A board analyser for our 7 Languages in 7 Weeks Prolog session

tictactoe.pl

%% A board is a list of states, each of which can be 'O', '' or 'X'
%% e.g. board('', '', '', '', '', '', '', '', '').

board('X', 'O', '', 'X', '', '', 'X', 'O', '').

row_one_win(X) :- board(X, X, X, _, _, _, _, _, _).
row_two_win(X) :- board(_, _, _, X, X, X, _, _, _).
row_three_win(X) :- board(_, _, _, _, _, _, X, X, X).
row_win(X) :- row_one_win(X) ; row_two_win(X) ; row_three_win(X).

column_one_win(X) :- board(X, _, _, X,_, _, X, _, _).
column_two_win(X) :- board(_, X, _, _, X, _, _, X, _).
column_three_win(X) :- board(_, _, X, _, _, X, _, _, X).
column_win(X) :- column_one_win(X) ; column_two_win(X) ; column_three_win(X).

leading_diagonal_win(X) :- board(X, _, _, _, X, _, _, _, X).
trailing_diagonal_win(X) :- board(_, _, X, _, X, _, X, _, _).
diagonal_win(X) :- leading_diagonal_win(X) ; trailing_diagonal_win(X).

win(X) :- (row_win(X) ; column_win(X) ; diagonal_win(X)), \=(X, '').

draw :- \+ win(X).

Line 20 is a win by (row OR column OR diagonal) AND X NOT EQUAL TO the empty string