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    }