org.hipparchus.ode.events

## Interface FieldODEEventHandler<T extends CalculusFieldElement<T>>

• Type Parameters:
T - the type of the field elements
All Known Implementing Classes:
FieldEventFilter

public interface FieldODEEventHandler<T extends CalculusFieldElement<T>>
This interface represents a handler for discrete events triggered during ODE integration.

Some events can be triggered at discrete times as an ODE problem is solved. This occurs for example when the integration process should be stopped as some state is reached (G-stop facility) when the precise date is unknown a priori, or when the derivatives have states boundaries crossings.

These events are defined as occurring when a g switching function sign changes.

Since events are only problem-dependent and are triggered by the independent time variable and the state vector, they can occur at virtually any time, unknown in advance. The integrators will take care to avoid sign changes inside the steps, they will reduce the step size when such an event is detected in order to put this event exactly at the end of the current step. This guarantees that step interpolation (which always has a one step scope) is relevant even in presence of discontinuities. This is independent from the stepsize control provided by integrators that monitor the local error (this event handling feature is available for all integrators, including fixed step ones).

• ### Method Summary

All Methods
Modifier and Type Method and Description
Action eventOccurred(FieldODEStateAndDerivative<T> state, boolean increasing)
Handle an event and choose what to do next.
T g(FieldODEStateAndDerivative<T> state)
Compute the value of the switching function.
default void init(FieldODEStateAndDerivative<T> initialState, T finalTime)
Initialize event handler at the start of an ODE integration.
default FieldODEState<T> resetState(FieldODEStateAndDerivative<T> state)
Reset the state prior to continue the integration.
• ### Method Detail

• #### init

default void init(FieldODEStateAndDerivative<T> initialState,
T finalTime)
Initialize event handler at the start of an ODE integration.

This method is called once at the start of the integration. It may be used by the event handler to initialize some internal data if needed.

The default implementation does nothing

Parameters:
initialState - initial time, state vector and derivative
finalTime - target time for the integration
• #### g

T g(FieldODEStateAndDerivative<T> state)
Compute the value of the switching function.

The discrete events are generated when the sign of this switching function changes. The integrator will take care to change the stepsize in such a way these events occur exactly at step boundaries. The switching function must be continuous in its roots neighborhood (but not necessarily smooth), as the integrator will need to find its roots to locate precisely the events.

Also note that the integrator expect that once an event has occurred, the sign of the switching function at the start of the next step (i.e. just after the event) is the opposite of the sign just before the event. This consistency between the steps must be preserved, otherwise exceptions related to root not being bracketed will occur.

This need for consistency is sometimes tricky to achieve. A typical example is using an event to model a ball bouncing on the floor. The first idea to represent this would be to have g(state) = h(state) where h is the height above the floor at time state.getTime(). When g(state) reaches 0, the ball is on the floor, so it should bounce and the typical way to do this is to reverse its vertical velocity. However, this would mean that before the event g(state) was decreasing from positive values to 0, and after the event g(state) would be increasing from 0 to positive values again. Consistency is broken here! The solution here is to have g(state) = sign * h(state), where sign is a variable with initial value set to +1. Each time eventOccurred method is called, sign is reset to -sign. This allows the g(state) function to remain continuous (and even smooth) even across events, despite h(state) is not. Basically, the event is used to fold h(state) at bounce points, and sign is used to unfold it back, so the solvers sees a g(state) function which behaves smoothly even across events.

This method is idempotent, that is calling this multiple times with the same state will result in the same value, with two exceptions. First, the definition of the g function may change when an event occurs on this handler, as in the above example. Second, the definition of the g function may change when the eventOccurred method of any other event handler in the same integrator returns Action.RESET_EVENTS, Action.RESET_DERIVATIVES, or Action.RESET_STATE.

Parameters:
state - current value of the independent time variable, state vector and derivative
Returns:
value of the g switching function
• #### eventOccurred

Action eventOccurred(FieldODEStateAndDerivative<T> state,
boolean increasing)
Handle an event and choose what to do next.

This method is called when the integrator has accepted a step ending exactly on a sign change of the function, just after the step handler itself is called (see below for scheduling). It allows the user to update his internal data to acknowledge the fact the event has been handled (for example setting a flag in the differential equations to switch the derivatives computation in case of discontinuity), or to direct the integrator to either stop or continue integration, possibly with a reset state or derivatives.

The scheduling between this method and the FieldODEStepHandler method handleStep(interpolator, isLast) is to call handleStep first and this method afterwards (this scheduling changed as of Hipparchus 2.0). This scheduling allows user code called by this method and user code called by step handlers to get values of the independent time variable consistent with integration direction.

Parameters:
state - current value of the independent time variable, state vector and derivative
increasing - if true, the value of the switching function increases when times increases around event (note that increase is measured with respect to physical time, not with respect to integration which may go backward in time)
Returns:
indication of what the integrator should do next, this value must be one of Action.STOP, Action.RESET_STATE, Action.RESET_DERIVATIVES, Action.RESET_EVENTS, or Action.CONTINUE
• #### resetState

default FieldODEState<T> resetState(FieldODEStateAndDerivative<T> state)
Reset the state prior to continue the integration.

This method is called after the step handler has returned and before the next step is started, but only when eventOccurred has itself returned the Action.RESET_STATE indicator. It allows the user to reset the state vector for the next step, without perturbing the step handler of the finishing step.

The default implementation returns its argument.

Parameters:
state - current value of the independent time variable, state vector and derivative
Returns:
reset state (note that it does not include the derivatives, they will be added automatically by the integrator afterwards)