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 — pl:prolog:pllib:best-first [2019/06/27 15:50] (aktualna) Linia 1: Linia 1: + ====== Best-first ====== + {{tag>​searching}} + ===== Description ===== + A best-first search program. + + **Source**: ​ PROLOG programming for artificial intelligence,​ 3rd Edition, Harlow, 2001, ISBN 0-201-40375-7. + ===== Download ===== + Program source code: {{best-first.pl}} + ===== Listing ===== + + % Figure 12.3  A best-first search program. + + :- op( 900, fy, not). + + % not Goal): negation as failure; ​ + %   Note: This is often available as a built-in predicate, + %   often written as prefix operator "​\+",​ e.g. \+ likes(mary,​snakes) + + not Goal  :- + Goal, !, fail + ; + true. + + % bestfirst( Start, Solution): Solution is a path from Start to a goal + + bestfirst( Start, Solution) :- + expand( [], l( Start, 0/0),  9999, _, yes, Solution). + %  Assume 9999 is greater than any f-value + + % expand( Path, Tree, Bound, Tree1, Solved, Solution): + %   Path is path between start node of search and subtree Tree, + %   Tree1 is Tree expanded within Bound, + %   if goal found then Solution is solution path and Solved = yes + + %  Case 1: goal leaf-node, construct a solution path + + expand( P, l( N, _), _, _, yes, [N|P]) ​ :- + ​goal(N). + + %  Case 2: leaf-node, f-value less than Bound + %  Generate successors and expand them within Bound. + + expand( P, l(N,F/G), Bound, Tree1, Solved, Sol)  :- + F  =<  Bound, + (  bagof( M/C, ( s(N,M,C), not member(M,P) ), Succ), ​ + ​!, ​                                   % Node N has successors + ​succlist( G, Succ, Ts),               % Make subtrees Ts + ​bestf( Ts, F1),                       % f-value of best successor + ​expand( P, t(N,​F1/​G,​Ts),​ Bound, Tree1, Solved, Sol) + ; + ​Solved = never                        % N has no successors - dead end + ) . + + %  Case 3: non-leaf, f-value less than Bound + %  Expand the most promising subtree; depending on + %  results, procedure continue will decide how to proceed + + expand( P, t(N,​F/​G,​[T|Ts]),​ Bound, Tree1, Solved, Sol)  :- + F  =<  Bound, + bestf( Ts, BF), min( Bound, BF, Bound1), ​         % Bound1 = min(Bound,​BF) + expand( [N|P], T, Bound1, T1, Solved1, Sol), + continue( P, t(N,​F/​G,​[T1|Ts]),​ Bound, Tree1, Solved1, Solved, Sol). + + %  Case 4: non-leaf with empty subtrees + %  This is a dead end which will never be solved + + expand( _, t(_,_,[]), _, _, never, _) :- !. + + %  Case 5:  f-value greater than Bound + %  Tree may not grow. + + expand( _, Tree, Bound, Tree, no, _)  :- + f( Tree, F), F > Bound. + + % continue( Path, Tree, Bound, NewTree, SubtreeSolved,​ TreeSolved, Solution) + + continue( _, _, _, _, yes, yes, Sol). + + continue( P, t(N,​F/​G,​[T1|Ts]),​ Bound, Tree1, no, Solved, Sol)  :- + insert( T1, Ts, NTs), + bestf( NTs, F1), + expand( P, t(N,​F1/​G,​NTs),​ Bound, Tree1, Solved, Sol). + + continue( P, t(N,​F/​G,​[_|Ts]),​ Bound, Tree1, never, Solved, Sol)  :- + bestf( Ts, F1), + expand( P, t(N,​F1/​G,​Ts),​ Bound, Tree1, Solved, Sol). + + % succlist( G0, [ Node1/​Cost1,​ ...], [ l(BestNode,​BestF/​G),​ ...]): + %   make list of search leaves ordered by their F-values + + succlist( _, [], []). + + succlist( G0, [N/C | NCs], Ts)  :- + G is G0 + C, + h( N, H),                             % Heuristic term h(N) + F is G + H, + succlist( G0, NCs, Ts1), + insert( l(N,F/G), Ts1, Ts). + + % Insert T into list of trees Ts preserving order w.r.t. f-values + + insert( T, Ts, [T | Ts])  :- + f( T, F), bestf( Ts, F1), + F  =<  F1, !. + + insert( T, [T1 | Ts], [T1 | Ts1])  :- + insert( T, Ts, Ts1). + + + % Extract f-value + + f( l(_,F/_), F).        % f-value of a leaf + + f( t(_,F/_,_), F).      % f-value of a tree + + bestf( [T|_], F)  :-    % Best f-value of a list of trees + f( T, F). + + bestf( [], 9999). ​      % No trees: bad f-value + ​ + min( X, Y, X)  :- + X  =<  Y, !. + + min( X, Y, Y). + ​ + ===== Comments =====
pl/prolog/pllib/best-first.txt · ostatnio zmienione: 2019/06/27 15:50 (edycja zewnętrzna)