You are given the following information, but you may prefer to do some research for yourself.
How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?
p19()-> put(months,{31,28,31,30,31,30,31,31,30,31,30,31}), io:format("~w~n",[p19({1,0,1900},0)]). p19({_,_,2001},R)->R; p19({D,M,1900},R)-> p19(p19_next({D,M,1900}),R); p19({0,M,Y},R)-> p19(p19_next({0,M,Y}),R+1); p19(Day,R)-> p19(p19_next(Day),R). p19_next({D,M,Y})-> {(D+element(M+1,get(months)) + if M==1,(Y rem 4) ==0->1; true->0 end ) rem 7,(M+1) rem 12, Y + if M==11->1; true->0 end}.
def p19 dm=[31,28,31,30,31,30,31,31,30,31,30,31] day=1 year=1900 month=0 count=0 while(year<2001) count+=1 if day==0 and year.between?(1901,2000) day+=dm[month] month=(month+1)%12 day+=1 if month==2 and year%4==0 and year > 1900 year+=1 if month==0 day=day%7 end puts count end