% \iffalse meta-comment
%
%% File: latex-lab-marginpar.dtx 
%% 
% Copyright (C) 2024 LaTeX Project
%
% It may be distributed and/or modified under the conditions of the
% LaTeX Project Public License (LPPL), either version 1.3c of this
% license or (at your option) any later version.  The latest version
% of this license is in the file
%
%    https://www.latex-project.org/lppl.txt
%
%
% The development version of the bundle can be found below
%
%    https://github.com/latex3/latex2e/required/latex-lab
%
% for those people who are interested or want to report an issue.
%
\def\ltlabmarginpardate{2024-08-06}
\def\ltlabmarginparversion{0.85c}

%<*driver>
\documentclass[kernel]{l3doc}
\EnableCrossrefs
\CodelineIndex
\begin{document}
  \DocInput{latex-lab-marginpar.dtx}
\end{document}
%</driver>
%
% \fi
%
% \title{The \textsf{latex-lab-marginpar} package\\
% Changes related to the tagging of margin notes}
% \author{\LaTeX{} Project\thanks{Initial implementation done by Ulrike Fischer}}
% \date{v\ltlabmarginparversion\ \ltlabmarginpardate}
%
% \maketitle
%
% \newcommand{\xt}[1]{\textsl{\textsf{#1}}}
% \newcommand{\TODO}[1]{\textbf{[TODO:} #1\textbf{]}}
% \newcommand{\docclass}{document class \marginpar{\raggedright document class
% customizations}}
%
% \providecommand\hook[1]{\texttt{#1\DescribeHook[noprint]{#1}}}
% \providecommand\socket[1]{\texttt{#1\DescribeSocket[noprint]{#1}}}
% \providecommand\plug[1]{\texttt{#1\DescribePlug[noprint]{#1}}}
%
% \NewDocElement[printtype=\textit{socket},idxtype=socket,idxgroup=Sockets]{Socket}{socketdecl}
% \NewDocElement[printtype=\textit{hook},idxtype=hook,idxgroup=Hooks]{Hook}{hookdecl}
% \NewDocElement[printtype=\textit{plug},idxtype=plug,idxgroup=Plugs]{Plug}{plugdecl}
%
% \begin{abstract}
% \end{abstract}
%
% \section{Introduction}
%
% This module contains changes to improve the tagging (in the standard classes) of
% margin notes created with the \cs{marginpar} command. 
%
% Such margin notes are rather small but nevertheless tagging is not trivial and poses
% a number of interesting problems both regarding the structure and the implementation.
% 
% \subsection{Structure}
% \cs{marginpar} creates small boxes in the margin of a page. 
% While they are technically floats they also typically 
% relate to the paragraph beside them, so their structure element should be placed near 
% such a paragraph.
%
% They can be tagged either as artifacts (if they are merely distracting decoration),
% as small headings before the paragraph, or as |Aside|. Unlike the PDF 1.7 fallback |Note|
% the structure |Aside| is not allowed inside |P|,
% so if |Aside| is used, it must be placed before or after the current |P| in the surrounding |text-unit|, 
% or it must split the |P|. Splitting is probably not so good as 
% |\marginpar| is often used somewhere in the middle of sentence. 
% The best default is probably to use |Aside|, find the parent |text-unit| and add it there. 
%
% \subsection{Implementation}
%
% |marginpar| has an optional argument which allows to define a different content 
% on left/right margins for odd/even pages (depending on twoside and \cs{reversemarginpar}).
%
% The current implementation stores *both* arguments in boxes and 
% decides in the output routine which
% one to use. This is quite problematic for tagging, as the unused box produces 
% structure objects, literals, labels, MCID numbers, 
% and perhaps OBJR-objects from links
% which must be later thrown away again (or are thrown away by the engine).
% While it is theoretically not impossible to create
% both boxes while taging is active but to keep everything on hold and insert
% the real structure only when the box is used it is in practice quite difficult,
% slow and error prone. 
% 
% There are a number of options to avoid this hassle.
%
% \begin{description}
% \item[Minimal tagging] One option is to disable tagging when the boxes 
% are built and when the box is used to surround them with a simple Aside.
% 
% \begin{description}
% \item[Pro:] easy to implement
% \item[Contra:] 
% Content that perhaps needed tagging (|\LaTeX| logo with /ActualText, images, links) is not tagged.
% \end{description}
%
% \item[Remaking the box]
% Tagging is deactivated when the boxes are created. 
% The arguments are stored and when the box should be used in the output routine
% (and when it is known which one will be used) the box is (re)created, now with proper tagging.
% 
% Contra: The content can change e.g. if it uses counters or macros that are redefine later before the
% output routine is called. As tagging is done later, it is not trivial to insert the 
% |Aside| in the right place in the paragraph structure.
%
% \item[Two pass compilation]
% A label is used to detect which argument/boxed is used and 
% tagging is activated only for this one.
% 
% Contra: costs a |label| and requires a two pass compilation (which is needed anyway). 
% This should be ok. |\marginnote| uses a label too, and the same
% label could also be used to resolve problems if a margin note 
% moves to a new page (and so replace the mparhack package). 
% \end{description}
% 
% The last option seems the best and is therefore implemented. 
%   
% 
% \section{Implementation}
%    \begin{macrocode}
%<*package>
%<@@=tag>
%    \end{macrocode}
%    \begin{macrocode}
\ProvidesExplPackage {latex-lab-testphase-marginpar} {\ltlabmarginpardate} {\ltlabmarginparversion}
  {Changes related to the tagging of the margin notes}
%    \end{macrocode}
% \subsection{Variables}
% We need a variable to make the label unique. 
% Todo: Not sure about the name.
% \begin{macro}{\g__kernel_marginpar_int}
%    \begin{macrocode}
\int_new:N \g__kernel_marginpar_int
%    \end{macrocode}
% \end{macro}
% 
% \subsection{Tagging sockets}
% 
% \begin{socketdecl}{tagsupport/marginpar/begin,tagsupport/marginpar/end}
%    \begin{macrocode}
\socket_new:nn {tagsupport/marginpar/begin}{0}
\socket_new:nn {tagsupport/marginpar/end}{0}
%    \end{macrocode}
% \end{socketdecl}
% 
% \begin{macro}{\@@_get_marginpar_parent:N,\@@_get_marginpar_parent_aux:nn}
% In a paragraph we must retrieve 
% the structure number of the surrounding Part structure
% TODO: if this is needed in more places this should 
% go into tagpdf.
%    \begin{macrocode}
\cs_new_protected:Npn\@@_get_marginpar_parent_aux:nn #1 #2  
 { 
   \str_if_eq:eeT{\use_ii:nn #1}{Part}{\seq_put_right:Nn\l_@@_tmpa_seq {#2}}
 }
\cs_new_protected:Npn \@@_get_marginpar_parent:N #1
 {
   \seq_clear:N\l_@@_tmpa_seq
   \seq_map_pairwise_function:NNN
     \g_@@_struct_tag_stack_seq
     \g_@@_struct_stack_seq
     \@@_get_marginpar_parent_aux:nn
   \seq_put_right:Nn\l_@@_tmpa_seq {2}
   \seq_get_left:NN \l_@@_tmpa_seq #1 
 }
\cs_generate_variant:Nn \tag_struct_begin:n {e}
%    \end{macrocode}
% \end{macro}
% \begin{plugdecl}{tagsupport/marginpar/begin,tagsupport/marginpar/end}
%    \begin{macrocode}
\socket_new_plug:nnn {tagsupport/marginpar/begin}{default}
  {
   \if_mode_horizontal:
    \tag_mc_end:
    \@@_get_marginpar_parent:N \l_@@_tmpa_tl
    \tag_struct_begin:e{tag=Aside,parent=\l_@@_tmpa_tl}%
   \else: 
    \tag_struct_begin:n{tag=Aside}%
   \fi:
  }
\socket_new_plug:nnn {tagsupport/marginpar/end}{default}
  {
    \tag_struct_end:   
    \if_mode_horizontal: \tag_mc_begin:n{} \fi:
  }
\socket_assign_plug:nn {tagsupport/marginpar/begin}{default}  
\socket_assign_plug:nn {tagsupport/marginpar/end}{default} 
%    \end{macrocode}
% \end{plugdecl}

% \subsection{Kernel command changes}
% \begin{macro}{\@savemarbox }
% We add sockets that add a tagging structure Aside:
%    \begin{macrocode}
\long\def \@savemarbox #1#2{%
  \UseTaggingSocket{marginpar/begin}
  \global\setbox #1%
    \color@vbox
      \vtop{%
        \hsize\marginparwidth
        \@parboxrestore
        \@marginparreset
        #2\par
        \@minipagefalse
        \outer@nobreak
        }%
    \color@endbox
   \UseTaggingSocket{marginpar/end}
}
%    \end{macrocode}
% \end{macro}
% 
% \begin{macro}{\@ympar}
% We must avoid that \cs{@xympar} creates tagging structure:
%    \begin{macrocode}
\long\def\@ympar#1{%
  \@savemarbox\@marbox{#1}%
  \global\setbox\@currbox\copy\@marbox
  \tag_stop:
  \@xympar
  \tag_start:}
%    \end{macrocode}
% \end{macro}
% 
% \begin{macro}{\@xmpar}
% \cs{@xmpar} is the command used if an optional argument is present.
% In this case we must stop tagging for \cs{@xympar} as above
% but also decide which of the two @savemarbox should have tagging active.
% We add a label to \cs{@marbox}. If that exist tagging is activated for this
% box else for the other one.
%    \begin{macrocode}
\long\def\@xmpar[#1]#2{%
  \int_gincr:N\g__kernel_marginpar_int
  \property_if_recorded:eTF { tag_marginpar-opt-\int_use:N\g__kernel_marginpar_int }
    { 
     \@savemarbox\@marbox {#1
       \property_record:ee 
         { tag_marginpar-opt-\int_use:N\g__kernel_marginpar_int }{page} 
       }%
     \tag_stop:
     \@savemarbox\@currbox{#2}%
     \tag_start:
    }
%    \end{macrocode}
% order matters! the tagged box should be first so that it can pick up 
% the correct text-unit number.
%    \begin{macrocode}
    {
     \@savemarbox\@currbox{#2}%         
     \tag_stop:
     \@savemarbox\@marbox{#1
       \property_record:ee 
         { tag_marginpar-opt-\int_use:N\g__kernel_marginpar_int }{page}}
     \tag_start: 
    } 
  \tag_stop:
  \@xympar
  \tag_start:
  }
%</package>
%    \end{macrocode}
% \end{macro}
%
%    \begin{macrocode}
%<*latex-lab>
\ProvidesFile{marginpar-latex-lab-testphase.ltx}
        [\ltlabmarginpardate\space v\ltlabmarginparversion\space
         Changes related to the tagging of the margin notes]

\RequirePackage{latex-lab-testphase-marginpar}

%</latex-lab>
%    \end{macrocode}
