 0.0) || (ga < 0.0 && gb > 0.0));

  260.         final double convergence = detector.getThreshold();
  261.         final int maxIterationCount = detector.getMaxIterationCount();
  262.         final BracketedUnivariateSolver<UnivariateFunction> solver =
  263.                 new BracketingNthOrderBrentSolver(0, convergence, 0, 5);

  265.         // event time, just at or before the actual root.
  266.         AbsoluteDate beforeRootT = null;
  267.         double beforeRootG = Double.NaN;
  268.         // time on the other side of the root.
  269.         // Initialized the the loop below executes once.
  270.         AbsoluteDate afterRootT = ta;
  271.         double afterRootG = 0.0;

  273.         // check for some conditions that the root finders don't like
  274.         // these conditions cannot not happen in the loop below
  275.         // the ga == 0.0 case is handled by the loop below
  276.         if (ta.equals(tb)) {
  277.             // both non-zero but times are the same. Probably due to reset state
  278.             beforeRootT = ta;
  279.             beforeRootG = ga;
  280.             afterRootT = shiftedBy(beforeRootT, convergence);
  281.             afterRootG = g(interpolator.getInterpolatedState(afterRootT));
  282.         } else if (ga != 0.0 && gb == 0.0) {
  283.             // hard: ga != 0.0 and gb == 0.0
  284.             // look past gb by up to convergence to find next sign
  285.             // throw an exception if g(t) = 0.0 in [tb, tb + convergence]
  286.             beforeRootT = tb;
  287.             beforeRootG = gb;
  288.             afterRootT = shiftedBy(beforeRootT, convergence);
  289.             afterRootG = g(interpolator.getInterpolatedState(afterRootT));
  290.         } else if (ga != 0.0) {
  291.             final double newGa = g(interpolator.getInterpolatedState(ta));
  292.             if (ga > 0 != newGa > 0) {
  293.                 // both non-zero, step sign change at ta, possibly due to reset state
  294.                 beforeRootT = ta;
  295.                 beforeRootG = newGa;
  296.                 afterRootT = minTime(shiftedBy(beforeRootT, convergence), tb);
  297.                 afterRootG = g(interpolator.getInterpolatedState(afterRootT));
  298.             }
  299.         }

  301.         // loop to skip through "fake" roots, i.e. where g(t) = g'(t) = 0.0
  302.         // executed once if we didn't hit a special case above
  303.         AbsoluteDate loopT = ta;
  304.         double loopG = ga;
  305.         while ((afterRootG == 0.0 || afterRootG > 0.0 == g0Positive) &&
  306.                 strictlyAfter(afterRootT, tb)) {
  307.             if (loopG == 0.0) {
  308.                 // ga == 0.0 and gb may or may not be 0.0
  309.                 // handle the root at ta first
  310.                 beforeRootT = loopT;
  311.                 beforeRootG = loopG;
  312.                 afterRootT = minTime(shiftedBy(beforeRootT, convergence), tb);
  313.                 afterRootG = g(interpolator.getInterpolatedState(afterRootT));
  314.             } else {
  315.                 // both non-zero, the usual case, use a root finder.
  316.                 // time zero for evaluating the function f. Needs to be final
  317.                 final AbsoluteDate fT0 = loopT;
  318.                 final UnivariateFunction f = dt -> {
  319.                     return g(interpolator.getInterpolatedState(fT0.shiftedBy(dt)));
  320.                 };
  321.                 // tb as a double for use in f
  322.                 final double tbDouble = tb.durationFrom(fT0);
  323.                 if (forward) {
  324.                     final Interval interval =
  325.                             solver.solveInterval(maxIterationCount, f, 0, tbDouble);
  326.                     beforeRootT = fT0.shiftedBy(interval.getLeftAbscissa());
  327.                     beforeRootG = interval.getLeftValue();
  328.                     afterRootT = fT0.shiftedBy(interval.getRightAbscissa());
  329.                     afterRootG = interval.getRightValue();
  330.                 } else {
  331.                     final Interval interval =
  332.                             solver.solveInterval(maxIterationCount, f, tbDouble, 0);
  333.                     beforeRootT = fT0.shiftedBy(interval.getRightAbscissa());
  334.                     beforeRootG = interval.getRightValue();
  335.                     afterRootT = fT0.shiftedBy(interval.getLeftAbscissa());
  336.                     afterRootG = interval.getLeftValue();
  337.                 }
  338.             }
  339.             // tolerance is set to less than 1 ulp
  340.             // assume tolerance is 1 ulp
  341.             if (beforeRootT.equals(afterRootT)) {
  342.                 afterRootT = nextAfter(afterRootT);
  343.                 afterRootG = g(interpolator.getInterpolatedState(afterRootT));
  344.             }
  345.             // check loop is making some progress
  346.             check((forward && afterRootT.compareTo(beforeRootT) > 0) ||
  347.                   (!forward && afterRootT.compareTo(beforeRootT) < 0));
  348.             // setup next iteration
  349.             loopT = afterRootT;
  350.             loopG = afterRootG;
  351.         }

  353.         // figure out the result of root finding, and return accordingly
  354.         if (afterRootG == 0.0 || afterRootG > 0.0 == g0Positive) {
  355.             // loop gave up and didn't find any crossing within this step
  356.             return false;
  357.         } else {
  358.             // real crossing
  359.             check(beforeRootT != null && !Double.isNaN(beforeRootG));
  360.             // variation direction, with respect to the integration direction
  361.             increasing = !g0Positive;
  362.             pendingEventTime = beforeRootT;
  363.             stopTime = beforeRootG == 0.0 ? beforeRootT : afterRootT;
  364.             pendingEvent = true;
  365.             afterEvent = afterRootT;
  366.             afterG = afterRootG;

  368.             // check increasing set correctly
  369.             check(afterG > 0 == increasing);
  370.             check(increasing == gb >= ga);

  372.             return true;
  373.         }

  375.     }

  377.     /**
  378.      * Get the next number after the given number in the current propagation direction.
  379.      *
  380.      * @param t input time
  381.      * @return t +/- 1 ulp depending on the direction.
  382.      */
  383.     private AbsoluteDate nextAfter(final AbsoluteDate t) {
  384.         return t.shiftedBy(forward ? +Precision.EPSILON : -Precision.EPSILON);
  385.     }

  387.     /** Get the occurrence time of the event triggered in the current
  388.      * step.
  389.      * @return occurrence time of the event triggered in the current
  390.      * step.
  391.      */
  392.     public AbsoluteDate getEventDate() {
  393.         return pendingEventTime;
  394.     }

  396.     /**
  397.      * Try to accept the current history up to the given time.
  398.      *
  399.      * <p> It is not necessary to call this method before calling {@link
  400.      * #doEvent(SpacecraftState)} with the same state. It is necessary to call this
  401.      * method before you call {@link #doEvent(SpacecraftState)} on some other event
  402.      * detector.
  403.      *
  404.      * @param state        to try to accept.
  405.      * @param interpolator to use to find the new root, if any.
  406.      * @return if the event detector has an event it has not detected before that is on or
  407.      * before the same time as {@code state}. In other words {@code false} means continue
  408.      * on while {@code true} means stop and handle my event first.
  409.      */
  410.     public boolean tryAdvance(final SpacecraftState state,
  411.                               final OrekitStepInterpolator interpolator) {
  412.         // check this is only called before a pending event.
  413.         check(!(pendingEvent && strictlyAfter(pendingEventTime, state.getDate())));

  415.         final AbsoluteDate t = state.getDate();

  417.         // just found an event and we know the next time we want to search again
  418.         if (strictlyAfter(t, earliestTimeConsidered)) {
  419.             return false;
  420.         }

  422.         final double g = g(state);
  423.         final boolean positive = g > 0;

  425.         // check for new root, pendingEventTime may be null if there is not pending event
  426.         if ((g == 0.0 && t.equals(pendingEventTime)) || positive == g0Positive) {
  427.             // at a root we already found, or g function has expected sign
  428.             t0 = t;
  429.             g0 = g; // g0Positive is the same
  430.             return false;
  431.         } else {
  432.             // found a root we didn't expect -> find precise location
  433.             return findRoot(interpolator, t0, g0, t, g);
  434.         }
  435.     }

  437.     /**
  438.      * Notify the user's listener of the event. The event occurs wholly within this method
  439.      * call including a call to {@link EventDetector#resetState(SpacecraftState)}
  440.      * if necessary.
  441.      *
  442.      * @param state the state at the time of the event. This must be at the same time as
  443.      *              the current value of {@link #getEventDate()}.
  444.      * @return the user's requested action and the new state if the action is {@link
  445.      * org.orekit.propagation.events.handlers.EventHandler.Action#RESET_STATE Action.RESET_STATE}.
  446.      * Otherwise the new state is {@code state}. The stop time indicates what time propagation
  447.      * should stop if the action is {@link
  448.      * org.orekit.propagation.events.handlers.EventHandler.Action#STOP Action.STOP}.
  449.      * This guarantees the integration will stop on or after the root, so that integration
  450.      * may be restarted safely.
  451.      */
  452.     public EventOccurrence doEvent(final SpacecraftState state) {
  453.         // check event is pending and is at the same time
  454.         check(pendingEvent);
  455.         check(state.getDate().equals(this.pendingEventTime));

  457.         final EventHandler.Action action = detector.eventOccurred(state, increasing == forward);
  458.         final SpacecraftState newState;
  459.         if (action == EventHandler.Action.RESET_STATE) {
  460.             newState = detector.resetState(state);
  461.         } else {
  462.             newState = state;
  463.         }
  464.         // clear pending event
  465.         pendingEvent     = false;
  466.         pendingEventTime = null;
  467.         // setup for next search
  468.         earliestTimeConsidered = afterEvent;
  469.         t0 = afterEvent;
  470.         g0 = afterG;
  471.         g0Positive = increasing;
  472.         // check g0Positive set correctly
  473.         check(g0 == 0.0 || g0Positive == (g0 > 0));
  474.         return new EventOccurrence(action, newState, stopTime);
  475.     }

  477.     /**
  478.      * Shift a time value along the current integration direction: {@link #forward}.
  479.      *
  480.      * @param t     the time to shift.
  481.      * @param delta the amount to shift.
  482.      * @return t + delta if forward, else t - delta. If the result has to be rounded it
  483.      * will be rounded to be before the true value of t + delta.
  484.      */
  485.     private AbsoluteDate shiftedBy(final AbsoluteDate t, final double delta) {
  486.         if (forward) {
  487.             final AbsoluteDate ret = t.shiftedBy(delta);
  488.             if (ret.durationFrom(t) > delta) {
  489.                 return ret.shiftedBy(-Precision.EPSILON);
  490.             } else {
  491.                 return ret;
  492.             }
  493.         } else {
  494.             final AbsoluteDate ret = t.shiftedBy(-delta);
  495.             if (t.durationFrom(ret) > delta) {
  496.                 return ret.shiftedBy(+Precision.EPSILON);
  497.             } else {
  498.                 return ret;
  499.             }
  500.         }
  501.     }

  503.     /**
  504.      * Get the time that happens first along the current propagation direction: {@link
  505.      * #forward}.
  506.      *
  507.      * @param a first time
  508.      * @param b second time
  509.      * @return min(a, b) if forward, else max (a, b)
  510.      */
  511.     private AbsoluteDate minTime(final AbsoluteDate a, final AbsoluteDate b) {
  512.         return (forward ^ (a.compareTo(b) > 0)) ? a : b;
  513.     }

  515.     /**
  516.      * Check the ordering of two times.
  517.      *
  518.      * @param t1 the first time.
  519.      * @param t2 the second time.
  520.      * @return true if {@code t2} is strictly after {@code t1} in the propagation
  521.      * direction.
  522.      */
  523.     private boolean strictlyAfter(final AbsoluteDate t1, final AbsoluteDate t2) {
  524.         if (t1 == null || t2 == null) {
  525.             return false;
  526.         } else {
  527.             return forward ? t1.compareTo(t2) < 0 : t2.compareTo(t1) < 0;
  528.         }
  529.     }

  531.     /**
  532.      * Same as keyword assert, but throw a {@link MathRuntimeException}.
  533.      *
  534.      * @param condition to check
  535.      * @throws MathRuntimeException if {@code condition} is false.
  536.      */
  537.     private void check(final boolean condition) throws MathRuntimeException {
  538.         if (!condition) {
  539.             throw new OrekitInternalError(null);
  540.         }
  541.     }

  543.     /**
  544.      * Class to hold the data related to an event occurrence that is needed to decide how
  545.      * to modify integration.
  546.      */
  547.     public static class EventOccurrence {

  549.         /** User requested action. */
  550.         private final EventHandler.Action action;
  551.         /** New state for a reset action. */
  552.         private final SpacecraftState newState;
  553.         /** The time to stop propagation if the action is a stop event. */
  554.         private final AbsoluteDate stopDate;

  556.         /**
  557.          * Create a new occurrence of an event.
  558.          *
  559.          * @param action   the user requested action.
  560.          * @param newState for a reset event. Should be the current state unless the
  561.          *                 action is {@link Action#RESET_STATE}.
  562.          * @param stopDate to stop propagation if the action is {@link Action#STOP}. Used
  563.          *                 to move the stop time to just after the root.
  564.          */
  565.         EventOccurrence(final EventHandler.Action action,
  566.                         final SpacecraftState newState,
  567.                         final AbsoluteDate stopDate) {
  568.             this.action = action;
  569.             this.newState = newState;
  570.             this.stopDate = stopDate;
  571.         }

  573.         /**
  574.          * Get the user requested action.
  575.          *
  576.          * @return the action.
  577.          */
  578.         public EventHandler.Action getAction() {
  579.             return action;
  580.         }

  582.         /**
  583.          * Get the new state for a reset action.
  584.          *
  585.          * @return the new state.
  586.          */
  587.         public SpacecraftState getNewState() {
  588.             return newState;
  589.         }

  591.         /**
  592.          * Get the new time for a stop action.
  593.          *
  594.          * @return when to stop propagation.
  595.          */
  596.         public AbsoluteDate getStopDate() {
  597.             return stopDate;
  598.         }

  600.     }

  602. }
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