![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])
return hash](https://image.slidesharecdn.com/hashing-130320085717-phpapp02/85/Hashing-8-320.jpg)
Hashing
- 1. Hashing
- 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
- 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?
- 4. Applications of hashing • File management – working out where to store records • Comparing complex values • Cryptography – creating digital signatures – eg: md5
- 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
- 6. Closed Hashing 1 23 32 44 End 2 3 4 33 Hash table is supplemented by 5 a linked list, which is used to store colliding entries. 6 7 Therefore, some values are found outside of the standard hash table (in the linked list)
- 7. ‘Open’ Hashing Some strategy is used to fit colliding entries in a predictable way inside the 1 23 existing table 2 32 3 44 For this to work, the size of 4 33 the table needs to be 5 significantly bigger than the total number of records 6 7 At least 3:2
- 8. 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]) return hash