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 * https://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 /*
19 * This is not the original file distributed by the Apache Software Foundation
20 * It has been modified by the Hipparchus project
21 */
22
23 package org.hipparchus.migration.ode.events;
24
25 import org.hipparchus.ode.ODEState;
26 import org.hipparchus.ode.ODEStateAndDerivative;
27 import org.hipparchus.ode.events.ODEEventDetector;
28 import org.hipparchus.ode.events.ODEEventHandler;
29
30 /** This interface represents a handler for discrete events triggered
31 * during ODE integration.
32 *
33 * <p>Some events can be triggered at discrete times as an ODE problem
34 * is solved. This occurs for example when the integration process
35 * should be stopped as some state is reached (G-stop facility) when the
36 * precise date is unknown a priori, or when the derivatives have
37 * discontinuities, or simply when the user wants to monitor some
38 * states boundaries crossings.
39 * </p>
40 *
41 * <p>These events are defined as occurring when a <code>g</code>
42 * switching function sign changes.</p>
43 *
44 * <p>Since events are only problem-dependent and are triggered by the
45 * independent <i>time</i> variable and the state vector, they can
46 * occur at virtually any time, unknown in advance. The integrators will
47 * take care to avoid sign changes inside the steps, they will reduce
48 * the step size when such an event is detected in order to put this
49 * event exactly at the end of the current step. This guarantees that
50 * step interpolation (which always has a one step scope) is relevant
51 * even in presence of discontinuities. This is independent from the
52 * stepsize control provided by integrators that monitor the local
53 * error (this event handling feature is available for all integrators,
54 * including fixed step ones).</p>
55 * @deprecated as of 1.0, replaced with {@link ODEEventDetector}
56 */
57 @Deprecated
58 public interface EventHandler extends ODEEventDetector {
59
60 /** {@inheritDoc} */
61 @Override
62 default void init(final ODEStateAndDerivative initialState, final double finalTime) {
63 init(initialState.getTime(), initialState.getPrimaryState(), finalTime);
64 }
65
66 /** {@inheritDoc} */
67 @Override
68 default double g(final ODEStateAndDerivative state) {
69 return g(state.getTime(), state.getPrimaryState());
70 }
71
72 /** {@inheritDoc} */
73 @Override
74 default ODEEventHandler getHandler() {
75 return new ODEEventHandler() {
76 /** {@inheritDoc} */
77 @Override
78 public org.hipparchus.ode.events.Action eventOccurred(ODEStateAndDerivative state, ODEEventDetector detector, boolean increasing) {
79 switch (EventHandler.this.eventOccurred(state.getTime(), state.getPrimaryState(), increasing)) {
80 case CONTINUE:
81 return org.hipparchus.ode.events.Action.CONTINUE;
82 case RESET_DERIVATIVES:
83 return org.hipparchus.ode.events.Action.RESET_DERIVATIVES;
84 case RESET_STATE:
85 return org.hipparchus.ode.events.Action.RESET_STATE;
86 default:
87 return org.hipparchus.ode.events.Action.STOP;
88 }
89 }
90
91 /** {@inheritDoc} */
92 @Override
93 public ODEState resetState(final ODEEventDetector detector, final ODEStateAndDerivative state) {
94 final double t = state.getTime();
95 final double[] y = state.getPrimaryState();
96 EventHandler.this.resetState(t, y);
97 return new ODEState(t, y);
98 }
99 };
100 }
101
102 /** Enumerate for actions to be performed when an event occurs.
103 * @deprecated as of 1.0, replaced with {@link org.hipparchus.ode.events.Action}
104 */
105 @Deprecated
106 enum Action {
107
108 /** Stop indicator.
109 * <p>This value should be used as the return value of the {@link
110 * #eventOccurred eventOccurred} method when the integration should be
111 * stopped after the event ending the current step.</p>
112 */
113 STOP,
114
115 /** Reset state indicator.
116 * <p>This value should be used as the return value of the {@link
117 * #eventOccurred eventOccurred} method when the integration should
118 * go on after the event ending the current step, with a new state
119 * vector (which will be retrieved thanks to the {@link #resetState
120 * resetState} method).</p>
121 */
122 RESET_STATE,
123
124 /** Reset derivatives indicator.
125 * <p>This value should be used as the return value of the {@link
126 * #eventOccurred eventOccurred} method when the integration should
127 * go on after the event ending the current step, with a new derivatives
128 * vector (which will be retrieved thanks to the {@link
129 * org.hipparchus.ode.OrdinaryDifferentialEquation#computeDerivatives}
130 * method).</p>
131 */
132 RESET_DERIVATIVES,
133
134 /** Continue indicator.
135 * <p>This value should be used as the return value of the {@link
136 * #eventOccurred eventOccurred} method when the integration should go
137 * on after the event ending the current step.</p>
138 */
139 CONTINUE;
140
141 }
142
143 /** Initialize event handler at the start of an ODE integration.
144 * <p>
145 * This method is called once at the start of the integration. It
146 * may be used by the event handler to initialize some internal data
147 * if needed.
148 * </p>
149 * @param t0 start value of the independent <i>time</i> variable
150 * @param y0 array containing the start value of the state vector
151 * @param t target time for the integration
152 */
153 void init(double t0, double[] y0, double t);
154
155 /** Compute the value of the switching function.
156
157 * <p>The discrete events are generated when the sign of this
158 * switching function changes. The integrator will take care to change
159 * the stepsize in such a way these events occur exactly at step boundaries.
160 * The switching function must be continuous in its roots neighborhood
161 * (but not necessarily smooth), as the integrator will need to find its
162 * roots to locate precisely the events.</p>
163 * <p>Also note that the integrator expect that once an event has occurred,
164 * the sign of the switching function at the start of the next step (i.e.
165 * just after the event) is the opposite of the sign just before the event.
166 * This consistency between the steps <strong>must</strong> be preserved,
167 * otherwise {@link org.hipparchus.exception.MathIllegalArgumentException
168 * exceptions} related to root not being bracketed will occur.</p>
169 * <p>This need for consistency is sometimes tricky to achieve. A typical
170 * example is using an event to model a ball bouncing on the floor. The first
171 * idea to represent this would be to have {@code g(t) = h(t)} where h is the
172 * height above the floor at time {@code t}. When {@code g(t)} reaches 0, the
173 * ball is on the floor, so it should bounce and the typical way to do this is
174 * to reverse its vertical velocity. However, this would mean that before the
175 * event {@code g(t)} was decreasing from positive values to 0, and after the
176 * event {@code g(t)} would be increasing from 0 to positive values again.
177 * Consistency is broken here! The solution here is to have {@code g(t) = sign
178 * * h(t)}, where sign is a variable with initial value set to {@code +1}. Each
179 * time {@link #eventOccurred(double, double[], boolean) eventOccurred} is called,
180 * {@code sign} is reset to {@code -sign}. This allows the {@code g(t)}
181 * function to remain continuous (and even smooth) even across events, despite
182 * {@code h(t)} is not. Basically, the event is used to <em>fold</em> {@code h(t)}
183 * at bounce points, and {@code sign} is used to <em>unfold</em> it back, so the
184 * solvers sees a {@code g(t)} function which behaves smoothly even across events.</p>
185
186 * @param t current value of the independent <i>time</i> variable
187 * @param y array containing the current value of the state vector
188 * @return value of the g switching function
189 */
190 double g(double t, double[] y);
191
192 /** Handle an event and choose what to do next.
193
194 * <p>This method is called when the integrator has accepted a step
195 * ending exactly on a sign change of the function, just <em>before</em>
196 * the step handler itself is called (see below for scheduling). It
197 * allows the user to update his internal data to acknowledge the fact
198 * the event has been handled (for example setting a flag in the {@link
199 * org.hipparchus.migration.ode.FirstOrderDifferentialEquations
200 * differential equations} to switch the derivatives computation in
201 * case of discontinuity), or to direct the integrator to either stop
202 * or continue integration, possibly with a reset state or derivatives.</p>
203
204 * <ul>
205 * <li>if {@link Action#STOP} is returned, the integration will be stopped,</li>
206 * <li>if {@link Action#RESET_STATE} is returned, the {@link #resetState
207 * resetState} method will be called once the step handler has
208 * finished its task, and the integrator will also recompute the
209 * derivatives,</li>
210 * <li>if {@link Action#RESET_DERIVATIVES} is returned, the integrator
211 * will recompute the derivatives,
212 * <li>if {@link Action#CONTINUE} is returned, no specific action will
213 * be taken (apart from having called this method) and integration
214 * will continue.</li>
215 * </ul>
216
217 * @param t current value of the independent <i>time</i> variable
218 * @param y array containing the current value of the state vector
219 * @param increasing if true, the value of the switching function increases
220 * when times increases around event (note that increase is measured with respect
221 * to physical time, not with respect to integration which may go backward in time)
222 * @return indication of what the integrator should do next, this
223 * value must be one of {@link Action#STOP}, {@link Action#RESET_STATE},
224 * {@link Action#RESET_DERIVATIVES} or {@link Action#CONTINUE}
225 */
226 Action eventOccurred(double t, double[] y, boolean increasing);
227
228 /** Reset the state prior to continue the integration.
229
230 * <p>This method is called after the step handler has returned and
231 * before the next step is started, but only when {@link
232 * #eventOccurred} has itself returned the {@link Action#RESET_STATE}
233 * indicator. It allows the user to reset the state vector for the
234 * next step, without perturbing the step handler of the finishing
235 * step. If the {@link #eventOccurred} never returns the {@link
236 * Action#RESET_STATE} indicator, this function will never be called, and it is
237 * safe to leave its body empty.</p>
238
239 * @param t current value of the independent <i>time</i> variable
240 * @param y array containing the current value of the state vector
241 * the new state should be put in the same array
242 */
243 void resetState(double t, double[] y);
244
245 }