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.math.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 $Revision: 1067500 $ $Date: 2011-02-05 21:11:30 +0100 (sam. 05 f??vr. 2011) $
047 * @since 1.2
048 */
049
050 public interface EventHandler {
051
052 /** Stop indicator.
053 * <p>This value should be used as the return value of the {@link
054 * #eventOccurred eventOccurred} method when the integration should be
055 * stopped after the event ending the current step.</p>
056 */
057 int STOP = 0;
058
059 /** Reset state indicator.
060 * <p>This value should be used as the return value of the {@link
061 * #eventOccurred eventOccurred} method when the integration should
062 * go on after the event ending the current step, with a new state
063 * vector (which will be retrieved thanks to the {@link #resetState
064 * resetState} method).</p>
065 */
066 int RESET_STATE = 1;
067
068 /** Reset derivatives indicator.
069 * <p>This value should be used as the return value of the {@link
070 * #eventOccurred eventOccurred} method when the integration should
071 * go on after the event ending the current step, with a new derivatives
072 * vector (which will be retrieved thanks to the {@link
073 * org.apache.commons.math.ode.FirstOrderDifferentialEquations#computeDerivatives}
074 * method).</p>
075 */
076 int RESET_DERIVATIVES = 2;
077
078 /** Continue indicator.
079 * <p>This value should be used as the return value of the {@link
080 * #eventOccurred eventOccurred} method when the integration should go
081 * on after the event ending the current step.</p>
082 */
083 int CONTINUE = 3;
084
085 /** Compute the value of the switching function.
086
087 * <p>The discrete events are generated when the sign of this
088 * switching function changes. The integrator will take care to change
089 * the stepsize in such a way these events occur exactly at step boundaries.
090 * The switching function must be continuous in its roots neighborhood
091 * (but not necessarily smooth), as the integrator will need to find its
092 * roots to locate precisely the events.</p>
093 *
094 * @param t current value of the independent <i>time</i> variable
095 * @param y array containing the current value of the state vector
096 * @return value of the g switching function
097 * @exception EventException if the switching function cannot be evaluated
098 */
099 double g(double t, double[] y) throws EventException;
100
101 /** Handle an event and choose what to do next.
102
103 * <p>This method is called when the integrator has accepted a step
104 * ending exactly on a sign change of the function, just <em>before</em>
105 * the step handler itself is called (see below for scheduling). It
106 * allows the user to update his internal data to acknowledge the fact
107 * the event has been handled (for example setting a flag in the {@link
108 * org.apache.commons.math.ode.FirstOrderDifferentialEquations
109 * differential equations} to switch the derivatives computation in
110 * case of discontinuity), or to direct the integrator to either stop
111 * or continue integration, possibly with a reset state or derivatives.</p>
112 *
113 * <ul>
114 * <li>if {@link #STOP} is returned, the step handler will be called
115 * with the <code>isLast</code> flag of the {@link
116 * org.apache.commons.math.ode.sampling.StepHandler#handleStep handleStep}
117 * method set to true and the integration will be stopped,</li>
118 * <li>if {@link #RESET_STATE} is returned, the {@link #resetState
119 * resetState} method will be called once the step handler has
120 * finished its task, and the integrator will also recompute the
121 * derivatives,</li>
122 * <li>if {@link #RESET_DERIVATIVES} is returned, the integrator
123 * will recompute the derivatives,
124 * <li>if {@link #CONTINUE} is returned, no specific action will
125 * be taken (apart from having called this method) and integration
126 * will continue.</li>
127 * </ul>
128 *
129 * <p>The scheduling between this method and the {@link
130 * org.apache.commons.math.ode.sampling.StepHandler StepHandler} method {@link
131 * org.apache.commons.math.ode.sampling.StepHandler#handleStep(
132 * org.apache.commons.math.ode.sampling.StepInterpolator, boolean)
133 * handleStep(interpolator, isLast)} is to call this method first and
134 * <code>handleStep</code> afterwards. This scheduling allows the integrator to
135 * pass <code>true</code> as the <code>isLast</code> parameter to the step
136 * handler to make it aware the step will be the last one if this method
137 * returns {@link #STOP}. As the interpolator may be used to navigate back
138 * throughout the last step (as {@link
139 * org.apache.commons.math.ode.sampling.StepNormalizer StepNormalizer}
140 * does for example), user code called by this method and user
141 * code called by step handlers may experience apparently out of order values
142 * of the independent time variable. As an example, if the same user object
143 * implements both this {@link EventHandler EventHandler} interface and the
144 * {@link org.apache.commons.math.ode.sampling.FixedStepHandler FixedStepHandler}
145 * interface, a <em>forward</em> integration may call its
146 * <code>eventOccurred</code> method with t = 10 first and call its
147 * <code>handleStep</code> method with t = 9 afterwards. Such out of order
148 * calls are limited to the size of the integration step for {@link
149 * org.apache.commons.math.ode.sampling.StepHandler variable step handlers} and
150 * to the size of the fixed step for {@link
151 * org.apache.commons.math.ode.sampling.FixedStepHandler fixed step handlers}.</p>
152 *
153 * @param t current value of the independent <i>time</i> variable
154 * @param y array containing the current value of the state vector
155 * @param increasing if true, the value of the switching function increases
156 * when times increases around event (note that increase is measured with respect
157 * to physical time, not with respect to integration which may go backward in time)
158 * @return indication of what the integrator should do next, this
159 * value must be one of {@link #STOP}, {@link #RESET_STATE},
160 * {@link #RESET_DERIVATIVES} or {@link #CONTINUE}
161 * @exception EventException if the event occurrence triggers an error
162 */
163 int eventOccurred(double t, double[] y, boolean increasing) throws EventException;
164
165 /** Reset the state prior to continue the integration.
166
167 * <p>This method is called after the step handler has returned and
168 * before the next step is started, but only when {@link
169 * #eventOccurred} has itself returned the {@link #RESET_STATE}
170 * indicator. It allows the user to reset the state vector for the
171 * next step, without perturbing the step handler of the finishing
172 * step. If the {@link #eventOccurred} never returns the {@link
173 * #RESET_STATE} indicator, this function will never be called, and it is
174 * safe to leave its body empty.</p>
175 *
176 * @param t current value of the independent <i>time</i> variable
177 * @param y array containing the current value of the state vector
178 * the new state should be put in the same array
179 * @exception EventException if the state cannot be reseted
180 */
181 void resetState(double t, double[] y) throws EventException;
182
183 }