Package org.apache.commons.math4.legacy.ode.events

Class EventFilter

  • All Implemented Interfaces:
    EventHandler
    public class EventFilter
extends Object
implements EventHandler
    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:
    3.2

    • Method Detail

      • init

        public void init​(double t0,
                 double[] y0,
                 double t)
        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.

        Specified by:
        init in interface EventHandler
        Parameters:
        t0 - start value of the independent time variable
        y0 - array containing the start value of the state vector
        t - target time for the integration

      • g

        public double g​(double t,
                double[] y)
        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(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.

        Specified by:
        g in interface EventHandler
        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

      • eventOccurred

        public EventHandler.Action eventOccurred​(double t,
                                         double[] y,
                                         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 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.

        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.

        Specified by:
        eventOccurred in interface EventHandler
        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 EventHandler.Action.STOP, EventHandler.Action.RESET_STATE, EventHandler.Action.RESET_DERIVATIVES or EventHandler.Action.CONTINUE