/* 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.Collections;
  import java.util.List;
  import java.util.Map;
  
  import org.orekit.errors.OrekitException;
  import org.orekit.errors.OrekitMessages;
  import org.orekit.orbits.Orbit;
  import org.orekit.propagation.Propagator;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.time.AbsoluteDate;
  
  /** Orbit propagator that adapts an underlying propagator, adding {@link
   * DifferentialEffect differential effects}.
   * <p>
   * This propagator is used when a reference propagator does not handle
   * some effects that we need. A typical example would be an ephemeris
   * that was computed for a reference orbit, and we want to compute a
   * station-keeping maneuver on top of this ephemeris, changing its
   * final state. The principal is to add one or more {@link
   * org.orekit.forces.maneuvers.SmallManeuverAnalyticalModel small maneuvers
   * analytical models} to it and use it as a new propagator, which takes the
   * maneuvers into account.
   * </p>
   * <p>
   * From a space flight dynamics point of view, this is a differential
   * correction approach. From a computer science point of view, this is
   * a use of the decorator design pattern.
   * </p>
   * @see Propagator
   * @see org.orekit.forces.maneuvers.SmallManeuverAnalyticalModel
   * @author Luc Maisonobe
   */
  public class AdapterPropagator extends AbstractAnalyticalPropagator {
  
      /** Interface for orbit differential effects. */
      public interface DifferentialEffect {
  
          /** Apply the effect to a {@link SpacecraftState spacecraft state}.
           * <p>
           * Applying the effect may be a no-op in some cases. A typical example
           * is maneuvers, for which the state is changed only for time <em>after</em>
           * the maneuver occurrence.
           * </p>
           * @param original original state <em>without</em> the effect
           * @return updated state at the same date, taking the effect
           * into account if meaningful
           */
          SpacecraftState apply(SpacecraftState original);
  
      }
  
      /** Underlying reference propagator. */
      private Propagator reference;
  
      /** Effects to add. */
      private List<DifferentialEffect> effects;
  
      /** Build a propagator from an underlying reference propagator.
       * <p>The reference propagator can be almost anything, numerical,
       * analytical, and even an ephemeris. It may already take some maneuvers
       * into account.</p>
       * @param reference reference propagator
       */
      public AdapterPropagator(final Propagator reference) {
          super(reference.getAttitudeProvider());
          this.reference = reference;
          this.effects = new ArrayList<DifferentialEffect>();
      }
  
      /** Add a differential effect.
       * @param effect differential effect
       */
      public void addEffect(final DifferentialEffect effect) {
          effects.add(effect);
      }
  
      /** Get the reference propagator.
       * @return reference propagator
       */
      public Propagator getPropagator() {
         return reference;
     }
 
     /** Get the differential effects.
      * @return differential effects models, as an unmodifiable list
      */
     public List<DifferentialEffect> getEffects() {
         return Collections.unmodifiableList(effects);
     }
 
     /** {@inheritDoc} */
     public SpacecraftState getInitialState() {
         return reference.getInitialState();
     }
 
     /** {@inheritDoc} */
     @Override
     public void resetInitialState(final SpacecraftState state) {
         reference.resetInitialState(state);
     }
 
     /** {@inheritDoc} */
     protected void resetIntermediateState(final SpacecraftState state, final boolean forward) {
         if (reference instanceof AbstractAnalyticalPropagator) {
             ((AbstractAnalyticalPropagator) reference).resetIntermediateState(state, forward);
         } else {
             throw new OrekitException(OrekitMessages.NON_RESETABLE_STATE);
         }
     }
 
     /** {@inheritDoc} */
     @Override
     protected SpacecraftState basicPropagate(final AbsoluteDate date) {
 
         // compute reference state
         SpacecraftState state = reference.propagate(date);
         final Map<String, double[]> before = state.getAdditionalStates();
 
         // add all the effects
         for (final DifferentialEffect effect : effects) {
             state = effect.apply(state);
         }
 
         // forward additional states from the reference propagator
         for (final Map.Entry<String, double[]> entry : before.entrySet()) {
             if (!state.hasAdditionalState(entry.getKey())) {
                 state = state.addAdditionalState(entry.getKey(), entry.getValue());
             }
         }
 
         return state;
 
     }
 
     /** {@inheritDoc} */
     protected Orbit propagateOrbit(final AbsoluteDate date) {
         return basicPropagate(date).getOrbit();
     }
 
     /** {@inheritDoc}*/
     protected double getMass(final AbsoluteDate date) {
         return basicPropagate(date).getMass();
     }
 
 }