Best-move minimax

Choosing the best move with the minimax algorithm

Source: The Art of Prolog

Program source code: best-move_minimax.pl

/*
   evaluate_and_choose(Moves,Position,Depth,Flag,Record,BestMove) :-
	Chooses the BestMove from the set of Moves from the current Position
	using the minimax algorithm searching Depth ply ahead.
	Flag indicates if we are currently minimizing or maximizing.
	Record records the current best move
*/
 
     evaluate_and_choose([Move|Moves],Position,D,MaxMin,Record,BestMove) :-
	move(Move,Position,Position1),
        minimax(D,Position1,MaxMin,MoveX,Value), 
	update(Move,Value,Record,Record1),
 	evaluate_and_choose(Moves,Position,D,MaxMin,Record1,BestMove).
     evaluate_and_choose([],Position,D,MaxMin,Record,Record).
 
     minimax(0,Position,MaxMin,Move,Value) :- 
	value(Position,V), 
	Value is V * MaxMin.
     minimax(D,Position,MaxMin,Move,Value) :-
	D > 0,
        findall(M,move(Position,M),Moves),
        D1 is D - 1,
        MinMax is -MaxMin,
        evaluate_and_choose(Moves,Position,D1,MinMax,(nil,-1000),(Move,Value)).
 
	update(Move,Value,(Move1,Value1),(Move1,Value1)) :- 
	   Value =< Value1.
	update(Move,Value,(Move1,Value1),(Move,Value)) :- 
	  Value > Value1.
 
%  Program 20.10   Choosing the best move with the minimax algorithm