A PERFECT HASHING INCREMENTAL SCHEMEFOR UNRANKED TREES USING PSEUDO-MINIMALAUTOMATA

We describe a technique that maps unranked trees to arbitraryhash codes using a bottom-up deterministic tree automaton(DTA). In contrast to other hashing techniques based on automata, ourprocedure builds a pseudo-minimal DTA for this purpose. A pseudominimalautomaton may be larger than the minimal one accepting thesame language but, in turn, it contains proper elements (states or transitionswhich are unique) for every input accepted by the automaton.Therefore, pseudo-minimal DTA are a suitable structure to implementstable hashing schemes, that is, schemes where the output for everykey can be determined prior to the automaton construction. We provideincremental procedures to build the pseudo-minimal DTA and themapping that associates an integer value to every transition that willbe used to compute the hash codes. This incremental construction allowsfor the incorporation of new trees and their hash codes withoutthe need to rebuild the whole DTA from scratch