1   /* Copyright 2002-2022 CS GROUP
2    * Licensed to CS GROUP (CS) under one or more
3    * contributor license agreements.  See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * CS licenses this file to You under the Apache License, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License.  You may obtain a copy of the License at
8    *
9    *   http://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the License for the specific language governing permissions and
15   * limitations under the License.
16   */
17  package org.orekit.propagation;
18  
19  /** This interface represents providers for additional state data beyond {@link SpacecraftState}.
20   * <p>
21   * {@link Propagator Propagators} generate {@link SpacecraftState states} that contain at
22   * least orbit, attitude, and mass. These states may however also contain {@link
23   * SpacecraftState#addAdditionalState(String, double...) additional states}. Instances of classes
24   * implementing this interface are intended to be registered to propagators so they can add these
25   * additional states incrementally after having computed the basic components
26   * (orbit, attitude and mass).
27   * </p>
28   * <p>
29   * Some additional states may depend on previous additional states to
30   * be already available the before they can be computed. It may even be impossible to compute some
31   * of these additional states at some time if they depend on conditions that are fulfilled only
32   * after propagation as started or some event has occurred. As the propagator builds the complete
33   * state incrementally, looping over the registered providers, it must call their {@link
34   * #getAdditionalState(SpacecraftState) getAdditionalState} methods in an order that fulfill these dependencies that
35   * may be time-dependent and are not related to the order in which the providers are registered to
36   * the propagator. This reordering is performed each time the complete state is built, using a yield
37   * mechanism. The propagator first pushes all providers in a stack and then empty the stack, one provider
38   * at a time, taking care to select only providers that do <em>not</em> {@link
39   * #yield(SpacecraftState) yield} when asked. Consider for example a case where providers A, B and C
40   * have been registered and provider B needs in fact the additional state generated by provider C. Then
41   * when a complete state is built, the propagator puts the three providers in a new stack, and then starts the incremental
42   * generation of additional states. It first checks provider A which does not yield so it is popped from
43   * the stack and the additional state it generates is added. Then provider B is checked, but it yields
44   * because state from provider C is not yet available. So propagator checks provider C which does not
45   * yield, so it is popped out of the stack and applied. At this stage, provider B is the only remaining one
46   * in the stack, so it is checked again, but this time it does not yield because the state from provider
47   * C is available as it has just been added, so provider B is popped from the stack and applied. The stack
48   * is now empty and the propagator can return the completed state.
49   * </p>
50   * <p>
51   * It is possible that at some stages in the propagation, a subset of the providers registered to a
52   * propagator all yied and cannot {@link #getAdditionalState(SpacecraftState) retrieve} their additional
53   * state. This happens for example during the initialization phase of a propagator that
54   * computes State Transition Matrices or Jacobian matrices. These features are managed as secondary equations
55   * in the ODE integrator, and initialized after the primary equations (which correspond to orbit) have
56   * been initialized. So when the primary equation are initialized, the providers that depend on the secondary
57   * state will all yield. This behavior is expected. Another case occurs when users set up additional states
58   * that induce a dependency loop (state A depending on state B which depends on state C which depends on
59   * state A). In this case, the three corresponding providers will wait for each other and indefinitely yield.
60   * This second case is a deadlock and results from a design error of the additional states management at
61   * application level. The propagator cannot know it in advance if a subset of providers that all yield is
62   * normal or not. So at propagator level, when either situation is detected, the propagator just gives up and
63   * returns the most complete state it was able to compute, without generating any error. Errors will indeed
64   * not be triggered in the first case (once the primary equations have been initialized, the secondary
65   * equations will be initialized too), and they will be triggered in the second case as soon as user attempts
66   * to retrieve an additional state that was not added.
67   * </p>
68   * @see org.orekit.propagation.Propagator
69   * @see org.orekit.propagation.integration.AdditionalDerivativesProvider
70   * @author Luc Maisonobe
71   */
72  public interface AdditionalStateProvider {
73  
74      /** Get the name of the additional state.
75       * @return name of the additional state (names containing "orekit"
76       * with any case are reserved for the library internal use)
77       */
78      String getName();
79  
80      /** Check if this provider should yield so another provider has an opportunity to add missing parts.
81       * <p>
82       * Decision to yield is often based on an additional state being {@link SpacecraftState#hasAdditionalState(String)
83       * already available} in the provided {@code state} (but it could theoretically also depend on
84       * an additional state derivative being {@link SpacecraftState#hasAdditionalStateDerivative(String)
85       * already available}, or any other criterion). If for example a provider needs the state transition
86       * matrix, it could implement this method as:
87       * </p>
88       * <pre>{@code
89       * public boolean yield(final SpacecraftState state) {
90       *     return !state.getAdditionalStates().containsKey("STM");
91       * }
92       * }</pre>
93       * <p>
94       * The default implementation returns {@code false}, meaning that state data can be
95       * {@link #getAdditionalState(SpacecraftState) generated} immediately.
96       * </p>
97       * @param state state to handle
98       * @return true if this provider should yield so another provider has an opportunity to add missing parts
99       * as the state is incrementally built up
100      * @since 11.1
101      */
102     default boolean yield(SpacecraftState state) {
103         return false;
104     }
105 
106     /** Get the additional state.
107      * @param state spacecraft state to which additional state should correspond
108      * @return additional state corresponding to spacecraft state
109      */
110     double[] getAdditionalState(SpacecraftState state);
111 
112 }