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/* 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;
}
}
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