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## -*- texinfo -*-
## @deftypefn  {} {@var{tf} =} issymmetric (@var{A})
## @deftypefnx {} {@var{tf} =} issymmetric (@var{A}, @var{tol})
## @deftypefnx {} {@var{tf} =} issymmetric (@var{A}, @qcode{"skew"})
## @deftypefnx {} {@var{tf} =} issymmetric (@var{A}, @qcode{"skew"}, @var{tol})
## Return true if @var{A} is a symmetric or skew-symmetric numeric 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 symmetry or @qcode{"skew"} for
## skew-symmetry.
##
## Background: A matrix is symmetric if the transpose of the matrix is equal
## to the original matrix: @w{@tcode{@var{A} == @var{A}.'}}.  If a tolerance
## is given then symmetry is determined by
## @code{norm (@var{A} - @var{A}.', Inf) / norm (@var{A}, Inf) < @var{tol}}.
##
## A matrix is skew-symmetric if the transpose of the matrix is equal to the
## negative of the original matrix: @w{@tcode{@var{A} == -@var{A}.'}}.  If a
## tolerance is given then skew-symmetry is determined by
## @code{norm (@var{A} + @var{A}.', Inf) / norm (@var{A}, Inf) < @var{tol}}.
## @seealso{ishermitian, isdefinite}
## @end deftypefn

function tf = issymmetric (A, skewopt = "nonskew", tol = 0)

  if (nargin < 1)
    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 ("issymmetric: second argument must be a non-negative scalar TOL, or one of the strings: 'skew' / 'nonskew'");
    endif
  endif

  ## Validate inputs
  if (! (isnumeric (A) || islogical (A) || ischar (A)))
    error ("issymmetric: A must be a numeric, logical, or character matrix");
  endif

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

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

  if (! issquare (A))
    tf = false;
    return;
  endif

  ## Calculate symmetry
  if (strcmp (skewopt, "nonskew"))
    if (tol == 0)
      ## check for exact symmetry
      tf = full (! any ((A != A.')(:)));
    else
      if (! isnumeric (A))
        ## Hack to allow norm to work.  Choose single to minimize memory.
        A = single (A);
      endif
      norm_x = norm (A, Inf);
      tf = norm_x == 0 || norm (A - A.', Inf) / norm_x <= tol;
    endif
  else
    ## skew symmetry
    if (tol == 0)
      tf = full (! any ((A != -A.')(:)));
    else
      if (! isnumeric (A))
        ## Hack to allow norm to work.  Choose single to minimize memory.
        A = single (A);
      endif
      norm_x = norm (A, Inf);
      tf = norm_x == 0 || norm (A + A.', Inf) / norm_x <= tol;
    endif
  endif

endfunction


%!assert (issymmetric (1))
%!assert (! issymmetric ([1, 2]))
%!assert (issymmetric ([]))
%!assert (issymmetric ([1, 2; 2, 1]))
%!assert (issymmetric ([1, 2.1; 2, 1.1], 0.2))
%!assert (issymmetric ([1, 2i; 2i, 1]))
%!assert (issymmetric (speye (100)), true)  # Return full logical value.
%!assert (! issymmetric ([0, 2; -2, 0], "nonskew"))
%!assert (issymmetric ([0, 2; -2, 0], "skew"))
%!assert (! issymmetric ([0, 2; -2, eps], "skew"))
%!assert (issymmetric ([0, 2; -2, eps], "skew", eps))
%!assert (issymmetric (logical (eye (2))))
%!assert (! issymmetric (logical ([1 1; 0 1])))
%!assert (issymmetric (logical ([1 1; 0 1]), 0.5))
%!assert (! issymmetric ("test"))
%!assert (issymmetric ("t"))
%!assert (issymmetric (["te"; "et"]))

## Test input validation
%!error <Invalid call> issymmetric ()
%!error <second argument must be> issymmetric (1, {"skew"})
%!error <A must be a numeric,.* matrix> issymmetric ({1})
%!error <SKEWOPT must be 'skew' or 'nonskew'> issymmetric (1, "foobar")
%!error <TOL must be a scalar .= 0> issymmetric (1, "skew", {1})
%!error <TOL must be a scalar .= 0> issymmetric (1, "skew", [1 1])
%!error <TOL must be a scalar .= 0> issymmetric (1, -1)