%D \module
%D   [       file=page-ffl,
%D        version=2018.01.04,
%D          title=\CONTEXT\ Page Macros,
%D       subtitle=Facing floats,
%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 / Facing floats}

%D The code below comes from a module made for Thomas Schmitz and is now part of the
%D core. A simple example is given here:
%D
%D \starttyping
%D \definefacingfloat
%D   [whatever]
%D
%D \setupfacingfloat
%D   [whatever]
%D   [style=bold,
%D    width=frame,
%D    offset=10pt,
%D    color=white]
%D
%D \setupfacingfloat
%D   [whatever:left]
%D   [background=color,
%D    backgroundcolor=red]
%D
%D \setupfacingfloat
%D   [whatever:right]
%D   [background=color,
%D    backgroundcolor=green]
%D
%D \startfacingfloat[whatever]
%D     {\dorecurse{10}{\samplefile{tufte} }}
%D     {\dorecurse{10}{\samplefile{ward}  }}
%D     {\dorecurse{10}{\samplefile{tufte} }}
%D     {\dorecurse{10}{\samplefile{ward}  }}
%D \stopfacingfloat
%D
%D \startfacingfloat[whatever]
%D     \startcontent \dorecurse{10}{\samplefile{tufte} } \stopcontent
%D     \startcontent \dorecurse{10}{\samplefile{ward}  } \stopcontent
%D     \startcontent \dorecurse{10}{\samplefile{tufte} } \stopcontent
%D     \startcontent \dorecurse{10}{\samplefile{ward}  } \stopcontent
%D \stopfacingfloat
%D
%D \flushfacingfloats[whatever]
%D
%D \dorecurse{10}{\samplefile{sapolsky} }
%D
%D \flushpendingtopcontent
%D
%D \stoptyping
%D
%D The idea is to flush related floats more or less in parallel.

\unprotect

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% \starttext
%
% \registertopcontent[3]\vbox{\framed[align=normal,width=\textwidth]{\samplefile{ward}}}
% \registertopcontent[3]\vbox{\framed[align=normal,width=\textwidth]{\samplefile{davis}}}
% \registertopcontent[5]\vbox{\framed[align=normal,width=\textwidth]{\samplefile{ward}}}
%
% \dorecurse{20}{\samplefile{tufte}\par}
%
% \stoptext

\newdimension\d_page_adapts_preroll
\newinteger  \c_page_adapts_pushed

% this assumes a constant textheight and no adaptations otherwise so we
% should block these for already set pages in adaptheight .. or we can
% make these independent (so not use adaptheight)

\def\page_adapts_layout_preroll#1%
  {\begingroup
   \global\d_page_adapts_preroll\zeropoint
   \def\page_adapts_layout_indeed##1%
     {\setupcurrentadaptlayout[\c!top=\zeropoint,##1]%
      \global\d_page_adapts_preroll{\adaptlayoutparameter\c!top}}
   \begincsname\??pageadaptations\number#1\endcsname
   \endgroup}

\permanent\protected\def\registertopcontent[#1]%
  {\begingroup
   \dowithnextbox
     {\page_adapts_layout_preroll{#1}%
      \scratchdimen{%
        \htdp\nextbox+\d_page_adapts_preroll
        \ifzeropt\d_page_adapts_preroll
          +\lineheight
        \fi
      }%
      \putboxincache{\v!page:\number#1}{+}\nextbox
      \expanded{\adaptlayout[\number#1][\c!top=\the\scratchdimen]}%
      \global\advanceby\c_page_adapts_pushed\plusone
      \endgroup}}

\def\page_otr_flush_top_content
  {\scratchcounter\getboxcountfromcache{\v!page:\the\realpageno}\relax
   \ifcase\scratchcounter\else
      \dorecurse\scratchcounter
        {\directboxfromcache{\v!page:\the\realpageno}\recurselevel
         \nointerlineskip
         \par}%
      \disposeboxesincache{\v!page:\the\realpageno}%
      \global\advanceby\c_page_adapts_pushed\minusone
      \nointerlineskip
   \fi}
\def\page_otr_flush_top_content{}

\permanent\protected\def\flushpendingtopcontent
  {\ifcase\c_page_adapts_pushed\else
     \null
     \page
     \expandafter\flushpendingtopcontent
  \fi}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% \let\page_check_weird_page\relax

\def\page_check_weird_page_indeed % for now only when facing floats
  {\ifdim\vsize>\zeropoint\else
      %\showmessage\m!layouts9{}%
      \writestatus\m!layouts{forcing zero height page}%
      \emptyhbox\page
   \fi}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% width:
%
% -- fit       : do nothing
% -- dimension : use that
% -- frame     : use hsize minus frame offsets (based on preroll)

\installcorenamespace {facingfloat}

\installframedcommandhandler \??facingfloat {facingfloat} \??facingfloat

\setupfacingfloat
  [\c!spaceinbetween=\v!big,
   \c!inbetween={\blank[\v!big]},
  %\c!page=\v!left, % not used
  %\c!style,
  %\c!color,
   \c!width=\v!fit]

\newinteger\c_defining_facing_float

\appendtoks
  \advanceby\c_defining_facing_float\plusone
  \ifcase\c_defining_facing_float\or
      \expanded{
        \definefacingfloat[\currentfacingfloat:\v!left ][\currentfacingfloat]%
        \definefacingfloat[\currentfacingfloat:\v!right][\currentfacingfloat]%
      }%
    \fi
  \advanceby\c_defining_facing_float\minusone
\to \everydefinefacingfloat

\newinteger\c_strc_floats_facing_saved
\newinteger\c_strc_floats_facing_flushed
\newbox    \b_strc_floats_facing_l
\newbox    \b_strc_floats_facing_r

\let\m_strc_floats_state\relax

\installcorenamespace {facingfloatflusher}

\def\strc_floats_facing_flush_indeed_step#1#2#3%
  {\ifnum#2<#3\relax
     \advanceby#2\plusone
     \donetrue
     \writestatus
       {facing}
       {page: \the\scratchcounterone, location: #1, blob: \the#2, max: \the#3}%
     \registertopcontent
       [\scratchcounterone]
       \hbox{\directboxfromcache{\currentfacingfloat:#1}{\the#2}}%
   \fi}

\def\strc_floats_facing_flush_indeed#1%
  {\begingroup
   \cdef\currentfacingfloat{#1}%
   \glet\page_check_weird_page\page_check_weird_page_indeed % for now only when facing floats
   \scratchcounterone  \realpageno
   \scratchcounterthree\getboxcountfromcache{\currentfacingfloat:\v!left }\relax
   \scratchcounterfour \getboxcountfromcache{\currentfacingfloat:\v!right}\relax
   \scratchcounterfive \zerocount
   \scratchcountersix  \zerocount
   % find first empty spread i.e. odd (left) and even (right) empty
   \ifdim\pagetotal>\zeropoint
     \advanceby\scratchcounterone\plusone
   \fi
   \ifodd\scratchcounterone
     \advanceby\scratchcounterone\plusone
   \fi
   \writestatus
     {facing}
     {page: \the\scratchcounterone, start checking}%
   \doloop{%
       \page_adapts_layout_preroll\scratchcounterone
       \ifzeropt\d_page_adapts_preroll
         % left empty
         \advanceby\scratchcounterone\plusone
         \page_adapts_layout_preroll\scratchcounterone
         \ifzeropt\d_page_adapts_preroll
           % right empty
           \advanceby\scratchcounterone\minusone
           \exitloop
         \fi
       \else
         \advanceby\scratchcounterone\plustwo
       \fi
   }
   \writestatus
     {facing}
     {page: \the\scratchcounterone, start flushing}%
   \doloop{%
     \ifodd\scratchcounterone
       \strc_floats_facing_flush_indeed_step\v!right\scratchcountersix \scratchcounterfour
     \else
       \strc_floats_facing_flush_indeed_step\v!left \scratchcounterfive\scratchcounterthree
     \fi
     \ifnum\scratchcountersix<\scratchcounterfour
        % more
     \orelse\ifnum\scratchcounterfive<\scratchcounterthree
        % more
     \else
       \exitloop
     \fi
     \advanceby\scratchcounterone\plusone
   }
   \disposeboxesincache{\currentfacingfloat:\v!right}%
   \disposeboxesincache{\currentfacingfloat:\v!left}%
   \page_check_weird_page_indeed
   \endgroup}

\permanent\protected\tolerant\def\flushfacingfloats[#1]%
  {\processcommalist[#1]\strc_floats_facing_flush_indeed}

\protected\def\strc_floats_facing_setup
  {\cdef\currentfacingfloat{\currentfacingfloat:\m_strc_floats_state}%
   \usefacingfloatstyleandcolor\c!style\v!color}

\protected\def\strc_floats_facing_collect
  {\ifx\m_strc_floats_state\v!left
     \ifvoid\nextbox\orelse\ifzeropt\wd\nextbox\else
       \ifvoid\b_strc_floats_facing_l
         \setbox\b_strc_floats_facing_l\box\nextbox
       \else
         \setbox\b_strc_floats_facing_l\vbox\bgroup
           \unvbox\b_strc_floats_facing_l
           \facingfloatparameter\c!inbetween
           \unvbox\nextbox
         \egroup
       \fi
     \fi
     \let\m_strc_floats_state\v!right
   \orelse\ifx\m_strc_floats_state\v!right
     \ifvoid\nextbox\orelse\ifzeropt\wd\nextbox\else
       \ifvoid\b_strc_floats_facing_r
         \setbox\b_strc_floats_facing_r\box\nextbox
       \else
         \setbox\b_strc_floats_facing_r\vbox\bgroup
           \unvbox\b_strc_floats_facing_r
           \facingfloatparameter\c!inbetween
           \unvbox\nextbox
         \egroup
       \fi
     \fi
     \let\m_strc_floats_state\v!left
   \else
     \let\m_strc_floats_state\v!left
   \fi}

\protected\def\strc_floats_facing_handle
  {\doifnextbgroupelse
     \strc_floats_facing_handle_indeed
     \strc_floats_facing_wrap_up}

\protected\def\strc_floats_facing_handle_indeed
  {\dowithnextboxcontent
     \strc_floats_facing_setup
     {\strc_floats_facing_collect\strc_floats_facing_handle}
     \vbox}

\permanent\protected\def\startfacingfloat[#1]%
  {\begingroup
   \cdef\currentfacingfloat{#1}%
   \edef\p_width{\facingfloatparameter\c!width}%
   \letfacingfloatparameter\c!width\v!fit
   \ifx\p_width\v!frame
      \setbox\scratchbox\hpack{\inheritedfacingfloatframed{}}%
      \advanceby\hsize-\wd\scratchbox
   \orelse\ifx\p_width\v!fit
      % whatever
   \else
      \hsize\p_width
   \fi
   \enforced\let\startcontent\bgroup
   \enforced\let\stopcontent\egroup
   \let\m_strc_floats_state\v!left
   \strc_floats_facing_handle}

\permanent\protected\def\stopfacingfloat
  {\endgroup}

\protected\def\strc_floats_facing_wrap_up
  {\edef\p_spaceinbetween{\facingfloatparameter\c!spaceinbetween}%
   \ifempty\p_spaceinbetween
     \scratchdimen\zeropoint
   \else
     \setbox\scratchbox\vbox{\directvspacing\p_spaceinbetween}%
     \scratchdimen\htdp\scratchbox
   \fi
   \ifvoid\b_strc_floats_facing_l\else
     \page_postprocessors_linenumbers_box\b_strc_floats_facing_l
   \fi
   \ifvoid\b_strc_floats_facing_r\else
     \page_postprocessors_linenumbers_box\b_strc_floats_facing_r
   \fi
   \doloop{%
     \strc_floats_facing_flush_wrap\b_strc_floats_facing_l\v!left
     \strc_floats_facing_flush_wrap\b_strc_floats_facing_r\v!right
     \global\advanceby\c_strc_floats_facing_saved\plusone
     \ifvoid\b_strc_floats_facing_l\relax\ifvoid\b_strc_floats_facing_r\relax
       \exitloop
     \fi\fi}}

\def\strc_floats_facing_flush_wrap#1#2%
  {\ifvoid#1\relax
     % todo
   \else
     \begingroup
     \setbox\scratchboxone\hpack\bgroup
       \cdef\currentfacingfloat{\currentfacingfloat:#2}%
       \inheritedfacingfloatframed{\strut}%
     \egroup
     \scratchdimenone{\textheight-\htdp\scratchboxone+\lineheight}%
     \dontcomplain
     \splittopskip\zeroskip
     \setbox\scratchbox\vsplit#1 upto \scratchdimenone
     \setsplitlisthtdp\scratchbox\strutht\strutdp
     \setbox\scratchbox\hpack\bgroup
       \cdef\currentfacingfloat{\currentfacingfloat:#2}%
       \inheritedfacingfloatframed{\box\scratchbox}%
     \egroup
     \setbox\scratchbox\vbox
       \ifdim\ht\scratchbox<{\textheight-\scratchdimen}%
         {\box\scratchbox\directvspacing\p_spaceinbetween}%
       \else
         to \textheight{\box\scratchbox\vss}%
       \fi
     \putboxincache{\currentfacingfloat:#2}{+}\scratchbox
     \endgroup
   \fi}

\protect \endinput