path: root/tex/context/base/core-grd.tex

%D \module
%D   [       file=core-grd,
%D        version=1998.03.10,
%D          title=\CONTEXT\ Core Macros,
%D       subtitle=Grid Snapping (Experimental),
%D         author=Hans Hagen,
%D           date=\currentdate,
%D      copyright={PRAGMA / Hans Hagen \& Ton Otten}]
%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 Core Macros / Grid Snapping}

\unprotect

%D Moved from supp-box:

%D \macros
%D  {startbaselinecorrection,baselinecorrection,
%D   showbaselinecorrection,offbaselinecorrection}
%D
%D Spacing around ruled boxes can get pretty messed up. The
%D next macro tries as good as possible to fix this.
%D
%D \startbuffer[1]
%D \startbaselinecorrection
%D \ruledhbox{Rule Brittanica}
%D \stopbaselinecorrection
%D \stopbuffer
%D
%D \typebuffer[1]
%D
%D The macros put some white space around the box:
%D
%D \haalbuffer[1]
%D
%D A simple alternative is \type {\baselinecorrection}, which
%D only looks at the previous line.
%D
%D \startbuffer[2]
%D \baselinecorrection
%D \ruledhbox{Rule Brittanica}
%D \baselinecorrection
%D \stopbuffer
%D
%D \typebuffer[2]
%D
%D This time the last preceding line gets a correction,%
%D dependant on the depth.
%D
%D \haalbuffer[2]
%D
%D One can make the correction visible by saying \type
%D {\showbaselinecorrection}. Part of the correction is
%D calculated from the dimensions of a~(. One can disble the
%D correction by calling \type {\offbaselinecorrection}.
%D
%D When visualize the first example looks like:
%D
%D {\showbaselinecorrection\haalbuffer[1]}
%D
%D and the second one comes out as:
%D
%D {\showbaselinecorrection\haalbuffer[2]}

\definecolor[GridLineColor][red]
\definecolor[GridTextColor][blue]

\def\setbaselinecorrections%
  {\setbox0\hbox{\setstrut\strut}%
   \setbox2\hbox{(}%
   \dimen0\ht0\advance\dimen0 -\ht2
   \ifdim\dimen0<\zeropoint\dimen0\zeropoint\fi
   \dimen2\dp0\advance\dimen2 -\dp2
   \ifdim\dimen2<\zeropoint\dimen2\zeropoint\fi
   \edef\thetopbaselinecorrection   {\the\dimen0}\dimen0-\dimen0
   \edef\thebotbaselinecorrection   {\the\dimen2}\dimen2-\dimen2
   \edef\thenegtopbaselinecorrection{\the\dimen0}%
   \edef\thenegbotbaselinecorrection{\the\dimen2}}

\def\dotopbaselinecorrection   {\kern\thetopbaselinecorrection}
\def\dobotbaselinecorrection   {\kern\thebotbaselinecorrection}
\def\donegtopbaselinecorrection{\kern\thenegtopbaselinecorrection}
\def\donegbotbaselinecorrection{\kern\thenegbotbaselinecorrection}

\def\showbaselinecorrection%
  {\def\dobaselinecorrection% visualization is not watertight!
     {\bgroup
      \kern-\prevdepth
      \setbox0=\null
      \wd0=\hsize\dp0=\dp\strutbox
      \nointerlineskip
      \ruledvbox{\box0}%
      \egroup
      \prevdepth=\dp\strutbox}%
   \def\dotopbaselinecorrection%
     {\hrule\!!height\thetopbaselinecorrection}%
   \def\dobotbaselinecorrection%
     {\hrule\!!height\thebotbaselinecorrection}}

\def\dobaselinecorrection%
  {\kern-\prevdepth
   \kern\dp\strutbox
   \prevdepth=\dp\strutbox}

\def\baselinecorrection%
  {\endgraf
   \ifvmode
     \ifdim\prevdepth<\maxdimen
       \ifdim\prevdepth<\zeropoint \else
         \ifdim\prevdepth<\dp\strutbox
           \dobaselinecorrection
         \fi
       \fi
     \fi
   \fi}

% Beware, keep this one as it is, see for instance module
% m-steps.tex, where we apply a \localhsize to the \vbox, in
% order to follow narrower and side floats !

\def\startbaselinecorrection%
  {\baselinecorrection
   \ifvmode
     \bgroup
     \setbox\scratchbox=\vbox\bgroup
     \ignorespaces
     \let\stopbaselinecorrection=\dostopbaselinecorrection
   \else
     \let\stopbaselinecorrection=\relax
   \fi}

%D We do a bit more checking than needed. The pageborder check
%D is not needed, but I want to look the visualization as good
%D as possible too.

\def\dostopbaselinecorrection% I have to check columns yet.
  {\endgraf
   \egroup
   \topbaselinecorrection
   \box\scratchbox
   \botbaselinecorrection
   \egroup}

\let\stopbaselinecorrection=\relax

\def\offbaselinecorrection% Can be used inside correction.
  {\def\startbaselinecorrection%
     {\bgroup
      \let\stopbaselinecorrection=\egroup}}

%D \macros
%D  {topbaselinecorrection,botbaselinecorrection}
%D
%D The actual top and bottom corrections are implemented as:

\def\topbaselinecorrection%
  {\ifvmode
     \bgroup
     \setbaselinecorrections
     \ifdim\pagegoal<\maxdimen
       \ifdim\pagetotal<\pagegoal
         \dimen2=\ht\scratchbox
         \advance\dimen2 by \dp\scratchbox
         \advance\dimen2 by \parskip
         \advance\dimen2 by \thetopbaselinecorrection
         \advance\dimen2 by \thebotbaselinecorrection
         \dimen0=\pagetotal
         \advance\dimen0 by \dimen2
%        \ifdim\dimen0<\pagegoal % does more harm than good
           \witruimte
           \nointerlineskip
           \dotopbaselinecorrection
%        \else
%          \ifbinnenkolommen      
%            % \vskip\dimen2           % this could definitely
%            % \penalty\outputpenalty  % be improved
%          \else
%            %                         % 
%            % \vfill\eject            % pretty old and wrong
%            %
%            % \nobreak                % needed for headings
%            % \vskip\parskip          % but often splits 
%            % \vskip\dimen2           % normal paragraphs
%            % \penalty\outputpenalty  % and therefore 
%            % \vskip-\dimen2          % obsolete
%            %
%            % do nothing, sub optimal spacing after headings 
%            % still to be sorted out; use manuals as test case
%          \fi
%        \fi
       \else                           % probably a preceding
         \witruimte                    % one-liner
         \nointerlineskip
         \dotopbaselinecorrection
       \fi
     \fi
     \egroup
   \fi}

\def\botbaselinecorrection%
  {\ifvmode
     \bgroup
     \setbaselinecorrections
     \dobotbaselinecorrection
     \allowbreak % new, otherwise problems when many in a row
     \prevdepth\dp\strutbox
     \egroup
   \fi}

%D An old one, still to be translated:
%D
%D \macros
%D   {getnoflines, getrawnoflines}
%D
%D Het commando \type{\getnoflines} converteert een hoogte
%D (dimensie) in een aantal regels en kent dit toe aan
%D \type{\noflines}.
%D
%D \starttypen
%D \getnoflines{dimensie}
%D \stoptypen
%D
%D Er wordt gedeeld door \type{\openlineheight} en een hoogte
%D van~0pt komt overeen met 0~regels. The raw alternative 
%D does not round. 

\newcount\noflines
\newdimen\noflinesheight

\def\dogetnoflines#1#2%
  {\noflinesheight=#2\relax
   \ifdim\noflinesheight=\zeropoint
     \noflines=0
   \else
     \divide\noflinesheight by \openlineheight
     \noflines=\noflinesheight
     #1\ifdim\noflines\openlineheight=#2\relax \else
       \advance\noflines by 1
     \fi\fi
   \fi}

\def\getnoflines   {\dogetnoflines\iftrue } % compensated 
\def\getrawnoflines{\dogetnoflines\iffalse} % no compensation 

%D Still very experimental and therefore undocumented.

\newif\ifgridsnapping  % UNDER DEVELOPMENT, USE WITH CARE
\newif\ifforcepresnap  \forcepresnaptrue  % false in mixed single/double
\newif\ifstrutsnapping \strutsnappingtrue % sometimes handy to be false

\def\positiveextrasnap {\gdef\extrasnapsign{+}}
\def\negativeextrasnap {\gdef\extrasnapsign{-}}

\def\extrasnapreset  {\global\chardef\@@extrasnap=0
                      \positiveextrasnap}
\def\extrasnapbefore {\global\chardef\@@extrasnap=1 }
\def\extrasnaparound {\global\chardef\@@extrasnap=2 }
\def\extrasnapafter  {\global\chardef\@@extrasnap=3 }

\def\enablepresnapcorrection  {\global\chardef\@@presnap=0 }
\def\disablepresnapcorrection {\global\chardef\@@presnap=1 }

\extrasnapreset \enablepresnapcorrection

\newif\iftracegridsnapping
\newif\ifshowfuzzyskips

\chardef\@@alignsnap   =0
\chardef\@@alignsnapbox=0

\let\presnapskip \!!zeropoint  \def\presnap {--}
\let\postsnapskip\!!zeropoint  \let\postsnap\presnap

\def\tracesnapping%
  {\iftracegridsnapping
     \llap
       {\infofont
        \doglobal\increment\currentsnap
        \color[GridLineColor]{\vl\presnapskip
                    \vl\presnap
                    \vl\postsnap
                    \vl\ifcase\@@extrasnap
                            00\or
               \extrasnapsign0\or
  \extrasnapsign\extrasnapsign\or
               0\extrasnapsign\fi
                    \vl\currentsnap\vl}}%
   \fi}

\def\snaptogrid% [#1]#2 -> #2 == \hbox|\vbox
  {\dosingleempty\dosnaptogrid}

\def\dosnaptogrid[#1]% 
  {\ifgridsnapping
     \iffirstargument\doifsomething{#1}{\verplaatsopgrid[#1]}\fi
     \expandafter\dodosnaptogrid
   \fi}

% \def\dodosnaptogrid% 
%   {\dowithnextbox
%      {\bgroup
%       \ifcase\@@alignsnap\else % 1=top 2=high 3=middle 4=low
%         \getnoflines{\ht\nextbox}
%         \setbox\nextbox=\vbox to \noflines\lineheight
%           {\ifnum\@@alignsnap=1 \kern\lineheight\kern-\topskip\fi
%            \ifnum\@@alignsnap>2 \vfill\fi
%            \box\nextbox
%            \ifnum\@@alignsnap<4 \vfill\fi}
%       \fi
%       \forgetall
%       \par
%       \ifvbox\nextbox
%         \setbox\nextbox=\hbox{\box\nextbox}% don't ask
%       \fi
%       \scratchskip=\lastskip
%       \edef\presnapskip{\the\lastskip}%
%       % mixing single/double columns sometimes goes wrong,
%       % check 'som' document
%       \ifbinnenkolommen
%         \forcepresnaptrue
%       \fi
%       \ifforcepresnap
%         \vbox{\hsize\zeropoint\strut}\nobreak\vskip-\openlineheight
%       \fi
%       %
%       \ifdim\ht\nextbox>\ht\strutbox
%         \scratchdimen=\ht\nextbox
%         \ifcase\@@presnap
%           \ifdim\scratchskip>\zeropoint\relax
%             \scratchcounter=\scratchskip
%             \advance\scratchcounter by -\openlineheight
%             \ifnum\scratchcounter<0
%               \scratchcounter=-\scratchcounter
%             \fi
%             \ifnum\scratchcounter<10 % \lastkip is about \openlineheight
%               \advance\scratchdimen by -\openstrutdepth
%               \edef\presnapskip{*\presnapskip}%
%             \else\ifdim\scratchskip>\openlineheight
%                                   %<\openlineheight \else
%               \advance\scratchdimen by -\openstrutdepth
%               \edef\presnapskip{*\presnapskip}%
%             \fi\fi
%           \fi
%         \fi
%         \getnoflines\scratchdimen
%         \advance\noflines by -1
%         \ifnum\noflines>0
%           \scratchdimen=\noflines\lineheight
%         \else
%           \scratchdimen=\zeropoint
%         \fi
%       \else
%         \scratchdimen=\zeropoint
%       \fi
%       \ifnum\@@extrasnap=1 \advance\scratchdimen by \extrasnapsign\lineheight \fi
%       \ifnum\@@extrasnap=2 \advance\scratchdimen by \extrasnapsign.5\lineheight \fi
%       \edef\presnap{\the\scratchdimen}%
%       \ifstrutsnapping
%         \ifdim\scratchdimen=\zeropoint\else\vskip\scratchdimen\fi
%       \fi
%       \ifdim\dp\nextbox>\dp\strutbox
%         \getnoflines{\dp\nextbox}%
%         \advance\noflines by -1
%         \ifnum\noflines>0
%           \scratchdimen=\noflines\lineheight
%         \else
%           \scratchdimen=\zeropoint
%         \fi
%       \else
%         \scratchdimen=\zeropoint
%       \fi
%       \ifnum\@@extrasnap=2 \advance\scratchdimen by \extrasnapsign.5\lineheight \fi
%       \ifnum\@@extrasnap=3 \advance\scratchdimen by \extrasnapsign\lineheight \fi
%       \edef\postsnap{\the\scratchdimen}%
%       \ifstrutsnapping
%         \ht\nextbox=\ht\strutbox
%         \dp\nextbox=\dp\strutbox
%       \else
%         \scratchdimen=\presnap
%         \advance\scratchdimen by \ht\strutbox
%         \ht\nextbox=\scratchdimen
%         \scratchdimen=\postsnap
%         \advance\scratchdimen by \dp\strutbox
%         \dp\nextbox=\scratchdimen
%       \fi
%       \hbox{\tracesnapping\box\nextbox}%
%       \ifstrutsnapping
%         \ifdim\scratchdimen=\zeropoint\else\vskip\scratchdimen\fi
%       \fi
%       \extrasnapreset
%       \enablepresnapcorrection
%       \global\chardef\@@alignsnap0
%       \egroup}}

\def\dodosnaptogrid
  {\dowithnextbox
     {\bgroup
      \ifcase\@@alignsnapbox\relax
        \ifcase\@@alignsnap\else % 1=top 2=high 3=middle 4=low
          \getnoflines{\ht\nextbox}
          \setbox\nextbox\vbox to \noflines\lineheight
            {\ifnum\@@alignsnap=1 \kern\lineheight\kern-\topskip\fi
             \ifnum\@@alignsnap>2 \vfill\fi
             \box\nextbox
             \ifnum\@@alignsnap<4 \vfill\fi}
        \fi
        \forgetall
        \par
        \ifvbox\nextbox
          \setbox\nextbox\hbox{\box\nextbox}% don't ask
        \fi
        \scratchskip\lastskip
        \edef\presnapskip{\the\lastskip}%
        % mixing single/double columns sometimes goes wrong,
        % check 'som' document
        \ifbinnenkolommen
          \forcepresnaptrue
        \fi
        \ifforcepresnap
          \vbox{\hsize\zeropoint\strut}\nobreak\vskip-\openlineheight
        \fi
        %
        \ifdim\ht\nextbox>\ht\strutbox
          \scratchdimen\ht\nextbox
          \ifcase\@@presnap
            \ifdim\scratchskip>\zeropoint\relax
              \scratchcounter\scratchskip
              \advance\scratchcounter -\openlineheight
              \ifnum\scratchcounter<0
                \scratchcounter-\scratchcounter
              \fi
              \ifnum\scratchcounter<10 % \lastkip is about \openlineheight
                \advance\scratchdimen -\openstrutdepth
                \edef\presnapskip{*\presnapskip}%
              \else\ifdim\scratchskip>\openlineheight
                                    %<\openlineheight \else
                \advance\scratchdimen -\openstrutdepth
                \edef\presnapskip{*\presnapskip}%
              \fi\fi
            \fi
          \fi
          \getnoflines\scratchdimen
            \advance\noflines -1
          \ifnum\noflines>0
            \scratchdimen\noflines\lineheight
          \else
            \scratchdimen\zeropoint
          \fi
        \else
          \scratchdimen\zeropoint
        \fi
        \ifnum\@@extrasnap=1 \advance\scratchdimen \extrasnapsign\lineheight \fi
        \ifnum\@@extrasnap=2 \advance\scratchdimen \extrasnapsign.5\lineheight \fi
        \edef\presnap{\the\scratchdimen}%
        \ifstrutsnapping
          \ifdim\scratchdimen=\zeropoint\else\vskip\scratchdimen\fi
        \fi
        \ifdim\dp\nextbox>\dp\strutbox
          \getnoflines{\dp\nextbox}%
          \advance\noflines -1
          \ifnum\noflines>0
            \scratchdimen\noflines\lineheight
          \else
            \scratchdimen\zeropoint
          \fi
        \else
          \scratchdimen\zeropoint
        \fi
        \ifnum\@@extrasnap=2 \advance\scratchdimen \extrasnapsign.5\lineheight \fi
        \ifnum\@@extrasnap=3 \advance\scratchdimen \extrasnapsign\lineheight \fi
        \edef\postsnap{\the\scratchdimen}%
        \ifstrutsnapping
          \ht\nextbox\ht\strutbox
          \dp\nextbox\dp\strutbox
        \else
          \scratchdimen\presnap
          \advance\scratchdimen \ht\strutbox
          \ht\nextbox\scratchdimen
          \scratchdimen\postsnap
          \advance\scratchdimen \dp\strutbox
          \dp\nextbox\scratchdimen
        \fi
        \hbox{\tracesnapping\box\nextbox}%
        \ifstrutsnapping
          \ifdim\scratchdimen=\zeropoint\else\vskip\scratchdimen\fi
        \fi
      \else
        \scratchdimen\ht\nextbox
       %\advance\scratchdimen\dp\nextbox % add dp as option 
        \ifcase\@@alignsnapbox\or
          \getrawnoflines\scratchdimen
        \else
          \getnoflines   \scratchdimen
        \fi  
        \scratchdimen\noflines\lineheight\relax
        \ifnum\pagetotal>\zeropoint\else % disable this as option 
          \advance\scratchdimen-\strutheight
          \advance\scratchdimen+\topskip
        \fi
        \setbox\nextbox\vbox to \scratchdimen
          {\ifnum\@@alignsnapbox>2 \vfill\fi % 3 4 
           \box\nextbox
           \ifnum\@@alignsnapbox<4 \vfill\fi % 2 3 
           \kern\zeropoint}% 
        \ifnum\@@alignsnapbox>2 \def\presnap {++}\fi 
        \ifnum\@@alignsnapbox<4 \def\postsnap{++}\fi 
        \setbox\nextbox\hbox{\tracesnapping\box\nextbox}%
        \dp\nextbox\zeropoint 
        \box\nextbox 
      \fi
      \extrasnapreset
      \enablepresnapcorrection
      \global\chardef\@@alignsnap0
      \global\chardef\@@alignsnapbox0
      \egroup}}

\def\snaptomathgrid% probably not working ok
  {\ifgridsnapping
     \dowithnextbox
       {\blanko[\v!regel]
        \snaptogrid\vbox{\box\nextbox}
        \blanko[\v!regel]}
     \vbox\bgroup
       \let\setdisplayskips=\relax
       \abovedisplayskip\zeropoint
       \abovedisplayshortskip\zeropoint
       \belowdisplayskip\zeropoint
       \belowdisplayshortskip\zeropoint
       \@EA\let\@EA\next
   \fi}

\def\topsnaptogrid%
  {\ifgridsnapping
     \dowithnextbox
       {\scratchdimen=\ht\nextbox
        \advance\scratchdimen by -\ht\strutbox
        \advance\scratchdimen by \topskip
        \ht\nextbox=\scratchdimen
        \dp\nextbox=\zeropoint
        \box\nextbox
        \kern\lineheight
        \kern-\topskip
        \nointerlineskip}
       \hbox
   \fi}

\def\centertogrid%
  {\ifgridsnapping
     \dowithnextbox
       {\bgroup
        \par
        \scratchdimen=\ht\nextbox
        \advance\scratchdimen by \dp\nextbox
        \getnoflines\scratchdimen
        \setbox\nextbox=\vbox to \noflines\lineheight
          {\forgetall
           \vskip\zeropoint plus \ht\nextbox
           \copy\nextbox
           \kern.5\dp\strutbox % VOORLOPIGE WAARDE
           \vskip\zeropoint plus \dp\nextbox}
        \noindent\snaptogrid\vbox{\box\nextbox}%
        \egroup}
       \hbox
   \fi}

\ifx\startbaselinecorrection\undefined \wait \fi % change order

\let\normalstartbaselinecorrection=\startbaselinecorrection

\def\startbaselinecorrection%
  {\ifgridsnapping
     \centertogrid\bgroup
     \let\stopbaselinecorrection=\egroup
   \else
     \normalstartbaselinecorrection
   \fi}

\def\setgridbox#1#2#3%
  {\setbox#1=\ruledvbox to #3 % given size 
     {\forgetall
      \offinterlineskip
      \hsize=#2
      \baselinerulefalse
      \ruledvbox % calculated size 
        {\getnoflines{#3}%
         \vskip\topskip
         \vskip-\ht\strutbox
         \scratchdimen=#2\advance\scratchdimen by \lineheight
         \dorecurse
           {\noflines}
           {\strut
            \hskip-.5\lineheight
            \rlap
              {\hskip\scratchdimen\hskip2pt\infofont
               \hbox to 1em{\hss\recurselevel}}%
            \vrule
              \!!height .5\testrulewidth
              \!!depth .5\testrulewidth
              \!!width \scratchdimen
            \par}}
      \vfill}}

%D Some intervention macros:

\def\gridwarning#1{\message{[beware of #1 extra snap]}}

% \def\doverplaatsopgrid[#1]%
%   {\ifgridsnapping
%      \global\chardef\@@alignsnap0
%      \processaction
%        [#1]
%        [  \v!boven=>\gridwarning+\positiveextrasnap\extrasnapbefore,
%           \v!onder=>\gridwarning+\positiveextrasnap\extrasnapafter,
%           \v!beide=>\positiveextrasnap\extrasnaparound,
%          -\v!boven=>\gridwarning-\negativeextrasnap\extrasnapbefore,
%          -\v!onder=>\gridwarning-\negativeextrasnap\extrasnapafter,
%          -\v!beide=>\negativeextrasnap\extrasnaparound,
%          \v!pagina=>\global\chardef\@@alignsnap1, % topskip
%            \v!hoog=>\global\chardef\@@alignsnap2, 
%          \v!midden=>\global\chardef\@@alignsnap3, 
%            \v!laag=>\global\chardef\@@alignsnap4, 
%           \v!reset=>\positiveextrasnap\extrasnapreset,
%         \s!default=>\positiveextrasnap\extrasnaparound,
%         \s!unknown=>\positiveextrasnap\extrasnapreset]%
%    \fi}

\def\doverplaatsopgrid[#1]% some day : speed up 
  {\ifgridsnapping
     \global\chardef\@@alignsnap0 
     \global\chardef\@@alignsnapbox0 
     \donefalse 
     \ExpandFirstAfter\processallactionsinset[#1]
       [  \v!boven=>\gridwarning+\positiveextrasnap\extrasnapbefore,
          \v!onder=>\gridwarning+\positiveextrasnap\extrasnapafter,
          \v!beide=>\positiveextrasnap\extrasnaparound,
         -\v!boven=>\gridwarning-\negativeextrasnap\extrasnapbefore,
         -\v!onder=>\gridwarning-\negativeextrasnap\extrasnapafter,
         -\v!beide=>\negativeextrasnap\extrasnaparound,
         \v!pagina=>\global\chardef\@@alignsnap1, % topskip
           \v!hoog=>\global\chardef\@@alignsnap2, 
         \v!midden=>\global\chardef\@@alignsnap3, 
           \v!laag=>\global\chardef\@@alignsnap4, 
        \v!passend=>\global\chardef\@@alignsnapbox1, % new 
           \v!ruim=>\global\chardef\@@alignsnapbox2, % new 
          \v!reset=>\positiveextrasnap\extrasnapreset,
           \v!geen=>\global\chardef\@@alignsnap0
                    \global\chardef\@@alignsnapbox0,
      \v!standaard=>\positiveextrasnap\extrasnaparound,
        \s!default=>\positiveextrasnap\extrasnaparound,
        \s!unknown=>\positiveextrasnap\extrasnapreset]%
   \fi}

\def\verplaatsopgrid%
  {\dosingleempty\doverplaatsopgrid}

\def\doplaatsopgrid[#1]%
  {\doverplaatsopgrid[#1]\snaptogrid\vbox}

\def\plaatsopgrid%
  {\dosingleempty\doplaatsopgrid}

%D Snapping is rather robust as long as we use whole lines.
%D Half lines of white space can however be handled when they
%D come in pairs. The corrections needed when crossing page
%D boundaries in the middle of such a pair, are handled by
%D macros that are (named) sort of fuzzy. This fuzzy mechanism
%D was written as an extension to the grid typesetting needed
%D for typesetting (part of) the \MAPS.
%D
%D \starttypen
%D \setuptyping
%D   [before={\blank[halfline]},
%D    after={\blank[halfline]}]
%D \stoptypen

\newif\iffuzzysnapdone
\newif\iffuzzysnapping
\newif\iffuzzysnapped

\chardef\fuzzysnappedleft=0 % ==1 when fuzzybegin still open

\newpersistentmark\fuzzymark % (!)
\newcount\fuzzymarker
\newbox\fuzzysnapbox
\newbox\fuzzysnapsplit

%\def\dosyncfuzzyvskip%
%  {\ifvmode\ifdim\lastskip<\lineheight\ifdim\lastskip>\zeropoint
%     \bgroup
%     \endgraf
%     \forgetall
%     \vbox{\strut}
%     \kern-2\lineheight
%     \nobreak
%     \vskip\lineheight 
%     \egroup
%   \fi\fi\fi}

%\def\fuzzyvskip#1%
%  {\iffuzzysnapdone
%     \endfuzzysnapping
%     \vskip#1\relax
%     \dosyncfuzzyvskip  % NEW
%     \global\fuzzysnapdonefalse
%   \else
%     \vskip#1\relax
%     \beginfuzzysnapping
%     \global\fuzzysnapdonetrue
%   \fi}

\def\dosyncfuzzyvskip%
  {\ifvmode\ifdim\lastskip<\lineheight\ifdim\lastskip>\zeropoint
     \bgroup
       \endgraf\forgetall\vbox{\strut}\nobreak\vskip\lineheight 
     \egroup
   \fi\fi\fi}

\def\fuzzyvskip#1%
  {\iffuzzysnapdone
     \dosyncfuzzyvskip  % NEWER
     \endfuzzysnapping
     \vskip#1\relax
     \global\fuzzysnapdonefalse
   \else
     \vskip#1\relax
     \beginfuzzysnapping
     \global\fuzzysnapdonetrue
   \fi}

\def\setfuzzymark#1#2#3% #1/#2 => error recovery
  {\ifgridsnapping
     \global\fuzzysnappingtrue
     \global\advance\fuzzymarker by \ifodd\fuzzymarker#1\else#2\fi
     \nobreak
     \ifshowfuzzyskips
       \hbox{\color[GridTextColor]
         {\llap{\infofont#3\vl\the\fuzzymarker}\nobreak
          \vrule\!!width\hsize\!!height.1\lineheight}}
       \nobreak
     \fi
    %[\the\fuzzymarker]
    %\expandafter\fuzzymark\expandafter{\the\fuzzymarker}%
     \expandafter\setmark\expandafter\fuzzymark\expandafter{\the\fuzzymarker}%
     \nobreak
  \fi}

\def\beginfuzzysnapping% odd
  {\setfuzzymark21\v!start}

\def\endfuzzysnapping% even
  {\setfuzzymark12\v!stop}

\def\removelastfuzzyvskip%
  {\ifgridsnapping
     \iffuzzysnapping
       \ifdim\lastskip<\openlineheight
       \else
         \removelastskip
       \fi
     \else
       \removelastskip
     \fi
   \else
     \removelastskip
   \fi}

\def\docheckfuzzysnap#1%
  {\bgroup
   \mindermeldingen
   \setbox\fuzzysnapbox=\copy#1\relax
   \setbox\fuzzysnapsplit=\vsplit\fuzzysnapbox to 1\lineheight
   \let\topfuzzymark=\empty % indeed here ... no real mark
   \getsplitmarks\fuzzymark
%   \ifcase0\topfuzzymark
   \ifcase0\getsplittopmark\fuzzymark
     \global\chardef\fuzzysnappedleft=0
     \global\fuzzysnappedfalse
%   \else\ifodd\topfuzzymark
   \else\ifodd\getsplittopmark\fuzzymark
     \global\chardef\fuzzysnappedleft=1
      \global\fuzzysnappedtrue
   \else
     \global\chardef\fuzzysnappedleft=2
      \global\fuzzysnappedtrue
   \fi\fi
   \iffuzzysnapped \else
     \doloop
       {\ifvoid\fuzzysnapbox
          \exitloop
        \else
          \setbox\fuzzysnapsplit=\vsplit\fuzzysnapbox to \lineheight
         %\let\topfuzzymark=\empty % ... but not here
          \getsplitmarks\fuzzymark
%          \ifcase0\topfuzzymark
          \ifcase0\getsplittopmark\fuzzymark
            % continue
%          \else\ifodd\topfuzzymark
          \else\ifodd\getsplittopmark\fuzzymark
            \exitloop
          \else
            \global\chardef\fuzzysnappedleft=1
            \global\fuzzysnappedtrue
            \exitloop
          \fi\fi
        \fi}%
   \fi
   \egroup}

\def\getfuzzysnapcorrection#1%
  {\global\let\presnapcorrection =\relax
   \global\let\postsnapcorrection=\relax
   \ifgridsnapping\iffuzzysnapping
     \docheckfuzzysnap{#1}%
     \iffuzzysnapped
       \iftracegridsnapping
         \gdef\presnapcorrection%
           {\color[GridLineColor]{\hrule\!!height.5\openlineheight\!!width\hsize}}%
       \else
         \gdef\presnapcorrection{\kern.5\openlineheight}%
       \fi
       \gdef\postsnapcorrection{\kern-.5\openlineheight}% get the height ok
     \fi
   \fi\fi}

\def\fuzzysnappedbox#1#2% \box<n> \unvbox<n>
  {\getfuzzysnapcorrection{#2}%
   \presnapcorrection
   #1#2%
   \postsnapcorrection}

\def\adaptfuzzypagegoal%
  {\ifgridsnapping\iffuzzysnapping\ifcase\fuzzysnappedleft\or % see dopagecontents
     \scratchdimen\pagegoal
     \advance\scratchdimen -.5\openlineheight
     \global\pagegoal\scratchdimen
     \global\advance\vsize -.5\openlineheight
     \global\chardef\fuzzysnappedleft0
   \fi\fi\fi}

\protect

\endinput