## Copyright (C) 2012 Rik Wehbring
## Copyright (C) 1995-2016 Kurt Hornik
## Copyright (C) 2022-2023 Andreas Bertsatos <abertsatos@biol.uoa.gr>
##
## This file is part of the statistics package for GNU Octave.
##
## 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, see <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn {statistics} {@var{p} =} tpdf (@var{x}, @var{df})
##
## Student's T probability density function (PDF).
##
## For each element of @var{x}, compute the probability density function (PDF)
## of the Student's T distribution with @var{df} degrees of freedom.  The size
## of @var{y} is the common size of @var{x} and @var{df}.  A scalar input
## functions as a constant matrix of the same size as the other input.
##
## Further information about the Student's T distribution can be found at
## @url{https://en.wikipedia.org/wiki/Student%27s_t-distribution}
##
## Input arguments must be @qcode{double} or @qcode{single}; integer, logical,
## and character arrays are rejected.  MATLAB accepts a character array and
## evaluates it at the character codes, which Octave deliberately does not,
## since a character array is an integer type and integers are refused too.
##
## @seealso{tcdf, tinv, trnd, tstat}
## @end deftypefn

function y = tpdf (x, df)

  ## Check for valid number of input arguments
  if (nargin < 2)
    error ("tpdf: function called with too few input arguments.");
  endif

  ## Check for common size of X and DF
  if (! isscalar (x) || ! isscalar (df))
    [retval, x, df] = common_size (x, df);
    if (retval > 0)
      error ("tpdf: X and DF must be of common size or scalars.");
    endif
  endif

  ## Check for X and DF being double or single
  if (! (isfloat (x) && isfloat (df)))
    error ("tpdf: X and DF must be double or single.");
  endif

  ## Check for X and DF being reals
  if (iscomplex (x) || iscomplex (df))
    error ("tpdf: X and DF must not be complex.");
  endif

  ## Check for class type
  if (isa (x, 'single') || isa (df, 'single'))
    y = zeros (size (x), 'single');
  else
    y = zeros (size (x));
  endif

  k = isnan (x) | ! (df > 0);
  y(k) = NaN;

  k = isfinite (x) & (df > 0) & (df < Inf);
  kinf = isfinite (x) & isinf (df);
  if (any (k))
    y(k) = exp (- (df(k) + 1) .* log (1 + x(k) .^ 2 ./ df(k)) / 2) ./ ...
           (sqrt (df(k)) .* beta (df(k)/2, 1/2));
  endif
  if (any (kinf))
    y(kinf) = normpdf (x(kinf));
  endif

endfunction

%!demo
%! ## Plot various PDFs from the Student's T distribution
%! x = -5:0.01:5;
%! y1 = tpdf (x, 1);
%! y2 = tpdf (x, 2);
%! y3 = tpdf (x, 5);
%! y4 = tpdf (x, Inf);
%! plot (x, y1, '-b', x, y2, '-g', x, y3, '-r', x, y4, '-m')
%! grid on
%! xlim ([-5, 5])
%! ylim ([0, 0.41])
%! legend ({'df = 1', 'df = 2', ...
%!          'df = 5', 'df = \infty'}, 'location', 'northeast')
%! title ('Student''s T PDF')
%! xlabel ('values in x')
%! ylabel ('density')

## Test output
%!test
%! x = rand (10,1);
%! y = 1./(pi * (1 + x.^2));
%! assert_equal (tpdf (x, 1), y, 5*eps);
%!shared x, y
%! x = [-Inf 0 0.5 1 Inf];
%! y = 1./(pi * (1 + x.^2));
%!assert_equal (tpdf (x, ones (1,5)), y, eps)
%!assert_equal (tpdf (x, 1), y, eps)
%!assert_equal (tpdf (x, [0 NaN 1 1 1]), [NaN NaN y(3:5)], eps)
%!assert_equal (tpdf (x, Inf), normpdf (x))

## Test class of input preserved
%!assert_equal (tpdf ([x, NaN], 1), [y, NaN], eps)
%!assert_equal (tpdf (single ([x, NaN]), 1), single ([y, NaN]), eps ('single'))
%!assert_equal (tpdf ([x, NaN], single (1)), single ([y, NaN]), eps ('single'))

## Test input validation
%!error<tpdf: function called with too few input arguments.> tpdf ()
%!error<tpdf: function called with too few input arguments.> tpdf (1)
%!error<tpdf: X and DF must be of common size or scalars.> ...
%! tpdf (ones (3), ones (2))
%!error<tpdf: X and DF must be of common size or scalars.> ...
%! tpdf (ones (2), ones (3))
%!error<tpdf: X and DF must be double or single.> tpdf (int32 (2), 2)
%!error<tpdf: X and DF must be double or single.> tpdf (true, 2)
%!error<tpdf: X and DF must be double or single.> tpdf ('a', 2)
%!error<tpdf: X and DF must not be complex.> tpdf (i, 2)
%!error<tpdf: X and DF must not be complex.> tpdf (2, i)
