Tic-Tac-Toe program in Prolog

% A Tic-Tac-Toe program in Prolog. S. Tanimoto, May 11, 2003.
% To play a game with the computer, type
% playo.
% To watch the computer play a game with itself, type
% selfgame.

% Predicates that define the winning conditions:

win(Board, Player) :- rowwin(Board, Player).
win(Board, Player) :- colwin(Board, Player).
win(Board, Player) :- diagwin(Board, Player).

rowwin(Board, Player) :- Board = [Player,Player,Player,,,,,,].
rowwin(Board, Player) :- Board = [,,,Player,Player,Player,,,].
rowwin(Board, Player) :- Board = [,,,,,,Player,Player,Player].

colwin(Board, Player) :- Board = [Player,,,Player,,,Player,,].
colwin(Board, Player) :- Board = [,Player,,,Player,,,Player,].
colwin(Board, Player) :- Board = [,,Player,,,Player,,,Player].

diagwin(Board, Player) :- Board = [Player,,,,Player,,,,Player].
diagwin(Board, Player) :- Board = [,,Player,,Player,,Player,,].

% Helping predicate for alternating play in a “self” game:

other(x,o).
other(o,x).

game(Board, Player) :- win(Board, Player), !, write([player, Player, wins]).
game(Board, Player) :-
other(Player,Otherplayer),
move(Board,Player,Newboard),
!,
display(Newboard),
game(Newboard,Otherplayer).

move([b,B,C,D,E,F,G,H,I], Player, [Player,B,C,D,E,F,G,H,I]).
move([A,b,C,D,E,F,G,H,I], Player, [A,Player,C,D,E,F,G,H,I]).
move([A,B,b,D,E,F,G,H,I], Player, [A,B,Player,D,E,F,G,H,I]).
move([A,B,C,b,E,F,G,H,I], Player, [A,B,C,Player,E,F,G,H,I]).
move([A,B,C,D,b,F,G,H,I], Player, [A,B,C,D,Player,F,G,H,I]).
move([A,B,C,D,E,b,G,H,I], Player, [A,B,C,D,E,Player,G,H,I]).
move([A,B,C,D,E,F,b,H,I], Player, [A,B,C,D,E,F,Player,H,I]).
move([A,B,C,D,E,F,G,b,I], Player, [A,B,C,D,E,F,G,Player,I]).
move([A,B,C,D,E,F,G,H,b], Player, [A,B,C,D,E,F,G,H,Player]).

display([A,B,C,D,E,F,G,H,I]) :- write([A,B,C]),nl,write([D,E,F]),nl,
write([G,H,I]),nl,nl.

selfgame :- game([b,b,b,b,b,b,b,b,b],x).

% Predicates to support playing a game with the user:

x_can_win_in_one(Board) :- move(Board, x, Newboard), win(Newboard, x).

% The predicate orespond generates the computer’s (playing o) reponse
% from the current Board.

orespond(Board,Newboard) :-
move(Board, o, Newboard),
win(Newboard, o),
!.
orespond(Board,Newboard) :-
move(Board, o, Newboard),
not(x_can_win_in_one(Newboard)).
orespond(Board,Newboard) :-
move(Board, o, Newboard).
orespond(Board,Newboard) :-
not(member(b,Board)),
!,
write(‘Cats game!’), nl,
Newboard = Board.

% The following translates from an integer description
% of x’s move to a board transformation.

xmove([b,B,C,D,E,F,G,H,I], 1, [x,B,C,D,E,F,G,H,I]).
xmove([A,b,C,D,E,F,G,H,I], 2, [A,x,C,D,E,F,G,H,I]).
xmove([A,B,b,D,E,F,G,H,I], 3, [A,B,x,D,E,F,G,H,I]).
xmove([A,B,C,b,E,F,G,H,I], 4, [A,B,C,x,E,F,G,H,I]).
xmove([A,B,C,D,b,F,G,H,I], 5, [A,B,C,D,x,F,G,H,I]).
xmove([A,B,C,D,E,b,G,H,I], 6, [A,B,C,D,E,x,G,H,I]).
xmove([A,B,C,D,E,F,b,H,I], 7, [A,B,C,D,E,F,x,H,I]).
xmove([A,B,C,D,E,F,G,b,I], 8, [A,B,C,D,E,F,G,x,I]).
xmove([A,B,C,D,E,F,G,H,b], 9, [A,B,C,D,E,F,G,H,x]).
xmove(Board, N, Board) :- write(‘Illegal move.’), nl.

% The 0-place predicate playo starts a game with the user.

playo :- explain, playfrom([b,b,b,b,b,b,b,b,b]).

explain :-
write(‘You play X by entering integer positions followed by a period.’),
nl,
display([1,2,3,4,5,6,7,8,9]).

playfrom(Board) :- win(Board, x), write(‘You win!’).
playfrom(Board) :- win(Board, o), write(‘I win!’).
playfrom(Board) :- read(N),
xmove(Board, N, Newboard),
display(Newboard),
orespond(Newboard, Newnewboard),
display(Newnewboard),
playfrom(Newnewboard).f

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