## Copyright (C) 1996-2018 John W. Eaton
## Copyright (C) 2009 VZLU Prague
##
## This file is part of Octave.
##
## Octave 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.
##
## Octave 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 Octave; see the file COPYING.  If not, see
## <https://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn  {} {} ishermitian (@var{A})
## @deftypefnx {} {} ishermitian (@var{A}, @var{tol})
## @deftypefnx {} {} ishermitian (@var{A}, @qcode{"skew"})
## @deftypefnx {} {} ishermitian (@var{A}, @qcode{"skew"}, @var{tol})
## Return true if @var{A} is a Hermitian or skew-Hermitian matrix within the
## tolerance specified by @var{tol}.
##
## The default tolerance is zero (uses faster code).
##
## The type of symmetry to check may be specified with the additional input
## @qcode{"nonskew"} (default) for regular Hermitian or @qcode{"skew"} for
## skew-Hermitian.
##
## Background: A matrix is Hermitian if the complex conjugate transpose of the
## matrix is equal to the original matrix: @w{@acode{@var{A} == @var{A}'}}.  If
## a tolerance is given then the calculation is
## @code{norm (@var{A} - @var{A}', Inf) / norm (@var{A}, Inf) < @var{tol}}.
##
## A matrix is skew-hermitian if the complex conjugate transpose of the matrix
## is equal to the negative of the original matrix:
## @w{@acode{@var{A} == -@var{A}'}}.  If a
## tolerance is given then the calculation is
## @code{norm (@var{A} + @var{A}', Inf) / norm (@var{A}, Inf) < @var{tol}}.
## @seealso{issymmetric, isdefinite}
## @end deftypefn

## Author: A. S. Hodel <scotte@eng.auburn.edu>
## Created: August 1993
## Adapted-By: jwe

function retval = ishermitian (A, skewopt = "nonskew", tol = 0)

  if (nargin < 1 || nargin > 3)
    print_usage ();
  endif

  if (nargin == 2)
    ## Decode whether second argument is skewopt or tol
    if (isnumeric (skewopt))
      tol = skewopt;
      skewopt = "nonskew";
    elseif (! ischar (skewopt))
      error ("ishermitian: second argument must be a non-negative scalar TOL, or one of the strings: 'skew' / 'nonskew'");
    endif 
  endif

  ## Validate inputs
  retval = (isnumeric (A) || islogical (A)) && issquare (A);
  if (! retval)
    return;
  endif

  if (! (strcmp (skewopt, "skew") || strcmp (skewopt, "nonskew")))
    error ("ishermitian: SKEWOPT must be 'skew' or 'nonskew'");
  endif

  if (! (isnumeric (tol) && isscalar (tol) && tol >= 0))
    error ("ishermitian: TOL must be a scalar >= 0");
  endif

  ## Calculate Hermitian-ness
  if (strcmp (skewopt, "nonskew"))
    if (tol == 0)
      ## check for exact symmetry
      retval = ! any ((A != A')(:));
    else
      norm_x = norm (A, Inf);
      retval = norm_x == 0 || norm (A - A', Inf) / norm_x <= tol;
    endif
  else
    ## skew-Hermitian
    if (tol == 0)
      retval = ! any ((A != -A')(:));
    else
      norm_x = norm (A, Inf);
      retval = norm_x == 0 || norm (A + A', Inf) / norm_x <= tol;
    endif
  endif

endfunction


%!assert (ishermitian (1))
%!assert (! ishermitian ([1, 2]))
%!assert (ishermitian ([]))
%!assert (ishermitian ([1, 2; 2, 1]))
%!assert (ishermitian ([1, 2.1; 2, 1.1], 0.2))
%!assert (ishermitian ([1, -2i; 2i, 1]))
%!assert (ishermitian (speye (100)))
%!assert (ishermitian (logical (eye (2))))
%!assert (ishermitian ([0, 2i; 2i, 0], "skew"))
%!assert (! ishermitian ([0, 2; -2, eps], "skew"))
%!assert (ishermitian ([0, 2; -2, eps], "skew", eps))

%!assert (! (ishermitian ("test")))
%!assert (! (ishermitian ("t")))
%!assert (! (ishermitian (["te"; "et"])))
%!assert (! ishermitian ({1}))
%!test
%! s.a = 1;
%! assert (! ishermitian (s));

## Test input validation
%!error ishermitian ()
%!error ishermitian (1,2,3,4)
%!error <second argument must be> ishermitian (1, {"skew"})
%!error <SKEWOPT must be 'skew' or 'nonskew'> ishermitian (1, "foobar")
%!error <SKEWOPT must be 'skew' or 'nonskew'> ishermitian (1, "foobar")
%!error <TOL must be a scalar .= 0> ishermitian (1, "skew", {1})
%!error <TOL must be a scalar .= 0> ishermitian (1, "skew", [1 1])
%!error <TOL must be a scalar .= 0> ishermitian (1, -1)