cheryl.pl

% May    15 - May    16 - May    19
% June   17 - June   18
% July   14 - July   16
% August 14 - August 15 - August 17

% July   14 - July   16
% August 14 - August 15 - August 17

% July   16

birthday(may,    15).
birthday(may,    16).
birthday(may,    19).

birthday(june,   17).
birthday(june,   18).

birthday(july,   14).
birthday(july,   16).

birthday(august, 14).
birthday(august, 15).
birthday(august, 17).

all_days(Days) :-
    findall(Day, birthday(_, Day), Days).

all_months(Months) :-
    findall(Month, birthday(Month, _), AllMonths),
    sort(AllMonths, Months).

member(X, [Y|Remainder], Remainder) :- X == Y, !.
member(X, [_|List], Remainder) :-
    member(X, List, Remainder).

not_member(_, []).
not_member(X, [Y|Remainder]) :- X \= Y, not_member(X, Remainder), !.

member_once(X, List) :-
    member(X, List, Remainder),
    not_member(X, Remainder), !.

unique(List, Result) :-
    findall(X, (member(X, List), member_once(X, List)), Result).

unique_days(Days) :-
    all_days(AllDays),
    unique(AllDays, Days).

bernard_cant_know_immediately(Months) :-
    unique_days(UniqueDays),
    all_months(AllMonths),
    findall(Month, (member(Day, UniqueDays), birthday(Month, Day)), ImpossibleMonths),
    findall(Month, (member(Month, AllMonths), not_member(Month, ImpossibleMonths)), Months).

days_for_months(Months, Days) :-
    findall(Day, (member(Month, Months), birthday(Month, Day)), AllDays),
    sort(AllDays, Days).

bar(Days, Month, Day) :-
    member(Day2, Days),
    birthday(Month, Day2),
    Day \= Day2.

foo(Months, Days, Month, Day) :-
    member(Day, Days),
    member(Month, Months),
    birthday(Month, Day),
    not(bar(Days, Month, Day)).

bernard_knows_now(Months, Month, Day) :-
    days_for_months(Months, AllDays),
    findall(Day, (member(Month1, Months),
                  member(Month2, Months),
                  Month1 \= Month2,
                  birthday(Month1, Day),
                  birthday(Month2, Day)),
            ImpossibleDays),
    findall(Day, (member(Day, AllDays), not_member(Day, ImpossibleDays)), PossibleDays),
    sort(PossibleDays, Days),
    foo(Months, Days, Month, Day).

cheryl(Month, Day) :-
    bernard_cant_know_immediately(Months),
    bernard_knows_now(Months, Month, Day).