## Copyright (C) 1995, 1996, 1997  Kurt Hornik
##
## 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, 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 file.  If not, write to the Free Software Foundation,
## 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

## -*- texinfo -*-
## @deftypefn {Function File} {} f_inv (@var{x}, @var{m}, @var{n})
## For each component of @var{x}, compute the quantile (the inverse of
## the CDF) at @var{x} of the F distribution with parameters @var{m} and
## @var{n}.
## @end deftypefn

## Author: KH <Kurt.Hornik@ci.tuwien.ac.at>
## Description: Quantile function of the F distribution

function inv = f_inv (x, m, n)

  if (nargin != 3)
    usage ("f_inv (x, m, n)");
  endif

  [retval, x, m, n] = common_size (x, m, n);
  if (retval > 0)
    error ("f_inv: x, m and n must be of common size or scalar");
  endif

  [r, c] = size (x);
  s = r * c;
  x = reshape (x, 1, s);
  m = reshape (m, 1, s);
  n = reshape (n, 1, s);
  inv = zeros (1, s);

  k = find ((x < 0) | (x > 1) | isnan (x) | !(m > 0) | !(n > 0));
  if (any (k))
    inv(k) = NaN * ones (1, length (k));
  endif

  k = find ((x == 1) & (m > 0) & (n > 0));
  if (any (k))
    inv(k) = Inf * ones (1, length (k));
  endif

  k = find ((x > 0) & (x < 1) & (m > 0) & (n > 0));
  if (any (k))
    inv(k) = ((1 ./ beta_inv (1 - x(k), n(k) / 2, m(k) / 2) - 1)
	      .* n(k) ./ m(k));
  endif

  ## should we really only allow for positive integer m, n?
  k = find ((m != round (m)) | (n != round (n)));
  if (any (k))
    warning ("f_inv: m and n should be positive integers");
    inv(k) = NaN * ones (1, length (k));
  endif

  inv = reshape (inv, r, c);

endfunction