topic Re: UUID generator in Discussions

UUID generator

vince_faller — Mon, 16 Sep 2019 20:13:39 GMT

I'm trying to make a standard guid, but hex() is giving me a weird value. I assume it's because of the encoding hex() uses.

Can anyone give me some clarity on the below script? Or if someone even just has a UUID function that would be even better.

Names default to here(1);
full_h = "123e4567-e89b-12d3-a456-426655440000";
h = substitute(h, "-", "");
n = hex to number(h);
h_again = hex(n);
n_again = hex to number(h);
show(h == h_again, n == n_again);

Thanks

Re: UUID generator

Craige_Hales — Mon, 16 Sep 2019 21:13:11 GMT

It looks like you already have a guid, and are trying to pack it into another representation.

guid = "123e4567-e89b-12d3-a456-426655440000";
guid = substitute(guid,"-","");
blob=hextoblob(guid);
show(hex(blob)); //  "123E4567E89B12D3A456426655440000";

it is not possible to keep the 16 bytes in two 8-byte doubles without hitting missing value issues; the 16 byte blob (or 32 character hex representation) seems reasonable. I don't think JMP exposes a function to create guids; the OS typically forms them from your machine's MAC address, time-of-day, and maybe some other information that should make them nearly unique.

HexToNumber in JSL has a second argument that specifies what sort of encoding the hex represents. I think it may make a special case for exactly 16 hex characters being a double and less than or equal to 8 being a 4-byte integer.

You might find it helpful to load the blob (above) into a matrix of 1-byte unsigned integers if you are trying to disassemble the guid into its components. (blobToMatrix function). You might roll your own guid too, https://superuser.com/questions/892700/is-there-a-way-to-get-only-the-ethernet-mac-address-via-command-prompt-in-window

https://en.wikipedia.org/wiki/Universally_unique_identifier

Re: UUID generator

vince_faller — Mon, 16 Sep 2019 21:23:16 GMT

I wasn't trying to pack it into a different representation. I was actually just trying to test with a random 128 bit number and noticing that two VERY different sized hex strings were coming out to be the same number and was trying to figure out how.

Or maybe more succinctly. Why is this true?

hextonumber("47B23E4567E89B13") == hextonumber("123e4567e89b12d3a456426655440000")

Re: UUID generator

Craige_Hales — Mon, 16 Sep 2019 21:36:34 GMT

OK, if you give it exacly 16 hex digits, it tries to make an IEEE double. Otherwise it builds an integer, probably in the 53-bit part of a double, and if there are more than about 53/4 (13?) hex digits it both loses precision and displays as if it were a double. None of that is useful for what you are trying to do.

Re: UUID generator

vince_faller — Mon, 16 Sep 2019 21:44:20 GMT

haha. I'll have to look that stuff up. Thanks for the help.

I could run under the assumption the user has powershell and just get it through that. I'd have to write a mac equivalent though.

Names default to here(1);
create_uuid = function({}, 
	{DEFAULT LOCAL},
	txt = "";
	if(hostis("Windows"), 
		logtxt = logcapture(
			txt = Run Program(executable("Powershell.exe"), 
				Options( {"[guid]::NewGuid()"} ),
				ReadFunction( "text" )
			);
		);
	, //else 
		throw("I don't think Apple has powershell");
	);
	
	if(txt != "", 
		uuid = regex(txt, "[\w\d]{8}-[\w\d]{4}-[\w\d]{4}-[\w\d]{4}-[\w\d]{12}");
		return(uuid);
	, // elif
		Contains(logtxt, "RunProgram Error"), 
		throw(trim(logtxt));
	, //else
		throw("Couldn't find UUID");
	);
);
create_uuid();

Re: UUID generator

Craige_Hales — Mon, 16 Sep 2019 21:53:43 GMT

And the answer to the question:

2.4249434048109e+37 is a double precision number, represented in hex as the left-hand-side of your question. The huge hex value on the right side is an integer that is approximately the same value. It would need a lot more than 53 or 64 bits to accurately represent it.

Maybe the hex to number function should warn about the loss of precision. @EvanMcCorkle

Re: UUID generator

vince_faller — Mon, 16 Sep 2019 21:51:33 GMT

Ahh, I see. Same reason why

hextonumber("123e4567e89b12d3a456426655440001") == hextonumber("123e4567e89b12d3a456426655440000")

Sorry if this was actually two different questions.

Re: UUID generator

ih — Tue, 17 Sep 2019 13:25:55 GMT

Here is a method to create a UUID:

newuuid = function({},
	//following procedure here: https://www.cryptosys.net/pki/uuid-rfc4122.html
	strout = "";
	//guid made from 16 hex bytes
	for(i=1, i<=16, i++,
		thisint = random integer(256) - 1;
		thisbyte = right(hex(thisint,"integer"),2);
		if( i == 7, //the four most significant bits of the of the 7th byte are 0100
			thisbyte = char(4) || right(thisbyte,1),
			i == 9, //the two most significant bits of the 9th byte are 01
			//decimal to binary
			bin = "";
			value = thisint;
			While(value,
				bin = Char(Modulo(Num(value), 2)) || bin;
				value = Floor(value / 2);
			);
			//fix digits
			bin = right(bin,8,"0");
			bin = "10" || right(bin, 6);
			//back to decimal
			dec = 0;
			While(length(bin) > 0,
				dec = dec + (num(left(bin, 1))*2)^(num(length(bin))-1);
				bin = right(bin, length(bin) - 1);
			);
			//to hex
			thisbyte = right( hex( dec, "integer" ), 2 );
		);
		strout = strout || thisbyte;
	);
	//format output
	substr(strout,1,8) || 
		"-" || substr(strout,9,4) || 
		"-" || substr(strout,13,4)|| 
		"-" || substr(strout,17,4) || 
		"-" || substr(strout,21,12);
);

newuuid();

Disclaimer: I believe I blindly followed this link and I didn't do extensive testing...