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1   /*
2    * Licensed to the Hipparchus project 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 Hipparchus project 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  package org.hipparchus.ode.nonstiff.interpolators;
19  
20  import org.hipparchus.ode.EquationsMapper;
21  import org.hipparchus.ode.ODEStateAndDerivative;
22  import org.hipparchus.ode.nonstiff.EulerIntegrator;
23  
24  /**
25   * This class implements a linear interpolator for step.
26   *
27   * <p>This interpolator computes dense output inside the last
28   * step computed. The interpolation equation is consistent with the
29   * integration scheme :</p>
30   * <ul>
31   *   <li>Using reference point at step start:<br>
32   *     y(t<sub>n</sub> + &theta; h) = y (t<sub>n</sub>) + &theta; h y'
33   *   </li>
34   *   <li>Using reference point at step end:<br>
35   *     y(t<sub>n</sub> + &theta; h) = y (t<sub>n</sub> + h) - (1-&theta;) h y'
36   *   </li>
37   * </ul>
38   *
39   * <p>where &theta; belongs to [0 ; 1] and where y' is the evaluation of
40   * the derivatives already computed during the step.</p>
41   *
42   * @see EulerIntegrator
43   */
44  
45  public class EulerStateInterpolator extends RungeKuttaStateInterpolator {
46  
47      /** Serializable version identifier. */
48      private static final long serialVersionUID = 20160328L;
49  
50      /** Simple constructor.
51       * @param forward integration direction indicator
52       * @param yDotK slopes at the intermediate points
53       * @param globalPreviousState start of the global step
54       * @param globalCurrentState end of the global step
55       * @param softPreviousState start of the restricted step
56       * @param softCurrentState end of the restricted step
57       * @param mapper equations mapper for the all equations
58       */
59      public EulerStateInterpolator(final boolean forward,
60                                    final double[][] yDotK,
61                                    final ODEStateAndDerivative globalPreviousState,
62                                    final ODEStateAndDerivative globalCurrentState,
63                                    final ODEStateAndDerivative softPreviousState,
64                                    final ODEStateAndDerivative softCurrentState,
65                                    final EquationsMapper mapper) {
66          super(forward, yDotK, globalPreviousState, globalCurrentState, softPreviousState, softCurrentState, mapper);
67      }
68  
69      /** {@inheritDoc} */
70      @Override
71      protected EulerStateInterpolator create(final boolean newForward, final double[][] newYDotK,
72                                              final ODEStateAndDerivative newGlobalPreviousState,
73                                              final ODEStateAndDerivative newGlobalCurrentState,
74                                              final ODEStateAndDerivative newSoftPreviousState,
75                                              final ODEStateAndDerivative newSoftCurrentState,
76                                              final EquationsMapper newMapper) {
77          return new EulerStateInterpolator(newForward, newYDotK,
78                                            newGlobalPreviousState, newGlobalCurrentState,
79                                            newSoftPreviousState, newSoftCurrentState,
80                                            newMapper);
81      }
82  
83      /** {@inheritDoc} */
84      @Override
85      protected ODEStateAndDerivative computeInterpolatedStateAndDerivatives(final EquationsMapper mapper,
86                                                                             final double time, final double theta,
87                                                                             final double thetaH, final double oneMinusThetaH) {
88          final double[] interpolatedState;
89          if (getGlobalPreviousState() != null && theta <= 0.5) {
90              interpolatedState       = previousStateLinearCombination(thetaH);
91          } else {
92              interpolatedState       = currentStateLinearCombination(-oneMinusThetaH);
93          }
94          final double[] interpolatedDerivatives = derivativeLinearCombination(1.0);
95  
96          return mapper.mapStateAndDerivative(time, interpolatedState, interpolatedDerivatives);
97  
98      }
99  
100 }