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## -*- texinfo -*-
## @deftypefn  {} {@var{hstr} =} dec2hex (@var{d})
## @deftypefnx {} {@var{hstr} =} dec2hex (@var{d}, @var{len})
## Return a string representing the conversion of the integer @var{d} to a
## hexadecimal (base16) number.
##
## If @var{d} is negative, return the hexadecimal equivalent of the two's
## complement binary value of @var{d}.
## If @var{d} is a matrix or cell array, return a string matrix with one row
## for each element in @var{d}, padded with leading zeros to the width of the
## largest value.
##
## The optional second argument, @var{len}, specifies the minimum number of
## digits in the result.
##
## Examples:
##
## @example
## @group
## dec2hex (2748)
##      @result{} "ABC"
##
## dec2hex (-2)
##      @result{} "FE"
## @end group
## @end example
##
## @seealso{hex2dec, dec2base, dec2bin}
## @end deftypefn

function hstr = dec2hex (d, len)

  if (nargin == 0)
    print_usage ();
  endif

  ## To avoid repeating a lot of code, including input validation, we call dec2bin.
  if (nargin == 2)
    d = dec2bin (d, len*4);
  else
    d = dec2bin (d);
  endif

  ## Left-pad with zeros to make the number of columns divisible by 4
  n = mod (columns (d), 4);
  if (n > 0)
    d = [repmat("0", rows(d), 4-n), d];
  endif

  d -= "0"; # convert to numeric
  d = d(:, 1:4:end) * 8 + d(:, 2:4:end) * 4 + d(:, 3:4:end) * 2 + d(:, 4:4:end);
  ## Elements of d are now in the range 0 to 15

  hstr = "0123456789ABCDEF"(d+1); # convert to char and return

endfunction


%!assert (dec2hex (2748), "ABC")
%!assert (dec2hex (2748, 5), "00ABC")
%!assert (dec2hex ([2748, 2746]), ["ABC"; "ABA"])
%!assert (dec2hex ({2748, 2746}), ["ABC"; "ABA"])
%!assert (dec2hex ({2748, 2746}, 4), ["0ABC"; "0ABA"])

## Test negative inputs
%!assert (dec2hex (-3), "FD")
%!assert (dec2hex (-3, 1), "FD")
%!assert (dec2hex (-3, 3), "0FD")
%!assert (dec2hex (-2^7 - 1), "FF7F")
%!assert (dec2hex (-2^15 - 1), "FFFF7FFF")
%!assert (dec2hex (-2^31 - 1), "FFFFFFFF7FFFFFFF")
%!assert (dec2hex (-2^52), "FFF0000000000000")
%!assert (dec2hex (-2^63), "8000000000000000")
%!assert (dec2hex (int64 (-2) ^ 63), "8000000000000000")
%!assert (dec2hex (int64 (-2) ^ 63 - 1), "8000000000000000")
%!assert (dec2hex (int64 (-2) ^ 63 + 1), "8000000000000001")
%!assert (dec2hex ([-1, -2; -3, -4]), ["FF"; "FD"; "FE"; "FC"])
%!assert (dec2hex ([1, 2; 3, -4]), ["01"; "03"; "02"; "FC"])
%!assert (dec2hex ({1, 2; 3, -4}), ["01"; "03"; "02"; "FC"])

## Test input validation
%!error <Invalid call> dec2hex ()