- All Implemented Interfaces:
public class EventFilter extends Object implements EventHandlerWrapper used to detect only increasing or decreasing events.
eventsare defined implicitly by a
g functioncrossing 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 negative values.
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 handlerin an instance of this class and provide this wrapping instance to the
ODE solverin order to avoid wasting time looking for uninteresting events. The wrapper will intercept the calls to the
g functionand to the
eventOccurredmethod in order to ignore uninteresting events. The wrapped regular
event handlerwill the see only the interesting events, i.e. either only
decreasingevents. the number of calls to the
g functionwill also be reduced.
All Methods Instance Methods Concrete Methods Modifier and Type Method Description
eventOccurred(double t, double y, boolean increasing)Handle an event and choose what to do next.
g(double t, double y)Compute the value of the switching function.
init(double t0, double y0, double t)Initialize event handler at the start of an ODE integration.
resetState(double t, double y)Reset the state prior to continue the integration.
public void init(double t0, double y0, double t)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.
public double g(double t, double y)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
exceptionsrelated 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 event
g(t)was decreasing from positive values to 0, and after the event
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
+1. Each time
signis reset to
-sign. This allows the
g(t)function to remain continuous (and even smooth) even across events, despite
h(t)is not. Basically, the event is used to fold
h(t)at bounce points, and
signis 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)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 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 equationsto 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
handleStepafterwards. This scheduling allows the integrator to pass
isLastparameter to the step handler to make it aware the step will be the last one if this method returns
EventHandler.Action.STOP. As the interpolator may be used to navigate back throughout the last step (as
StepNormalizerdoes 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
EventHandlerinterface and the
FixedStepHandlerinterface, a forward integration may call its
eventOccurredmethod with t = 10 first and call its
handleStepmethod with t = 9 afterwards. Such out of order calls are limited to the size of the integration step for
variable step handlersand to the size of the fixed step for
fixed step handlers.
- Specified by:
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)
- indication of what the integrator should do next, this
value must be one of
public void resetState(double t, double y)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
EventHandler.eventOccurred(double, double, boolean)has itself returned the
EventHandler.Action.RESET_STATEindicator. 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_STATEindicator, this function will never be called, and it is safe to leave its body empty.