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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
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17  package org.orekit.propagation.events;
18  
19  import org.orekit.propagation.SpacecraftState;
20  import org.orekit.propagation.events.handlers.EventHandler;
21  import org.orekit.propagation.events.intervals.AdaptableInterval;
22  import org.orekit.time.AbsoluteDate;
23  
24  /** This interface represents space-dynamics aware events detectors.
25   *
26   * <p>It mirrors the {@link org.hipparchus.ode.events.ODEEventHandler
27   * ODEEventHandler} 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   * EventHandler#eventOccurred(SpacecraftState, EventDetector, boolean) eventOccurred}
57   * 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   * @author Luc Maisonobe
63   * @author V&eacute;ronique Pommier-Maurussane
64   */
65  public interface EventDetector {
66  
67      /** Initialize event detector at the start of a propagation.
68       * <p>
69       * This method is called once at the start of the propagation. It
70       * may be used by the event handler to initialize some internal data
71       * if needed.
72       * </p>
73       * <p>
74       * The default implementation initializes the handler.
75       * </p>
76       * @param s0 initial state
77       * @param t target time for the integration
78       *
79       */
80      default void init(final SpacecraftState s0, final AbsoluteDate t) {
81          getHandler().init(s0, t, this);
82      }
83  
84      /** Reset the event detector during propagation when the state is modified by an event or an additional data provider.
85       * <p>
86       * The default implementation does nothing.
87       * </p>
88       * @param state current state
89       * @param target target time for the integration
90       * @since 13.0
91       */
92      default void reset(final SpacecraftState state, final AbsoluteDate target) {
93          // nothing by default
94      }
95  
96      /**
97       * Method returning true if and only if the detection function g does not depend on dependent variables,
98       * just the independent one i.e. time. This information is used for performance in propagation.
99       * @return flag
100      * @since 13.1
101      */
102     default boolean dependsOnTimeOnly() {
103         return false;
104     }
105 
106     /** Compute the value of the switching function.
107      * This function must be continuous (at least in its roots neighborhood),
108      * as the integrator will need to find its roots to locate the events.
109      * @param s the current state information: date, kinematics, attitude
110      * @return value of the switching function
111      */
112     double g(SpacecraftState s);
113 
114     /** Get the convergence threshold in the event time search.
115      * @return convergence threshold (s)
116      */
117     default double getThreshold() {
118         return getDetectionSettings().getThreshold();
119     }
120 
121     /** Get maximal time interval between switching function checks.
122      * @return maximal time interval (s) between switching function checks
123      */
124     default AdaptableInterval getMaxCheckInterval() {
125         return getDetectionSettings().getMaxCheckInterval();
126     }
127 
128     /** Get maximal number of iterations in the event time search.
129      * @return maximal number of iterations in the event time search
130      */
131     default int getMaxIterationCount() {
132         return getDetectionSettings().getMaxIterationCount();
133     }
134 
135     /** Get the handler.
136      * @return event handler to call at event occurrences
137      * @since 12.0
138      */
139     EventHandler getHandler();
140 
141     /**
142      * This method finalizes the event detector's job.
143      * @param state state at propagation end
144      * @since 12.2
145      */
146     default void finish(final SpacecraftState state) {
147         getHandler().finish(state, this);
148     }
149 
150     /**
151      * Getter for the settings.
152      * @return detection settings
153      * @since 12.2
154      */
155     default EventDetectionSettings getDetectionSettings() {
156         return EventDetectionSettings.getDefaultEventDetectionSettings();
157     }
158 }