# rulesAI($x_or_o, @board);
# Returns index of chosen move
# @board = 9-element array, initialized to ($c,$c,$c,$c,$c,$c,$c,$c,$c)
# $c - character representing unoccupied square
# sub legals() - accepts a @board, returns list of unoccupied spaces on it
# sub check() - accepts a @board, returns "x" or "o" if x or o won, -1 if tie, 0 otherwise

sub rulesAI {
	my $mv = -1;
	my $col = shift;
	my @choices = legals(@_);
	# priorities:
	# 1. Win, if possible
	foreach $m (@choices) {
		$_[$m] = $col;
		if (check(@_) =~ /$col/) { $mv = $m; last; }
		$_[$m] = $c;
	}
	unless ($mv == -1) { return $mv; }
	# 2. Block opponent, if necessary
	my $opcol = ($col =~ /x/) ? 'o' : 'x';
	foreach $m (@choices) {
		$_[$m] = $opcol;
		if (check(@_) =~ /$opcol/) { $mv = $m; last; }
		$_[$m] = $c;
	}
	unless ($mv == -1) { return $mv; }
	# 3. Set up a trap
	# 4. Choose at random
	return $choices[(rand$#choices+1) % ($#choices+1)];
}