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/*
Calendar program:
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.
*/
% Last 10000 FoolsDays arithmetic benchmark:
cal(ShowResult):-
N = 10000,
day_of_week(1993,4,9,Day),
( ShowResult = true ->
write('April 9, 1993 is: '),
write(Day),
write(' fools_days = '), write(N), nl
; true),
empty_loop(N),
full_loop(N).
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(_).
% 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).
/*
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).
% benchmark interface
benchmark(ShowResult) :-
cal(ShowResult).
:- include(common).
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