go:-go(10000).

go(N):-
	day_of_week(1992,12,1,Day),
	statistics,
	write('dec 1, 1992 is: '),write(Day),nl,
	time(T0),
	empty_loop(N),
	time(T1),
	full_loop(N),
	time(T2),
	T is (T2-T1)-(T1-T0),
	write('BMARK_cal'=[fools_days=N,time(T=(T2-T1)-(T1-T0))]),nl.

time(T):-statistics(runtime,[T,_]).

range(Min,Min,Max):-Min=<Max.
range(I,Min,Max):-
        Min<Max,
        Min1 is Min+1,
        range(I,Min1,Max).

empty_loop(Y):-range(_,1,Y),true,fail.
empty_loop(_).

full_loop(Max):-
	range(Year,1,Max),
	day_of_week(Year,4,1,_FoolsDay),
	fail.
full_loop(_).

/*
This algorithm was published in comp.programming and comes from
    Andy Lowry, lowry@watson.ibm.com, (914) 784-7925
    IBM Research, P.O. Box 704, Yorktown Heights, NY 10598

Original Prolog version by Peter Ludemann,
Optimized for BinProlog by Paul Tarau

First, there is a special key value for each month and a
correction factor for January and February in leap years.
*/

cal_key( 1, 6, 1).
cal_key( 2, 2, 1).
cal_key( 3, 2, 0).
cal_key( 4, 5, 0).
cal_key( 5, 0, 0).
cal_key( 6, 3, 0).
cal_key( 7, 5, 0).
cal_key( 8, 1, 0).
cal_key( 9, 4, 0).
cal_key(10, 6, 0).
cal_key(11, 2, 0).
cal_key(12, 4, 0).
cal_key(jan, 6, 1).
cal_key(feb, 2, 1).
cal_key(mar, 2, 0).
cal_key(apr, 5, 0).
cal_key(may, 0, 0).
cal_key(jun, 3, 0).
cal_key(jul, 5, 0).
cal_key(aug, 1, 0).
cal_key(sep, 4, 0).
cal_key(oct, 6, 0).
cal_key(nov, 2, 0).
cal_key(dec, 4, 0).
cal_key('January', 6, 1).
cal_key('February', 2, 1).
cal_key('March', 2, 0).
cal_key('April', 5, 0).
cal_key('May', 0, 0).
cal_key('June', 3, 0).
cal_key('July', 5, 0).
cal_key('August', 1, 0).
cal_key('September',4, 0).
cal_key('October', 6, 0).
cal_key('November',2, 0).
cal_key('December', 4, 0).

% Next, we associate a number with each day of the week:

dow(0, sun).
dow(1, mon).
dow(2, tue).
dow(3, wed).
dow(4, thu).
dow(5, fri).
dow(6, sat).

% The day of week computation is rather arcane, but it works.
% Note the correction for leap years.

day_of_week(Year,Month,Day, DayOfWeek) :-
        cal_key(Month, Key, LeapC),
	compute_it(Year,Day,Key,LeapC,DayOfWeek).

compute_it(Year,Day,Key,LeapC,DayOfWeek):-
        Century is Year // 100,
        YearInCentury is Year - Century * 100,
        DOW0 is (Century * 5 + Century // 4 +
                 YearInCentury + YearInCentury // 4 +
                 Day + Key) 
                mod 7,
        leap_year(Year,DOW0,LeapC,DayOfWeek).

% A leap year is any year which is divisible by 4; if it is also
% divisible by 100 then it must also be divisible by 400 (thus,
% 1600 and 2000 are leap years; 1700, 1800, and 1900 are not).

leap_year(Year,DOW0,_,DayOfWeek) :-
	0 =\= Year mod 4,!,
	dow(DOW0,DayOfWeek).
leap_year(Year,DOW0,LeapC,DayOfWeek):-
	0 =\= Year mod 100,!,
	DOW is DOW0-LeapC,
	dow(DOW,DayOfWeek).
leap_year(Year,DOW0,_,DayOfWeek):-
	0 =\= Year mod 400,!,
	dow(DOW0,DayOfWeek).
leap_year(_,DOW0,LeapC,DayOfWeek):-
	DOW is DOW0-LeapC,
	dow(DOW,DayOfWeek).