Class EventFilter
 java.lang.Object

 org.apache.commons.math4.legacy.ode.events.EventFilter

 All Implemented Interfaces:
EventHandler
public class EventFilter extends Object implements EventHandler
Wrapper used to detect only increasing or decreasing events.General
events
are defined implicitly by ag 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 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 handler
in an instance of this class and provide this wrapping instance to theODE solver
in order to avoid wasting time looking for uninteresting events. The wrapper will intercept the calls to theg function
and to theeventOccurred
method in order to ignore uninteresting events. The wrapped regularevent handler
will the see only the interesting events, i.e. either onlyincreasing
events ordecreasing
events. the number of calls to theg function
will also be reduced. Since:
 3.2


Nested Class Summary

Nested classes/interfaces inherited from interface org.apache.commons.math4.legacy.ode.events.EventHandler
EventHandler.Action


Constructor Summary
Constructors Constructor Description EventFilter(EventHandler rawHandler, FilterType filter)
Wrap anevent handler
.

Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description EventHandler.Action
eventOccurred(double t, double[] y, boolean increasing)
Handle an event and choose what to do next.double
g(double t, double[] y)
Compute the value of the switching function.void
init(double t0, double[] y0, double t)
Initialize event handler at the start of an ODE integration.void
resetState(double t, double[] y)
Reset the state prior to continue the integration.



Constructor Detail

EventFilter
public EventFilter(EventHandler rawHandler, FilterType filter)
Wrap anevent handler
. Parameters:
rawHandler
 event handler to wrapfilter
 filter to use


Method Detail

init
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.
 Specified by:
init
in interfaceEventHandler
 Parameters:
t0
 start value of the independent time variabley0
 array containing the start value of the state vectort
 target time for the integration

g
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
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 timet
. Wheng(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 eventg(t)
was decreasing from positive values to 0, and after the eventg(t)
would be increasing from 0 to positive values again. Consistency is broken here! The solution here is to haveg(t) = sign * h(t)
, where sign is a variable with initial value set to+1
. Each timeeventOccurred
is called,sign
is reset tosign
. This allows theg(t)
function to remain continuous (and even smooth) even across events, despiteh(t)
is not. Basically, the event is used to foldh(t)
at bounce points, andsign
is used to unfold it back, so the solvers sees ag(t)
function which behaves smoothly even across events. Specified by:
g
in interfaceEventHandler
 Parameters:
t
 current value of the independent time variabley
 array containing the current value of the state vector Returns:
 value of the g switching function

eventOccurred
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 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. if
EventHandler.Action.STOP
is returned, the step handler will be called with theisLast
flag of thehandleStep
method set to true and the integration will be stopped,  if
EventHandler.Action.RESET_STATE
is returned, theresetState
method will be called once the step handler has finished its task, and the integrator will also recompute the derivatives,  if
EventHandler.Action.RESET_DERIVATIVES
is returned, the integrator will recompute the derivatives,  if
EventHandler.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
StepHandler
methodhandleStep(interpolator, isLast)
is to call this method first andhandleStep
afterwards. This scheduling allows the integrator to passtrue
as theisLast
parameter to the step handler to make it aware the step will be the last one if this method returnsEventHandler.Action.STOP
. As the interpolator may be used to navigate back throughout the last step (asStepNormalizer
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 thisEventHandler
interface and theFixedStepHandler
interface, a forward integration may call itseventOccurred
method with t = 10 first and call itshandleStep
method with t = 9 afterwards. Such out of order calls are limited to the size of the integration step forvariable step handlers
and to the size of the fixed step forfixed step handlers
. Specified by:
eventOccurred
in interfaceEventHandler
 Parameters:
t
 current value of the independent time variabley
 array containing the current value of the state vectorincreasing
 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
EventHandler.Action.STOP
,EventHandler.Action.RESET_STATE
,EventHandler.Action.RESET_DERIVATIVES
orEventHandler.Action.CONTINUE
 if

resetState
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 theEventHandler.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. If theEventHandler.eventOccurred(double, double[], boolean)
never returns theEventHandler.Action.RESET_STATE
indicator, this function will never be called, and it is safe to leave its body empty. Specified by:
resetState
in interfaceEventHandler
 Parameters:
t
 current value of the independent time variabley
 array containing the current value of the state vector the new state should be put in the same array

