+/**
+ * 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))";
+ }
+}
+