/* statistics/quantiles_source.c

 * 

 * Copyright (C) 1996, 1997, 1998, 1999, 2000, 2007 Jim Davies, 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.

 */

double

FUNCTION(gsl_stats,quantile_from_sorted_data) (const BASE sorted_data[], 

                                               const size_t stride,

                                               const size_t n,

                                               const double f)

{

  const double index = f * (n - 1) ;

  const size_t lhs = (int)index ;

  const double delta = index - lhs ;

  double result;

  if (n == 0)

    return 0.0 ;

  if (lhs == n - 1)

    {

      result = sorted_data[lhs * stride] ;

    }

  else 

    {

      result = (1 - delta) * sorted_data[lhs * stride] + delta * sorted_data[(lhs + 1) * stride] ;

    }

  return result ;

}