| /* linalg/givens.c |
| * |
| * Copyright (C) 1996, 1997, 1998, 1999, 2000 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 2 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; |
| } |
| } |
