--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/weight/probability/ad_weight_probability.inc	Thu Apr 02 15:28:21 2009 +0000
@@ -0,0 +1,57 @@
+<?php
+// $Id: ad_weight_probability.inc,v 1.1.4.3 2009/02/28 05:48:42 jeremy Exp $
+
+/**
+ * @file
+ * A plug in for the ad.module, allowing an admin to set the probability that
+ * a given advertisement will be displayed.
+ *
+ * Copyright (c) 2008-2009.
+ *  Jeremy Andrews <jeremy@tag1consulting.com>.
+ */
+
+function ad_weight_probability_cache_filter($ads) {
+  $display = array();
+  if (is_array($ads)) {
+    $probability = array();
+    $cache = adserve_cache('get_cache', 'weight');
+    foreach ($ads as $aid) {
+      $probability[] = isset($cache['probability'][$aid]) ? $cache['probability'][$aid] : 0;
+    }
+    $gcd = ad_weight_probability_gcd($probability);
+    _debug_echo("ad_weight_probability cache_filter gcd($gcd)");
+    foreach ($ads as $aid) {
+      $weight = (isset($cache['probability'][$aid]) && $gcd) ? $cache['probability'][$aid] / $gcd : 0;
+      _debug_echo("ad_weight_probability cache_filter aid($aid) weight($weight)");
+      for ($i = 1; $i <= $weight; $i++) {
+        $display[] = $aid;
+      }
+    }
+  }
+  return $display;
+}
+
+/**
+ * Returns the greatest common divisor of an array of integers.
+ */
+function ad_weight_probability_gcd($integers) {
+  $gcd = array_shift($integers);
+
+  while (!empty($integers)) {
+    $gcd = _ad_weight_probability_gcd($gcd, array_shift($integers));
+  }
+  return $gcd;
+}
+
+/**
+ * Helper function to calculate the greatest common divisor using the Euclidean
+ * algorithm (http://en.wikipedia.org/wiki/Euclidean_algorithm).
+ */
+function _ad_weight_probability_gcd($a, $b) {
+  if ($b == 0) {
+    return $a;
+  }
+  else {
+    return _ad_weight_probability_gcd($b, $a % $b);
+  }
+}