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yap-6.3/Logtalk/examples/searching/best_first1.lgt
pmoura b77427df89 Updated to Logtalk 2.9.1
git-svn-id: https://yap.svn.sf.net/svnroot/yap/trunk@211 b08c6af1-5177-4d33-ba66-4b1c6b8b522a
2001-12-05 23:57:48 +00:00

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:- object(best_first(Threshold),
instantiates(heuristic_search(Threshold))).
:- info([
version is 1.0,
authors is 'Paulo Moura',
date is 1998/3/23,
comment is 'Best first heuristic state space search strategy.',
parnames is ['Threshold']]).
:- uses(list).
:- private(expand/8).
:- private(succlist/5).
:- private(bestf/3).
:- private(continue/9).
:- private(f/2).
:- private(insert/4).
search(Space, State, Threshold, Solution, Cost) :-
expand([], l(State, 0/0), Threshold, _, yes, Path, Space, Cost),
list::reverse(Path, Solution).
expand(Path, l(State,Cost/_), _, _, yes, [State|Path], Space, Cost) :-
Space::goal_state(State).
expand(Path, l(State,F/G), Threshold, Tree, Solved, Solution, Space, Cost) :-
F =< Threshold,
(bagof(Next/Cost2,
(Space::next_state(State, Next, Cost2), \+ list::member(Next, Path)),
Successors) ->
succlist(G, Successors, Trees, Threshold, Space),
bestf(Trees, F2, Threshold),
expand(Path, t(State, F2/G, Trees), Threshold, Tree, Solved, Solution, Space, Cost)
;
Solved = never).
expand(Path, t(State, F/G,[Tree| Trees]), Threshold, Tree3, Solved, Solution, Space, Cost) :-
F =< Threshold,
bestf(Trees, Threshold2, Threshold),
expand([State|Path], Tree, Threshold2, Tree2, Solved2, Solution, Space, Cost),
continue(Path, t(State, F/G, [Tree2| Trees]), Threshold, Tree3, Solved2, Solved, Solution, Space, Cost).
expand(_, t(_, _, []), _, _, never, _, _, _) :-
!.
expand(_, Tree, Threshold, Tree, no, _, _, _) :-
f(Tree, F),
F > Threshold.
continue(_, _, _, _, yes, yes, _, _, _).
continue(Path, t(State, _/G, [Tree| Trees]), Threshold, Tree2, no, Solved, Solution, Space, Cost) :-
insert(Tree, Trees, NewTrees, Threshold),
bestf(NewTrees, F, Threshold),
expand(Path, t(State, F/G, NewTrees), Threshold, Tree2, Solved, Solution, Space, Cost).
continue(Path,t(State, _/G, [_| Trees]), Threshold, Tree2, never, Solved, Solution, Space, Cost) :-
bestf(Trees, F, Threshold),
expand(Path, t(State, F/G, Trees), Threshold, Tree2, Solved, Solution, Space, Cost).
succlist(_, [], [], _, _).
succlist(G0, [State/Cost| Rest], Trees, Threshold, Space) :-
G is G0 + Cost,
Space::heuristic(State, H),
F is G + H,
succlist(G0, Rest, Trees2, Threshold, Space),
insert(l(State, F/G), Trees2, Trees, Threshold).
insert(Tree, [], [Tree], _) :-
!.
insert(Tree, Trees, [Tree| Trees], Threshold) :-
f(Tree, F),
bestf(Trees, F2, Threshold),
F =< F2,
!.
insert(Tree, [Tree1| Trees], [Tree1| Trees1], Threshold) :-
insert(Tree, Trees, Trees1, Threshold).
f(l(_, F/_), F).
f(t(_, F/_, _), F).
bestf([Tree| _], F, _) :-
f(Tree, F).
bestf([], Threshold, Threshold).
:- end_object.