/* Copyright 2002-2021 CS GROUP
    * Licensed to CS GROUP (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.analytical;
  
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.Collections;
  import java.util.Comparator;
  import java.util.List;
  import java.util.PriorityQueue;
  import java.util.Queue;
  
  import org.hipparchus.exception.MathRuntimeException;
  import org.hipparchus.ode.events.Action;
  import org.orekit.attitudes.Attitude;
  import org.orekit.attitudes.AttitudeProvider;
  import org.orekit.errors.OrekitException;
  import org.orekit.errors.OrekitInternalError;
  import org.orekit.frames.Frame;
  import org.orekit.orbits.Orbit;
  import org.orekit.propagation.AbstractPropagator;
  import org.orekit.propagation.AdditionalStateProvider;
  import org.orekit.propagation.BoundedPropagator;
  import org.orekit.propagation.EphemerisGenerator;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.propagation.events.EventDetector;
  import org.orekit.propagation.events.EventState;
  import org.orekit.propagation.events.EventState.EventOccurrence;
  import org.orekit.propagation.sampling.OrekitStepInterpolator;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.utils.PVCoordinatesProvider;
  import org.orekit.utils.TimeStampedPVCoordinates;
  
  /** Common handling of {@link org.orekit.propagation.Propagator} methods for analytical propagators.
   * <p>
   * This abstract class allows to provide easily the full set of {@link
   * org.orekit.propagation.Propagator Propagator} methods, including all propagation
   * modes support and discrete events support for any simple propagation method. Only
   * two methods must be implemented by derived classes: {@link #propagateOrbit(AbsoluteDate)}
   * and {@link #getMass(AbsoluteDate)}. The first method should perform straightforward
   * propagation starting from some internally stored initial state up to the specified target date.
   * </p>
   * @author Luc Maisonobe
   */
  public abstract class AbstractAnalyticalPropagator extends AbstractPropagator {
  
      /** Provider for attitude computation. */
      private PVCoordinatesProvider pvProvider;
  
      /** Start date of last propagation. */
      private AbsoluteDate lastPropagationStart;
  
      /** End date of last propagation. */
      private AbsoluteDate lastPropagationEnd;
  
      /** Initialization indicator of events states. */
      private boolean statesInitialized;
  
      /** Indicator for last step. */
      private boolean isLastStep;
  
      /** Event steps. */
      private final Collection<EventState<?>> eventsStates;
  
      /** Build a new instance.
       * @param attitudeProvider provider for attitude computation
       */
      protected AbstractAnalyticalPropagator(final AttitudeProvider attitudeProvider) {
          setAttitudeProvider(attitudeProvider);
          pvProvider           = new LocalPVProvider();
          lastPropagationStart = AbsoluteDate.PAST_INFINITY;
          lastPropagationEnd   = AbsoluteDate.FUTURE_INFINITY;
          statesInitialized    = false;
          eventsStates         = new ArrayList<>();
      }
  
      /** {@inheritDoc} */
      @Override
      public EphemerisGenerator getEphemerisGenerator() {
          return () -> new BoundedPropagatorView(lastPropagationStart, lastPropagationEnd);
      }
  
      /** {@inheritDoc} */
      public <T extends EventDetector> void addEventDetector(final T detector) {
          eventsStates.add(new EventState<>(detector));
     }
 
     /** {@inheritDoc} */
     public Collection<EventDetector> getEventsDetectors() {
         final List<EventDetector> list = new ArrayList<>();
         for (final EventState<?> state : eventsStates) {
             list.add(state.getEventDetector());
         }
         return Collections.unmodifiableCollection(list);
     }
 
     /** {@inheritDoc} */
     public void clearEventsDetectors() {
         eventsStates.clear();
     }
 
     /** {@inheritDoc} */
     public SpacecraftState propagate(final AbsoluteDate start, final AbsoluteDate target) {
         try {
 
             initializePropagation();
 
             lastPropagationStart = start;
 
             final boolean isForward = target.compareTo(start) >= 0;
             SpacecraftState state   = updateAdditionalStates(basicPropagate(start));
 
             // initialize event detectors
             for (final EventState<?> es : eventsStates) {
                 es.init(state, target);
             }
 
             // initialize step handlers
             getMultiplexer().init(state, target);
 
             // iterate over the propagation range, need loop due to reset events
             statesInitialized = false;
             isLastStep = false;
             do {
 
                 // attempt to advance to the target date
                 final SpacecraftState previous = state;
                 final SpacecraftState current = updateAdditionalStates(basicPropagate(target));
                 final OrekitStepInterpolator interpolator =
                         new BasicStepInterpolator(isForward, previous, current);
 
                 // accept the step, trigger events and step handlers
                 state = acceptStep(interpolator, target);
 
                 // Update the potential changes in the spacecraft state due to the events
                 // especially the potential attitude transition
                 state = updateAdditionalStates(basicPropagate(state.getDate()));
 
             } while (!isLastStep);
 
             // return the last computed state
             lastPropagationEnd = state.getDate();
             setStartDate(state.getDate());
             return state;
 
         } catch (MathRuntimeException mrte) {
             throw OrekitException.unwrap(mrte);
         }
     }
 
     /** Accept a step, triggering events and step handlers.
      * @param interpolator interpolator for the current step
      * @param target final propagation time
      * @return state at the end of the step
      * @exception MathRuntimeException if an event cannot be located
      */
     protected SpacecraftState acceptStep(final OrekitStepInterpolator interpolator,
                                          final AbsoluteDate target)
         throws MathRuntimeException {
 
         SpacecraftState        previous   = interpolator.getPreviousState();
         final SpacecraftState  current    = interpolator.getCurrentState();
         OrekitStepInterpolator restricted = interpolator;
 
 
         // initialize the events states if needed
         if (!statesInitialized) {
 
             if (!eventsStates.isEmpty()) {
                 // initialize the events states
                 for (final EventState<?> state : eventsStates) {
                     state.reinitializeBegin(interpolator);
                 }
             }
 
             statesInitialized = true;
 
         }
 
         // search for next events that may occur during the step
         final int orderingSign = interpolator.isForward() ? +1 : -1;
         final Queue<EventState<?>> occurringEvents = new PriorityQueue<>(new Comparator<EventState<?>>() {
             /** {@inheritDoc} */
             @Override
             public int compare(final EventState<?> es0, final EventState<?> es1) {
                 return orderingSign * es0.getEventDate().compareTo(es1.getEventDate());
             }
         });
 
         boolean doneWithStep = false;
         resetEvents:
         do {
 
             // Evaluate all event detectors for events
             occurringEvents.clear();
             for (final EventState<?> state : eventsStates) {
                 if (state.evaluateStep(interpolator)) {
                     // the event occurs during the current step
                     occurringEvents.add(state);
                 }
             }
 
             do {
 
                 eventLoop:
                 while (!occurringEvents.isEmpty()) {
 
                     // handle the chronologically first event
                     final EventState<?> currentEvent = occurringEvents.poll();
 
                     // get state at event time
                     SpacecraftState eventState = restricted.getInterpolatedState(currentEvent.getEventDate());
 
                     // restrict the interpolator to the first part of the step, up to the event
                     restricted = restricted.restrictStep(previous, eventState);
 
                     // try to advance all event states to current time
                     for (final EventState<?> state : eventsStates) {
                         if (state != currentEvent && state.tryAdvance(eventState, interpolator)) {
                             // we need to handle another event first
                             // remove event we just updated to prevent heap corruption
                             occurringEvents.remove(state);
                             // add it back to update its position in the heap
                             occurringEvents.add(state);
                             // re-queue the event we were processing
                             occurringEvents.add(currentEvent);
                             continue eventLoop;
                         }
                     }
                     // all event detectors agree we can advance to the current event time
 
                     // handle the first part of the step, up to the event
                     getMultiplexer().handleStep(restricted);
 
                     // acknowledge event occurrence
                     final EventOccurrence occurrence = currentEvent.doEvent(eventState);
                     final Action action = occurrence.getAction();
                     isLastStep = action == Action.STOP;
 
                     if (isLastStep) {
 
                         // ensure the event is after the root if it is returned STOP
                         // this lets the user integrate to a STOP event and then restart
                         // integration from the same time.
                         final SpacecraftState savedState = eventState;
                         eventState = interpolator.getInterpolatedState(occurrence.getStopDate());
                         restricted = restricted.restrictStep(savedState, eventState);
 
                         // handle the almost zero size last part of the final step, at event time
                         getMultiplexer().handleStep(restricted);
                         getMultiplexer().finish(restricted.getCurrentState());
 
                     }
 
                     if (isLastStep) {
                         // the event asked to stop integration
                         return eventState;
                     }
 
                     if (action == Action.RESET_DERIVATIVES || action == Action.RESET_STATE) {
                         // some event handler has triggered changes that
                         // invalidate the derivatives, we need to recompute them
                         final SpacecraftState resetState = occurrence.getNewState();
                         resetIntermediateState(resetState, interpolator.isForward());
                         return resetState;
                     }
                     // at this point action == Action.CONTINUE or Action.RESET_EVENTS
 
                     // prepare handling of the remaining part of the step
                     previous = eventState;
                     restricted = new BasicStepInterpolator(restricted.isForward(), eventState, current);
 
                     if (action == Action.RESET_EVENTS) {
                         continue resetEvents;
                     }
 
                     // at this point action == Action.CONTINUE
                     // check if the same event occurs again in the remaining part of the step
                     if (currentEvent.evaluateStep(restricted)) {
                         // the event occurs during the current step
                         occurringEvents.add(currentEvent);
                     }
 
                 }
 
                 // last part of the step, after the last event. Advance all detectors to
                 // the end of the step. Should only detect a new event here if an event
                 // modified the g function of another detector. Detecting such events here
                 // is unreliable and RESET_EVENTS should be used instead. Might as well
                 // re-check here because we have to loop through all the detectors anyway
                 // and the alternative is to throw an exception.
                 for (final EventState<?> state : eventsStates) {
                     if (state.tryAdvance(current, interpolator)) {
                         occurringEvents.add(state);
                     }
                 }
 
             } while (!occurringEvents.isEmpty());
 
             doneWithStep = true;
         } while (!doneWithStep);
 
         isLastStep = target.equals(current.getDate());
 
         // handle the remaining part of the step, after all events if any
         getMultiplexer().handleStep(restricted);
         if (isLastStep) {
             getMultiplexer().finish(restricted.getCurrentState());
         }
 
         return current;
 
     }
 
     /** Get the mass.
      * @param date target date for the orbit
      * @return mass mass
      */
     protected abstract double getMass(AbsoluteDate date);
 
     /** Get PV coordinates provider.
      * @return PV coordinates provider
      */
     public PVCoordinatesProvider getPvProvider() {
         return pvProvider;
     }
 
     /** Reset an intermediate state.
      * @param state new intermediate state to consider
      * @param forward if true, the intermediate state is valid for
      * propagations after itself
      */
     protected abstract void resetIntermediateState(SpacecraftState state, boolean forward);
 
     /** Extrapolate an orbit up to a specific target date.
      * @param date target date for the orbit
      * @return extrapolated parameters
      */
     protected abstract Orbit propagateOrbit(AbsoluteDate date);
 
     /** Propagate an orbit without any fancy features.
      * <p>This method is similar in spirit to the {@link #propagate} method,
      * except that it does <strong>not</strong> call any handler during
      * propagation, nor any discrete events, not additional states. It always
      * stop exactly at the specified date.</p>
      * @param date target date for propagation
      * @return state at specified date
      */
     protected SpacecraftState basicPropagate(final AbsoluteDate date) {
         try {
 
             // evaluate orbit
             final Orbit orbit = propagateOrbit(date);
 
             // evaluate attitude
             final Attitude attitude =
                 getAttitudeProvider().getAttitude(pvProvider, date, orbit.getFrame());
 
             // build raw state
             return new SpacecraftState(orbit, attitude, getMass(date));
 
         } catch (OrekitException oe) {
             throw new OrekitException(oe);
         }
     }
 
     /** Internal PVCoordinatesProvider for attitude computation. */
     private class LocalPVProvider implements PVCoordinatesProvider {
 
         /** {@inheritDoc} */
         public TimeStampedPVCoordinates getPVCoordinates(final AbsoluteDate date, final Frame frame) {
             return propagateOrbit(date).getPVCoordinates(frame);
         }
 
     }
 
     /** {@link BoundedPropagator} view of the instance. */
     private class BoundedPropagatorView extends AbstractAnalyticalPropagator implements BoundedPropagator {
 
         /** Min date. */
         private final AbsoluteDate minDate;
 
         /** Max date. */
         private final AbsoluteDate maxDate;
 
         /** Simple constructor.
          * @param startDate start date of the propagation
          * @param endDate end date of the propagation
          */
         BoundedPropagatorView(final AbsoluteDate startDate, final AbsoluteDate endDate) {
             super(AbstractAnalyticalPropagator.this.getAttitudeProvider());
             super.resetInitialState(AbstractAnalyticalPropagator.this.getInitialState());
             if (startDate.compareTo(endDate) <= 0) {
                 minDate = startDate;
                 maxDate = endDate;
             } else {
                 minDate = endDate;
                 maxDate = startDate;
             }
 
             try {
                 // copy the same additional state providers as the original propagator
                 for (AdditionalStateProvider provider : AbstractAnalyticalPropagator.this.getAdditionalStateProviders()) {
                     addAdditionalStateProvider(provider);
                 }
             } catch (OrekitException oe) {
                 // as the providers are already compatible with each other,
                 // this should never happen
                 throw new OrekitInternalError(null);
             }
 
         }
 
         /** {@inheritDoc} */
         public AbsoluteDate getMinDate() {
             return minDate;
         }
 
         /** {@inheritDoc} */
         public AbsoluteDate getMaxDate() {
             return maxDate;
         }
 
         /** {@inheritDoc} */
         protected Orbit propagateOrbit(final AbsoluteDate target) {
             return AbstractAnalyticalPropagator.this.propagateOrbit(target);
         }
 
         /** {@inheritDoc} */
         public double getMass(final AbsoluteDate date) {
             return AbstractAnalyticalPropagator.this.getMass(date);
         }
 
         /** {@inheritDoc} */
         public TimeStampedPVCoordinates getPVCoordinates(final AbsoluteDate date, final Frame frame) {
             return propagate(date).getPVCoordinates(frame);
         }
 
         /** {@inheritDoc} */
         public void resetInitialState(final SpacecraftState state) {
             super.resetInitialState(state);
             AbstractAnalyticalPropagator.this.resetInitialState(state);
         }
 
         /** {@inheritDoc} */
         protected void resetIntermediateState(final SpacecraftState state, final boolean forward) {
             AbstractAnalyticalPropagator.this.resetIntermediateState(state, forward);
         }
 
         /** {@inheritDoc} */
         public SpacecraftState getInitialState() {
             return AbstractAnalyticalPropagator.this.getInitialState();
         }
 
         /** {@inheritDoc} */
         public Frame getFrame() {
             return AbstractAnalyticalPropagator.this.getFrame();
         }
 
     }
 
     /** Internal class for local propagation. */
     private class BasicStepInterpolator implements OrekitStepInterpolator {
 
         /** Previous state. */
         private final SpacecraftState previousState;
 
         /** Current state. */
         private final SpacecraftState currentState;
 
         /** Forward propagation indicator. */
         private final boolean forward;
 
         /** Simple constructor.
          * @param isForward integration direction indicator
          * @param previousState start of the step
          * @param currentState end of the step
          */
         BasicStepInterpolator(final boolean isForward,
                               final SpacecraftState previousState,
                               final SpacecraftState currentState) {
             this.forward         = isForward;
             this.previousState   = previousState;
             this.currentState    = currentState;
         }
 
         /** {@inheritDoc} */
         @Override
         public SpacecraftState getPreviousState() {
             return previousState;
         }
 
         /** {@inheritDoc} */
         @Override
         public boolean isPreviousStateInterpolated() {
             // no difference in analytical propagators
             return false;
         }
 
         /** {@inheritDoc} */
         @Override
         public SpacecraftState getCurrentState() {
             return currentState;
         }
 
         /** {@inheritDoc} */
         @Override
         public boolean isCurrentStateInterpolated() {
             // no difference in analytical propagators
             return false;
         }
 
         /** {@inheritDoc} */
         @Override
         public SpacecraftState getInterpolatedState(final AbsoluteDate date) {
 
             // compute the basic spacecraft state
             final SpacecraftState basicState = basicPropagate(date);
 
             // add the additional states
             return updateAdditionalStates(basicState);
 
         }
 
         /** {@inheritDoc} */
         @Override
         public boolean isForward() {
             return forward;
         }
 
         /** {@inheritDoc} */
         @Override
         public BasicStepInterpolator restrictStep(final SpacecraftState newPreviousState,
                                                   final SpacecraftState newCurrentState) {
             return new BasicStepInterpolator(forward, newPreviousState, newCurrentState);
         }
 
     }
 
 }