org.apache.commons.math.ode.events

Interface EventHandler

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 (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).

Since:

1.2

Field Summary

Fields

Modifier and Type	Field and Description
static int	CONTINUE Continue indicator.
static int	RESET_DERIVATIVES Reset derivatives indicator.
static int	RESET_STATE Reset state indicator.
static int	STOP Stop indicator.

Method Summary

Modifier and Type	Method and Description
int	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	resetState(double t, double[] y) Reset the state prior to continue the integration.

Field Detail

STOP

static final int STOP

Stop indicator.

This value should be used as the return value of the eventOccurred method when the integration should be stopped after the event ending the current step.

See Also:

Constant Field Values

RESET_STATE

static final int RESET_STATE

Reset state indicator.

This value should be used as the return value of the eventOccurred method when the integration should go on after the event ending the current step, with a new state vector (which will be retrieved thanks to the resetState method).

See Also:

Constant Field Values

RESET_DERIVATIVES

static final int RESET_DERIVATIVES

Reset derivatives indicator.

This value should be used as the return value of the eventOccurred method when the integration should go on after the event ending the current step, with a new derivatives vector (which will be retrieved thanks to the FirstOrderDifferentialEquations.computeDerivatives(double, double[], double[]) method).

See Also:

Constant Field Values

CONTINUE

static final int CONTINUE

Continue indicator.

This value should be used as the return value of the eventOccurred method when the integration should go on after the event ending the current step.

See Also:

Constant Field Values

Method Detail

g

double g(double t,
         double[] y)
  throws EventException

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.

Parameters:

t - current value of the independent time variable

y - array containing the current value of the state vector

Returns:

value of the g switching function

Throws:

EventException - if the switching function cannot be evaluated

eventOccurred

int eventOccurred(double t,
                  double[] y,
                  boolean increasing)
           throws EventException

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 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,
• if 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,
• if RESET_DERIVATIVES is returned, the integrator will recompute the derivatives,
• if 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 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.

Parameters:

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)

Returns:

indication of what the integrator should do next, this value must be one of STOP, RESET_STATE, RESET_DERIVATIVES or CONTINUE

Throws:

EventException - if the event occurrence triggers an error

resetState

void resetState(double t,
                double[] y)
         throws EventException

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 eventOccurred(double, double[], boolean) has itself returned the 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 RESET_STATE indicator, this function will never be called, and it is safe to leave its body empty.

Parameters:

t - current value of the independent time variable

y - array containing the current value of the state vector the new state should be put in the same array

Throws:

EventException - if the state cannot be reseted