Five weekends: Difference between revisions

=={{header|Prolog}}==

Works with SWI-Prolog;

<lang Prolog>main() :-

weekends(1900, 2100, FiveWeekendList, RemainderWeekendList),

length(FiveWeekendList, FiveLen),

maplist(write, ["Total five weekend months:", FiveLen, '\n']),

slice(FiveWeekendList, 5, FirstFiveList),

maplist(write, ["First five {year,month} pairs:", FirstFiveList, '\n']),

slice(FiveWeekendList, -5, LastFiveList),

maplist(write, ["Last five {year,month} pairs:", LastFiveList, '\n']),

maplist(take_year, FiveWeekendList, FiveYearList),

list_to_set(FiveYearList, FiveYearSet),

maplist(take_year, RemainderWeekendList, RemainderYearList),

list_to_set(RemainderYearList, RemainderYearSet),

subtract(RemainderYearSet, FiveYearSet, NonFiveWeekendSet),

length(NonFiveWeekendSet, NonFiveWeekendLen),

maplist(write, ["Total years with no five weekend months:", NonFiveWeekendLen, '\n']),

writeln(NonFiveWeekendSet).

weekends(StartYear, EndYear, FiveWeekendList, RemainderWeekendList) :-

numlist(StartYear, EndYear, YearList),

numlist(1, 12, MonthList),

pair(YearList, MonthList, YearMonthList),

partition(has_five_weekends, YearMonthList, FiveWeekendList, RemainderWeekendList).

has_five_weekends({Year, Month}) :-

long_month(Month),

starts_on_a_friday(Year, Month).

starts_on_a_friday(Year, Month) :-

Date = date(Year, Month, 1),

day_of_the_week(Date, DayOfTheWeek),

DayOfTheWeek == 5.

take_year({Year, _}, Year).

long_month(1).

long_month(3).

long_month(5).

long_month(7).

long_month(8).

long_month(10).

long_month(12).

% Helpers

% https://stackoverflow.com/a/7739806

pair(L1, L2, Pairs):-findall({A,B}, (member(A, L1), member(B, L2)), Pairs).

slice(_, 0, []).

slice(List, N, NList):-

N < 0,

N1 is abs(N),

last_n_elements(List, N1, NList).

slice(List, N, NList):-

N > 0,

first_n_elements(List, N, NList).

first_n_elements(List, N, FirstN):-

length(FirstN, N),

append(FirstN, _, List).

last_n_elements(List, N, LastN) :-

length(LastN, N),

append(_, LastN, List).

</lang>

{{out}}

<pre>?- main.

Total five weekend months:201

First five {year,month} pairs:[{1901,3},{1902,8},{1903,5},{1904,1},{1904,7}]

Last five {year,month} pairs:[{2097,3},{2098,8},{2099,5},{2100,1},{2100,10}]

Total years with no five weekend months:29

[1900,1906,1917,1923,1928,1934,1945,1951,1956,1962,1973,1979,1984,1990,2001,2007,2012,2018,2029,2035,2040,2046,2057,2063,2068,2074,2085,2091,2096]

true .</pre>