Apparatus, systems, and methods operate to obtain data from a first array constructed from a directed acyclic graph formed as a prefix tree having key strings associated with a plurality of interconnected nodes, including branch nodes coupled via branches to sibling nodes and child nodes. Reference numbers are assigned to nodes in a monotonic progression as the prefix tree is traversed along the plurality of nodes. Sibling nodes are assigned reference numbers before child nodes, and child nodes are assigned reference numbers according to the order of appearance of key string characters. The first array comprises the key strings ordered according to the reference numbers. A second array can be formed as a linear searchable index derived from data in the first array, with elements of the second array comprising the reference numbers. Additional apparatus, systems, and methods are disclosed.