Master Baboon

I wrote a simple game to learn how to store on and retrieve data from a server-side database. The rules are simple: you have to guess 2/3 of the average of the guesses of all the players. So, for example, if you think that the other players guessed on average 100, your guess should be 67. The 2/3 rule is there to avoid the trivial strategy of  submitting the same number over and over again. According to game theory, the game has a Nash equilibrium at 0, which in this case does not correspond to a rational strategy. Wikipedia has some information and links about the game. Give it a try:

The score you see after submitting is the distance from your guess to the true 2/3 of the average. Since you get some information about your and others’ score, it should be possible to find a strategy to win in a small number of trials… Let me know if you find one!

Communicating with the server

To implement the game, I created a MySQL table in a database on the server that is used to store the players’ name and guess. The database is accessed by calling a server-side PHP script from the game Actionscript client.

The PHP script gets the player’s information and stores it in the database; then it computes the 2/3 of the average of all entries, and returns the score and rank of the current player, along with the top scores in the database. The script receives the data as a HTTP request (the ?var1=value1&var2=value2 thing you see in many dynamical web pages), and returns an XML fragment.

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<?php



// submit name, guess, ip, time to the Guess 2/3 database



// input variables:

// name: player's name

// guess: the guessed 2/3 of the average

// ntop: the number of top scores to be sent back



// this is how to obtain and verify the HTTP variables

if (!isset($_REQUEST["name"])) { die("Argument 'name' needed."); }

$name   = $_REQUEST["name"];

if (!isset($_REQUEST["guess"])) { die("Argument 'guess' needed."); }

$guess  = $_REQUEST["guess"];

if (!isset($_REQUEST["ntop"])) { die("Argument 'ntop' needed."); }

$ntop   = $_REQUEST["ntop"];

// some server data

$ip     = $_SERVER["REMOTE_ADDR"];

// the request time is used as unique identifier for every entry

$time   = $_SERVER["REQUEST_TIME"];



$dbh=mysql_connect ("localhost", "username", "password")

    or die ('Error connecting to the database:' . mysql_error());

mysql_select_db ("dabasename");



// insert new entry in database

// ============================



// this is the MySQL query to insert a new guess in the database

$sql = "INSERT INTO Guess23(name, guess, ip, time)

    VALUES ('{$name}', '{$guess}', '{$ip}', '{$time}')";

mysql_query($sql) or die (mysql_error());



// compute top scores and return XML fragment with top results

// ===========================================================



// compute 2/3 of the average

// SQL query to compute 2/3 of the average of the 'guess' column

$sql = 'SELECT (avg(guess)*2/3) AS avg FROM `Guess23`';

$qry = mysql_query($sql);

$avg = mysql_fetch_row($qry);



// SQL query to get all guesses so far and sort them by distance

// from the real target value, here called 'diff'

$sql = "SELECT name, guess, time, abs(guess-'$avg[0]') AS diff FROM `Guess23` ORDER BY diff ASC";

$hit = mysql_query($sql) or die(mysql_error());



// the following loop generates the XML code with the top scores

// in the loop, we also look for the current player's entry to

// determine her ranking

$currentrank = 0;

$rank = 1;

print "<table>\n";

// top scores

while (($row = mysql_fetch_array($hit)) && ($currentrank==0 || $rank<=$ntop)) {

  if ($rank<=$ntop) {

    print "<entry rank='{$rank}'>\n";

    print "\t<name>{$row['name']}</name>\n";

    print "\t<score>{$row['diff']}</score>\n";

    print "</entry>\n";

  }

  if ($row['time'] == $time) {

    $currentrank = $rank;

    $currentscore = $row['diff'];

  }

  $rank = $rank + 1;

}



// current score

print "<current rank='{$currentrank}'>\n\t<name>{$name}</name>\n\t<score>{$currentscore}</score>\n</current>";

print "</table>\n";



mysql_close();



?>

Calling the script from AS3 is relativelty easy. The remote call is done by a URLLoader object; the actual request is stored in a URLRequest object, and the request variables are wrapped in a URLVariables instance. I had an annoying problem with testing the code: for some reason, Flash always caches the requests. I looked around and it seems that the only workaround is to add a dummy variable to the request with a value that changes constantly, e.g., the current time.

This is the code for the communication with the server:

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public function submitGuess(name:String, guess:uint, ntop:int):void {

    var request:URLRequest = new URLRequest("http://www.masterbaboon.com/php/guess23/guess23submit.php");

    var httpHeader : URLRequestHeader = new URLRequestHeader("pragma", "no-cache");

    request.requestHeaders.push(httpHeader);



    // request variables: name of player, guess, number of top scores to return

    var vars:URLVariables = new URLVariables();

    vars.name = name;

    vars.guess = guess;

    vars.ntop = ntop;

    // this is to trick AS3 into not caching the URL request

    vars.nocache = Number(new Date().getTime());



    request.method = URLRequestMethod.GET;

    request.data = vars;



    // send to server and call loadingComplete when complete

    var loader:URLLoader = new URLLoader();



    // the results of the request is not returned immediately, but

    // we can monitor the COMPLETE event dispatched by the loader

    loader.addEventListener(Event.COMPLETE, loadingComplete);

    try {

        loader.load(request);

    } catch (e:Error) {

        trace("An error has occurred while communicating with the server.");

        trace(e);

    }

}



public function loadingComplete(e:Event):void {

    // this is how to access the data returned from the request

    data = XML(e.target.data);

    // switch frame

    gotoAndStop("highScoreFrame");

}

You’re welcome to download the whole code.