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Hashing

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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

- 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?

- File management – working out where to store records
- Comparing complex values
- Cryptography – creating digital signatures – eg: md5

- Where the hash value returned for two keys is the same.
- What to do?
- Open hashing
- Closed hashing
- Deleting

- The 2/3rds rule

- “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