%D \module
%D   [       file=math-ext,
%D        version=2006.01.14,
%D          title=\CONTEXT\ Math Macros,
%D       subtitle=Extra Macros,
%D         author={Hans Hagen \& Taco Hoekwater},
%D           date=\currentdate,
%D      copyright=\PRAGMA]
%C
%C This module is part of the \CONTEXT\ macro||package and is
%C therefore copyrighted by \PRAGMA. See mreadme.pdf for
%C details.

%M \ifx\startalign\undefined \input math-ext.tex \fi

% some simple math constructs (based on user requests)
%
% preliminary

\unprotect

% adapted stuff

% we need to make this plain code a bit more 'readable'

\def\openup
  {\afterassignment\dopenup\scratchdimen=}

\def\dopenup
  {\advance\lineskip     \scratchdimen
   \advance\baselineskip \scratchdimen
   \advance\lineskiplimit\scratchdimen}

\def\jot{.25\bodyfontsize} % plain tex: 3 pt (todo: better name)

\newif\ifdt@p

\def\displ@y % \presetdisplayspacing or so
  {\global\dt@ptrue
   \openup\jot
   \mathsurround\zeropoint
   \everycr{\noalign{%
      \ifdt@p
        \global\dt@pfalse
        \ifdim\prevdepth>-\thousandpoint
          \vskip-\lineskiplimit
          \vskip\normallineskiplimit
        \fi
      \else
        \penalty\interdisplaylinepenalty
      \fi}}}

% context (hooks)

\let\normaldispl@y\displ@y

\def\displ@y{\resetdisplaymatheq\normaldispl@y}

\let\presetdisplaymath\displ@y

% plain tex value: \centering = 0pt plus 1000pt minus 1000pt
% plain tex valye: \jot = 3pt

% n>1 #### needed, strange # interaction (maybe we need a toks in recurse ane no macro)

\def\buildeqalign
  {\scratchtoks\emptytoks
   \dorecurse{\mathalignparameter\c!m}
     {\ifnum\recurselevel>\plusone
        \appendtoks
          \tabskip\mathalignparameter\c!distance&\tabskip\zeropoint
        \to\scratchtoks
      \fi
      \expanded{\scratchtoks{\the\scratchtoks\the\!!toksa}}%
      \dorecurse{\numexpr\mathalignparameter\c!n-\plusone\relax}
        {\expanded{\scratchtoks{\the\scratchtoks\the\!!toksb}}}}%
   \expanded{\scratchtoks{\the\scratchtoks\the\!!toksc}}}

\def\forgetalign
  {\tabskip\zeropoint\everycr\emptytoks}

\def\eqalign#1% why no halign here, probably because of displaywidth
  {\!!toksa{\strut\hfil$\displaystyle{##}$}%
   \!!toksb{&$\displaystyle{{}##}$\hfil}%
   \!!toksc\emptytoks
   \buildeqalign
   \null\,\vcenter
     {\openup\jot
      \mathsurround\zeropoint
      \expandafter \ialign \expandafter {\the\scratchtoks\crcr#1\crcr}%
   }\,}

\def\prepareeqalignno
  {\!!toksa{\hfil$\forgetalign\displaystyle{##}$\tabskip\zeropoint}%
   \!!toksb{&$\forgetalign\displaystyle{{}##}$\tabskip\zeropoint}%
   \!!toksc{\hfil\tabskip\centering
            &\llap{$\forgetalign##$}\tabskip\zeropoint}%
   \buildeqalign
   \presetdisplaymath
   \tabskip\centering}

\def\prepareleqalignno
  {\!!toksa{\hfil$\forgetalign\displaystyle{##}$\tabskip\zeropoint}%
   \!!toksb{&$\forgetalign\displaystyle{{}##}$\tabskip\zeropoint}%
   \!!toksc{\hfil\tabskip\centering
            &\kern-\displaywidth\rlap{$\forgetalign##$}\tabskip\displaywidth}%
   \buildeqalign
   \presetdisplaymath
   \tabskip\centering}

\def\eqalignno#1%
  {\prepareeqalignno
   \halign to \displaywidth \expandafter {\the\scratchtoks\crcr#1\crcr}}

\def\leqalignno#1%
  {\prepareleqaligno
   \halign to \displaywidth \expandafter {\the\scratchtoks\crcr#1\crcr}}

\def\aligneqalignno
  {\prepareeqalignno
   \halign to \displaywidth \expandafter \bgroup\the\scratchtoks\crcr}

\def\alignleqalignno
  {\prepareleqalignno
   \halign to \displaywidth \expandafter \bgroup\the\scratchtoks\crcr}

\def\finishalignno
  {\crcr\egroup}

\definesystemvariable{eq}

\def\setupmathalign
  {\dodoubleempty\dosetupmathalign}

\def\dosetupmathalign[#1][#2]%
  {\ifsecondargument
     \getparameters[\??eq#1][#2]%
   \else
     \getparameters[\??eq][#1]%
   \fi}

\let\currentmathalign\empty

\def\mathalignparameter#1%
  {\executeifdefined{\??eq\currentmathalign#1}{\executeifdefined{\??eq#1}\empty}}

\setupmathalign
  [\c!n=2,
   \c!m=1,
   \c!distance=1em]

\def\numberedeqalign
  {\doifelse\@@fmlocation\v!left\alignleqalignno\aligneqalignno}

\def\doxxdoubleempty#1#2%
  {\ifx#2[\expandafter\dodoxxdoubleempty\else\expandafter\noxxdoubleempty\fi#1#2}

\def\dodoxxdoubleempty#1[#2]#3%
  {\ifx#3[\else\expandafter\nonoxxdoubleempty\fi#1[#2]#3}

\def\noxxdoubleempty      #1{#1[][]}
\def\nonoxxdoubleempty#1[#2]{#1[#2][]}

\def\doalignNR[#1][#2]%
  {\doifsomething{#1}
     {\doifelse{#1}{+}
        {\doformulanumber[][#2][]{}}
        {\doformulanumber[#1][#2][]{}}}\cr}

\def\dostartmathalign[#1][#2]%
  {\edef\currentmathalign{#1}%
   \doifassignmentelse{#2}{\setupmathalign[#1][#2]}\donothing
   \def\NC{\def\NC####1{&####1}}%
   \def\EQ{&=}%
   \def\NR{&\doxxdoubleempty\doalignNR}%
   % amstex compatibility mode: (ugly)
   \def\notag{\def\\{&\crcr}}%
   \doifelse{#2}{*}{\def\\{&\crcr}}{\def\\{&\doalignNR[+][]\crcr}}%
   % end of compatibility mode
   \numberedeqalign}

\let\dostopmathalign\finishalignno

\def\definemathalign
  {\dodoubleempty\dodefinemathalign}

\def\dodefinemathalign[#1]% [#2]%
  {\setvalue{\e!start#1}{\dodoubleempty\dostartmathalign[#1]}%
   \setvalue{\e!stop #1}{\dostopmathalign}%
   \setupmathalign[#1]}% [#2]

\definemathalign[align] % default case

% \def\startsplit
%   {\startalign[*]} % no number by default
%
% \def\stopsplit
%   {&\doalignNR[+][]\crcr % for a number on last line
%    \stopalign}

%D \startbuffer
%D \placeformula \startformula \eqalignno {
%D  a &= b & \formulanumber \cr
%D  c &= d \cr
%D    &= e \cr
%D    &= f & \formulanumber
%D } \stopformula
%D \stopbuffer
%D
%D \typebuffer \getbuffer
%D
%D \startbuffer
%D \placeformula \startformula \startalign
%D \NC  a \EQ b \NR[+]
%D \NC  c \EQ d \NR
%D \NC    \EQ f \NR[for:demo-a-1]
%D \NC    \EQ g \NR[for:demo-a-2][a]
%D \NC    \EQ h \NR[for:demo-a-3][b]
%D \NC    \EQ i \NR
%D \stopalign \stopformula
%D \stopbuffer
%D
%D \typebuffer \getbuffer
%D
%D \startbuffer
%D \placeformula \startformula \startalign
%D \NC a \EQ b \NR[+]
%D \NC c \EQ d \NR
%D \NC   \EQ f \NR
%D \NC   \EQ g \NR
%D \NC   \EQ h \NR
%D \NC   \EQ i \NR[+]
%D \stopalign \stopformula
%D \stopbuffer
%D
%D \typebuffer \getbuffer
%D
%D \startbuffer
%D \placeformula \startformula \startalign
%D a &= b \\
%D c &= d \notag \\
%D   &= e \notag \\
%D   &= f \\
%D \stopalign \stopformula
%D \stopbuffer
%D
%D \typebuffer \getbuffer
%D
%D \startbuffer
%D \placeformula \startformula \startalign
%D a &= b \NR[+]
%D c &= d \NR
%D   &= e \NR
%D   &= f \NR[+]
%D \stopalign \stopformula
%D \stopbuffer
%D
%D \typebuffer \getbuffer
%D
%D \startbuffer
%D \placeformula \startformula \startalign
%D \NC  a \NC \eq  b \NR[+]
%D \NC  c \NC \neq d \NR
%D \NC    \NC \neq f \NR[for:demo-b-1]
%D \NC    \NC \geq g \NR[for:demo-b-2][a]
%D \NC    \NC \leq h \NR[for:demo-b-3][b]
%D \NC    \NC \neq i \NR
%D \stopalign \stopformula
%D \stopbuffer
%D
%D \typebuffer \getbuffer
%D
%D \startbuffer
%D \placeformula \startformula \startalign[*]
%D a &= b \\
%D c &= d \\
%D   &= e \\
%D   &= f \\
%D \stopalign \stopformula
%D \stopbuffer
%D
%D \typebuffer \getbuffer
%D
%D \startbuffer
%D \placeformula \startformula \startalign
%D     x &= y \\
%D     a &= b \\
%D \stopalign \stopformula
%D \stopbuffer
%D
%D \typebuffer \getbuffer
%D
%D \startbuffer
%D \placeformula \startformula \startalign[m=3]
%D     x &= y & x &= y & z &= t \\
%D     a &= b & p &= q & w &= s \\
%D \stopalign \stopformula
%D \stopbuffer
%D
%D \typebuffer \getbuffer
%D
%D \startbuffer
%D \placeformula \startformula \startalign[m=3,distance=0pt]
%D     x &= y &= x &= y &= z &= t \\
%D     a &= b &= p &= q &= w &= s \\
%D \stopalign \stopformula
%D \stopbuffer
%D
%D \typebuffer \getbuffer
%D
%D \startbuffer
%D \placeformula \startformula \startalign[n=5,distance=0pt]
%D     x &= yy &= xx &= yy &= zz \\
%D     a &= b  &= p  &= q  &= w  \\
%D \stopalign \stopformula
%D \stopbuffer
%D
%D \typebuffer \getbuffer

\protect \endinput