How to create random number for a document only on first build then reuse it in subsequent builds?

Question

I want to create unique ID numbers for my documents, and am doing so using a random number generator like this:

\usepackage[first=1, last=100000000]{lcg}
\newcommand{\random}{\rand\arabic{rand}}

However, really I would like to create a random number once on first build of the document and then have the same number used from then on in subsequent builds. Can I do this? and if so, how?

Answer 1

You can save the generated random number to a file named \jobname.rid, then every time you build the document, it reads that file. If you delete the .rid file then a new random number is generated:

\begin{filecontents*}{myclass.cls}
\LoadClass{article}

% ID code
\ExplSyntaxOn
% Create a variable "\c_crobar_id_file_str" that contains the .rid file name:
\str_const:Nx \c_crobar_id_file_str { \c_sys_jobname_str . rid }
% Create a macro "\crobar@set@ID" that assigns its argument to the integer "\c_crobar_random_int":
\cs_new_protected:Npn \crobar@set@ID #1
  { \int_const:Nn \c_crobar_random_int {#1} }
% Check if the .rid file exists:
\file_if_exist:nTF { \c_crobar_id_file_str }
  {
    % If it does, input it:
    \file_input:n { \c_crobar_id_file_str }
  }
  {
    % If the file doesn't exist, create one and assign using \crobar@set@ID
    \crobar@set@ID { \int_rand:nn { 1 } { 100000000 } }
    % Then open the .rid file for writing (iow = IO/Write)
    \iow_open:Nn \g_tmpa_iow { \c_crobar_id_file_str }
    % Now write "\crobar@set@ID{<random number>}" to the .rid file
    \iow_now:Nx \g_tmpa_iow
      { \crobar@set@ID { \int_use:N \c_crobar_random_int } }
    % And close the .rid file
    \iow_close:N \g_tmpa_iow
  }
% Finally, create a user-level command \random that writes the integer \c_crobar_random_int
\cs_new:Npn \random { \int_to_arabic:n { \c_crobar_random_int } }
\ExplSyntaxOff
% ID code
\end{filecontents*}


\documentclass{myclass}
\begin{document}
This document ID is \random.
\end{document}

Running the document on my machine resulted in:

Answer 2

Not an answer, but extending the idea of an ID into the visual space:

Since a glyph in a font is (mapable to) a code-number, setting the randomizer to a Unicode codeblock, say, \int_rand:nn { 19968 } { 40938 }, running it multiple times and typesetting it in an appropriate font, a many-digit ID number can become an ID "phrase":

There are many blocks:

Possible use-case could be to replace GUIDs or larger, representing them in a more compact form.

Answer 3

Try to load \jobname.rid, and if it fails, create a random ID to a command \docID and store its definition in \jobname.rid.

\documentclass{report}

\usepackage[first=1, last=100000000]{lcg}

\IfFileExists{\jobname.rid}
  {\input{\jobname.rid}}
  {\rand
   \def\docID{\arabic{rand}}
   % writing \def\docID{<ID>} to \jobname.rid
   \newwrite\docIDoutput
   \immediate\openout\docIDoutput=\jobname.rid
   \write\docIDoutput{\protect\def\protect\docID{\docID}}}

\begin{document}

ID: \docID

\end{document}

Caution: random seed seams to depend on time up to minute (so it is the same in runs within the same minute).