# ttt:   play tic-tac-toe
# usage: ttt
# 2000-03-13 erikt@uia.ua.ac.be

$true = 1;
$false = 0;
$gamesPlayed = 0;
$continue = $true;
$computerScore = 0;
$humanScore = 0;
$computerStarts = "";

# this loop allows us to play a series of games
while ($continue) {
   $movesPlayed = 0;
   $maxMovesPlayed = 9;
   $gameWon = $false;
   
   # The field contains nine squares which can be identified as follows:
   # 0..1..2
   # 3..4..5
   # 6..7..8 
   # In the move lists the value $true means that the player has occupied
   # the square. 
   @computerMoves = 
      ($false,$false,$false,$false,$false,$false,$false,$false,$false);
   @humanMoves = 
      ($false,$false,$false,$false,$false,$false,$false,$false,$false);
   @occupied = 
       ($false,$false,$false,$false,$false,$false,$false,$false,$false);
   
   # The field contains eight rows which can be identified as follows:
   # 0  1  2  3  4    row 0 contains squares 0, 4, 8
   #    0..1..2  5    row 1 contains squares 0, 3, 6
   #    3..4..5  6    row 2 contains squares 1, 4, 7
   #    6..7..8  7    row 3 contains squares 2, 5, 8
   #                  row 4 contains squares 2, 4, 6
   #                  row 5 contains squares 0, 1, 2
   #                  row 6 contains squares 3, 4, 5
   #                  row 7 contains squares 6, 7, 8
   # Each square is represented by a number in which ones and zeroes 
   # indicate in what rows the square appears. 
   @fieldValues = ( 11000100,  100100,   11100,
                     1000010,10101010,   10010,
                     1001001,  100001,10010001);
   # The numbers will be added to the field value of the player that 
   # occupies them. As soon as one of these numbers contains a 3 then
   # the player has three-in-a-row
   $computerField = 0;
   $humanField = 0;
   
   # let the computer play the first move, if necessary
   if ($computerStarts !~ /^[01]$/) {
      print "Do you want to start (y/n)? ";
      chomp($answer = <>);
      $computerStarts = $answer ne "y";
   }
   if ($computerStarts) {
      # compute a computer move: choose a random number until it is ok
      $move = $maxMovesPlayed;
      $move = int(rand(9)) while (not(defined($occupied[$move])) or
                                  $occupied[$move]);
      print "computer playes ",$move+1,"\n";
      $computerMoves[$move] = $true;
      $occupied[$move] = $true;
      $computerField += $fieldValues[$move];
      $movesPlayed++;
      # for next game
      $computerStarts = $false;
   } else { $computerStarts = $true; }
   
   # while the game has not ended, read a human move
   while ($movesPlayed < $maxMovesPlayed and not($gameWon)) {
      # print the field
      for ($i=0;$i<$maxMovesPlayed;$i++) {
         if (not($occupied[$i])) { print ". "; }
         elsif ($computerMoves[$i]) { print "x "; }
         elsif ($humanMoves[$i]) { print "o "; }
         else { print "cannot happen\n"; exit(1); }
         if ($i == 3*(int(($i+1)/3))-1) { print "\n"; }
      }
      # read a human move: accept numbers 1-9!
      $move = "";
      while ($move !~ /^[1-9]$/ or $occupied[$move-1]) {
         print "Please enter a move (1-9): ";
         chomp($move = <>);
      }
      # go back to numbers 0-8
      $move--;
      $humanMoves[$move] = $true;
      $occupied[$move] = $true;
      $humanField += $fieldValues[$move];
      $movesPlayed++;
      if ($humanField =~ /3/) { 
         print "Human wins!\n"; 
         $humanScore++;
         $gameWon = $true; 
      }
   
      # if the game has not ended, play a computer move
      if ($movesPlayed < $maxMovesPlayed and not($gameWon)) {
         $move = $maxMovesPlayed;
         $move = int(rand(9)) while (not(defined($occupied[$move])) or
                                     $occupied[$move]);
         print "computer playes ",$move+1,"\n";
         $computerMoves[$move] = $true;
         $occupied[$move] = $true;
         $computerField += $fieldValues[$move];
         $movesPlayed++;
         if ($computerField =~ /3/) { 
            print "Computer wins!\n";
            $computerScore++;
            $gameWon = $true; 
         }
      }
   }
   
   # print the final field
   for ($i=0;$i<$maxMovesPlayed;$i++) {
      if (not($occupied[$i])) { print ". "; }
      elsif ($computerMoves[$i]) { print "x "; }
      elsif ($humanMoves[$i]) { print "o "; }
      else { print "cannot happen\n"; exit(1); }
      if ($i == 3*(int(($i+1)/3))-1) { print "\n"; }
   }
   if (not($gameWon)) { print "Draw.\n"; }

   # print results
   $gamesPlayed++;
   print "\n$gamesPlayed game";
   print "s" if ($gamesPlayed != 1);
   print " played: Computer $computerScore - Human $humanScore\n";
   print "\nContinue playing? (y/n)";
   chomp($answer = <>);
   $continue = $false if ($answer eq "n");
}
exit(0);