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  /**
21   * This class is used in the junit tests for the ODE integrators.
22  
23   * <p>This specific problem is the following differential equation :
24   * <pre>
25   *    y' = -y
26   * </pre>
27   * the solution of this equation is a simple exponential function :
28   * <pre>
29   *   y (t) = y (t0) exp (t0-t)
30   * </pre>
31   * </p>
32  
33   */
34  class TestProblem1
35    extends TestProblemAbstract {
36  
37    /** theoretical state */
38    private double[] y;
39  
40    /**
41     * Simple constructor.
42     */
43    public TestProblem1() {
44      super();
45      double[] y0 = { 1.0, 0.1 };
46      setInitialConditions(0.0, y0);
47      setFinalConditions(4.0);
48      double[] errorScale = { 1.0, 1.0 };
49      setErrorScale(errorScale);
50      y = new double[y0.length];
51    }
52   
53    /**
54     * Copy constructor.
55     * @param problem problem to copy
56     */
57    public TestProblem1(TestProblem1 problem) {
58      super(problem);
59      y = (double[]) problem.y.clone();
60    }
61  
62    /**
63     * Clone operation.
64     * @return a copy of the instance
65     */
66    public Object clone() {
67      return new TestProblem1(this);
68    }
69  
70    public void doComputeDerivatives(double t, double[] y, double[] yDot) {
71  
72      // compute the derivatives
73      for (int i = 0; i < n; ++i)
74        yDot[i] = -y[i];
75  
76    }
77  
78    public double[] computeTheoreticalState(double t) {
79      double c = Math.exp (t0 - t);
80      for (int i = 0; i < n; ++i) {
81        y[i] = c * y0[i];
82      }
83      return y;
84    }
85  
86  }