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 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.commons.math.ode.events;

import org.apache.commons.math.ode.FirstOrderDifferentialEquations;
import org.apache.commons.math.ode.sampling.StepHandler;

/** This interface represents a handler for discrete events triggered
 * during ODE integration.
 *
 * <p>Some events can be triggered at discrete times as an ODE problem
 * is solved. These occurs for example when the integration process
 * should be stopped as some state is reached (G-stop facility) when the
 * precise date is unknown a priori, or when the derivatives have
 * discontinuities, or simply when the user wants to monitor some
 * states boundaries crossings.
 * </p>
 * 
 * <p>These events are defined as occurring when a <code>g</code>
 * switching function sign changes.</p>
 *
 * <p>Since events are only problem-dependent and are triggered by the
 * independent <i>time</i> variable and the state vector, they can
 * occur at virtually any time, unknown in advance. The integrators will
 * take care to avoid sign changes inside the steps, they will reduce
 * the step size when such an event is detected in order to put this
 * event exactly at the end of the current step. This guarantees that
 * step interpolation (which always has a one step scope) is relevant
 * even in presence of discontinuities. This is independent from the
 * stepsize control provided by integrators that monitor the local
 * error (this event handling feature is available for all integrators,
 * including fixed step ones).</p>
 *
 * @version $Revision$ $Date$
 * @since 1.2
 */

public interface EventHandler  {

  /** Stop indicator.
   * <p>This value should be used as the return value of the {@link
   * #eventOccurred eventOccurred} method when the integration should be
   * stopped after the event ending the current step.</p>
   */
  public static final int STOP = 0;

  /** Reset state indicator.
   * <p>This value should be used as the return value of the {@link
   * #eventOccurred eventOccurred} method when the integration should
   * go on after the event ending the current step, with a new state
   * vector (which will be retrieved thanks to the {@link #resetState
   * resetState} method).</p>
   */
  public static final int RESET_STATE = 1;

  /** Reset derivatives indicator.
   * <p>This value should be used as the return value of the {@link
   * #eventOccurred eventOccurred} method when the integration should
   * go on after the event ending the current step, with a new derivatives
   * vector (which will be retrieved thanks to the {@link
   * FirstOrderDifferentialEquations#computeDerivatives} method).</p>
   */
  public static final int RESET_DERIVATIVES = 2;

  /** Continue indicator.
   * <p>This value should be used as the return value of the {@link
   * #eventOccurred eventOccurred} method when the integration should go
   * on after the event ending the current step.</p>
   */
  public static final int CONTINUE = 3;

  /** Compute the value of the switching function.

   * <p>The discrete events are generated when the sign of this 
   * switching function changes. The integrator will take care to change
   * the stepsize in such a way these events occur exactly at step boundaries.
   * The switching function must be continuous in its roots neighborhood
   * (but not necessarily smooth), as the integrator will need to find its
   * roots to locate precisely the events.</p>

   * @param t current value of the independent <i>time</i> variable
   * @param y array containing the current value of the state vector
   * @return value of the g switching function
   * @exception EventException if the switching function cannot be evaluated
   */
  public double g(double t, double[] y) throws EventException;

  /** Handle an event and choose what to do next.

   * <p>This method is called when the integrator has accepted a step
   * ending exactly on a sign change of the function, just before the
   * step handler itself is called. It allows the user to update his
   * internal data to acknowledge the fact the event has been handled
   * (for example setting a flag in the {@link
   * FirstOrderDifferentialEquations differential equations} to switch
   * the derivatives computation in case of discontinuity), or to
   * direct the integrator to either stop or continue integration,
   * possibly with a reset state or derivatives.</p>

   * <ul>
   *   <li>if {@link #STOP} is returned, the step handler will be called
   *   with the <code>isLast</code> flag of the {@link
   *   StepHandler#handleStep handleStep} method set to true and the
   *   integration will be stopped,</li>
   *   <li>if {@link #RESET_STATE} is returned, the {@link #resetState
   *   resetState} method will be called once the step handler has
   *   finished its task, and the integrator will also recompute the
   *   derivatives,</li>
   *   <li>if {@link #RESET_DERIVATIVES} is returned, the integrator
   *   will recompute the derivatives,
   *   <li>if {@link #CONTINUE} is returned, no specific action will
   *   be taken (apart from having called this method) and integration
   *   will continue.</li>
   * </ul>

   * @param t current value of the independent <i>time</i> variable
   * @param y array containing the current value of the state vector
   * @param increasing if true, the value of the switching function increases
   * when times increases around event (note that increase is measured with respect
   * to physical time, not with respect to integration which may go backward in time)
   * @return indication of what the integrator should do next, this
   * value must be one of {@link #STOP}, {@link #RESET_STATE},
   * {@link #RESET_DERIVATIVES} or {@link #CONTINUE}
   * @exception EventException if the event occurrence triggers an error
   */
  public int eventOccurred(double t, double[] y, boolean increasing) throws EventException;
  
  /** Reset the state prior to continue the integration.

   * <p>This method is called after the step handler has returned and
   * before the next step is started, but only when {@link
   * #eventOccurred} has itself returned the {@link #RESET_STATE}
   * indicator. It allows the user to reset the state vector for the
   * next step, without perturbing the step handler of the finishing
   * step. If the {@link #eventOccurred} never returns the {@link
   * #RESET_STATE} indicator, this function will never be called, and it is
   * safe to leave its body empty.</p>

   * @param t current value of the independent <i>time</i> variable
   * @param y array containing the current value of the state vector
   * the new state should be put in the same array
   * @exception EventException if the state cannot be reseted
   */
  public void resetState(double t, double[] y) throws EventException;

}
