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zsolve_quadratic.c

/* poly/zsolve_quadratic.c
 * 
 * Copyright (C) 1996, 1997, 1998, 1999, 2000 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.
 */

/* complex_solve_quadratic.c - finds complex roots of a x^2 + b x + c = 0 */

#include <config.h>
#include <math.h>
#include <gsl/gsl_complex.h>
#include <gsl/gsl_poly.h>

int
gsl_poly_complex_solve_quadratic (double a, double b, double c,
                                  gsl_complex *z0, gsl_complex *z1)
{
  double disc = b * b - 4 * a * c;

  if (a == 0) /* Handle linear case */
    {
      if (b == 0)
        {
          return 0;
        }
      else
        {
          GSL_REAL(*z0) = -c / b;
          GSL_IMAG(*z0) = 0;
          return 1;
        };
    }

  if (disc > 0)
    {
      if (b == 0)
        {
          double s = fabs (0.5 * sqrt (disc) / a);
          GSL_REAL (*z0) = -s;
          GSL_IMAG (*z0) = 0;
          GSL_REAL (*z1) = s;
          GSL_IMAG (*z1) = 0;
        }
      else
        {
          double sgnb = (b > 0 ? 1 : -1);
          double temp = -0.5 * (b + sgnb * sqrt (disc));
          double r1 = temp / a;
          double r2 = c / temp;

          if (r1 < r2)
            {
              GSL_REAL (*z0) = r1;
              GSL_IMAG (*z0) = 0;
              GSL_REAL (*z1) = r2;
              GSL_IMAG (*z1) = 0;
            }
          else
            {
              GSL_REAL (*z0) = r2;
              GSL_IMAG (*z0) = 0;
              GSL_REAL (*z1) = r1;
              GSL_IMAG (*z1) = 0;
            }
        }
      return 2;
    }
  else if (disc == 0)
    {
      GSL_REAL (*z0) = -0.5 * b / a;
      GSL_IMAG (*z0) = 0;

      GSL_REAL (*z1) = -0.5 * b / a;
      GSL_IMAG (*z1) = 0;

      return 2;
    }
  else
    {
      double s = fabs (0.5 * sqrt (-disc) / a);

      GSL_REAL (*z0) = -0.5 * b / a;
      GSL_IMAG (*z0) = -s;

      GSL_REAL (*z1) = -0.5 * b / a;
      GSL_IMAG (*z1) = s;

      return 2;
    }
}

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