%% calendarweek.tex
%% 2006 (C) Taco Hoekwater, public domain
%%
%%  Usage:
%%   \calendarweek{30}{12}{2006}
%%
%% Watch out: Dec 29 can be week 1 of the next year; and Jan 3 can be
%% week 53 of the previous year.

\def\Expr#1{\the\numexpr #1\relax}

\def\Modulonumber#1#2{\Expr{#2-((((#2+(#1/2))/#1)-1)*#1)}}
\def\Divisionnumber#1#2{\Expr{(2*#2-#1)/(2*#1)}}

\def\Mod#1#2{\Modulonumber{\Expr{#2}}{\Expr{#1}}}
\def\Div#1#2{\Divisionnumber{\Expr{#2}}{\Expr{#1}}}

\def\Jday#1#2#3%
    {\Expr{#1+\Div{((153*(\Expr{#2+(12*(\Div{14-#2}{12}))-3}))+2)}
     {5}+365*(\Expr{#3+4800-(\Div{14-#2}{12})})+
      (\Div{\Expr{#3+4800-(\Div{14-#2}{12})}}{4})-
      (\Div{\Expr{#3+4800-(\Div{14-#2}{12})}}{100})+
      (\Div{\Expr{#3+4800-(\Div{14-#2}{12})}}{400})-32045 }}

\def\cwhlp#1#2#3%
    {\Expr{\Mod {\Mod {\Mod {\Expr
     {\Jday{#1}{#2}{#3}+31741-\Mod{\Jday{#1}{#2}{#3}}{7}}}%
     {146097}}{36524}}{1461}}}

\def\calendarweek#1#2#3%
  {\Expr{\Expr{\Div{\Expr{\Mod{\cwhlp{#1}{#2}{#3}-
	\Expr{\Div{\cwhlp{#1}{#2}{#3}}{1460}}}{365}+
         \Expr{\Div{\cwhlp{#1}{#2}{#3}}{1460}}}}{7} +1}}}

\endinput