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 018package org.apache.commons.math3.ode.events; 019 020 021/** This interface represents a handler for discrete events triggered 022 * during ODE integration. 023 * 024 * <p>Some events can be triggered at discrete times as an ODE problem 025 * is solved. This occurs for example when the integration process 026 * should be stopped as some state is reached (G-stop facility) when the 027 * precise date is unknown a priori, or when the derivatives have 028 * discontinuities, or simply when the user wants to monitor some 029 * states boundaries crossings. 030 * </p> 031 * 032 * <p>These events are defined as occurring when a <code>g</code> 033 * switching function sign changes.</p> 034 * 035 * <p>Since events are only problem-dependent and are triggered by the 036 * independent <i>time</i> variable and the state vector, they can 037 * occur at virtually any time, unknown in advance. The integrators will 038 * take care to avoid sign changes inside the steps, they will reduce 039 * the step size when such an event is detected in order to put this 040 * event exactly at the end of the current step. This guarantees that 041 * step interpolation (which always has a one step scope) is relevant 042 * even in presence of discontinuities. This is independent from the 043 * stepsize control provided by integrators that monitor the local 044 * error (this event handling feature is available for all integrators, 045 * including fixed step ones).</p> 046 * 047 * @since 1.2 048 */ 049 050public interface EventHandler { 051 052 /** Enumerate for actions to be performed when an event occurs. */ 053 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 * <p>Also note that the integrator expect that once an event has occurred, 111 * the sign of the switching function at the start of the next step (i.e. 112 * just after the event) is the opposite of the sign just before the event. 113 * This consistency between the steps <string>must</strong> be preserved, 114 * otherwise {@link org.apache.commons.math3.exception.NoBracketingException 115 * exceptions} related to root not being bracketed will occur.</p> 116 * <p>This need for consistency is sometimes tricky to achieve. A typical 117 * example is using an event to model a ball bouncing on the floor. The first 118 * idea to represent this would be to have {@code g(t) = h(t)} where h is the 119 * height above the floor at time {@code t}. When {@code g(t)} reaches 0, the 120 * ball is on the floor, so it should bounce and the typical way to do this is 121 * to reverse its vertical velocity. However, this would mean that before the 122 * event {@code g(t)} was decreasing from positive values to 0, and after the 123 * event {@code g(t)} would be increasing from 0 to positive values again. 124 * Consistency is broken here! The solution here is to have {@code g(t) = sign 125 * * h(t)}, where sign is a variable with initial value set to {@code +1}. Each 126 * time {@link #eventOccurred(double, double[], boolean) eventOccurred} is called, 127 * {@code sign} is reset to {@code -sign}. This allows the {@code g(t)} 128 * function to remain continuous (and even smooth) even across events, despite 129 * {@code h(t)} is not. Basically, the event is used to <em>fold</em> {@code h(t)} 130 * at bounce points, and {@code sign} is used to <em>unfold</em> it back, so the 131 * solvers sees a {@code g(t)} function which behaves smoothly even across events.</p> 132 133 * @param t current value of the independent <i>time</i> variable 134 * @param y array containing the current value of the state vector 135 * @return value of the g switching function 136 */ 137 double g(double t, double[] y); 138 139 /** Handle an event and choose what to do next. 140 141 * <p>This method is called when the integrator has accepted a step 142 * ending exactly on a sign change of the function, just <em>before</em> 143 * the step handler itself is called (see below for scheduling). It 144 * allows the user to update his internal data to acknowledge the fact 145 * the event has been handled (for example setting a flag in the {@link 146 * org.apache.commons.math3.ode.FirstOrderDifferentialEquations 147 * differential equations} to switch the derivatives computation in 148 * case of discontinuity), or to direct the integrator to either stop 149 * or continue integration, possibly with a reset state or derivatives.</p> 150 151 * <ul> 152 * <li>if {@link Action#STOP} is returned, the step handler will be called 153 * with the <code>isLast</code> flag of the {@link 154 * org.apache.commons.math3.ode.sampling.StepHandler#handleStep handleStep} 155 * method set to true and the integration will be stopped,</li> 156 * <li>if {@link Action#RESET_STATE} is returned, the {@link #resetState 157 * resetState} method will be called once the step handler has 158 * finished its task, and the integrator will also recompute the 159 * derivatives,</li> 160 * <li>if {@link Action#RESET_DERIVATIVES} is returned, the integrator 161 * will recompute the derivatives, 162 * <li>if {@link Action#CONTINUE} is returned, no specific action will 163 * be taken (apart from having called this method) and integration 164 * will continue.</li> 165 * </ul> 166 167 * <p>The scheduling between this method and the {@link 168 * org.apache.commons.math3.ode.sampling.StepHandler StepHandler} method {@link 169 * org.apache.commons.math3.ode.sampling.StepHandler#handleStep( 170 * org.apache.commons.math3.ode.sampling.StepInterpolator, boolean) 171 * handleStep(interpolator, isLast)} is to call this method first and 172 * <code>handleStep</code> afterwards. This scheduling allows the integrator to 173 * pass <code>true</code> as the <code>isLast</code> parameter to the step 174 * handler to make it aware the step will be the last one if this method 175 * returns {@link Action#STOP}. As the interpolator may be used to navigate back 176 * throughout the last step (as {@link 177 * org.apache.commons.math3.ode.sampling.StepNormalizer StepNormalizer} 178 * does for example), user code called by this method and user 179 * code called by step handlers may experience apparently out of order values 180 * of the independent time variable. As an example, if the same user object 181 * implements both this {@link EventHandler EventHandler} interface and the 182 * {@link org.apache.commons.math3.ode.sampling.FixedStepHandler FixedStepHandler} 183 * interface, a <em>forward</em> integration may call its 184 * <code>eventOccurred</code> method with t = 10 first and call its 185 * <code>handleStep</code> method with t = 9 afterwards. Such out of order 186 * calls are limited to the size of the integration step for {@link 187 * org.apache.commons.math3.ode.sampling.StepHandler variable step handlers} and 188 * to the size of the fixed step for {@link 189 * org.apache.commons.math3.ode.sampling.FixedStepHandler fixed step handlers}.</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 * @param increasing if true, the value of the switching function increases 194 * when times increases around event (note that increase is measured with respect 195 * to physical time, not with respect to integration which may go backward in time) 196 * @return indication of what the integrator should do next, this 197 * value must be one of {@link Action#STOP}, {@link Action#RESET_STATE}, 198 * {@link Action#RESET_DERIVATIVES} or {@link Action#CONTINUE} 199 */ 200 Action eventOccurred(double t, double[] y, boolean increasing); 201 202 /** Reset the state prior to continue the integration. 203 204 * <p>This method is called after the step handler has returned and 205 * before the next step is started, but only when {@link 206 * #eventOccurred} has itself returned the {@link Action#RESET_STATE} 207 * indicator. It allows the user to reset the state vector for the 208 * next step, without perturbing the step handler of the finishing 209 * step. If the {@link #eventOccurred} never returns the {@link 210 * Action#RESET_STATE} indicator, this function will never be called, and it is 211 * safe to leave its body empty.</p> 212 213 * @param t current value of the independent <i>time</i> variable 214 * @param y array containing the current value of the state vector 215 * the new state should be put in the same array 216 */ 217 void resetState(double t, double[] y); 218 219}