#!/usr/bin/perl

use strict;
use warnings;

my $board = "123456789";
my @wins = qw/ 123 456 789 147 258 369 159 357 /;

sub board {
	print "\n$_" for ($board =~ /(...)/g);
	if ($_ = shift) { print; exit; }
}

sub move { # Returns 0 for invalid move or piece, 1 for valid
	my $piece = shift; # X or O
	my $move  = shift; # 1 through 9
	return 0 if $piece !~ /^[XO]$/ or $move !~ /^[1-9]$/;
	return 0 unless $board =~ s/$move/$piece/;
	for (0..7) {
		board ("\n$piece wins!\n") if $wins[$_] =~ s/$move/$piece/ && $wins[$_] eq $piece x 3;
	}
	board ("\nTie!\n") if $board !~ /\d/;
	return 1;
}

sub check {
	my $piece = shift; # X or O
	my $move  = shift; # 1 through 9
	return 0 if $piece !~ /^[XO]$/ or $move !~ /^[1-9]$/;
	return grep { /$piece/ && /$move/ && /\d${piece}?\d/ } @wins;
}

sub player {
	my $retval = 0;
	my $move;
	until ( $retval ) {
		print "\nMove : ";
		chomp ($move = <>);
		$retval = move('X', $move);
	}
}

sub bot {
	return if move('O', 5);
	my $move;
	my @open = ($board =~ /(\d)/g);
	for $move (@open) { return move('O',$move) if grep {/$move/ && /O\d?O/} @wins; }
	for $move (@open) { return move('O',$move) if grep {/$move/ && /X\d?X/} @wins; }
	for (@open) { return move('O',$_) if ( check('O', $_) > 1 ); }
	for $move (@open) {
		if ( my @seq = check('O', $move) ) {
			my ($opposite) = $seq[0] =~ /([^O$move])/;
			return move('O', $move) if ( check('X', $opposite) < 2 );
		}
	}
	move ('O', $open[0]);
}

while (1) {
	board;
	player;
	bot;
}