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public interface EventHandler
This interface represents a handler for discrete events triggered during ODE integration.
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 discontinuities, or simply when the user wants to monitor some 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).
Nested Class Summary  

static class 
EventHandler.Action
Enumerate for actions to be performed when an event occurs. 
Method Summary  

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. 
Method Detail 

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.
t0
 start value of the independent time variabley0
 array containing the start value of the state vectort
 target time for the integrationdouble 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 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 t
. When 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 eventOccurred
is called,
sign
is 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 sign
is used to unfold it back, so the
solvers sees a g(t)
function which behaves smoothly even across events.
t
 current value of the independent time variabley
 array containing the current value of the state vector
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.STOP
is returned, the step handler will be called
with the isLast
flag of the handleStep
method set to true and the integration will be stopped,EventHandler.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,EventHandler.Action.RESET_DERIVATIVES
is returned, the integrator
will recompute the derivatives,
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
method handleStep(interpolator, isLast)
is to call this method first and
handleStep
afterwards. This scheduling allows the integrator to
pass true
as the isLast
parameter 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 StepNormalizer
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
FixedStepHandler
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 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)
EventHandler.Action.STOP
, EventHandler.Action.RESET_STATE
,
EventHandler.Action.RESET_DERIVATIVES
or EventHandler.Action.CONTINUE
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 eventOccurred(double, double[], boolean)
has itself returned the EventHandler.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 the 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.
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


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