1. A cache data structure, comprising: a hash array comprising one or more index elements, each index element comprising a host tree pointer and a hash key derived from a stored URL; and one or more host trees depending from said host tree pointers of the index elements of the hash array, each host tree comprising one or more tree nodes each holding URL data representing stored URLs and associated category codes; and an age list to list each of the tree nodes by age, wherein the age list comprises, within each tree node, a next pointer and a previous pointer which refer to a next older tree node and a previous newer tree node, respectively.