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 * 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 /*
19 * This is not the original file distributed by the Apache Software Foundation
20 * It has been modified by the Hipparchus project
21 */
22 package org.hipparchus.analysis.integration.gauss;
23
24 import org.hipparchus.analysis.UnivariateFunction;
25 import org.hipparchus.exception.MathIllegalArgumentException;
26 import org.hipparchus.util.Pair;
27
28 /**
29 * This class's implements {@link #integrate(UnivariateFunction) integrate}
30 * method assuming that the integral is symmetric about 0.
31 * This allows to reduce numerical errors.
32 *
33 */
34 public class SymmetricGaussIntegrator extends GaussIntegrator {
35 /**
36 * Creates an integrator from the given {@code points} and {@code weights}.
37 * The integration interval is defined by the first and last value of
38 * {@code points} which must be sorted in increasing order.
39 *
40 * @param points Integration points.
41 * @param weights Weights of the corresponding integration nodes.
42 * @throws MathIllegalArgumentException if the {@code points} are not
43 * sorted in increasing order.
44 * @throws MathIllegalArgumentException if points and weights don't have the same length
45 */
46 public SymmetricGaussIntegrator(double[] points,
47 double[] weights)
48 throws MathIllegalArgumentException {
49 super(points, weights);
50 }
51
52 /**
53 * Creates an integrator from the given pair of points (first element of
54 * the pair) and weights (second element of the pair.
55 *
56 * @param pointsAndWeights Integration points and corresponding weights.
57 * @throws MathIllegalArgumentException if the {@code points} are not
58 * sorted in increasing order.
59 *
60 * @see #SymmetricGaussIntegrator(double[], double[])
61 */
62 public SymmetricGaussIntegrator(Pair<double[], double[]> pointsAndWeights)
63 throws MathIllegalArgumentException {
64 this(pointsAndWeights.getFirst(), pointsAndWeights.getSecond());
65 }
66
67 /**
68 * {@inheritDoc}
69 */
70 @Override
71 public double integrate(UnivariateFunction f) {
72 final int ruleLength = getNumberOfPoints();
73
74 if (ruleLength == 1) {
75 return getWeight(0) * f.value(0d);
76 }
77
78 final int iMax = ruleLength / 2;
79 double s = 0;
80 double c = 0;
81 for (int i = 0; i < iMax; i++) {
82 final double p = getPoint(i);
83 final double w = getWeight(i);
84
85 final double f1 = f.value(p);
86 final double f2 = f.value(-p);
87
88 final double y = w * (f1 + f2) - c;
89 final double t = s + y;
90
91 c = (t - s) - y;
92 s = t;
93 }
94
95 if (ruleLength % 2 != 0) {
96 final double w = getWeight(iMax);
97
98 final double y = w * f.value(0d) - c;
99 final double t = s + y;
100
101 s = t;
102 }
103
104 return s;
105 }
106 }