/* Contributed in the public domain.
    * 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.Collection;
  import java.util.Map.Entry;
  import java.util.NavigableMap;
  import java.util.TreeMap;
  
  import org.orekit.attitudes.Attitude;
  import org.orekit.attitudes.AttitudeProvider;
  import org.orekit.attitudes.InertialProvider;
  import org.orekit.errors.OrekitException;
  import org.orekit.errors.OrekitMessages;
  import org.orekit.frames.Frame;
  import org.orekit.orbits.Orbit;
  import org.orekit.propagation.BoundedPropagator;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.utils.TimeStampedPVCoordinates;
  
  /**
   * A {@link BoundedPropagator} that covers a larger time span from several constituent
   * propagators that cover shorter time spans.
   *
   * @author Evan Ward
   * @see #AggregateBoundedPropagator(Collection)
   */
  public class AggregateBoundedPropagator extends AbstractAnalyticalPropagator
          implements BoundedPropagator {
  
      /** Constituent propagators. */
      private final NavigableMap<AbsoluteDate, BoundedPropagator> propagators;
  
      /**
       * Create a propagator by concatenating several {@link BoundedPropagator}s.
       *
       * @param propagators that provide the backing data for this instance. There must be
       *                    at least one propagator in the collection. If there are gaps
       *                    between the {@link BoundedPropagator#getMaxDate()} of one
       *                    propagator and the {@link BoundedPropagator#getMinDate()} of the
       *                    next propagator an exception may be thrown by any method of this
       *                    class at any time. If there are overlaps between the the {@link
       *                    BoundedPropagator#getMaxDate()} of one propagator and the {@link
       *                    BoundedPropagator#getMinDate()} of the next propagator then the
       *                    propagator with the latest {@link BoundedPropagator#getMinDate()}
       *                    is used.
       */
      public AggregateBoundedPropagator(
              final Collection<? extends BoundedPropagator> propagators) {
          super(defaultAttitude(propagators));
          this.propagators = new TreeMap<>();
          for (final BoundedPropagator propagator : propagators) {
              this.propagators.put(propagator.getMinDate(), propagator);
          }
          super.resetInitialState(
                  this.propagators.firstEntry().getValue().getInitialState());
      }
  
      /**
       * Helper function for the constructor.
       * @param propagators to consider.
       * @return attitude provider.
       */
      private static AttitudeProvider defaultAttitude(
              final Collection<? extends BoundedPropagator> propagators) {
          // this check is needed here because it can't be before the super() call in the
          // constructor.
          if (propagators.isEmpty()) {
              throw new OrekitException(OrekitMessages.NOT_ENOUGH_PROPAGATORS);
          }
          return new InertialProvider(propagators.iterator().next().getFrame());
      }
  
      @Override
      protected SpacecraftState basicPropagate(final AbsoluteDate date) {
          // #589 override this method for a performance benefit,
          // getPropagator(date).propagate(date) is only called once
  
          // do propagation
          final SpacecraftState state = getPropagator(date).propagate(date);
  
          // evaluate attitude
          final Attitude attitude =
                  getAttitudeProvider().getAttitude(this, date, state.getFrame());
 
         // build raw state
         if (state.isOrbitDefined()) {
             return new SpacecraftState(
                     state.getOrbit(), attitude, state.getMass(), state.getAdditionalStates());
         } else {
             return new SpacecraftState(
                     state.getAbsPVA(), attitude, state.getMass(), state.getAdditionalStates());
         }
     }
 
     @Override
     public TimeStampedPVCoordinates getPVCoordinates(final AbsoluteDate date,
                                                      final Frame frame) {
         return getPropagator(date).getPVCoordinates(date, frame);
     }
 
     @Override
     protected Orbit propagateOrbit(final AbsoluteDate date) {
         return getPropagator(date).propagate(date).getOrbit();
     }
 
     @Override
     public AbsoluteDate getMinDate() {
         return propagators.firstEntry().getValue().getMinDate();
     }
 
     @Override
     public AbsoluteDate getMaxDate() {
         return propagators.lastEntry().getValue().getMaxDate();
     }
 
     @Override
     protected double getMass(final AbsoluteDate date) {
         return getPropagator(date).propagate(date).getMass();
     }
 
     @Override
     public SpacecraftState getInitialState() {
         return propagators.firstEntry().getValue().getInitialState();
     }
 
     @Override
     protected void resetIntermediateState(final SpacecraftState state,
                                           final boolean forward) {
         throw new OrekitException(OrekitMessages.NON_RESETABLE_STATE);
     }
 
     @Override
     public void resetInitialState(final SpacecraftState state) {
         throw new OrekitException(OrekitMessages.NON_RESETABLE_STATE);
     }
 
     /**
      * Get the propagator to use for the given date.
      *
      * @param date of query
      * @return propagator to use on date.
      */
     private BoundedPropagator getPropagator(final AbsoluteDate date) {
         final Entry<AbsoluteDate, BoundedPropagator> entry = propagators.floorEntry(date);
         if (entry != null) {
             return entry.getValue();
         } else {
             // let the first propagator throw the exception
             return propagators.firstEntry().getValue();
         }
     }
 
 }