Wikipedia
A hash function is any function that can be used to map data of arbitrary size to fixed-size values The values returned by a hash function are called hash values, hash codes, digests, or simply hashes The values are usually used to index a fixed-size table called a hash table Use of a hash function to index a hash table is called hashing or scatter storage addressing Hash functions and their associated hash tables are used in data storage and retrieval applications to access data in a small and nearly constant time per retrieval, and require an amount of storage space only fractionally greater than the total space required for the data or records themselves
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