Static Perfect Hashing

Data Structure AlgorithmsAnalysis of AlgorithmsAlgorithms

Definition of Perfect Hashing

Perfect hashing is defined as a model of hashing in which any set of n elements can be stored in a hash table of equal size and can have lookups performed in constant time. It was specifically invented and discussed by Fredman, Komlos and Szemeredi (1984) and has therefore been nicknamed as "FKS Hashing".

Definition of Static Hashing

Static Hashing defines another form of the hashing problem which permits users to accomplish lookups on a finalized dictionary set (that means all objects in the dictionary are final as well as not changing).

Application

Since static hashing needs that the database, its objects and reference remain the same its applications are limited. Databases which contain information which experiences rare change are also eligible as it would only require a full rehash of the whole database on rare occasion. Various examples of this hashing scheme include sets of words and definitions of specific languages, sets of significant data for an organization's personnel, etc.

Implementation

In the static case, we are provided a set with a total of p entries, each one associated with a unique key, ahead of time. Fredman, Komlós and Szemerédi select a first-level hash table with size s = 2(p-1) buckets. To construct, p entries are separated into q buckets by the top-level hashing function, where q = 2(p-1). Then for each bucket with r entries, a second-level table is allocated with r2 slots, and its hash function is chosen at random from a universal hash function set so that it becomes collision-free and stored alongside the hash table. If the hash function randomly chosen creates a table with collisions, a new hash function is randomly chosen until a collision-free table can be guaranteed. At last, with the collision-free hash, the r entries are hashed into the second-level table.

raja
Published on 03-Jan-2020 05:48:16
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