public interface ODEEventHandler
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 (G-stop 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 problem-dependent 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).
org.hipparchus.ode.events| 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.
|
default void |
init(ODEStateAndDerivative initialState,
double finalTime)
Initialize event handler at the start of an ODE integration.
|
default ODEState |
resetState(ODEStateAndDerivative state)
Reset the state prior to continue the integration.
|
default 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
initialState - initial time, state vector and derivativefinalTime - target time for the integrationdouble 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.
state - current value of the independent time variable, state vector
and derivativeorg.hipparchus.ode.eventsAction 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 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.
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,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, 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 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
ODEFixedStepHandler
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.
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.CONTINUEdefault ODEState resetState(ODEStateAndDerivative state)
This method is called after the step handler has returned and
before the next step is started, but only when 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.
state - current value of the independent time variable, state vector
and derivativeCopyright © 2016–2020 Hipparchus.org. All rights reserved.