/* Copyright 2002-2024 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.estimation.measurements.generation;
  
  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.Comparator;
  import java.util.HashMap;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Map;
  import java.util.SortedSet;
  import java.util.TreeSet;
  
  import org.orekit.estimation.measurements.ObservableSatellite;
  import org.orekit.estimation.measurements.ObservedMeasurement;
  import org.orekit.propagation.Propagator;
  import org.orekit.propagation.PropagatorsParallelizer;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.propagation.sampling.MultiSatStepHandler;
  import org.orekit.propagation.sampling.OrekitStepHandler;
  import org.orekit.propagation.sampling.OrekitStepInterpolator;
  import org.orekit.propagation.sampling.StepHandlerMultiplexer;
  import org.orekit.time.AbsoluteDate;
  
  
  /** Main generator for {@link ObservedMeasurement observed measurements}.
   * @author Luc Maisonobe
   * @since 9.3
   */
  public class Generator {
  
      /** Observable satellites.
       * @since 12.0
       */
      private final List<ObservableSatellite> observableSatellites;
  
      /** Propagators. */
      private final List<Propagator> propagators;
  
      /** Schedulers for multiple satellites measurements. */
      private final List<Scheduler<? extends ObservedMeasurement<?>>> multiSatSchedulers;
  
      /** Schedulers for single satellite measurements. */
      private final Map<ObservableSatellite, List<Scheduler<? extends ObservedMeasurement<?>>>> singleSatSchedulers;
  
      /** Subscribers for generated measurements events.
       * @since 12.0
       */
      private final List<GeneratedMeasurementSubscriber> subscribers;
  
      /** Build a generator with no sequences generator.
       */
      public Generator() {
          this.observableSatellites = new ArrayList<>();
          this.propagators          = new ArrayList<>();
          this.multiSatSchedulers   = new ArrayList<>();
          this.singleSatSchedulers  = new HashMap<>();
          this.subscribers          = new ArrayList<>();
      }
  
      /** Add a propagator.
       * @param propagator to add
       * @return satellite satellite propagated by the propagator
       */
      public ObservableSatellite addPropagator(final Propagator propagator) {
          final ObservableSatellite os = new ObservableSatellite(propagators.size());
          observableSatellites.add(os);
          propagators.add(propagator);
          return os;
      }
  
      /** Get a registered propagator.
       * @param satellite satellite propagated by the propagator {@link #addPropagator(Propagator)}
       * @return propagator corresponding to satellite
       */
      public Propagator getPropagator(final ObservableSatellite satellite) {
          return propagators.get(satellite.getPropagatorIndex());
      }
  
      /** Add a sequences generator for a specific measurement type.
       * @param scheduler sequences generator to add
       * @param <T> the type of the measurement
       */
      public <T extends ObservedMeasurement<T>> void addScheduler(final Scheduler<T> scheduler) {
         final ObservableSatellite[] satellites = scheduler.getBuilder().getSatellites();
         if (satellites.length == 1) {
             // this scheduler manages only one satellite
             // we can let the individual propagator handle it
             List<Scheduler<? extends ObservedMeasurement<?>>> list = singleSatSchedulers.get(satellites[0]);
             if (list == null) {
                 list = new ArrayList<>();
                 singleSatSchedulers.put(satellites[0], list);
             }
             list.add(scheduler);
         } else {
             // this scheduler manages several satellites at once
             // we need to handle it at top level
             multiSatSchedulers.add(scheduler);
         }
     }
 
     /** Add a subscriber.
      * @param subscriber to add
      * @see GatheringSubscriber
      * @since 12.0
      */
     public void addSubscriber(final GeneratedMeasurementSubscriber subscriber) {
         subscribers.add(subscriber);
     }
 
     /** Generate measurements.
      * @param start start of the measurements time span
      * @param end end of the measurements time span
      */
     public void generate(final AbsoluteDate start, final AbsoluteDate end) {
 
         // set up top level handler
         final MultipleSatGeneratorHandler globalHandler =
                         new MultipleSatGeneratorHandler(multiSatSchedulers, subscribers,
                                                         observableSatellites, end.isAfterOrEqualTo(start));
 
         // set up low level handlers
         for (final Map.Entry<ObservableSatellite, List<Scheduler<? extends ObservedMeasurement<?>>>> entry : singleSatSchedulers.entrySet()) {
             final StepHandlerMultiplexer multiplexer = propagators.get(entry.getKey().getPropagatorIndex()).getMultiplexer();
             for (final Scheduler<?> scheduler : entry.getValue()) {
                 multiplexer.add(new SingleSatGeneratorHandler<>(scheduler, globalHandler));
             }
         }
 
         // prepare parallelized generation
         final PropagatorsParallelizer parallelizer = new PropagatorsParallelizer(propagators, globalHandler);
 
         // generate the measurements
         parallelizer.propagate(start, end);
 
         // clean up low level handlers
         for (final Map.Entry<ObservableSatellite, List<Scheduler<? extends ObservedMeasurement<?>>>> entry : singleSatSchedulers.entrySet()) {
             // we need to clean up the step handlers in two loops to avoid concurrent modification exception
             final StepHandlerMultiplexer multiplexer = propagators.get(entry.getKey().getPropagatorIndex()).getMultiplexer();
             final List<OrekitStepHandler> toBeRemoved = new ArrayList<>();
             for (final OrekitStepHandler handler : multiplexer.getHandlers()) {
                 if (handler instanceof SingleSatGeneratorHandler &&
                     ((SingleSatGeneratorHandler<?>) handler).globalHandler == globalHandler) {
                     toBeRemoved.add(handler);
                 }
             }
             for (final OrekitStepHandler handler : toBeRemoved) {
                 multiplexer.remove(handler);
             }
         }
 
     }
 
     /** Handler for measurements generation steps, single satellite case.
      * <p>
      * These handlers are called from the individual propagators threads.
      * This means they generate measurements in parallel.
      * </p>
      * @param <T> the type of the measurement
      * @since 12.0
      */
     private static class SingleSatGeneratorHandler<T extends ObservedMeasurement<T>> implements OrekitStepHandler {
 
         /** Scheduler. */
         private final Scheduler<T> scheduler;
 
         /** Satellite related to this scheduler. */
         private final ObservableSatellite satellite;
 
         /** Global handler. */
         private final MultipleSatGeneratorHandler globalHandler;
 
         /** Simple constructor.
          * @param scheduler scheduler
          * @param globalHandler global handler
          */
         SingleSatGeneratorHandler(final Scheduler<T> scheduler, final MultipleSatGeneratorHandler globalHandler) {
             this.scheduler     = scheduler;
             this.satellite     = scheduler.getBuilder().getSatellites()[0];
             this.globalHandler = globalHandler;
         }
 
         /** {@inheritDoc} */
         @Override
         public void init(final SpacecraftState state0, final AbsoluteDate t) {
             scheduler.init(state0.getDate(), t);
         }
 
         /** {@inheritDoc} */
         @Override
         public void handleStep(final OrekitStepInterpolator interpolator) {
             globalHandler.addMeasurements(scheduler.generate(Collections.singletonMap(satellite, interpolator)));
         }
 
     }
 
     /** Handler for measurements generation steps.
      * <p>
      * This handler is called from the propagator parallelizer thread.
      * The parallelizer thread is called after the individual propagators thread,
      * which may already have produced measurements ahead of time, so we must
      * take care than within each step we handle only the measurements that belong
      * to this step.
      * </p>
      */
     private static class MultipleSatGeneratorHandler implements MultiSatStepHandler {
 
         /** Sequences generators. */
         private final List<Scheduler<? extends ObservedMeasurement<?>>> schedulers;
 
         /** Subscribers for generated measurements events.
          * @since 12.0
          */
         private final List<GeneratedMeasurementSubscriber> subscribers;
 
         /** Observable satellites.
          * @since 12.0
          */
         private final List<ObservableSatellite> observableSatellites;
 
         /** Storage for sorted measurements within one step.
          * @since 12.0
          */
         private final SortedSet<ObservedMeasurement<?>> generated;
 
         /** Forward generation indicator.
          * @since 12.0
          */
         private final boolean forward;
 
         /** Simple constructor.
          * @param schedulers sequences generators
          * @param subscribers subscribers for generated measurements events
          * @param observableSatellites observable satellites
          * @param forward if true, generation is forward
          * @since 12.0
          */
         MultipleSatGeneratorHandler(final List<Scheduler<? extends ObservedMeasurement<?>>> schedulers,
                                     final List<GeneratedMeasurementSubscriber> subscribers,
                                     final List<ObservableSatellite> observableSatellites, final boolean forward) {
 
             // measurements comparator, consistent with generation direction
             final Comparator<ObservedMeasurement<?>> comparator = forward ? Comparator.naturalOrder() : Comparator.reverseOrder();
 
             this.schedulers           = schedulers;
             this.subscribers          = subscribers;
             this.observableSatellites = observableSatellites;
             this.generated            = new TreeSet<>(comparator);
             this.forward              = forward;
 
         }
 
         /** {@inheritDoc} */
         @Override
         public void init(final List<SpacecraftState> states0, final AbsoluteDate t) {
 
             final AbsoluteDate start = states0.get(0).getDate();
 
             // initialize schedulers
             for (final Scheduler<?> scheduler : schedulers) {
                 scheduler.init(start, t);
             }
 
             // initialize subscribers
             for (final GeneratedMeasurementSubscriber subscriber : subscribers) {
                 subscriber.init(start, t);
             }
 
         }
 
         /** {@inheritDoc} */
         @Override
         public void handleStep(final List<OrekitStepInterpolator> interpolators) {
 
             // prepare interpolators map
             final Map<ObservableSatellite, OrekitStepInterpolator> interpolatorsMap =
                             new HashMap<>(interpolators.size());
             for (int i = 0; i < interpolators.size(); ++i) {
                 interpolatorsMap.put(observableSatellites.get(i), interpolators.get(i));
             }
             final AbsoluteDate lastDate = interpolators.get(0).getCurrentState().getDate();
 
             synchronized (generated) {
 
                 // generate measurements, looping over schedulers
                 for (final Scheduler<? extends ObservedMeasurement<?>> scheduler : schedulers) {
                     generated.addAll(scheduler.generate(interpolatorsMap));
                 }
 
                 // now that we have all measurements properly sorted, we can feed them to subscribers
                 for (final Iterator<ObservedMeasurement<?>> iterator = generated.iterator(); iterator.hasNext();) {
                     final ObservedMeasurement<?> measurement = iterator.next();
                     if (forward == lastDate.isAfterOrEqualTo(measurement)) {
                         // this measurement belongs to the current step
                         for (final GeneratedMeasurementSubscriber subscriber : subscribers) {
                             subscriber.handleGeneratedMeasurement(measurement);
                         }
                         iterator.remove();
                     } else {
                         // this measurement belongs to an upcoming step ; we don't handle it yet as more
                         // intermediate measurements may be produced by low level propagators threads
                         break;
                     }
                 }
 
             }
 
         }
 
         /** {@inheritDoc} */
         public void finish(final List<SpacecraftState> finalStates) {
             synchronized (generated) {
                 for (final ObservedMeasurement<?> measurement : generated) {
                     for (final GeneratedMeasurementSubscriber subscriber : subscribers) {
                         subscriber.handleGeneratedMeasurement(measurement);
                     }
                 }
                 generated.clear();
             }
         }
 
         /** Add measurements performed by a low level handler.
          * @param measurements measurements to add
          * @since 12.0
          */
         private void addMeasurements(final SortedSet<? extends ObservedMeasurement<?>> measurements) {
             synchronized (generated) {
                 generated.addAll(measurements);
             }
         }
 
     }
 
 }