| /* |
| * 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.math4.legacy.ode.events; |
| |
| import org.apache.commons.math4.legacy.core.RealFieldElement; |
| import org.apache.commons.math4.legacy.ode.FieldODEState; |
| import org.apache.commons.math4.legacy.ode.FieldODEStateAndDerivative; |
| |
| /** 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> |
| * |
| * @param <T> the type of the field elements |
| * @since 3.6 |
| */ |
| public interface FieldEventHandler<T extends RealFieldElement<T>> { |
| |
| /** 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 initialState initial time, state vector and derivative |
| * @param finalTime target time for the integration |
| */ |
| void init(FieldODEStateAndDerivative<T> initialState, T finalTime); |
| |
| /** 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 <strong>must</strong> be preserved, |
| * otherwise {@link org.apache.commons.math4.legacy.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(FieldODEStateAndDerivative, boolean) eventOccurred} |
| * method 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 state current value of the independent <i>time</i> variable, state vector |
| * and derivative |
| * @return value of the g switching function |
| */ |
| T g(FieldODEStateAndDerivative<T> state); |
| |
| /** 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.math4.legacy.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.math4.legacy.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.math4.legacy.ode.sampling.FieldStepHandler FieldStepHandler} method {@link |
| * org.apache.commons.math4.legacy.ode.sampling.FieldStepHandler#handleStep( |
| * org.apache.commons.math4.legacy.ode.sampling.FieldStepInterpolator, 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, 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 FieldEventHandler FieldEventHandler} interface and the |
| * {@link org.apache.commons.math4.legacy.ode.sampling.FieldStepHandler FieldStepHandler} |
| * interface, a <em>forward</em> integration may call its |
| * {code eventOccurred} method with t = 10 first and call its |
| * {code 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.math4.legacy.ode.sampling.FieldStepHandler variable step handlers}.</p> |
| |
| * @param state current value of the independent <i>time</i> variable, state vector |
| * and derivative |
| * @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(FieldODEStateAndDerivative<T> state, 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(FieldODEStateAndDerivative, boolean) 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(FieldODEStateAndDerivative, boolean) |
| * 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 state current value of the independent <i>time</i> variable, state vector |
| * and derivative |
| * @return reset state (note that it does not include the derivatives, they will |
| * be added automatically by the integrator afterwards) |
| */ |
| FieldODEState<T> resetState(FieldODEStateAndDerivative<T> state); |
| |
| } |
