001 /*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements. See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License. You may obtain a copy of the License at
008 *
009 * http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017
018 package org.apache.commons.math3.ode.events;
019
020 /** This interface represents a handler for discrete events triggered
021 * during ODE integration.
022 *
023 * <p>Some events can be triggered at discrete times as an ODE problem
024 * is solved. This occurs for example when the integration process
025 * should be stopped as some state is reached (G-stop facility) when the
026 * precise date is unknown a priori, or when the derivatives have
027 * discontinuities, or simply when the user wants to monitor some
028 * states boundaries crossings.
029 * </p>
030 *
031 * <p>These events are defined as occurring when a <code>g</code>
032 * switching function sign changes.</p>
033 *
034 * <p>Since events are only problem-dependent and are triggered by the
035 * independent <i>time</i> variable and the state vector, they can
036 * occur at virtually any time, unknown in advance. The integrators will
037 * take care to avoid sign changes inside the steps, they will reduce
038 * the step size when such an event is detected in order to put this
039 * event exactly at the end of the current step. This guarantees that
040 * step interpolation (which always has a one step scope) is relevant
041 * even in presence of discontinuities. This is independent from the
042 * stepsize control provided by integrators that monitor the local
043 * error (this event handling feature is available for all integrators,
044 * including fixed step ones).</p>
045 *
046 * @version $Id: EventHandler.java 1416643 2012-12-03 19:37:14Z tn $
047 * @since 1.2
048 */
049
050 public interface EventHandler {
051
052 /** Enumerate for actions to be performed when an event occurs. */
053 public enum Action {
054
055 /** Stop indicator.
056 * <p>This value should be used as the return value of the {@link
057 * #eventOccurred eventOccurred} method when the integration should be
058 * stopped after the event ending the current step.</p>
059 */
060 STOP,
061
062 /** Reset state indicator.
063 * <p>This value should be used as the return value of the {@link
064 * #eventOccurred eventOccurred} method when the integration should
065 * go on after the event ending the current step, with a new state
066 * vector (which will be retrieved thanks to the {@link #resetState
067 * resetState} method).</p>
068 */
069 RESET_STATE,
070
071 /** Reset derivatives indicator.
072 * <p>This value should be used as the return value of the {@link
073 * #eventOccurred eventOccurred} method when the integration should
074 * go on after the event ending the current step, with a new derivatives
075 * vector (which will be retrieved thanks to the {@link
076 * org.apache.commons.math3.ode.FirstOrderDifferentialEquations#computeDerivatives}
077 * method).</p>
078 */
079 RESET_DERIVATIVES,
080
081 /** Continue indicator.
082 * <p>This value should be used as the return value of the {@link
083 * #eventOccurred eventOccurred} method when the integration should go
084 * on after the event ending the current step.</p>
085 */
086 CONTINUE;
087
088 }
089
090 /** Initialize event handler at the start of an ODE integration.
091 * <p>
092 * This method is called once at the start of the integration. It
093 * may be used by the event handler to initialize some internal data
094 * if needed.
095 * </p>
096 * @param t0 start value of the independent <i>time</i> variable
097 * @param y0 array containing the start value of the state vector
098 * @param t target time for the integration
099 */
100 void init(double t0, double[] y0, double t);
101
102 /** Compute the value of the switching function.
103
104 * <p>The discrete events are generated when the sign of this
105 * switching function changes. The integrator will take care to change
106 * the stepsize in such a way these events occur exactly at step boundaries.
107 * The switching function must be continuous in its roots neighborhood
108 * (but not necessarily smooth), as the integrator will need to find its
109 * roots to locate precisely the events.</p>
110
111 * @param t current value of the independent <i>time</i> variable
112 * @param y array containing the current value of the state vector
113 * @return value of the g switching function
114 */
115 double g(double t, double[] y);
116
117 /** Handle an event and choose what to do next.
118
119 * <p>This method is called when the integrator has accepted a step
120 * ending exactly on a sign change of the function, just <em>before</em>
121 * the step handler itself is called (see below for scheduling). It
122 * allows the user to update his internal data to acknowledge the fact
123 * the event has been handled (for example setting a flag in the {@link
124 * org.apache.commons.math3.ode.FirstOrderDifferentialEquations
125 * differential equations} to switch the derivatives computation in
126 * case of discontinuity), or to direct the integrator to either stop
127 * or continue integration, possibly with a reset state or derivatives.</p>
128
129 * <ul>
130 * <li>if {@link Action#STOP} is returned, the step handler will be called
131 * with the <code>isLast</code> flag of the {@link
132 * org.apache.commons.math3.ode.sampling.StepHandler#handleStep handleStep}
133 * method set to true and the integration will be stopped,</li>
134 * <li>if {@link Action#RESET_STATE} is returned, the {@link #resetState
135 * resetState} method will be called once the step handler has
136 * finished its task, and the integrator will also recompute the
137 * derivatives,</li>
138 * <li>if {@link Action#RESET_DERIVATIVES} is returned, the integrator
139 * will recompute the derivatives,
140 * <li>if {@link Action#CONTINUE} is returned, no specific action will
141 * be taken (apart from having called this method) and integration
142 * will continue.</li>
143 * </ul>
144
145 * <p>The scheduling between this method and the {@link
146 * org.apache.commons.math3.ode.sampling.StepHandler StepHandler} method {@link
147 * org.apache.commons.math3.ode.sampling.StepHandler#handleStep(
148 * org.apache.commons.math3.ode.sampling.StepInterpolator, boolean)
149 * handleStep(interpolator, isLast)} is to call this method first and
150 * <code>handleStep</code> afterwards. This scheduling allows the integrator to
151 * pass <code>true</code> as the <code>isLast</code> parameter to the step
152 * handler to make it aware the step will be the last one if this method
153 * returns {@link Action#STOP}. As the interpolator may be used to navigate back
154 * throughout the last step (as {@link
155 * org.apache.commons.math3.ode.sampling.StepNormalizer StepNormalizer}
156 * does for example), user code called by this method and user
157 * code called by step handlers may experience apparently out of order values
158 * of the independent time variable. As an example, if the same user object
159 * implements both this {@link EventHandler EventHandler} interface and the
160 * {@link org.apache.commons.math3.ode.sampling.FixedStepHandler FixedStepHandler}
161 * interface, a <em>forward</em> integration may call its
162 * <code>eventOccurred</code> method with t = 10 first and call its
163 * <code>handleStep</code> method with t = 9 afterwards. Such out of order
164 * calls are limited to the size of the integration step for {@link
165 * org.apache.commons.math3.ode.sampling.StepHandler variable step handlers} and
166 * to the size of the fixed step for {@link
167 * org.apache.commons.math3.ode.sampling.FixedStepHandler fixed step handlers}.</p>
168
169 * @param t current value of the independent <i>time</i> variable
170 * @param y array containing the current value of the state vector
171 * @param increasing if true, the value of the switching function increases
172 * when times increases around event (note that increase is measured with respect
173 * to physical time, not with respect to integration which may go backward in time)
174 * @return indication of what the integrator should do next, this
175 * value must be one of {@link Action#STOP}, {@link Action#RESET_STATE},
176 * {@link Action#RESET_DERIVATIVES} or {@link Action#CONTINUE}
177 */
178 Action eventOccurred(double t, double[] y, boolean increasing);
179
180 /** Reset the state prior to continue the integration.
181
182 * <p>This method is called after the step handler has returned and
183 * before the next step is started, but only when {@link
184 * #eventOccurred} has itself returned the {@link Action#RESET_STATE}
185 * indicator. It allows the user to reset the state vector for the
186 * next step, without perturbing the step handler of the finishing
187 * step. If the {@link #eventOccurred} never returns the {@link
188 * Action#RESET_STATE} indicator, this function will never be called, and it is
189 * safe to leave its body empty.</p>
190
191 * @param t current value of the independent <i>time</i> variable
192 * @param y array containing the current value of the state vector
193 * the new state should be put in the same array
194 */
195 void resetState(double t, double[] y);
196
197 }