1. Dr. Marina Gavrilova CPSC 335 Computer Science University of Calgary
Canada

2. Outline
Perfect Hashing
Cichelli’s Algorithm

3. Perfect Hashing
A perfect hashing function maps a key into a unique address. If the range of potential addresses is the same as the number of keys, the function is a minimal (in space) perfect hashing function.
What makes perfect hashing distinctive is that it is a process for mapping a key space to a unique address in a smaller address space, that is
hash (key) unique address
Not only does a perfect hashing function improve retrieval performance, but a minimal perfect hashing function would provide 100 percent storage utilization.

4. Perfect Hashing Process of creating a perfect hash function
A general form of a perfect hashing function is:
p.hash (key) =(h0(key) + g[h1(key)] + g[h2(key)] mod N

5. Cichelli’s Algorithm
In Cichelli’s algorithm, the component functions are:
h0 = length (key)
h1 = first_character (key)
h2 = last_character (key)
and g = T (x)
where T is the table of values associated with individual characters x which may apply in a key.
The time consuming part of Cichelli’s algorithm is determining T.

6. Cichelli’s Algorithm
Table 1: Values associated with the characters of the Pascal reserved words
When we apply the Cichelli’s perfect hashing function to the keyword begin using table 1, we can get –
The keyword begin would be stored in location 33. Since the hash values run from 2 through 37 for this set of data, the hash function is a minimal perfect hashing function.

7. Cichelli’s Algorithm
Cichelli’s algorithm for determining a perfect hash function

8. Thank You