org.apache.commons.math3.ode.events

## Class EventFilter

- java.lang.Object
- - org.apache.commons.math3.ode.events.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

- - ### Nested Class Summary

    - ### Nested classes/interfaces inherited from interface org.apache.commons.math3.ode.events.EventHandler

      `EventHandler.Action`

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `EventFilter(EventHandler rawHandler, FilterType filter)` Wrap an `event handler`.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `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.

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Constructor Detail

    - #### EventFilter

          public EventFilter(EventHandler rawHandler,
                     FilterType filter)

      Wrap an `event handler`.
      Parameters:
        `rawHandler` - event handler to wrap
        `filter` - filter to use

  - ### 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.

      - if `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,
      - if `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,
      - if `EventHandler.Action.RESET_DERIVATIVES` is returned, the integrator will recompute the derivatives,
      - if `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`.

      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`

    - #### resetState

          public void resetState(double t,
                        double[] y)

      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 `EventHandler.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 `EventHandler.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.

      Specified by:
        `resetState` in interface `EventHandler`
      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

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