%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    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 \dorecurse{10}{\samplefile{sapolsky} }
%D \stoptyping
%D
%D The idea is to flush related floats more or less in parallel.

\unprotect

\installcorenamespace {facingfloat}

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

\setupfacingfloat
  [\c!spaceinbetween=\v!big,
   \c!inbetween={\blank[\v!big]},
  %\c!style,
  %\c!color,
   \c!page=\v!yes]

\appendtoks
    \ifx\currentfacingfloatparent\empty
        \normalexpanded{\definefacingfloat[\currentfacingfloat:\v!left ][\currentfacingfloat]}%
        \normalexpanded{\definefacingfloat[\currentfacingfloat:\v!right][\currentfacingfloat]}%
    \fi
\to \everydefinefacingfloat

\newcount\c_strc_floats_saved
\newcount\c_strc_floats_flushed

\newbox\b_strc_floats_facing_l
\newbox\b_strc_floats_facing_r

\let\m_strc_floats_state\relax

\def\strc_floats_facing_flush
  {\ifnum\c_strc_floats_flushed<\c_strc_floats_saved
     \strc_floats_facing_flush_indeed % less tracing
   \fi}

\def\strc_floats_facing_flush_indeed
  {\global\advance\c_strc_floats_flushed\plusone
   \floatingpenalty\zerocount
   \insert\namedinsertionnumber\s!topfloat\bgroup
     \forgetall
     \ifconditional\c_page_one_top_of_insert
       \ifconditional\c_page_one_correct_top_insert
         \topskipcorrection % [xx] new: see icare topbleed
         \kern-\lineskip
         \par
         \prevdepth\maxdimen
       \fi
     \fi
     \directboxfromcache{\currentfacingfloat}{\number\c_strc_floats_flushed}%
     \vkern\s_page_one_between_top_insert
   \egroup
   \ifnum\c_strc_floats_saved=\c_strc_floats_flushed
     \global\c_strc_floats_saved  \zerocount
     \global\c_strc_floats_flushed\zerocount
     \resetboxesincache{\currentfacingfloat}%
   \fi}

\unexpanded\def\strc_floats_facing_setup
  {\edef\currentfacingfloat{\currentfacingfloat:\m_strc_floats_state}%
   \usefacingfloatstyleandcolor\c!style\v!color}

\unexpanded\def\strc_floats_facing_collect
  {\ifx\m_strc_floats_state\v!left
     \ifvoid\nextbox\else\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\fi
     \let\m_strc_floats_state\v!right
   \else\ifx\m_strc_floats_state\v!right
     \ifvoid\nextbox\else\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\fi
     \let\m_strc_floats_state\v!left
   \else
     \let\m_strc_floats_state\v!left
   \fi\fi}

\unexpanded\def\strc_floats_facing_handle
  {\strc_floats_facing_collect
   \doifnextbgroupelse
     \strc_floats_facing_handle_indeed
     \strc_floats_wrap_up}

\unexpanded\def\strc_floats_facing_handle_indeed
  {\dowithnextboxcontent
     \strc_floats_facing_setup
     \strc_floats_facing_handle
     \vbox}

\unexpanded\def\startfacingfloat[#1]%
  {\begingroup
   % todo: \usepageparameter
   \edef\p_page{\facingfloatparameter\c!page}%
   \ifx\p_page\empty
     \page[\p_page]%
   \fi
   %
   \let\startcontent\bgroup
   \let\stopcontent\egroup
   \def\currentfacingfloat{#1}%
   \strc_floats_facing_handle}

\unexpanded\def\stopfacingfloat
  {\endgroup}

\unexpanded\def\strc_floats_wrap_up
  {\edef\p_spaceinbetween{\facingfloatparameter\c!spaceinbetween}%
   \ifx\p_spaceinbetween\empty
     \scratchdimen\zeropoint
   \else
     \setbox\scratchbox\vbox{\directvspacing\p_spaceinbetween}%
     \scratchdimen\htdp\scratchbox
   \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
     \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\scratchbox\vsplit#1 upto \textheight
     \setbox\scratchbox\hpack\bgroup
       \edef\currentfacingfloat{\currentfacingfloat:#2}%
       \inheritedfacingfloatframed{\box\scratchbox}%
     \egroup
     \ifdim\ht\scratchbox<\dimexpr\textheight-\scratchdimen\relax
        \setbox\scratchbox\vbox{\box\scratchbox\directvspacing\p_spaceinbetween}%
     \else
        \setbox\scratchbox\vbox to \textheight{\box\scratchbox\vss}%
     \fi
     \global\advance\c_strc_floats_saved\plusone
     \putboxincache{\currentfacingfloat}{\number\c_strc_floats_saved}\scratchbox
     \endgroup
   \fi}

\appendtoks
    \strc_floats_facing_flush
\to \everyafteroutput

\protect \endinput