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.math4.legacy.ode.nonstiff;
19
20 import org.apache.commons.math4.legacy.core.Field;
21 import org.apache.commons.math4.legacy.core.RealFieldElement;
22 import org.apache.commons.math4.legacy.ode.FieldEquationsMapper;
23 import org.apache.commons.math4.legacy.ode.FieldODEStateAndDerivative;
24 import org.apache.commons.math4.legacy.core.MathArrays;
25
26 /**
27 * This class implements a second order Runge-Kutta integrator for
28 * Ordinary Differential Equations.
29 *
30 * <p>This method is an explicit Runge-Kutta method, its Butcher-array
31 * is the following one :
32 * <pre>
33 * 0 | 0 0
34 * 1/2 | 1/2 0
35 * |----------
36 * | 0 1
37 * </pre>
38 *
39 * @see EulerFieldIntegrator
40 * @see ClassicalRungeKuttaFieldIntegrator
41 * @see GillFieldIntegrator
42 * @see ThreeEighthesFieldIntegrator
43 * @see LutherFieldIntegrator
44 *
45 * @param <T> the type of the field elements
46 * @since 3.6
47 */
48
49 public class MidpointFieldIntegrator<T extends RealFieldElement<T>> extends RungeKuttaFieldIntegrator<T> {
50
51 /** Simple constructor.
52 * Build a midpoint integrator with the given step.
53 * @param field field to which the time and state vector elements belong
54 * @param step integration step
55 */
56 public MidpointFieldIntegrator(final Field<T> field, final T step) {
57 super(field, "midpoint", step);
58 }
59
60 /** {@inheritDoc} */
61 @Override
62 public T[] getC() {
63 final T[] c = MathArrays.buildArray(getField(), 1);
64 c[0] = getField().getOne().multiply(0.5);
65 return c;
66 }
67
68 /** {@inheritDoc} */
69 @Override
70 public T[][] getA() {
71 final T[][] a = MathArrays.buildArray(getField(), 1, 1);
72 a[0][0] = fraction(1, 2);
73 return a;
74 }
75
76 /** {@inheritDoc} */
77 @Override
78 public T[] getB() {
79 final T[] b = MathArrays.buildArray(getField(), 2);
80 b[0] = getField().getZero();
81 b[1] = getField().getOne();
82 return b;
83 }
84
85 /** {@inheritDoc} */
86 @Override
87 protected MidpointFieldStepInterpolator<T>
88 createInterpolator(final boolean forward, T[][] yDotK,
89 final FieldODEStateAndDerivative<T> globalPreviousState,
90 final FieldODEStateAndDerivative<T> globalCurrentState,
91 final FieldEquationsMapper<T> mapper) {
92 return new MidpointFieldStepInterpolator<>(getField(), forward, yDotK,
93 globalPreviousState, globalCurrentState,
94 globalPreviousState, globalCurrentState,
95 mapper);
96 }
97 }