public class EventFilter extends Object implements EventHandler
events are defined implicitly
g function crossing
zero. This function needs to be continuous in the event neighborhood,
and its sign must remain consistent between events. This implies that
during an ODE integration, events triggered are alternately events
for which the function increases from negative to positive values,
and events for which the function decreases from positive to
Sometimes, users are only interested in one type of event (say increasing events for example) and not in the other type. In these cases, looking precisely for all events location and triggering events that will later be ignored is a waste of computing time.
Users can wrap a regular
event handler in
an instance of this class and provide this wrapping instance to
in order to avoid wasting time looking for uninteresting events.
The wrapper will intercept the calls to the
g function and to the
eventOccurred method in order to ignore uninteresting events. The
event handler will the see only
the interesting events, i.e. either only
increasing events or
decreasing events. the number of calls to the
g function will also be reduced.
|Constructor and Description|
|Modifier and Type||Method and Description|
Handle an event and choose what to do next.
Compute the value of the switching function.
Initialize event handler at the start of an ODE integration.
Reset the state prior to continue the integration.
public void init(double t0, double y0, double t)
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.
public double g(double t, double y)
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,
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(t) = h(t) where h is the
height above the floor at time
g(t) 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
g(t) was decreasing from positive values to 0, and after the
g(t) would be increasing from 0 to positive values again.
Consistency is broken here! The solution here is to have
g(t) = sign
* h(t), where sign is a variable with initial value set to
eventOccurred is called,
sign is reset to
-sign. This allows the
function to remain continuous (and even smooth) even across events, despite
h(t) is not. Basically, the event is used to fold
at bounce points, and
sign is used to unfold it back, so the
solvers sees a
g(t) function which behaves smoothly even across events.
public EventHandler.Action eventOccurred(double t, double y, boolean increasing)
This method is called when the integrator has accepted a step
ending exactly on a sign change of the function, just before
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.
EventHandler.Action.STOPis returned, the step handler will be called with the
isLastflag of the
handleStepmethod set to true and the integration will be stopped,
EventHandler.Action.RESET_STATEis returned, the
resetStatemethod will be called once the step handler has finished its task, and the integrator will also recompute the derivatives,
EventHandler.Action.RESET_DERIVATIVESis returned, the integrator will recompute the derivatives,
EventHandler.Action.CONTINUEis returned, no specific action will be taken (apart from having called this method) and integration will continue.
The scheduling between this method and the
handleStep(interpolator, isLast) is to call this method first and
handleStep afterwards. This scheduling allows the integrator to
true as the
isLast parameter to the step
handler to make it aware the step will be the last one if this method
EventHandler.Action.STOP. As the interpolator may be used to navigate back
throughout the last step (as
does for example), user code called by this method and user
code called by step handlers may experience apparently out of order values
of the independent time variable. As an example, if the same user object
implements both this
EventHandler interface and the
interface, a forward integration may call its
eventOccurred method with t = 10 first and call its
handleStep method with t = 9 afterwards. Such out of order
calls are limited to the size of the integration step for
variable step handlers and
to the size of the fixed step for
fixed step handlers.
t- current value of the independent time variable
y- array containing the current value of the state vector
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)
public void resetState(double t, double y)
This method is called after the step handler has returned and
before the next step is started, but only when
EventHandler.eventOccurred(double, double, boolean) has itself returned the
indicator. It allows the user to reset the state vector for the
next step, without perturbing the step handler of the finishing
step. If the
EventHandler.eventOccurred(double, double, boolean) never returns the
EventHandler.Action.RESET_STATE indicator, this function will never be called, and it is
safe to leave its body empty.
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