Я сделал программу пролога, чтобы показать мне, как лучше всего размещать материалы на DVD. Вопросы находятся в комментариях к коду для справки, который я вставлю ниже, но все сводится к следующему:
Is there a sort of inverted cut operator to make it search for more although it already matches? See fitexact, something like fitexact(Size,Sum,L,L):-Sum
What's the best way to keep track of already processed movies? I retract them but wonder how to do it without that.
fitfuzzy использует конструкцию
ifthen. Я не уверен, что о них думать, это странно в прологе. Однако попытка сделать его рекурсивным оставила меня ужасно сбитым с толку :)
% given a list of movies and sizes, try to fit them all onto dvd's
% wasting as little space as possible.
% set the dvd size
dvdsize(4812).
% sum of all movies in the db
movies_size(Size) :- findall(S, movie(_,S), LS), sum_list(LS,Size).
% count of all movies in the db
movies_count(Count) :- findall(S, movie(_,S), LS), length(LS,Count).
% see which ones fit exactly
% this is where i got into trouble, the original idea was to make
% it like the fuzzy search below but i don't understand how i can
% express 'when there are no more movies which make an exact fit,
% and the sum is smaller then the dvdsize the list is ok too'.
fitexact(Movies) :- dvdsize(Size), fitexact(Size, 0, [], Movies).
% Stop when there's a perfect fit
% so here i tried Size,Sum and Sum<Size in the body. That obviously
% doesn't work since it instantly matches.
fitexact(Size, Size, Movies, Movies).
% since otherwise the same movies show up on different dvd's i
% thought it would be best to delete them after they fitted.
% if I don't want to do that, what's the best way to make sure once
% a movie is processed it won't show up again? Should it have an extra
% flag like processed(movie(name,size,processed) or should i assert
% done dvd's and see if they're already in them? I wonder how long this
% all would take since it's already quite slow as is.
%% :-
%% forall(member(Movie,Movies), retract(movie(Movie,_))). %%, !.
% Otherwise keep filling
fitexact(Size, Sum, Acc, Movies) :-
movie(Movie, MovieSize),
\+ member(Movie, Acc), % no doubles!
NewSum is Sum + MovieSize,
NewSum =< Size,
fitexact(Size, NewSum, [Movie|Acc], Movies).
removedvd(DVD) :-
forall(member(Movie,DVD),retract(movie(Movie,_))).
% do a fuzzy fit, try exact fits with decreasing size when
% there are no exact fits.
fitfuzzy(DVD) :- dvdsize(Size), fitfuzzy(DVD,Size,0).
fitfuzzy(_,Size,Size) :- movies_size(Size), !.
fitfuzzy(_,Size,Size) :- dvdsize(Size), !.
fitfuzzy(DVD,Size,Wasted) :-
CheckSize is Size - Wasted,
% this feels like a horrible way to do this. I very much like suggestions
% about how to make it recursive or better in general.
( fitexact(CheckSize, 0, [], DVD)
-> removedvd(DVD)
; NewWasted is Wasted + 1,
write('fitfuzzy: Increasing wasted space to '), write(NewWasted), nl,
fitfuzzy(DVD,Size,NewWasted)
).
status :-
movies_count(MoviesLeft),
movies_size(MoviesSize),
write('Movies left: '), write(MoviesLeft), nl,
write('Size left : '), write(MoviesSize), nl.
burnloop :-
movies_count(C), C>0,
fitfuzzy(DVD),
status,
write('DVD = '), print(DVD),nl, nl,
burnloop.
% movies.db contains a list of movie(Name,Size). statements. It also
% must have :- dynamic(movie/2). on top for retract to work.
go :-
['movies.db'],
burnloop.
