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## -*- texinfo -*-
## @deftypefn  {} {@var{map} =} prism ()
## @deftypefnx {} {@var{map} =} prism (@var{n})
## Create color colormap.  This colormap cycles through red, orange, yellow,
## green, blue and violet with each index change.
##
## The argument @var{n} must be a scalar.
## If @var{n} is not specified the length of the current colormap is used.  If
## there is no current colormap the default value of 256 is used.
## @seealso{colormap}
## @end deftypefn

function map = prism (n)

  if (nargin == 1)
    if (! (isscalar (n) && isreal (n) && n == fix (n)))
      error ("prism: N must be a scalar integer");
    endif
    n = double (n);
  else
    hf = get (0, "currentfigure");
    if (! isempty (hf))
      n = rows (get (hf, "colormap"));
    else
      n = 256;
    endif
  endif
  if (n == 1)
    map = [1 0 0];
  elseif (n > 1)
    C = [1, 0, 0; 1, 1/2, 0; 1, 1, 0; 0, 1, 0; 0, 0, 1; 2/3, 0, 1];
    map = C(rem (0:(n-1), 6) + 1, :);
  else
    map = zeros (0, 3);
  endif

endfunction


%!demo
%! ## Show the 'prism' colormap profile and as an image
%! cmap = prism (18); # 6 colors, therefore cycle 3 times
%! subplot (2, 1, 1);
%!  rgbplot (cmap, "composite");
%! subplot (2, 1, 2);
%!  rgbplot (cmap);


%!assert (size (prism ()), [256, 3])
%!assert (size (prism (16)), [16, 3])

%!assert (prism (1), [1, 0, 0])
%!assert (prism (true), double ([1, 0, 0]))
%!assert (prism (char (1)), double ([1, 0, 0]))
%!assert (prism (int32 (1)), double ([1, 0, 0]))

%!assert (prism (0), zeros (0, 3))
%!assert (prism (-1), zeros (0, 3))

%!test
%! a = [1, 0, 0; 1, 1/2, 0; 1, 1, 0; 0, 1, 0; 0, 0, 1; 2/3, 0, 1];
%! nr = rows (a);
%! assert (prism (3*nr), [a;a;a]);

## Input validation
%!error <N must be a scalar integer> prism ("foo")
%!error <N must be a scalar integer> prism ([1, 2, 3])
%!error <N must be a scalar integer> prism ({1, 2, 3})