T
 the type of the field elementspublic interface FieldODEEventHandler<T extends CalculusFieldElement<T>>
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 (Gstop 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 problemdependent 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).
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.

default void init(FieldODEStateAndDerivative<T> initialState, T finalTime)
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
initialState
 initial time, state vector and derivativefinalTime
 target time for the integrationT g(FieldODEStateAndDerivative<T> state)
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 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
.
state
 current value of the independent time variable, state vector
and derivativeAction eventOccurred(FieldODEStateAndDerivative<T> state, boolean increasing)
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.
Action.STOP
is returned, the integration will be stopped,Action.RESET_STATE
is returned, the resetState
method will be called once the step handler has
finished its task, and the integrator will also recompute the
derivatives,Action.RESET_DERIVATIVES
is returned, the integrator
will recompute the derivatives,
Action.RESET_EVENTS
is returned, the integrator
will recheck all event handlers,
Action.CONTINUE
is returned, no specific action will
be taken (apart from having called this method) and integration
will continue.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.
state
 current value of the independent time variable, state vector
and derivativeincreasing
 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)Action.STOP
, Action.RESET_STATE
,
Action.RESET_DERIVATIVES
, Action.RESET_EVENTS
, or
Action.CONTINUE
default FieldODEState<T> resetState(FieldODEStateAndDerivative<T> state)
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.
state
 current value of the independent time variable, state vector
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