104 lines
2.7 KiB
Plaintext
104 lines
2.7 KiB
Plaintext
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:- object(best_first(Threshold),
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instantiates(heuristic_search(Threshold))).
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:- info([
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version is 1.0,
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authors is 'Paulo Moura',
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date is 1998/3/23,
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comment is 'Best first heuristic state space search strategy.',
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parnames is ['Threshold']]).
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:- uses(list).
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:- private(expand/8).
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:- private(succlist/5).
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:- private(bestf/3).
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:- private(continue/9).
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:- private(f/2).
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:- private(insert/4).
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search(Space, State, Threshold, Solution, Cost) :-
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expand([], l(State, 0/0), Threshold, _, yes, Path, Space, Cost),
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list::reverse(Path, Solution).
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expand(Path, l(State,Cost/_), _, _, yes, [State|Path], Space, Cost) :-
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Space::goal_state(State).
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expand(Path, l(State,F/G), Threshold, Tree, Solved, Solution, Space, Cost) :-
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F =< Threshold,
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(bagof(Next/Cost,
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(Space::next_state(State, Next, Cost), \+ list::member(Next, Path)),
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Successors) ->
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succlist(G, Successors, Trees, Threshold, Space),
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bestf(Trees, F2, Threshold),
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expand(Path, t(State, F2/G, Trees), Threshold, Tree, Solved, Solution, Space, Cost)
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;
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Solved = never).
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expand(Path, t(State, F/G,[Tree| Trees]), Threshold, Tree3, Solved, Solution, Space, Cost) :-
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F =< Threshold,
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bestf(Trees, Threshold2, Threshold),
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expand([State|Path], Tree, Threshold2, Tree2, Solved2, Solution, Space, Cost),
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continue(Path, t(State, F/G, [Tree2| Trees]), Threshold, Tree3, Solved2, Solved, Solution, Space, Cost).
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expand(_, t(_, _, []), _, _, never, _, _, _) :-
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!.
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expand(_, Tree, Threshold, Tree, no, _, _, _) :-
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f(Tree, F),
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F > Threshold.
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continue(_, _, _, _, yes, yes, _, _, _).
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continue(Path, t(State, _/G, [Tree| Trees]), Threshold, Tree2, no, Solved, Solution, Space, Cost) :-
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insert(Tree, Trees, NewTrees, Threshold),
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bestf(NewTrees, F, Threshold),
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expand(Path, t(State, F/G, NewTrees), Threshold, Tree2, Solved, Solution, Space, Cost).
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continue(Path,t(State, _/G, [_| Trees]), Threshold, Tree2, never, Solved, Solution, Space, Cost) :-
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bestf(Trees, F, Threshold),
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expand(Path, t(State, F/G, Trees), Threshold, Tree2, Solved, Solution, Space, Cost).
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succlist(_, [], [], _, _).
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succlist(G0, [State/Cost| Rest], Trees, Threshold, Space) :-
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G is G0 + Cost,
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Space::heuristic(State, H),
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F is G + H,
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succlist(G0, Rest, Trees2, Threshold, Space),
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insert(l(State, F/G), Trees2, Trees, Threshold).
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insert(Tree, [], [Tree], _) :-
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!.
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insert(Tree, Trees, [Tree| Trees], Threshold) :-
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f(Tree, F),
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bestf(Trees, F2, Threshold),
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F =< F2,
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!.
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insert(Tree, [Tree1| Trees], [Tree1| Trees1], Threshold) :-
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insert(Tree, Trees, Trees1, Threshold).
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f(l(_, F/_), F).
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f(t(_, F/_, _), F).
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bestf([Tree| _], F, _) :-
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f(Tree, F).
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bestf([], Threshold, Threshold).
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:- end_object.
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