/* cheb/integ.c

 * 

 * Copyright (C) 1996, 1997, 1998, 1999, 2000 Gerard Jungman

 * 

 * 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.

 */

#include <config.h>

#include <stdlib.h>

#include <gsl/gsl_math.h>

#include <gsl/gsl_errno.h>

#include <gsl/gsl_chebyshev.h>

int gsl_cheb_calc_integ(gsl_cheb_series * integ, const gsl_cheb_series * f)

{

  const size_t n = f->order + 1;

  const double con = 0.25 * (f->b - f->a);

  if(integ->order != f->order) 

    {

      GSL_ERROR ("order of chebyshev series must be equal", GSL_ENOMEM);

    }

  /* set the other parameters in the chebyshev struct */

  integ->a = f->a;

  integ->b = f->b;

  /* FIXME:  should probably set integ->f[] as well */

  if(n == 1) {

    integ->c[0] = 0.;

  }

  else if(n == 2) {

    integ->c[1] = con * f->c[0];

    integ->c[0] = 2.0 * integ->c[1];

  }

  else {

    double sum = 0.0;

    double fac = 1.0;

    size_t i;

    for(i=1; i<=n-2; i++) {

      integ->c[i] = con * (f->c[i-1] - f->c[i+1])/((double)i);

      sum += fac * integ->c[i];

      fac = -fac;

    }

    integ->c[n-1] = con * f->c[n-2]/(n-1.0);

    sum += fac * integ->c[n-1];

    integ->c[0] = 2.0 * sum;

  }

  return GSL_SUCCESS;

}