## 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.

## usage:  gamma_pdf (x, a, b)
##
## For each element of x, return the probability density function (PDF)
## at x of the Gamma distribution with parameters a and b.
  
## Author:  TT <Teresa.Twaroch@ci.tuwien.ac.at>
## Description:  PDF of the Gamma distribution

function pdf = gamma_pdf (x, a, b)
  
  if (nargin != 3)
    usage ("gamma_pdf (x, a, b)");
  endif

  [retval, x, a, b] = common_size (x, a, b);
  if (retval > 0)
    error ("gamma_pdf:  x, a and b must be of common size or scalars");
  endif

  [r, c] = size (x);
  s = r * c;
  x   = reshape (x, s, 1);
  a   = reshape (a, s, 1);
  b   = reshape (b, s, 1);
  pdf = zeros (s, 1);
  
  k = find (!(a > 0) | !(b > 0) | isnan (x));
  if any (k)
    pdf (k) = NaN * ones (length (k), 1);
  endif

  k = find ((x > 0) & (a > 0) & (b > 0));
  if any (k)
    pdf (k) = (b(k) .^ a(k)) .* (x(k) .^ (a(k) - 1)) ...
	.* exp(-b(k) .* x(k)) ./ gamma (a(k));
  endif

  pdf = reshape (pdf, r, c);
  
endfunction
