[metapost] luamplib and randomseed

[Dohyun Kim]

2021년 8월 2일 (월) 오전 7:56, Toby Thurston <thurston at eml.cc>님이 작성:
>
> There appear to be a couple of small problems with the implementation of `randomseed` in luamplib.
>
> First one is that the default value appears only to change every minute.
>
> If I run this program through `mpost`
>
>   prologues:=3;outputtemplate:="rs.eps";
>   beginfig(1);
>   label.top(decimal normaldeviate, origin);
>   label.bot(decimal minute, origin);
>   endfig;
>   end
>
> Then I get a different value for the random number every time, and the minute
> value changes every minute (as expected).
>
> But if I run the same program through lualatex — LuaHBTeX, 1.13.0 (TL 2021)
>
>   \documentclass{standalone} \usepackage{luamplib} \begin{document} \begin{mplibcode}
>   beginfig(1);
>   label.top(decimal normaldeviate, origin);
>   label.bot(decimal minute, origin);
>   endfig;
>   \end{mplibcode} \end{document}
>
> then the random number only changes when the minute changes....
>
> Apparently the luamplib version is using the old default randomseed of day + minute of day despite
> the fact that the relevant bit of mp.w says this:
>
>     The global variable |random_seed| was introduced in version 0.9,
>     for the sole reason of stressing the fact that the initial value of the
>     random seed is system-dependant. The initialization code below will initialize
>     this variable to |(internal[mp_time] div unity)+internal[mp_day]|, but this
>     is not good enough on modern fast machines that are capable of running
>     multiple MetaPost processes within the same second.
>
> Could we have the default seed to set to something that is "good enough"?  Or at least something
> that is consistent with `mpost` ?
>

Next version of luamplib will feed random integer values to the
random_seed key of every new mplib instance.
For the time being, you can do it by yourself by declaring as follows:

        randomseed := runscript"mp.print(math.random(4095))";

Best,
-- 
Dohyun Kim
Seoul, Republic of Korea