%D \module
%D   [       file=page-spr,
%D        version=2002.11.11,
%D          title=\CONTEXT\ Page Macros,
%D       subtitle=Spreading,
%D         author=Hans Hagen,
%D           date=\currentdate,
%D      copyright={PRAGMA ADE \& \CONTEXT\ Development Team}]
%C
%C This module is part of the \CONTEXT\ macro||package and is
%C therefore copyrighted by \PRAGMA. See mreadme.pdf for
%C details.

\writestatus{loading}{ConTeXt Page Macros / Spreading}

% This module is experimental and not yet official!

\unprotect

\newbox        \b_page_spread_content
\newconditional\c_page_spread_busy
\newconditional\c_page_spread_once     % when true only one flush (writes etc)

\permanent\overloaded\protected\def\normalsettextpagecontent#1#2#3% #2 and #3 will disappear
  {\setbox#1\hpack
     {\setlayoutcomponentattribute{\v!page:\v!text}%
      \vpack \layoutcomponentboxattribute to \textheight
        {\offinterlineskip
         \freezetextwidth
         \hsize\textwidth   % local variant of \sethsize
         \boxmaxdepth\maxdepth
         \page_otr_flush_top_content
         \noindent          % content can be < \hsize
         \page_otr_command_package_contents#2#3}}%
   \dp#1\zeropoint
   \ifconditional\c_page_spread_busy
      \normalsettextpagecontent_spread{#1}%
   \else
      \normalsettextpagecontent_normal{#1}%
   \fi}

\def\normalsettextpagecontent_normal#1%
  {\ifdim\wd#1=\makeupwidth\else
     \setbox#1\hpack to \makeupwidth
       {\hss\box#1\hss}% never change the \hss's
   \fi}

\def\normalsettextpagecontent_spread#1%
  {\setbox#1\hpack to \makeupwidth
     {\ifvoid\b_page_spread_content
        \ifconditional\c_page_spread_once
          \box#1%
        \else
          \global\setbox\b_page_spread_content\box#1%
          \copy\b_page_spread_content
        \fi
        \hss % left page
      \else
        \hss % right page
        \ifarrangingpages
          % writes don't matter anyway
        \orelse\ifconditional\c_page_spread_once
          \global\setbox\b_page_spread_content\emptyhbox
          \wd\b_page_spread_content\makeupwidth
          \ht\b_page_spread_content\textheight
        \else
          % writes can interfere (todo: write stripper, but how about hyperlinks)
        \fi
        \box\b_page_spread_content
      \fi}}

\protected\def\page_spread_flush
  {\ifconditional\c_page_spread_busy \ifvoid\b_page_spread_content\else
     % this page will be discarded later
     \emptyhbox \page
   \fi \fi }

\permanent\protected\def\startspread
  {\unless\ifdoublesided
     \expandafter\page_spread_start_nop
   \orelse\ifconditional\c_page_spread_busy
     \expandafter\page_spread_start_nop
   \else
     \expandafter\page_spread_start_yes
   \fi}

\aliased\let\stopspread\relax

\protected\def\page_spread_start_yes
  {\bgroup
   \page[\v!left]%
  %\setsystemmode{spread}%
   \c_page_spread_busy\conditionaltrue
   \normalexpanded{\setuplayout[\c!textwidth=\the\dimexpr2\textwidth+2\backspace\relax]}%
   \enforced\let\stopspread\page_spread_stop_yes}

\permanent\protected\def\page_spread_stop_yes
  {\kern\zeropoint
   \page
   \setuplayout[\c!textwidth=\textwidth]%
   \page[\v!left]%
   \egroup}

\protected\def\page_spread_start_nop
  {\bgroup
   \let\stopspread\page_spread_stop_nop}

\protected\def\page_spread_stop_nop
  {\egroup}

\protect \endinput

% texexec --arr --pdf test
%
% \setuplayout[width=middle]
% \setuppapersize[A4][A3,landscape]
% \setuppagenumbering[alternative=doublesided]
% \setuparranging[2UP]
%
% \starttext
%
% \dorecurse{3}{\input tufte }
%
% \startspread
%   \dorecurse{10}{\input tufte }
% \stopspread
%
% \dorecurse{3}{\input tufte }
%
% \stoptext