/* Copyright 2002-2016 CS Systèmes d'Information
    * Licensed to CS Systèmes d'Information (CS) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * CS 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 java.io.Serializable;
  
  import org.orekit.errors.OrekitException;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.propagation.events.handlers.EventHandler.Action;
  import org.orekit.time.AbsoluteDate;
  
  /** This interface represents space-dynamics aware events detectors.
   *
   * <p>It mirrors the {@link org.hipparchus.ode.events.ODEEventHandler
   * ODEEventHandler} interface from <a href="https://hipparchus.org/">
   * Hipparchus</a> but provides a space-dynamics interface to the
   * methods.</p>
   *
   * <p>Events detectors are a useful solution to meet the requirements
   * of propagators concerning discrete conditions. The state of each
   * event detector is queried by the integrator at each step. When the
   * sign of the underlying g switching function changes, the step is rejected
   * and reduced, in order to make sure the sign changes occur only at steps
   * boundaries.</p>
   *
   * <p>When step ends exactly at a switching function sign change, the corresponding
   * event is triggered, by calling the {@link #eventOccurred(SpacecraftState, boolean)}
   * method. The method can do whatever it needs with the event (logging it, performing
   * some processing, ignore it ...). The return value of the method will be used by
   * the propagator to stop or resume propagation, possibly changing the state vector.<p>
   *
   * @author Luc Maisonobe
   * @author V&eacute;ronique Pommier-Maurussane
   */
  public interface EventDetector extends Serializable {
  
      /** 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>
       * <p>
       * The default implementation does nothing
       * </p>
       * @param s0 initial state
       * @param t target time for the integration
       */
      default void init(SpacecraftState s0, AbsoluteDate t) {
          // nothing by default
      }
  
      /** 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
       * @exception OrekitException if some specific error occurs
       */
      double g(SpacecraftState s) throws OrekitException;
  
      /** Get the convergence threshold in the event time search.
       * @return convergence threshold (s)
       */
      double getThreshold();
  
      /** Get maximal time interval between switching function checks.
       * @return maximal time interval (s) between switching function checks
       */
      double getMaxCheckInterval();
  
      /** Get maximal number of iterations in the event time search.
       * @return maximal number of iterations in the event time search
       */
      int getMaxIterationCount();
  
      /** Handle the event.
       * @param s SpaceCraft state to be used in the evaluation
       * @param increasing with the event occurred in an "increasing" or "decreasing" slope direction
       * @return the Action that the calling detector should pass back to the evaluation system
       * @exception OrekitException if some specific error occurs
       * @since 7.0
       */
      Action eventOccurred(SpacecraftState s, boolean increasing) throws OrekitException;
  
     /** Reset the state prior to continue propagation.
      * <p>This method is called after the step handler has returned and
      * before the next step is started, but only when {@link
      * #eventOccurred} has itself returned the {@link Action#RESET_STATE}
      * indicator. It allows the user to reset the state for the next step,
      * without perturbing the step handler of the finishing step. If the
      * {@link #eventOccurred} never returns the {@link Action#RESET_STATE}
      * indicator, this function will never be called, and it is safe to simply return null.</p>
      * <p>
      * The default implementation simply returns its argument.
      * </p>
      * @param oldState old state
      * @return new state
      * @exception OrekitException if the state cannot be reseted
      * @since 7.0
      */
     default SpacecraftState resetState(SpacecraftState oldState) throws OrekitException {
         return oldState;
     }
 
 }