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11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
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14   * See the License for the specific language governing permissions and
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17  package org.orekit.propagation.events;
18  
19  import org.hipparchus.CalculusFieldElement;
20  import org.orekit.propagation.FieldSpacecraftState;
21  import org.orekit.propagation.events.handlers.FieldEventHandler;
22  import org.orekit.time.FieldAbsoluteDate;
23  
24  /** This interface represents space-dynamics aware events detectors.
25   *
26   * <p>It mirrors the {@link org.hipparchus.ode.events.FieldODEEventHandler
27   * FieldODEEventHandler} interface from <a href="https://hipparchus.org/">
28   * Hipparchus</a> but provides a space-dynamics interface to the
29   * methods.</p>
30   *
31   * <p>Events detectors are a useful solution to meet the requirements
32   * of propagators concerning discrete conditions. The state of each
33   * event detector is queried by the propagator from time to time, at least
34   * once every {@link #getMaxCheckInterval() max check interval} but it may
35   * be more frequent. When the sign of the underlying g switching function
36   * changes, a root-finding algorithm is run to precisely locate the event,
37   * down to a configured {@link #getThreshold() convergence threshold}. The
38   * {@link #getMaxCheckInterval() max check interval} is therefore devoted to
39   * separate roots and is often much larger than the  {@link #getThreshold()
40   * convergence threshold}.</p>
41   *
42   * <p>The physical meaning of the g switching function is not really used
43   * by the event detection algorithms. Its varies from event detector to
44   * event detector. One example would be a visibility detector that could use the
45   * angular elevation of the satellite above horizon as a g switching function.
46   * In this case, the function would switch from negative to positive when the
47   * satellite raises above horizon and it would switch from positive to negative
48   * when it sets backs below horizon. Another example would be an apside detector
49   * that could use the dot product of position and velocity. In this case, the
50   * function would switch from negative to positive when the satellite crosses
51   * periapsis and it would switch from positive to negative when the satellite
52   * crosses apoapsis.</p>
53   *
54   * <p>When the precise state at which the g switching function changes has been
55   * located, the corresponding event is triggered, by calling the {@link
56   * FieldEventHandler#eventOccurred(FieldSpacecraftState, FieldEventDetector, boolean)
57   * eventOccurred} method from the associated {@link #getHandler() handler}.
58   * The method can do whatever it needs with the event (logging it, performing
59   * some processing, ignore it ...). The return value of the method will be used by
60   * the propagator to stop or resume propagation, possibly changing the state vector.</p>
61   *
62   * @param <T> type of the field element
63   * @author Luc Maisonobe
64   * @author V&eacute;ronique Pommier-Maurussane
65   */
66  public interface FieldEventDetector <T extends CalculusFieldElement<T>> {
67  
68      /** Initialize event handler at the start of a propagation.
69       * <p>
70       * This method is called once at the start of the propagation. It
71       * may be used by the event handler to initialize some internal data
72       * if needed.
73       * </p>
74       * <p>
75       * The default implementation does nothing
76       * </p>
77       * @param s0 initial state
78       * @param t target time for the integration
79       *
80       */
81      default void init(FieldSpacecraftState<T> s0,
82                        FieldAbsoluteDate<T> t) {
83          // nothing by default
84      }
85  
86      /** Compute the value of the switching function.
87       * This function must be continuous (at least in its roots neighborhood),
88       * as the integrator will need to find its roots to locate the events.
89       * @param s the current state information: date, kinematics, attitude
90       * @return value of the switching function
91       */
92      T g(FieldSpacecraftState<T> s);
93  
94      /** Get the convergence threshold in the event time search.
95       * @return convergence threshold (s)
96       */
97      T getThreshold();
98  
99      /** Get maximal time interval between switching function checks.
100      * @return maximal time interval (s) between switching function checks
101      */
102     FieldAdaptableInterval<T> getMaxCheckInterval();
103 
104     /** Get maximal number of iterations in the event time search.
105      * @return maximal number of iterations in the event time search
106      */
107     int getMaxIterationCount();
108 
109     /** Get the handler.
110      * @return event handler to call at event occurrences
111      * @since 12.0
112      */
113     FieldEventHandler<T> getHandler();
114 
115 }