COMPRESSION OF GROUPED DATA

An electronic reference device provides the ability to access all members of any one of a plurality of groups of data items in response to a user input of any one member of the group being accessed. The reference device includes a memory having a data item list, a token uniquely identifying each member of the data item list, and a plurality of groups of the data item list. Certain members of said data item list being a member of one or more of the groups and certain members of the data item list being a member of none of the groups. An ordered search list consists of each member of the data item list which is a member of at least one of the groups. A delta value is associated with each member of the ordered search list. The sum of the delta values up through any one of the members of the ordered search list provides the value of the token for the corresponding member of the data item list. Each member of the search list has a next-entry pointer identifying another member of the search list. All members of the search list that are linked by a set of inter-connecting next-entry pointers correspond to members of the data item list from the same one of the groups.