##########################
                       #         calendar       #
                       ##########################
#                    Returns the day of the week in 
#                   either Gregorian or Julian, with
#                       Gregorian as the default.
#                        Uses a formula due to Gauss.
#
#copyleft David Joyner, 2-3-97
##################################################################

##
##day of the week
##
day_of_week:=proc(month::integer,day::integer,year::integer)
local w,d,m,c,y,leap;
d:=day;
m:=month_num(month,day,year);
if nargs=4 and args[4]=`J` then
  c:=cal_num(month,day,year,`J`);
 else
  c:=cal_num(month,day,year);
fi;
y:=year_num(month,day,year);
leap:=floor(year/4)*4;
if ((month=1 or month=2) and leap=year) then y:=y-1; fi;
w:=`mod`(d+m+c+y,7);
if w=1 then RETURN(`Sunday`); fi;
if w=2 then RETURN(`Monday`); fi;
if w=3 then RETURN(`Tuesday`); fi;
if w=4 then RETURN(`Wednesday`); fi;
if w=5 then RETURN(`Thursday`); fi;
if w=6 then RETURN(`Friday`); fi;
if w=0 then RETURN(`Saturday`); fi;
end:

month_num:=proc(month::integer,day::integer,year::integer)
if month=1 then RETURN(0); fi;
if month=2 then RETURN(3); fi;
if month=3 then RETURN(3); fi;
if month=4 then RETURN(6); fi;
if month=5 then RETURN(1); fi;
if month=6 then RETURN(4); fi;
if month=7 then RETURN(6); fi;
if month=8 then RETURN(2); fi;
if month=9 then RETURN(5); fi;
if month=10 then RETURN(0); fi;
if month=11 then RETURN(3); fi;
if month=12 then RETURN(5); fi;
end:

cal_num:=proc(month::integer,day::integer,year::integer)
local y;
y:=floor(year/100);

if nargs=4 and args[4]=`J` then ###signifies Julian calendar
################### - the default is Gregorian calendar
 if member(y,{0,7,14}) then 
  print(`Julian calendar`); 
  RETURN(5); 
 fi;
 if member(y,{1,8,15}) then 
  print(`Julian calendar`); 
  RETURN(4); 
 fi;
 if member(y,{2,9,16}) then 
  print(`Julian calendar`); 
  RETURN(3); 
 fi;
 if member(y,{3,10,17}) then 
  print(`Julian calendar`); 
  RETURN(2); 
 fi;
 if member(y,{4,11,18}) then 
  print(`Julian calendar`); 
  RETURN(1); 
 fi;
 if member(y,{5,12,19}) then 
  print(`Julian calendar`); 
  RETURN(0); 
 fi;
 if member(y,{6,13,20}) then 
  print(`Julian calendar`); 
  RETURN(6); 
 fi;
fi; ##end if `J`
if member(y,{15,19,23}) then 
  print(`Gregorian calendar`); 
  RETURN(1); 
fi;
if member(y,{16,20,24}) then 
  print(`Gregorian calendar`); 
  RETURN(0); 
fi;
if member(y,{17,21,25}) then 
  print(`Gregorian calendar`); 
  RETURN(5); 
fi;
if member(y,{18,22,26}) then 
  print(`Gregorian calendar`); 
  RETURN(3); 
fi;
ERROR(`The Gregorian calendar began in 1582. Use the "J" option for the Julian calendar.`);
end:

year_num:=proc(month::integer,day::integer,year::integer)
local y;
y:=year-floor(year/100)*100;
if member(y,{0,6,17,23,28,34,45,51,56,62,73,79,84,90}) then 
  RETURN(0); 
fi;
if member(y,{1,7,12,18,29,35,40,46,57,63,68,74,85,91,96}) then 
  RETURN(1); 
fi;
if member(y,{2,13,19,24,30,41,47,52,58,69,75,80,86,97}) then 
  RETURN(2); 
fi;
if member(y,{3,8,14,25,31,36,42,53,59,64,70,81,87,92,98}) then 
  RETURN(3); 
fi;
if member(y,{9,15,20,26,37,43,48,54,65,71,76,82,93,99}) then 
  RETURN(4); 
fi;
if member(y,{4,10,21,27,32,38,49,55,60,66,77,83,88,94}) then 
  RETURN(5); 
fi;
if member(y,{5,11,16,22,33,39,44,50,61,67,72,78,89,95}) then 
  RETURN(6); 
fi;
end:

save `calendar.m`;