public class EventFilter extends Object implements ODEEventHandler
General events
are defined implicitly
by a 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
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
the ODE solver
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
wrapped regular 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 

EventFilter(ODEEventHandler rawHandler,
FilterType filter)
Wrap an
event handler . 
Modifier and Type  Method and Description 

Action 
eventOccurred(ODEStateAndDerivative state,
boolean increasing)
Handle an event and choose what to do next.

double 
g(ODEStateAndDerivative state)
Compute the value of the switching function.

void 
init(ODEStateAndDerivative initialState,
double finalTime)
Initialize event handler at the start of an ODE integration.

ODEState 
resetState(ODEStateAndDerivative state)
Reset the state prior to continue the integration.

public EventFilter(ODEEventHandler rawHandler, FilterType filter)
event handler
.rawHandler
 event handler to wrapfilter
 filter to usepublic void init(ODEStateAndDerivative initialState, double 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
init
in interface ODEEventHandler
initialState
 initial time, state vector and derivativefinalTime
 target time for the integrationpublic double g(ODEStateAndDerivative 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 for the integrator to detect an event the sign of the switching function must have opposite signs just before and after the event. If this consistency is not preserved the integrator may not detect any events.
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
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
.
g
in interface ODEEventHandler
state
 current value of the independent time variable, state vector
and derivativeorg.hipparchus.ode.events
public Action eventOccurred(ODEStateAndDerivative 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 ODEStepHandler
method handleStep(interpolator)
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.
eventOccurred
in interface ODEEventHandler
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
public ODEState resetState(ODEStateAndDerivative state)
This method is called after the step handler has returned and
before the next step is started, but only when ODEEventHandler.eventOccurred(org.hipparchus.ode.ODEStateAndDerivative, boolean)
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.
resetState
in interface ODEEventHandler
state
 current value of the independent time variable, state vector
and derivativeCopyright © 20162021 CS GROUP. All rights reserved.