1 /* 2 * Licensed to the Apache Software Foundation (ASF) under one or more 3 * contributor license agreements. See the NOTICE file distributed with 4 * this work for additional information regarding copyright ownership. 5 * The ASF licenses this file to You under the Apache License, Version 2.0 6 * (the "License"); you may not use this file except in compliance with 7 * the License. You may obtain a copy of the License at 8 * 9 * http://www.apache.org/licenses/LICENSE-2.0 10 * 11 * Unless required by applicable law or agreed to in writing, software 12 * distributed under the License is distributed on an "AS IS" BASIS, 13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 14 * See the License for the specific language governing permissions and 15 * limitations under the License. 16 */ 17 18 package org.apache.commons.math.ode; 19 20 import java.io.Serializable; 21 22 import org.apache.commons.math.FunctionEvaluationException; 23 24 /** This interface represents a switching function. 25 * 26 * <p>A switching function allows to handle discrete events in 27 * integration problems. These events occur for example when the 28 * integration process should be stopped as some value is reached 29 * (G-stop facility), or when the derivatives have 30 * discontinuities, or simply when the user wants to monitor some 31 * states boundaries crossings. These events are traditionally defined 32 * as occurring when a <code>g</code> function sign changes, hence 33 * the name <em>switching functions</em>.</p> 34 * 35 * <p>Since events are only problem-dependent and are triggered by the 36 * independent <i>time</i> variable and the state vector, they can 37 * occur at virtually any time, unknown in advance. The integrators will 38 * take care to avoid sign changes inside the steps, they will reduce 39 * the step size when such an event is detected in order to put this 40 * event exactly at the end of the current step. This guarantees that 41 * step interpolation (which always has a one step scope) is relevant 42 * even in presence of discontinuities. This is independent from the 43 * stepsize control provided by integrators that monitor the local 44 * error (this feature is available on all integrators, including 45 * fixed step ones).</p> 46 * 47 * @version $Revision: 620312 $ $Date: 2008-02-10 12:28:59 -0700 (Sun, 10 Feb 2008) $ 48 * @since 1.2 49 */ 50 51 public interface SwitchingFunction extends Serializable { 52 53 /** Stop indicator. 54 * <p>This value should be used as the return value of the {@link 55 * #eventOccurred eventOccurred} method when the integration should be 56 * stopped after the event ending the current step.</p> 57 */ 58 public static final int STOP = 0; 59 60 /** Reset state indicator. 61 * <p>This value should be used as the return value of the {@link 62 * #eventOccurred eventOccurred} method when the integration should 63 * go on after the event ending the current step, with a new state 64 * vector (which will be retrieved thanks to the {@link #resetState 65 * resetState} method).</p> 66 */ 67 public static final int RESET_STATE = 1; 68 69 /** Reset derivatives indicator. 70 * <p>This value should be used as the return value of the {@link 71 * #eventOccurred eventOccurred} method when the integration should 72 * go on after the event ending the current step, with a new derivatives 73 * vector (which will be retrieved thanks to the {@link 74 * FirstOrderDifferentialEquations#computeDerivatives} method).</p> 75 */ 76 public static final int RESET_DERIVATIVES = 2; 77 78 /** Continue indicator. 79 * <p>This value should be used as the return value of the {@link 80 * #eventOccurred eventOccurred} method when the integration should go 81 * on after the event ending the current step.</p> 82 */ 83 public static final int CONTINUE = 3; 84 85 /** Compute the value of the switching function. 86 87 * <p>Discrete events are generated when the sign of this function 88 * changes, the integrator will take care to change the stepsize in 89 * such a way these events occur exactly at step boundaries. This 90 * function must be continuous (at least in its roots neighborhood), 91 * as the integrator will need to find its roots to locate the events.</p> 92 93 * @param t current value of the independent <i>time</i> variable 94 * @param y array containing the current value of the state vector 95 * @return value of the g function 96 * @exception FunctionEvaluationException if the value of the function 97 * cannot be evaluated 98 */ 99 public double g(double t, double[] y) throws FunctionEvaluationException; 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 before the 105 * step handler itself is called. It allows the user to update his 106 * internal data to acknowledge the fact the event has been handled 107 * (for example setting a flag in the {@link 108 * FirstOrderDifferentialEquations differential equations} to switch 109 * the derivatives computation in case of discontinuity), or to 110 * direct the integrator to either stop or continue integration, 111 * 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 * StepHandler#handleStep handleStep} method set to true and the 117 * 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 * @param t current value of the independent <i>time</i> variable 130 * @param y array containing the current value of the state vector 131 * @return indication of what the integrator should do next, this 132 * value must be one of {@link #STOP}, {@link #RESET_STATE}, 133 * {@link #RESET_DERIVATIVES} or {@link #CONTINUE} 134 */ 135 public int eventOccurred(double t, double[] y); 136 137 /** Reset the state prior to continue the integration. 138 139 * <p>This method is called after the step handler has returned and 140 * before the next step is started, but only when {@link 141 * #eventOccurred} has itself returned the {@link #RESET_STATE} 142 * indicator. It allows the user to reset the state vector for the 143 * next step, without perturbing the step handler of the finishing 144 * step. If the {@link #eventOccurred} never returns the {@link 145 * #RESET_STATE} indicator, this function will never be called, and it is 146 * safe to leave its body empty.</p> 147 148 * @param t current value of the independent <i>time</i> variable 149 * @param y array containing the current value of the state vector 150 * the new state should be put in the same array 151 */ 152 public void resetState(double t, double[] y); 153 154 }