added the elo calculation method

ag/include/elo.inc.php

@@ -0,0 +1,41 @@
+<?php
+/*
+ *
+ * @copyright (c) 2009 animegame.eu
+ * @license http://www.gnu.org/licenses/gpl-3.0.html GNU General Public Licence
+ *
+ */
+
+
+/**
+ * Calculates the elo value out of the given parameters
+ * @param int ownElo the own elo value
+ * @param int otherElo the others elo value
+ * @param int result the result (4:1 = 3, 2:3 = -1)
+ * @param int maxResult the maximum value result can have
+ */
+function calculateElo($ownElo, $otherElo, $result, $maxResult) {
+	$erw = 1 / (1 + pow(10, ($otherElo - $ownElo)/400));
+	$sw = ($result + $maxResult) / ($maxResult * 2) ;
+	return round($ownElo + 50 * ($sw - $erw));
+}
+
+/*
+echo calculateElo(1000, 1000, 5, 5) . '<br>';
+echo calculateElo(1000, 1000, -5,5) . '<br>';
+echo calculateElo(900,1100,5,5) . '<br>';
+echo calculateElo(900,1100,-5,5) . '<br>';
+echo calculateElo(1100,900,5,5) . '<br>';
+echo calculateElo(1100,900,-5,5) . '<br>';
+echo calculateElo(2000,500,5,5) . '<br>';
+echo calculateElo(2000,500,-5,5) . '<br>';
+
+for($i=1000;$i<2000;$i++) {
+	if($i == calculateElo($i, 1000, 5, 5)) {
+		echo 'ranking that does not provide points against 1000 is ' .$i . '<br>';
+		break;
+	}
+}
+*/
+
+?>