Ticket 2141: bugs with weighted popularity lists across year boundary.

Consolidated several separate implementations of the same weighting algorithm into common_sql_weight() and fixed some bugs...
For MySQL, now using timestampdiff() instead of subtraction for the comparison, so we get sane results when the year doesn't match, and utc_timestamp() rather than now() so we don't get negative ages for recent items with local server timezone.
Unknown whether the same problems affect PostgreSQL, but note that it lacks the timestampdiff() SQL function.

lib/util.php

@@ -908,6 +908,26 @@ function common_sql_date($datetime)
     return strftime('%Y-%m-%d %H:%M:%S', $datetime);
 }
 
+/**
+ * Return an SQL fragment to calculate an age-based weight from a given
+ * timestamp or datetime column.
+ *
+ * @param string $column name of field we're comparing against current time
+ * @param integer $dropoff divisor for age in seconds before exponentiation
+ * @return string SQL fragment
+ */
+function common_sql_weight($column, $dropoff)
+{
+    if (common_config('db', 'type') == 'pgsql') {
+        // PostgreSQL doesn't support timestampdiff function.
+        // @fixme will this use the right time zone?
+        // @fixme does this handle cross-year subtraction correctly?
+        return "sum(exp(-extract(epoch from (now() - $column)) / $dropoff))";
+    } else {
+        return "sum(exp(timestampdiff(second, utc_timestamp(), $column) / $dropoff))";
+    }
+}
+
 function common_redirect($url, $code=307)
 {
     static $status = array(301 => "Moved Permanently",