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The Apache Commons Math project is a library of lightweight, self-contained mathematics and statistics components addressing the most common practical problems not immediately available in the Java programming language or commons-lang.

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 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
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 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
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 *      http://www.apache.org/licenses/LICENSE-2.0
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package org.apache.commons.math3.ode.events;


/** 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 */ public interface EventHandler { /** Enumerate for actions to be performed when an event occurs. */ public enum Action { /** Stop indicator. *

This value should be used as the return value of the {@link * #eventOccurred eventOccurred} method when the integration should be * stopped after the event ending the current step.

*/ STOP, /** Reset state indicator. *

This value should be used as the return value of the {@link * #eventOccurred 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 {@link #resetState * resetState} method).

*/ RESET_STATE, /** Reset derivatives indicator. *

This value should be used as the return value of the {@link * #eventOccurred 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 {@link * org.apache.commons.math3.ode.FirstOrderDifferentialEquations#computeDerivatives} * method).

*/ RESET_DERIVATIVES, /** Continue indicator. *

This value should be used as the return value of the {@link * #eventOccurred eventOccurred} method when the integration should go * on after the event ending the current step.

*/ CONTINUE; } /** 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. *

* @param t0 start value of the independent time variable * @param y0 array containing the start value of the state vector * @param t target time for the integration */ void init(double t0, double[] y0, double t); /** 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 {@link org.apache.commons.math3.exception.NoBracketingException * 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 {@code g(t) = h(t)} where h is the * height above the floor at time {@code t}. When {@code 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 {@code g(t)} was decreasing from positive values to 0, and after the * event {@code g(t)} would be increasing from 0 to positive values again. * Consistency is broken here! The solution here is to have {@code g(t) = sign * * h(t)}, where sign is a variable with initial value set to {@code +1}. Each * time {@link #eventOccurred(double, double[], boolean) eventOccurred} is called, * {@code sign} is reset to {@code -sign}. This allows the {@code g(t)} * function to remain continuous (and even smooth) even across events, despite * {@code h(t)} is not. Basically, the event is used to fold {@code h(t)} * at bounce points, and {@code sign} is used to unfold it back, so the * solvers sees a {@code g(t)} function which behaves smoothly even across events.

* @param t current value of the independent time variable * @param y array containing the current value of the state vector * @return value of the g switching function */ double g(double t, double[] y); /** 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 {@link * org.apache.commons.math3.ode.FirstOrderDifferentialEquations * 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 {@link Action#STOP} is returned, the step handler will be called * with the isLast flag of the {@link * org.apache.commons.math3.ode.sampling.StepHandler#handleStep handleStep} * method set to true and the integration will be stopped,
  • *
  • if {@link Action#RESET_STATE} is returned, the {@link #resetState * resetState} method will be called once the step handler has * finished its task, and the integrator will also recompute the * derivatives,
  • *
  • if {@link Action#RESET_DERIVATIVES} is returned, the integrator * will recompute the derivatives, *
  • if {@link 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 {@link * org.apache.commons.math3.ode.sampling.StepHandler StepHandler} method {@link * org.apache.commons.math3.ode.sampling.StepHandler#handleStep( * org.apache.commons.math3.ode.sampling.StepInterpolator, boolean) * 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 {@link Action#STOP}. As the interpolator may be used to navigate back * throughout the last step (as {@link * org.apache.commons.math3.ode.sampling.StepNormalizer 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 {@link EventHandler EventHandler} interface and the * {@link org.apache.commons.math3.ode.sampling.FixedStepHandler 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 {@link * org.apache.commons.math3.ode.sampling.StepHandler variable step handlers} and * to the size of the fixed step for {@link * org.apache.commons.math3.ode.sampling.FixedStepHandler fixed step handlers}.

* @param t current value of the independent time variable * @param y array containing the current value of the state vector * @param 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) * @return indication of what the integrator should do next, this * value must be one of {@link Action#STOP}, {@link Action#RESET_STATE}, * {@link Action#RESET_DERIVATIVES} or {@link Action#CONTINUE} */ Action eventOccurred(double t, double[] y, boolean increasing); /** 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 {@link * #eventOccurred} has itself returned the {@link 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 {@link #eventOccurred} never returns the {@link * Action#RESET_STATE} indicator, this function will never be called, and it is * safe to leave its body empty.

* @param t current value of the independent time variable * @param y array containing the current value of the state vector * the new state should be put in the same array */ void resetState(double t, double[] y); }




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