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+/*
+ * 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
+ *
+ * http://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.
+ */
+package org.apache.commons.math3.analysis.integration.gauss;
+
+import org.apache.commons.math3.exception.DimensionMismatchException;
+import org.apache.commons.math3.util.Pair;
+
+/**
+ * Factory that creates Gauss-type quadrature rule using Legendre polynomials.
+ * In this implementation, the lower and upper bounds of the natural interval
+ * of integration are -1 and 1, respectively.
+ * The Legendre polynomials are evaluated using the recurrence relation
+ * presented in <a href="http://en.wikipedia.org/wiki/Abramowitz_and_Stegun">
+ * Abramowitz and Stegun, 1964</a>.
+ *
+ * @since 3.1
+ */
+public class LegendreRuleFactory extends BaseRuleFactory<Double> {
+ /** {@inheritDoc} */
+ @Override
+ protected Pair<Double[], Double[]> computeRule(int numberOfPoints)
+ throws DimensionMismatchException {
+
+ if (numberOfPoints == 1) {
+ // Break recursion.
+ return new Pair<Double[], Double[]>(new Double[] { 0d },
+ new Double[] { 2d });
+ }
+
+ // Get previous rule.
+ // If it has not been computed yet it will trigger a recursive call
+ // to this method.
+ final Double[] previousPoints = getRuleInternal(numberOfPoints - 1).getFirst();
+
+ // Compute next rule.
+ final Double[] points = new Double[numberOfPoints];
+ final Double[] weights = new Double[numberOfPoints];
+
+ // Find i-th root of P[n+1] by bracketing.
+ final int iMax = numberOfPoints / 2;
+ for (int i = 0; i < iMax; i++) {
+ // Lower-bound of the interval.
+ double a = (i == 0) ? -1 : previousPoints[i - 1].doubleValue();
+ // Upper-bound of the interval.
+ double b = (iMax == 1) ? 1 : previousPoints[i].doubleValue();
+ // P[j-1](a)
+ double pma = 1;
+ // P[j](a)
+ double pa = a;
+ // P[j-1](b)
+ double pmb = 1;
+ // P[j](b)
+ double pb = b;
+ for (int j = 1; j < numberOfPoints; j++) {
+ final int two_j_p_1 = 2 * j + 1;
+ final int j_p_1 = j + 1;
+ // P[j+1](a)
+ final double ppa = (two_j_p_1 * a * pa - j * pma) / j_p_1;
+ // P[j+1](b)
+ final double ppb = (two_j_p_1 * b * pb - j * pmb) / j_p_1;
+ pma = pa;
+ pa = ppa;
+ pmb = pb;
+ pb = ppb;
+ }
+ // Now pa = P[n+1](a), and pma = P[n](a) (same holds for b).
+ // Middle of the interval.
+ double c = 0.5 * (a + b);
+ // P[j-1](c)
+ double pmc = 1;
+ // P[j](c)
+ double pc = c;
+ boolean done = false;
+ while (!done) {
+ done = b - a <= Math.ulp(c);
+ pmc = 1;
+ pc = c;
+ for (int j = 1; j < numberOfPoints; j++) {
+ // P[j+1](c)
+ final double ppc = ((2 * j + 1) * c * pc - j * pmc) / (j + 1);
+ pmc = pc;
+ pc = ppc;
+ }
+ // Now pc = P[n+1](c) and pmc = P[n](c).
+ if (!done) {
+ if (pa * pc <= 0) {
+ b = c;
+ pmb = pmc;
+ pb = pc;
+ } else {
+ a = c;
+ pma = pmc;
+ pa = pc;
+ }
+ c = 0.5 * (a + b);
+ }
+ }
+ final double d = numberOfPoints * (pmc - c * pc);
+ final double w = 2 * (1 - c * c) / (d * d);
+
+ points[i] = c;
+ weights[i] = w;
+
+ final int idx = numberOfPoints - i - 1;
+ points[idx] = -c;
+ weights[idx] = w;
+ }
+ // If "numberOfPoints" is odd, 0 is a root.
+ // Note: as written, the test for oddness will work for negative
+ // integers too (although it is not necessary here), preventing
+ // a FindBugs warning.
+ if (numberOfPoints % 2 != 0) {
+ double pmc = 1;
+ for (int j = 1; j < numberOfPoints; j += 2) {
+ pmc = -j * pmc / (j + 1);
+ }
+ final double d = numberOfPoints * pmc;
+ final double w = 2 / (d * d);
+
+ points[iMax] = 0d;
+ weights[iMax] = w;
+ }
+
+ return new Pair<Double[], Double[]>(points, weights);
+ }
+}