AdditionalStateProvider.java
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* Unless required by applicable law or agreed to in writing, software
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package org.orekit.propagation;
import org.orekit.time.AbsoluteDate;
/** This interface allows to modify {@link SpacecraftState} and set up additional state data.
* <p>
* {@link Propagator Propagators} generate {@link SpacecraftState states} that contain at
* least orbit, attitude, and mass. These states may however also contain {@link
* SpacecraftState#addAdditionalState(String, double...) additional states}. Instances of classes
* implementing this interface are intended to be registered to propagators so they can either
* modify the basic components (orbit, attitude and mass) or add additional states incrementally
* after having computed the basic components.
* </p>
* <p>
* Some additional states may depend on previous additional states to
* be already available the before they can be computed. It may even be impossible to compute some
* of these additional states at some time if they depend on conditions that are fulfilled only
* after propagation as started or some event has occurred. As the propagator builds the complete
* state incrementally, looping over the registered providers, it must call their {@link
* #update(SpacecraftState) update} methods in an order that fulfill these dependencies that
* may be time-dependent and are not related to the order in which the providers are registered to
* the propagator. This reordering is performed each time the complete state is built, using a yield
* mechanism. The propagator first pushes all providers in a stack and then empty the stack, one provider
* at a time, taking care to select only providers that do <em>not</em> {@link
* #yields(SpacecraftState) yield} when asked. Consider for example a case where providers A, B and C
* have been registered and provider B needs in fact the additional state generated by provider C. Then
* when a complete state is built, the propagator puts the three providers in a new stack, and then starts the incremental
* generation of additional states. It first checks provider A which does not yield so it is popped from
* the stack and the additional state it generates is added. Then provider B is checked, but it yields
* because state from provider C is not yet available. So propagator checks provider C which does not
* yield, so it is popped out of the stack and applied. At this stage, provider B is the only remaining one
* in the stack, so it is checked again, but this time it does not yield because the state from provider
* C is available as it has just been added, so provider B is popped from the stack and applied. The stack
* is now empty and the propagator can return the completed state.
* </p>
* <p>
* It is possible that at some stages in the propagation, a subset of the providers registered to a
* propagator all yield and cannot {@link #update(SpacecraftState) update} the state. This happens
* for example during the initialization phase of a propagator that
* computes State Transition Matrices or Jacobian matrices. These features are managed as secondary equations
* in the ODE integrator, and initialized after the primary equations (which correspond to orbit) have
* been initialized. So when the primary equation are initialized, the providers that depend on the secondary
* state will all yield. This behavior is expected. Another case occurs when users set up additional states
* that induce a dependency loop (state A depending on state B which depends on state C which depends on
* state A). In this case, the three corresponding providers will wait for each other and indefinitely yield.
* This second case is a deadlock and results from a design error of the additional states management at
* application level. The propagator cannot know it in advance if a subset of providers that all yield is
* normal or not. So at propagator level, when either situation is detected, the propagator just gives up and
* returns the most complete state it was able to compute, without generating any error. Errors will indeed
* not be triggered in the first case (once the primary equations have been initialized, the secondary
* equations will be initialized too), and they will be triggered in the second case as soon as user attempts
* to retrieve an additional state that was not added.
* </p>
* @see org.orekit.propagation.Propagator
* @see org.orekit.propagation.integration.AdditionalDerivativesProvider
* @see AbstractStateModifier
* @author Luc Maisonobe
*/
public interface AdditionalStateProvider {
/** Get the name of the additional state.
* <p>
* If a provider just modifies one of the basic elements (orbit, attitude
* or mass) without adding any new state, it should return the empty string
* as its name.
* </p>
* @return name of the additional state (names containing "orekit"
* with any case are reserved for the library internal use)
*/
String getName();
/** Initialize the additional state provider at the start of propagation.
* @param initialState initial state information at the start of propagation
* @param target date of propagation
* @since 11.2
*/
default void init(final SpacecraftState initialState, final AbsoluteDate target) {
// nothing by default
}
/** Check if this provider should yield so another provider has an opportunity to add missing parts.
* <p>
* Decision to yield is often based on an additional state being {@link SpacecraftState#hasAdditionalState(String)
* already available} in the provided {@code state} (but it could theoretically also depend on
* an additional state derivative being {@link SpacecraftState#hasAdditionalStateDerivative(String)
* already available}, or any other criterion). If for example a provider needs the state transition
* matrix, it could implement this method as:
* </p>
* <pre>{@code
* public boolean yields(final SpacecraftState state) {
* return !state.getAdditionalStates().containsKey("STM");
* }
* }</pre>
* <p>
* The default implementation returns {@code false}, meaning that state data can be
* {@link #getAdditionalState(SpacecraftState) generated} immediately.
* </p>
* @param state state to handle
* @return true if this provider should yield so another provider has an opportunity to add missing parts
* as the state is incrementally built up
* @since 11.1
*/
default boolean yields(SpacecraftState state) {
return false;
}
/** Get the additional state.
* @param state spacecraft state to which additional state should correspond
* @return additional state corresponding to spacecraft state
*/
double[] getAdditionalState(SpacecraftState state);
/** Update a state.
* @param state spacecraft state to update
* @return updated state
* @since 12.1
*/
default SpacecraftState update(final SpacecraftState state) {
return state.addAdditionalState(getName(), getAdditionalState(state));
}
}