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Diffstat (limited to 'src/main/java/org/apache/commons/math3/ode/events/EventHandler.java')
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diff --git a/src/main/java/org/apache/commons/math3/ode/events/EventHandler.java b/src/main/java/org/apache/commons/math3/ode/events/EventHandler.java new file mode 100644 index 0000000..58533bc --- /dev/null +++ b/src/main/java/org/apache/commons/math3/ode/events/EventHandler.java @@ -0,0 +1,219 @@ +/* + * Licensed to the Apache Software Foundation (ASF) under one or more + * contributor license agreements. See the NOTICE file distributed with + * this work for additional information regarding copyright ownership. + * 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 + * + * http://www.apache.org/licenses/LICENSE-2.0 + * + * Unless required by applicable law or agreed to in writing, software + * distributed under the License is distributed on an "AS IS" BASIS, + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + * See the License for the specific language governing permissions and + * limitations under the License. + */ + +package org.apache.commons.math3.ode.events; + + +/** This interface represents a handler for discrete events triggered + * during ODE integration. + * + * <p>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. + * </p> + * + * <p>These events are defined as occurring when a <code>g</code> + * switching function sign changes.</p> + * + * <p>Since events are only problem-dependent and are triggered by the + * independent <i>time</i> 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).</p> + * + * @since 1.2 + */ + +public interface EventHandler { + + /** Enumerate for actions to be performed when an event occurs. */ + enum Action { + + /** Stop indicator. + * <p>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.</p> + */ + STOP, + + /** Reset state indicator. + * <p>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).</p> + */ + RESET_STATE, + + /** Reset derivatives indicator. + * <p>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).</p> + */ + RESET_DERIVATIVES, + + /** Continue indicator. + * <p>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.</p> + */ + CONTINUE; + + } + + /** Initialize event handler at the start of an ODE integration. + * <p> + * 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. + * </p> + * @param t0 start value of the independent <i>time</i> 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. + + * <p>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.</p> + * <p>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 <string>must</strong> be preserved, + * otherwise {@link org.apache.commons.math3.exception.NoBracketingException + * exceptions} related to root not being bracketed will occur.</p> + * <p>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 <em>fold</em> {@code h(t)} + * at bounce points, and {@code sign} is used to <em>unfold</em> it back, so the + * solvers sees a {@code g(t)} function which behaves smoothly even across events.</p> + + * @param t current value of the independent <i>time</i> 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. + + * <p>This method is called when the integrator has accepted a step + * ending exactly on a sign change of the function, just <em>before</em> + * 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.</p> + + * <ul> + * <li>if {@link Action#STOP} is returned, the step handler will be called + * with the <code>isLast</code> flag of the {@link + * org.apache.commons.math3.ode.sampling.StepHandler#handleStep handleStep} + * method set to true and the integration will be stopped,</li> + * <li>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,</li> + * <li>if {@link Action#RESET_DERIVATIVES} is returned, the integrator + * will recompute the derivatives, + * <li>if {@link Action#CONTINUE} is returned, no specific action will + * be taken (apart from having called this method) and integration + * will continue.</li> + * </ul> + + * <p>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 + * <code>handleStep</code> afterwards. This scheduling allows the integrator to + * pass <code>true</code> as the <code>isLast</code> 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 <em>forward</em> integration may call its + * <code>eventOccurred</code> method with t = 10 first and call its + * <code>handleStep</code> 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}.</p> + + * @param t current value of the independent <i>time</i> 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. + + * <p>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.</p> + + * @param t current value of the independent <i>time</i> 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); + +} |