/* linalg/givens.c * * Copyright (C) 1996, 1997, 1998, 1999, 2000, 2007 Gerard Jungman, Brian Gough * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; either version 3 of the License, or (at * your option) any later version. * * This program is distributed in the hope that it will be useful, but * WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU * General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */ /* Generate a Givens rotation (cos,sin) which takes v=(x,y) to (|v|,0) From Golub and Van Loan, "Matrix Computations", Section 5.1.8 */ inline static void create_givens (const double a, const double b, double *c, double *s) { if (b == 0) { *c = 1; *s = 0; } else if (fabs (b) > fabs (a)) { double t = -a / b; double s1 = 1.0 / sqrt (1 + t * t); *s = s1; *c = s1 * t; } else { double t = -b / a; double c1 = 1.0 / sqrt (1 + t * t); *c = c1; *s = c1 * t; } }

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