1. A data structure for storage in a computer memory, said data structure accessible by an application program being executed on a data processing system, said data structure comprising: a root pointer; and a digital tree pointed to by said root pointer, comprising a first plurality of nodes arranged hierarchically, a second plurality of said nodes including: a branch node selected from the group consisting of a linear, bitmap and uncompressed branch node selected according to a number of populated subexpanses and an overall status of the digital tree, and a leaf node selected from the group consisting of linear and bitmap leaf nodes, each holding a plurality of indexes and containing only undecoded index bits according to a level of the leaf in the digital tree and a number of indexes in the leaf.