CONCEPTS.t2t

Minimal Perfect Hash Functions - Introduction


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==Basic Concepts==

Suppose [figs/img14.png] is a universe of //keys//.
Let [figs/img15.png] be a //hash function// that maps the keys from [figs/img14.png] to a given interval of integers [figs/img16.png].
Let [figs/img17.png] be a set of [figs/img8.png] keys from [figs/img14.png].
Given a key [figs/img18.png], the hash function [figs/img7.png] computes an 
integer in [figs/img19.png] for the storage or retrieval of [figs/img11.png] in 
a //hash table//.
Hashing methods for //non-static sets// of keys can be used to construct
data structures storing [figs/img20.png] and supporting membership queries
"[figs/img18.png]?" in expected time [figs/img21.png].
However, they involve a certain amount of wasted space owing to unused
locations in the table and waisted time to resolve collisions when
two keys are hashed to the same table location.

For //static sets// of keys it is possible to compute a function
to find any key in a table in one probe; such hash functions are called
//perfect//. 
More precisely, given a set of keys [figs/img20.png], we shall say that a 
hash function [figs/img15.png] is a //perfect hash function// 
for [figs/img20.png] if [figs/img7.png] is an injection on [figs/img20.png],
that is, there are no //collisions// among the keys in [figs/img20.png]: 
if [figs/img11.png] and [figs/img22.png] are in [figs/img20.png] and [figs/img23.png], 
then [figs/img24.png].
Figure 1(a) illustrates a perfect hash function.
Since no collisions occur, each key can be retrieved from the table
with a single probe.
If [figs/img25.png], that is, the table has the same size as [figs/img20.png],
then we say that [figs/img7.png] is a //minimal perfect hash function// 
for [figs/img20.png].
Figure 1(b) illustrates a minimal perfect hash function.
Minimal perfect hash functions totally avoid the problem of wasted
space and time. A perfect hash function [figs/img7.png] is //order preserving// 
if the keys in [figs/img20.png] are arranged in some given order 
and [figs/img7.png] preserves this order in the hash table.

 |                [figs/img26.png]
 | **Figure 1:** (a) Perfect hash function. (b) Minimal perfect hash function.

Minimal perfect hash functions are widely used for memory efficient
storage and fast retrieval of items from static sets, such as words in natural
languages, reserved words in programming languages or interactive systems,
universal resource locations (URLs) in Web search engines, or item sets in
data mining techniques.

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