org.hipparchus.ode.events

Class FieldEventFilter<T extends CalculusFieldElement<T>>

• Type Parameters:
T - the type of the field elements
All Implemented Interfaces:
FieldODEEventHandler<T>

public class FieldEventFilter<T extends CalculusFieldElement<T>>
extends Object
implements FieldODEEventHandler<T>
Wrapper used to detect only increasing or decreasing events.

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.

Since:
2.0
• Constructor Summary

Constructors
Constructor and Description
FieldEventFilter(Field<T> field, FieldODEEventHandler<T> rawHandler, FilterType filter)
• Method Summary

All Methods
Modifier and Type Method and Description
Action eventOccurred(FieldODEStateAndDerivative<T> state, boolean increasing)
Handle an event and choose what to do next.
T g(FieldODEStateAndDerivative<T> state)
Compute the value of the switching function.
void init(FieldODEStateAndDerivative<T> initialState, T finalTime)
Initialize event handler at the start of an ODE integration.
FieldODEState<T> resetState(FieldODEStateAndDerivative<T> state)
Reset the state prior to continue the integration.
• Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
• Constructor Detail

• FieldEventFilter

public FieldEventFilter(Field<T> field,
FieldODEEventHandler<T> rawHandler,
FilterType filter)
Parameters:
field - field to which array elements belong
rawHandler - event handler to wrap
filter - filter to use
• Method Detail

• init

public void init(FieldODEStateAndDerivative<T> initialState,
T finalTime)
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.

The default implementation does nothing

Specified by:
init in interface FieldODEEventHandler<T extends CalculusFieldElement<T>>
Parameters:
initialState - initial time, state vector and derivative
finalTime - target time for the integration
• g

public T g(FieldODEStateAndDerivative<T> state)
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(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 method 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.

Specified by:
g in interface FieldODEEventHandler<T extends CalculusFieldElement<T>>
Parameters:
state - current value of the independent time variable, state vector and derivative
Returns:
value of the g switching function
• eventOccurred

public Action eventOccurred(FieldODEStateAndDerivative<T> state,
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 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.

The scheduling between this method and the FieldODEStepHandler method handleStep(interpolator, isLast) 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.

Specified by:
eventOccurred in interface FieldODEEventHandler<T extends CalculusFieldElement<T>>
Parameters:
state - current value of the independent time variable, state vector and derivative
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 Action.STOP, Action.RESET_STATE, Action.RESET_DERIVATIVES, Action.RESET_EVENTS, or Action.CONTINUE
• resetState

public FieldODEState<T> resetState(FieldODEStateAndDerivative<T> state)
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 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.

Specified by:
resetState in interface FieldODEEventHandler<T extends CalculusFieldElement<T>>
Parameters:
state - current value of the independent time variable, state vector and derivative
Returns:
reset state (note that it does not include the derivatives, they will be added automatically by the integrator afterwards)