function str = encodevarname(str,varargin)
%
%    newname = encodevarname(name)
%
%    Encode an invalid variable name using a hex-format for bi-directional
%    conversions. 

%    This function is sensitive to the default charset
%    settings in MATLAB, please call feature('DefaultCharacterSet','utf8')
%    to set the encoding to UTF-8 before calling this function.
%
%    author: Qianqian Fang (q.fang <at> neu.edu)
%
%    input:
%        name: a string, can be either a valid or invalid variable name
%
%    output:
%        newname: a valid variable name by converting the leading non-ascii
%              letter into "x0xHH_" and non-ascii letters into "_0xHH_"
%              format, where HH is the ascii (or Unicode) value of the
%              character.
%
%              if the encoded variable name CAN NOT be longer than 63, i.e. 
%              the maximum variable name specified by namelengthmax, and
%              one uses the output of this function as a struct or variable
%              name, the name will be trucated at 63. Please consider using
%              the name as a containers.Map key, which does not have such
%              limit.
%
%    example:
%        encodevarname('_a')   % returns x0x5F_a
%        encodevarname('a_')   % returns a_ as it is a valid variable name
%        encodevarname('变量')  % returns 'x0xE58F98__0xE9878F_' 
%
%    this file is part of EasyH5 Toolbox: https://github.com/NeuroJSON/easyh5
%
%    License: GPLv3 or 3-clause BSD license, see https://github.com/NeuroJSON/easyh5 for details
%

    if(~isvarname(str(1)))
        if(exist('unicode2native','builtin'))
            str=sprintf('x0x%s_%s',sprintf('%X',unicode2native(str(1))),str(2:end));
        else
            str=sprintf('x0x%X_%s',char(str(1))+0,str(2:end));
        end
    end
    if(isvarname(str))
        return;
    end
    if(exist('unicode2native','builtin'))
        str=regexprep(str,'([^0-9A-Za-z_])','_0x${sprintf(''%X'',unicode2native($1))}_');
    else
        cpos=regexp(str,'[^0-9A-Za-z_]');
        if(isempty(cpos))
            return;
        end
        str0=str;
        pos0=[0 cpos(:)' length(str)];
        str='';
        for i=1:length(cpos)
            str=[str str0(pos0(i)+1:cpos(i)-1) sprintf('_0x%X_',str0(cpos(i))+0)];
        end
        if(cpos(end)~=length(str))
            str=[str str0(pos0(end-1)+1:pos0(end))];
        end
    end
end
