CPSC 335

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CPSC 335

Dr. Marina Gavrilova Computer Science University
of Calgary Canada
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Outline

• Perfect Hashing
• Cichellis Algorithm

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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.

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Perfect Hashing
Process of creating a perfect hash function A
general form of a perfect hashing function is
p.hash (key) (h0(key) gh1(key)
gh2(key) mod N
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Cichellis Algorithm

• In Cichellis 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 Cichellis algorithm
  is determining T.

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Cichellis Algorithm
Table 1 Values associated with the characters of
the Pascal reserved words

• When we apply the Cichellis 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.

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Cichellis Algorithm
Cichellis algorithm for determining a perfect
hash function
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Thank You