    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    }