## Copyright (C) 2024 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} =} tlspdf (@var{x}, @var{mu}, @var{sigma}, @var{nu})
##
## Location-scale Student's T probability density function (PDF).
##
## For each element of @var{x}, compute the probability density function (PDF)
## of the location-scale Student's T distribution with location parameter
## @var{mu}, scale parameter @var{sigma}, and @var{nu} degrees of freedom.  The
## size of @var{y} is the common size of @var{x}, @var{mu}, @var{sigma}, and
## @var{nu}. A scalar input functions as a constant matrix of the same size as
## the other inputs.
##
## Further information about the location-scale Student's T distribution can be
## found at
## @url{https://en.wikipedia.org/wiki/Student%27s_t-distribution#Location-scale_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{tlscdf, tlsinv, tlsrnd, tlsfit, tlslike, tlsstat}
## @end deftypefn

function y = tlspdf (x, mu, sigma, nu)

  ## Check for valid number of input arguments
  if (nargin < 4)
    error ("tlspdf: function called with too few input arguments.");
  endif

  ## Check for common size of X, MU, SIGMA, and NU
  if (! isscalar (x) || ! isscalar (mu) || ! isscalar (sigma) || ! isscalar (nu))
    [err, x, mu, sigma, nu] = common_size (x, mu, sigma, nu);
    if (err > 0)
      error ("tlspdf: X, MU, SIGMA, and NU must be of common size or scalars.");
    endif
  endif

  ## Check for X, MU, SIGMA, and NU being double or single
  if (! (isfloat (x) && isfloat (mu) && isfloat (sigma) && isfloat (nu)))
    error ("tlspdf: X, MU, SIGMA, and NU must be double or single.");
  endif

  ## Check for X, MU, SIGMA, and NU being reals
  if (iscomplex (x) || iscomplex (mu) || iscomplex (sigma) || iscomplex (nu))
    error ("tlspdf: X, MU, SIGMA, and NU must not be complex.");
  endif

  ## Check for class type
  if (isa (x, 'single') || isa (mu, 'single') ||
      isa (sigma, 'single') || isa (nu, 'single'))
    cls = 'single';
  else
    cls = 'double';
  endif

  ## Force invalid SIGMA parameter to NaN
  sigma(sigma <= 0) = NaN;

  ## Call tpdf to do the work
  y = tpdf ((x - mu) ./ sigma, nu) ./ sigma;

  ## Force class type
  y = cast (y, cls);

endfunction

%!demo
%! ## Plot various PDFs from the Student's T distribution
%! x = -8:0.01:8;
%! y1 = tlspdf (x, 0, 1, 1);
%! y2 = tlspdf (x, 0, 2, 2);
%! y3 = tlspdf (x, 3, 2, 5);
%! y4 = tlspdf (x, -1, 3, Inf);
%! plot (x, y1, '-b', x, y2, '-g', x, y3, '-r', x, y4, '-m')
%! grid on
%! xlim ([-8, 8])
%! ylim ([0, 0.41])
%! legend ({'mu = 0, sigma = 1, nu = 1', 'mu = 0, sigma = 2, nu = 2', ...
%!          'mu = 3, sigma = 2, nu = 5', 'mu = -1, sigma = 3, nu = \infty'}, ...
%!         'location', 'northwest')
%! title ('Location-scale 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 (tlspdf (x, 0, 1, 1), y, 5*eps);
%! assert_equal (tlspdf (x+5, 5, 1, 1), y, 5*eps);
%! assert_equal (tlspdf (x.*2, 0, 2, 1), y./2, 5*eps);
%!shared x, y
%! x = [-Inf 0 0.5 1 Inf];
%! y = 1./(pi * (1 + x.^2));
%!assert_equal (tlspdf (x, 0, 1, ones (1,5)), y, eps)
%!assert_equal (tlspdf (x, 0, 1, 1), y, eps)
%!assert_equal (tlspdf (x, 0, 1, [0 NaN 1 1 1]), [NaN NaN y(3:5)], eps)
%!assert_equal (tlspdf (x, 0, 1, Inf), normpdf (x))

## Test class of input preserved
%!assert_equal (class (tlspdf ([x, NaN], 1, 1, 1)), "double")
%!assert_equal (class (tlspdf (single ([x, NaN]), 1, 1, 1)), "single")
%!assert_equal (class (tlspdf ([x, NaN], single (1), 1, 1)), "single")
%!assert_equal (class (tlspdf ([x, NaN], 1, single (1), 1)), "single")
%!assert_equal (class (tlspdf ([x, NaN], 1, 1, single (1))), "single")

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