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All versions of course get faster and use less memory as the board fills up. The faster versions should generate a move in under 10 seconds as soon as 2 moves have been made.

All versions of course get faster and use less memory as the board fills up. The faster versions should generate a move in under 10 seconds as soon as 2 or 3 moves have been made.

#!/usr/bin/perl
sub f{
    if ($p++ % 100000 == 0) {
        local $| = 1;
        print ".";
    }
y/XO/OX/,$@=-$@while$|-=/(@{[map{(O.".{$_}O"x3)=~s%O%Z|$`X$'|Z%gr}0,3..5]})(?{$@++})^|$/osx;$@<=>0||s%-%$_="$`O$'";$a{$_}//=&f+1or return 1%eeg&&/1/-1}

# Driver
my $tomove = "X";
my $move = 0;
@board = ("----\n") x 4;
while (1) {
    print "Current board after move $move ($tomove to move):\n  ABCD\n";
    for my $i (1..4) {
        print "$i $board[$i-1]";
    }
    print "Enter a move like B4, PASS (not a valid move, just for setup) or just press enter to let the program make suggestions\n";
    my $input = <> // die "Unexpected EOF";
    if ($input eq "\n") {
        $_ = join "", @board;
        tr/OX/XO/ if $tomove eq "O";
        $p = 0;
        $@="";
        %a = ();
        my $start = time();
        my $result = f;
        if ($result == 1) {
            tr/OX/XO/ if $tomove eq "O";
            tr/012/-/;
        } else {
            tr/OX/XO/ if $tomove eq "X";
            tr/012/123/;
        }
        $result = -$result if $tomove eq "O";
        my $period = time() - $start;
        print "\nSuggested moves (evaluated $p positions in $period seconds, predicted result for X: $result):\n$_";
        redo;
    } elsif ($input =~ /^pass$/i) {
        # Do nothing
    } elsif (my ($x, $y) = $input =~ /^([A-D])([1-4])$/) {
        $x = ord($x) - ord("A");
        --$y;
        my $ch = substr($board[$y],$x, 1);
        if ($ch ne "-") {
            print "Position already has $ch. Try again\n";
            redo;
        }
        substr($board[$y],$x, 1) = $tomove;
    } else {
        print "Cannot parse move. Try again\n";
        redo;
    }
    $tomove =~ tr/OX/XO/;
    ++$move;
}

#!/usr/bin/perl
sub f{
    if ($p++ % 100000 == 0) {
        local $| = 1;
        print ".";
    }
y/XO/OX/,$@=-$@while$|-=/(@{[map{(O.".{$_}O"x3)=~s%O%Z|$`X$'|Z%gr}0,3..5]})(?{$@++})^|$/osx;$@<=>0||s%-%$_="$`O$'";$a{$_}//=&f+1or return 1%eeg&&/1/-1}

# Driver
my $tomove = "X";
my $move = 0;
@board = ("----\n") x 4;
while (1) {
    print "Current board after move $move ($tomove to move):\n  ABCD\n";
    for my $i (1..4) {
        print "$i $board[$i-1]";
    }
    print "Enter a move like B4, PASS (not a valid move, just for setup) or just press enter to let the program make suggestions\n";
    my $input = <> // exit;
    if ($input eq "\n") {
        $_ = join "", @board;
        tr/OX/XO/ if $tomove eq "O";
        $p = 0;
        $@="";
        %a = ();
        my $start = time();
        my $result = f;
        if ($result == 1) {
            tr/OX/XO/ if $tomove eq "O";
            tr/012/-/;
        } else {
            tr/OX/XO/ if $tomove eq "X";
            tr/012/123/;
        }
        $result = -$result if $tomove eq "O";
        my $period = time() - $start;
        print "\nSuggested moves (evaluated $p positions in $period seconds, predicted result for X: $result):\n$_";
        redo;
    } elsif ($input =~ /^pass$/i) {
        # Do nothing
    } elsif (my ($x, $y) = $input =~ /^([A-D])([1-4])$/) {
        $x = ord($x) - ord("A");
        --$y;
        my $ch = substr($board[$y],$x, 1);
        if ($ch ne "-") {
            print "Position already has $ch. Try again\n";
            redo;
        }
        substr($board[$y],$x, 1) = $tomove;
    } else {
        print "Cannot parse move. Try again\n";
        redo;
    }
    $tomove =~ tr/OX/XO/;
    ++$move;
}

That last one uses 0 for a win (don't confuse with O), 1 for a draw and 2 for a loss.

All versions of course get faster and use less memory as the board fills up.

If you put $\= (cost: 3 bytes) just before the $@<=>0 then each output board will be followed by the status of the whole board: 1 for X wins, 0 for draw and -1 for loss.

That last one uses 0 for a win (don't confuse with O), 1 for a draw and 2 for a loss. The output of this one is also more confusing. It fills in the winning move for X in case of a win without color swap, but if the input game was already won is still does the color swap and does not fill in any move.

All versions of course get faster and use less memory as the board fills up. The faster versions should generate a move in under 10 seconds as soon as 2 moves have been made.

If you put $\= (cost: 3 bytes) just before the $@<=>0 then each output board will be followed by the status of the whole board: 1 for X wins, 0 for draw and -1 for loss.

Here is an interactive driver based on the fastest version mentioned above. The driver has no logic for when the game is finished so you have to stop yourself. The golfed code knows though. If the suggested move returns with no - replaced by anything the game is over.

#!/usr/bin/perl
sub f{
    if ($p++ % 100000 == 0) {
        local $| = 1;
        print ".";
    }
y/XO/OX/,$@=-$@while$|-=/(@{[map{(O.".{$_}O"x3)=~s%O%Z|$`X$'|Z%gr}0,3..5]})(?{$@++})^|$/osx;$@<=>0||s%-%$_="$`O$'";$a{$_}//=&f+1or return 1%eeg&&/1/-1}

# Driver
my $tomove = "X";
my $move = 0;
@board = ("----\n") x 4;
while (1) {
    print "Current board after move $move ($tomove to move):\n  ABCD\n";
    for my $i (1..4) {
        print "$i $board[$i-1]";
    }
    print "Enter a move like B4, PASS (not a valid move, just for setup) or just press enter to let the program make suggestions\n";
    my $input = <> // die "Unexpected EOF";
    if ($input eq "\n") {
        $_ = join "", @board;
        tr/OX/XO/ if $tomove eq "O";
        $p = 0;
        $@="";
        %a = ();
        my $start = time();
        my $result = f;
        if ($result == 1) {
            tr/OX/XO/ if $tomove eq "O";
            tr/012/-/;
        } else {
            tr/OX/XO/ if $tomove eq "X";
            tr/012/123/;
        }
        $result = -$result if $tomove eq "O";
        my $period = time() - $start;
        print "\nSuggested moves (evaluated $p positions in $period seconds, predicted result for X: $result):\n$_";
        redo;
    } elsif ($input =~ /^pass$/i) {
        # Do nothing
    } elsif (my ($x, $y) = $input =~ /^([A-D])([1-4])$/) {
        $x = ord($x) - ord("A");
        --$y;
        my $ch = substr($board[$y],$x, 1);
        if ($ch ne "-") {
            print "Position already has $ch. Try again\n";
            redo;
        }
        substr($board[$y],$x, 1) = $tomove;
    } else {
        print "Cannot parse move. Try again\n";
        redo;
    }
    $tomove =~ tr/OX/XO/;
    ++$move;
}