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Hashing







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Hashing. What is hashing. Simply, generating a numeric key using an algorithm (hash function) Definition: A function that maps keys to integers, usually to get an even distribution on a smaller set of values. The very simplest hash function is to use the modulus operator %
Hashing

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Slide 1

Hashing

Slide 2

What is hashing

  • Simply, generating a numeric key using an algorithm (hash function)

  • Definition: A function that maps keys to integers, usually to get an even distribution on a smaller set of values.

  • The very simplest hash function is to use the modulus operator %

  • Input range % key range

Slide 3

Input and key range

  • Example. We want to store 7 digit telephone numbers so that they can be quickly retrieved.

    • Number of expected entries = 100

    • Range of telephone numbers = 0 – 9999999

      Simple hashing algorithm

      hash = inputNumber % 100

      What’s the effect?

Slide 4

Applications of hashing

  • File management – working out where to store records

  • Comparing complex values

  • Cryptography – creating digital signatures – eg: md5

Slide 5

Collisions

  • Where the hash value returned for two keys is the same.

  • What to do?

    • Open hashing

    • Closed hashing

    • Deleting

  • The 2/3rds rule

Slide 6

DJB Hash function

  • “An algorithm produced by Professor Daniel J. Bernstein and shown first to the world on the usenet newsgroup comp.lang.c. It is one of the most efficient hash functions ever published. “

    def DJBHash(key):

    hash = 5381

    for i in range(len(key)):

    hash = ((hash << 5) + hash) + ord(key[i])

    returnhash


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