/*
    * 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
    *
    *      https://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.
   */
  
  /*
   * This is not the original file distributed by the Apache Software Foundation
   * It has been modified by the Hipparchus project
   */
  
  package org.hipparchus.ode.nonstiff;
  
  import org.hipparchus.analysis.UnivariateFunction;
  import org.hipparchus.analysis.solvers.BracketedUnivariateSolver;
  import org.hipparchus.analysis.solvers.BracketingNthOrderBrentSolver;
  import org.hipparchus.ode.ODEStateAndDerivative;
  import org.hipparchus.ode.OrdinaryDifferentialEquation;
  import org.hipparchus.ode.events.AbstractODEDetector;
  import org.hipparchus.ode.events.Action;
  import org.hipparchus.ode.events.AdaptableInterval;
  import org.hipparchus.ode.events.ODEEventDetector;
  import org.hipparchus.ode.events.ODEEventHandler;
  
  
  public class StepProblem extends AbstractODEDetector<StepProblem> implements OrdinaryDifferentialEquation {
  
      private double rateBefore;
      private double rateAfter;
      private double rate;
      private double switchTime;
  
      public StepProblem(final AdaptableInterval maxCheck, final double threshold, final int maxIter,
                         double rateBefore, double rateAfter, double switchTime) {
          this(maxCheck, maxIter, new BracketingNthOrderBrentSolver(0, threshold, 0, 5),
               new LocalHandler(), rateBefore, rateAfter, switchTime);
      }
  
      private StepProblem(final AdaptableInterval maxCheck, final int maxIter,
                          final BracketedUnivariateSolver<UnivariateFunction> solver,
                          final ODEEventHandler handler,
                          final double rateBefore, final double rateAfter,
                          final double switchTime) {
          super(maxCheck, maxIter, solver, handler);
          this.rateBefore = rateBefore;
          this.rateAfter  = rateAfter;
          this.switchTime = switchTime;
          setRate(rateBefore);
      }
  
      protected StepProblem create(AdaptableInterval newMaxCheck, int newMaxIter,
                                   BracketedUnivariateSolver<UnivariateFunction> newSolver,
                                   ODEEventHandler newHandler) {
          return new StepProblem(newMaxCheck, newMaxIter, newSolver, newHandler,
                                 rateBefore, rateAfter, switchTime);
      }
  
      public double[] computeDerivatives(double t, double[] y) {
          return new double[] {
              rate
          };
      }
  
      public int getDimension() {
          return 1;
      }
  
      public void setRate(double rate) {
          this.rate = rate;
      }
  
      public void init(double t0, double[] y0, double t) {
      }
  
      private static class LocalHandler implements ODEEventHandler {
          public Action eventOccurred(ODEStateAndDerivative state, ODEEventDetector detector, boolean increasing) {
              final StepProblem sp = (StepProblem) detector;
              sp.setRate(sp.rateAfter);
              return Action.RESET_DERIVATIVES;
          }
      }
  
      public double g(ODEStateAndDerivative s) {
          return s.getTime() - switchTime;
      }
  
  }